Recent advances in li metal anode protection for high performance lithium-sulfur batteries

Abstract

Lithium-sulfur batteries (LSBs) have garnered significant attention as a promising next-generation rechargeable battery, offering superior energy density and cost-effectiveness. However, the commercialization of LSBs faces several challenges, including the ionic/electronic insulating nature of the active materials, lithium polysulfide (LiPS) shuttle effect, volume expansion/contraction of the cathode, and issues with Li metal anode. Despite numerous efforts to address these challenges, previous studies have predominantly been conducted under mild conditions such as high electrolyte-to-sulfur (E/S) ratio, low sulfur loading, and excess Li metal, which cover issues related to Li metal anode. However, for realizing high-energy–density LSBs, practical conditions such as low E/S ratio, high sulfur loading, and limited Li metal are essential. Under these conditions, the increased current on Li metal and higher LiPS concentration exacerbate issues with Li metal anode such as dendrite growth, dead Li, high reactivity with electrolyte, and high reactivity with LiPSs. These problems lead to rapid failure of Li metal, significantly impacting the electrochemical performance of LSBs. Consequently, protecting Li metal anode is crucial for the practical LSBs. This paper introduces the challenges associated with Li metal anode in LSBs and reviews research focused on protecting Li metal anode in each battery component: anode, electrolyte, cathode, and separator/interlayer. Finally, we discuss future research directions of each component towards practical LSBs.

Graphical Abstract

1 Introduction

The depletion of fossil fuels and the need for carbon neutrality has significantly intensified the interest in sustainable electrical energy conversion and storage devices [1, 2]. Over the past three decades, Li-ion batteries (LIBs) based on intercalated composite cathodes and anodes have been widely used as rechargeable batteries, thus dominating the battery market [3, 4]. However, the theoretical capacity of cathode materials such as LiCoO₂ and LiFePO₄ used in LIBs is around 200 mAh g⁻¹, and they are currently approaching their limits [5]. Consequently, with the expanding market for electric vehicles (EVs) and energy storage systems (ESSs), there has been a recent surge in research interest in high-energy–density next-generation rechargeable batteries [6,7,8,9,10,11,12]. Lithium-sulfur batteries (LSBs) utilize sulfur as the cathode material, exhibiting a specific capacity of 1675 mAh g⁻¹, and Li metal as the anode material with an ultrahigh theoretical capacity of 3860 mAh g⁻¹, which makes LSBs a promising candidate for achieving an energy density of LSB cells above 500 Wh kg⁻¹ [13,14,15]. Furthermore, sulfur is cost-effective and environmentally friendly, making LSBs competitive with the next-generation rechargeable batteries [16, 17].

Despite these advantages, LSBs face several intrinsic challenges that hinder their commercialization due to: (1) The ionic/electronic insulating nature of elemental sulfur (S₈) and the final discharge products, lithium sulfides (Li₂S₂/Li₂S), leads to retarded kinetics and limited sulfur utilization [18, 19]; (2) 80% volume changes at the cathode during charging and discharging lead to the detachment of active materials from the cathode, resulting in capacity decay in LSBs [20, 21]; (3) Lithium polysulfide (LiPS) intermediates dissolve in the electrolyte, inducing LiPS shuttle effects that reduce Coulombic efficiency (CE) [22, 23]; 4) The high reactivity of Li metal anodes with the electrolyte and LiPSs results in rapid electrolyte depletion and unstable electrolyte-anode interfaces [24, 25]. Over the past decade, research on LSBs has primarily focused on addressing issues related to cathode active materials, such as LiPS shuttle effects, volume change, and the insulating properties of the cathode [26,27,28,29,30,31]. However, most of these studies have been conducted under ideal conditions, such as low sulfur loading (~ 1.5 mg cm⁻²), high electrolyte-to-sulfur (E/S) ratio (> 10 µL mg⁻¹), and excess Li metal anode (> 200 µm) [32, 33]. Under these conditions, the issues associated with Li metal anodes are covered, allowing LSBs to exhibit minimal capacity decay (> 95%) even after several hundred of cycles (~ 1000 cycles) [34, 35]. However, such conditions are not conducive to exploiting the high-energy–density advantages of LSBs.

To achieve LSBs with an energy density of beyond 500 Wh kg⁻¹, practical conditions of high sulfur loading (> 5.0 mg cm⁻²), low E/S ratio (< 3.0 µL mg⁻¹), and limited Li anode (< 50 µm) are essential [36]. Under such conditions, the issues with Li metal are exacerbated, leading to a significant capacity decay within 100 cycles [37]. Moreover, the morphology of Li after cycling under practical conditions reveals a severely uneven surface, in stark contrast to that observed under mild conditions. This indicates that Li metal is a critical factor influencing the performance of LSBs under practical conditions. The prominence of Li metal issues under harsh conditions can be summarized as follows. (1) With increased sulfur loading, the amplified current imposed on Li metal results in uneven Li deposition/stripping and substantial volumetric changes [38,39,40]. (2) A high concentration of LiPSs (> 6.0 M [S] species) in the electrolyte driven by high sulfur loading and low E/S ratio induces severe shuttle effects, promoting side reactions between Li and LiPSs [41, 42]. (3) With a low negative-to-positive electrode ratio (N/P ratio < 2) and E/S ratio, limited amounts of Li and the electrolyte cannot withstand continuous side reactions, leading to rapid cell failure [43, 44]. These issues, coupled with the intrinsic challenges of Li metal, such as dendrite growth and the formation of dead Li, synergistically contribute to the rapid capacity decay and premature cell failure [45, 46].

Consequently, to realize high-energy–density LSBs, it is necessary to focus on protecting Li metal. This paper discusses the issues related to Li metal in LSBs and reviews various strategies for protecting Li metal across battery components: anodes, electrolytes, cathodes, and separators/interlayers. Finally, we present research directions for each component to realize practical LSBs.

2 Electrochemistry of LSBs

Typical LSBs consist of an elemental sulfur cathode, a Li metal anode, a separator, and an ether-based electrolyte (primarily 1 M lithium bis (trifluoromethanesulfonyl) imide (LiTFSI) in DOL (1,3-dioxolane) / DME (1,2-dimethoxyethane) (1:1, by volume) with 2 wt. % LiNO₃), and operate based on the conversion reactions between sulfur and Li (Fig. 1a) [47]. During discharging, elemental sulfur reacts with Li⁺ to form lithium sulfide (Li₂S), which decomposes back into Li and sulfur during charging (Eq. (1)). This reaction mechanism, which involves the participation of 16 electrons, endows LSBs with a theoretical capacity of 1675 mAh g⁻¹, significantly exceeding the capacity of cathode materials used in LIBs by over five times [48, 49].

$${\text{S}}_{{8\left( {\text{s}} \right)}} + 16{\text{ Li}}^{ + } + 16{\text{ e}}^{ - } \leftrightarrow 8{\text{ Li}}_{2} {\text{S}}_{{\left( {\text{s}} \right)}}$$

(1)

Fig. 1

a Schematic representation of conventional LSBs (Reprinted with permission from [47], 2021, Wiley–VCH). b Typical charging and discharging voltage profile of LSBs (Reprinted with permission from [5], 2020, Wiley–VCH)

In LSBs, the electrochemical processes are considerably complex. During the discharge process, elemental sulfur transforms into a long-chain LiPS intermediate (Li₂S_x, 4 ≤ x ≤ 8) which is soluble in ether-based solvents and is eventually converted into lithium sulfides (Li₂S₂/Li₂S), following a “solid–liquid-solid” conversion mechanism [5]. The transition from sulfur to various sulfur compounds, including insoluble lithium sulfides and soluble LiPSs, plays a critical role in defining the electrochemical characteristics of LSBs [50]. The discharge mechanism of the LSBs can be categorized into four distinct stages (Fig. 1b).

In stage I, solid sulfur reacts with Li and electrons from the Li metal anode to form soluble long-chain Li₂S₈ via a solid–liquid reaction (Eq. (2)). This reaction occurs at a relatively high voltage of approximately 2.3 V, and is characterized by a slightly sloping plateau. Here, elemental sulfur in the cathode dissolves in Li₂S₈, causing the cathode to become porous, leading to volumetric contraction [51].

$${\text{S}}_{{8\left( {\text{s}} \right)}} + 2{\text{ Li}}^{ + } + 2{\text{ e}}^{ - } \to {\text{Li}}_{2} {\text{S}}_{{8\left( {\text{l}} \right)}}$$

(2)

The stage II involves the continuous reduction of Li₂S₈ to a liquid-state Li₂S₄ (Eqs. (3) and (4)), which constitute a “liquid–liquid” reaction. This stage leads to an increase in the electrolyte viscosity owing to the increasing concentration of LiPSs in the electrolyte, resulting in a voltage peak at the end of the second stage owing to an increased overpotential [52]. Stages I and II collectively contributed one-fourth (419 mAh g⁻¹) of the total theoretical capacity of the LSBs.

$$3{\text{ Li}}_{2} {\text{S}}_{{8\left( {\text{l}} \right)}} + 2{\text{ Li}}^{ + } + 2{\text{ e}}^{ - } \to 4{\text{ Li}}_{2} {\text{S}}_{{6\left( {\text{l}} \right)}}$$

(3)

$$2{\text{ Li}}_{2} {\text{S}}_{{6\left( {\text{l}} \right)}} + 2{\text{ Li}}^{ + } + 2{\text{ e}}^{ - } \to 3{\text{ Li}}_{2} {\text{S}}_{{4\left( {\text{l}} \right)}}$$

(4)

In the stage III, liquid Li₂S₄ is reduced to solid-state lithium sulfides (Li₂S₂/Li₂S), following a “liquid–solid” reaction (Eqs. (5) and (6)). This process forms a long second plateau in the discharge curve, occurring around 2.1 V.

$${\text{Li}}_{2} {\text{S}}_{{4\left( {\text{l}} \right)}} + 2{\text{ Li}}^{ + } + 2{\text{ e}}^{ - } \to 2{\text{ Li}}_{2} {\text{S}}_{{2\left( {\text{s}} \right)}}$$

(5)

$${\text{Li}}_{2} {\text{S}}_{{4\left( {\text{l}} \right)}} + 6{\text{ Li}}^{ + } + 6{\text{ e}}^{ - } \to 4{\text{ Li}}_{2} {\text{S}}_{{\left( {\text{s}} \right)}}$$

(6)

Stage IV involves the transformation of residual Li₂S₂ into the final product, Li₂S, which is characterized by a “solid–solid” reaction (Eq. (7)). Consequently, this stage is associated with high overpotential and slow kinetics, leading to a rapid voltage drop [53]. Stages III and IV contributed three-fourths (1256 mAh g⁻¹) to the total capacity.

$${\text{Li}}_{2} {\text{S}}_{{2\left( {\text{s}} \right)}} + 2{\text{ Li}}^{ + } + 2{\text{ e}}^{ - } \to 2{\text{ Li}}_{2} {\text{S}}_{{\left( {\text{s}} \right)}}$$

(7)

During charging, the reverse reaction of the discharge process occurs: solid-state Li₂S is oxidized, going through intermediate soluble LiPSs, and ultimately converting back to S₈. The small voltage peak observed in the initial part of charging was attributed to the overpotential occurring during the decomposition of Li₂S [54].

3 Challenges of Li metal anode in LSBs

The use of Li metal anodes presents various inherent challenges, including Li dendrite formation, dead Li, uneven surface morphology, electrolyte depletion, and gassing problems. These issues become more pronounced when LiPSs dissolve in the electrolyte in LSBs, thereby diminishing the cycle stability and safety (Fig. 2a). Furthermore, as discussed previously, Li degradation occurs more rapidly and severely in LSBs under practical conditions. This section introduces the problems associated with Li metal in LSBs and categorizes them into three main types: Li dendrites & dead Li, high reactivity with electrolyte & LiPSs.

Fig. 2

a Schematic illustration of challenges of Li metal anode in LSBs. b Schematic diagram of Li dendrite stripping models: tip-stripping model, base-stripping model, and tip-/base-stripping model (Reprinted with permission from [63], 2021, John Wiley and Sons)

3.1 Li dendrite and dead Li

The formation of Li dendrites and dead Li is the most severe challenge to the stability and safety of Li metal anode [55,56,57]. Li dendrites are thin branching structures that can form on the surface of anodes, causing a low CE, capacity decay, and short circuits. These dendrites pose several challenges for the operation of LSBs. First, compared with Li with flat surfaces, Li dendrites have greater specific surface areas, inducing undesirable reactions at the surface. These side reactions consume electrolytes and metallic Li, resulting in poor cycle life of LSBs. Second, repeated side reactions can destroy the dendritic structure, causing Li to lose contact with the anode. When Li⁺ lose contact, they are unable to participate in the electrochemical reaction, becoming “dead Li” which cannot return to the anode [55]. Finally, the Li dendrites can penetrate the polymer separator and reach the cathode, causing safety hazards in short circuits [58]. More dangerously, short circuits typically happen with battery thermal runaway, causing spontaneous combustion and explosions.

Under electrodeposition conditions, dendrite growth is an unavoidable phenomenon for metal anodes such as Li, Zn, Cu, Ag, and other metals due to their 'host-less' property, unlike graphite anode. The “Space Charge Model” theory, proposed by Chazalviel in 1990, revealed how the rapid depletion of ions near the electrode surface causes space charge and electric field to arise in the electrode/electrolyte interface, accelerating the deposit of massive Li⁺ [59]. This phenomenon is related to the gradient in the Li⁺ concentration between the two electrodes during the electroplating process, which may be increased by the applied current, as shown in Eq. (8).

$$\frac{\partial {\text{C}}}{\partial {\text{x}}}\left({\text{x}}\right)= \frac{{\text{J}}{\upmu }_{{\text{a}}}}{{\text{eD}}({\upmu }_{{\text{a}}}+{\upmu }_{{{\text{Li}}}^{+}})}$$

(8)

where J referred to the current density, e is the elementary charge, D is the ion diffusion coefficient, ${\upmu }_{{\text{a}}}$ and ${\upmu }_{{{\text{Li}}}^{+}}$ are the mobility of anion and Li⁺, respectively.

The ionic concentration gradients remained constant when the concentration gradient was less than 2C₀/L, resulting in smooth deposition. However, when the gradient exceeded 2C₀/L, the formation of a local space charge eventually destroyed the potential balance on the electrode surface after a certain period. This period is the “Sands Time, τ” [60]:

$$\uptau =\mathrm{ \pi D}{(\frac{{{\text{C}}}_{0}{\text{e}}}{2{{\text{Jt}}}_{{\text{a}}}})}^{2}$$

(9)

$${{\text{t}}}_{{\text{a}}}=\frac{{\upmu }_{{\text{a}}}}{{\upmu }_{{\text{a}}}+{\upmu }_{{{\text{Li}}}^{+}}}$$

(10)

where C₀ is the initial concentration of Li salts, L is the distance between the electrodes, and t_a is the transference number of anions. In LSBs, where high current is applied to the Li metal anode due to the high capacity of the cathode active material, the Sands time decreases according to Eq. (9), which leads to an environment that exacerbates dendrite formation.

The Li dendrites formed by this mechanism can break during cycling and form dead Li. Dead Li is defined as Li that has lost contact with the electrode and is therefore unable to participate in electrochemical reaction [56, 61, 62]. This hindered the transport of Li and decreased the CE, resulting in a poor cycle life. Dead Li is formed not only by broken dendrites but also by uneven current distribution during the stripping process. As LSB are systems that start from the stripping process, unlike LIB, it is crucial to prevent the formation of dead Li during discharge. Figure 2b shows the formation of dead Li during the stripping [63]. The Li metal is electrochemically oxidized to Li⁺ during stripping, which departs from the anode and moves through the solid electrolyte interphase (SEI) layer to the bulk electrolyte. Thus, when controlling the stripping process, it is crucial to consider the rates of Li⁺ diffusion inside the anode, ionic diffusion through the SEI layer, and reaction at the SEI-electrolyte interface. However, in LSBs, LiPSs are rapidly reduced at the anode to form an SEI layer composed of components with low Li⁺ conductivity, such as Li₂S₂/Li₂S (1.90 × 10^–26 S cm⁻¹), which makes uniform stripping challenging. During the stripping process, stripping occurs locally at defect sites within the insulating layer, resulting in the formation of pits on the Li surface. In subsequent plating, electric charges are higher in areas of high curvature, leading to Li nucleation and growth within the pit mouth [64, 65]. This mechanism results in a porous Li structure. Several studies have reported that this pitting mechanism significantly induces the formation of dead Li and Li dendrite growth [66, 67].

As shown in Fig. 2b, three dissolving sites–tip, base, and combined tip/base–were used to discuss the stripping electrochemistry. In the tip-stripping model, Li dendrites maintain good contact with the substrate during the stripping process, preventing the formation of dead Li. This is ideal for dendritic stripping. However, the base-stripping model is a typical phenomenon because greater curvature at the dendrite neck tends to accumulate more electron densities, leading to faster rates of Li dissolution. After dendritic breakage, the electrolyte corrodes the newly exposed Li surface, resulting in the accumulation of a SEI layer with low electrical conductivity around the broken Li. Consequently, Li is no longer electrically connected to the anode and cannot participate in the electrochemical reaction. Additionally, based on the tip-/base-stripping model, the dendritic tip and base are considered to be the active areas for dendritic dissolution, which also results in dead Li.

3.2 High reactivity with electrolyte and LiPSs

The ultralow reduction potential (-3.04 V vs. standard hydrogen electrode (SHE)) of the Li metal anode makes it highly reactive with electrolyte. In 1971, Dey made the initial study of the thin film formed through the reaction of electrolyte and Li [68], and Peled described it as “SEI” in 1979 [69]. Goodenough et al. clarified the link between the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) of electrolytes and SEI production on electrodes [70]. The chemical potential of Li metal is well above the LUMO of practical organic electrolyte solvent molecules and anions. Within milliseconds or less, a significant reduction process between the free electrons in Li and the electrolyte begins when bare Li is exposed to the electrolyte. This severe parasitic reaction limits the CE of Li stripping/plating cycles by causing an irreversible loss of Li and the electrolyte. Moreover, the decomposition of DOL and DME generates gases such as CH₄ and H₂. Gas evolution causes the pouch cell to expand, resulting in safety issues [71, 72]. In conclusion, it is essential to suppress the reactivity of Li to improve cycle life by inducing a robust SEI layer.

Calendar aging due to corrosion also occurs because of the high reactivity of Li metal, which has also recently been reported to cause loss of Li metal and shorten its cycle life [73,74,75]. In the Li metal anode, unlike the graphite anode, each electrodeposition cycle exposes fresh Li metal surfaces. Consequently, the SEI layer was reformed at each cycle so that the potential of the Li metal was 0 V vs. Li/Li⁺ in the open circuit. This makes calendar aging of the Li metal anode critical. It has been reported that Li metal usually experiences a loss of approximately 2–3% of its capacity after 24 h of calendar aging [75]. Especially in an environment with a limited amount of electrolyte and Li at the practical level of the pouch cell, it is critical to suppress calendar aging to improve stability.

In LSBs, the LiPS intermediates are soluble in ether-based electrolytes. During battery operation, high-order LiPSs dissolve from the cathode and diffuse toward the anode owing to concentration gradients [76]. Owing to the strong reductive capability of Li metal and the oxidative nature of LiPSs, spontaneous reduction of LiPSs occurs at the anode, forming soluble low-order LiPSs or insoluble Li₂S₂/Li₂S which are deposited on the Li metal [77]. The produced low-order LiPSs subsequently diffuse back to the cathode, a process known as the “LiPS shuttle effect” [78]. The phenomenon of soluble LiPSs migrating between the sulfur cathode and Li metal anode leads to substantial degradation of the Li metal anode through severe corrosion, as well as a notable reduction in the active materials, making “dead sulfur” [79]. In an electrolyte composed of DOL, DME, and LiTFSI, LiPSs preferentially react with Li compared to the other components, as evidenced by the calculation of the Gibbs free energy change [80]. This led to the formation of a Li₂S₂/Li₂S-rich SEI layer on the Li surface. This phenomenon is more pronounced under practical conditions of high sulfur loading and low E/S ratios, where the concentration of LiPSs in the electrolyte increases dramatically.

However, Li₂S₂/Li₂S, with their low ionic conductivity (1.90 × 10^–26 S cm⁻¹), are inadequate as SEI layer components, increasing interfacial resistance due to their inability to facilitate Li⁺ transfer [81]. Therefore, Li stripping predominantly occurs at the defect sites of the insulating layers, forming uneven and discrete pits, a situation exacerbated under a high current density driven by high sulfur loading [82]. During the subsequent charging process, LiPSs react with Li dendrites to form mossy Li [83]. The increased surface area of this morphology leads to the formation of a thin SEI layer, allowing easy penetration of LiPSs and reaction with the newly plated Li [36]. In summary, the presence of LiPSs in the electrolyte intensifies the issues of dendrite growth, dead Li, electrolyte decomposition, and gas evolution, as discussed in Sects.  3. 1 and 3. 2. This contributes to the formation of a new Li₂S₂/Li₂S layer, perpetuating a vicious cycle, and ultimately leading to the rapid failure of the Li metal. Moreover, the reaction of LiPSs at the anode contributes to self-discharge, leading to a reduction in the open-circuit voltage (OCV) and capacity decay [84].

To summarize, in LSBs, high current density and the formation of Li₂S₂/Li₂S-rich SEI layer from the reduction of LiPSs on the Li metal anode lead to severely uneven Li stripping/plating. Consequently, substantial Li dendrite growth and the formation of dead Li occur at the anode. Additionally, in LSBs where Li stripping occurs first, the Li structure becomes porous. This leads to a mossy Li surface, continuously exposing new surfaces that react with the electrolyte and LiPSs, resulting in gassing problems, depletion of electrolyte and Li, continuous loss of active material, and further uneven Li growth again. These phenomena are especially pronounced under the practical conditions of LSBs, characterized by low E/S and N/P ratios, ultimately accelerating cell failure. Therefore, protecting the Li metal is essential for the commercialization of LSBs. Strategies for protecting the Li metal include suppressing the LiPS shuttle effect and directly stabilizing it [85,86,87]. In the following section, we review the various strategies employed for each part (anode, electrolyte, cathode, and separator/interlayer) of the battery and present research directions for each component toward achieving LSBs with high-energy–density and stability.

4 Li metal anode engineering

The most direct approach for addressing anode-related issues in LSBs is to engineer Li metal anode [88,89,90,91]. Previous studies have primarily focused on two strategies: (1) introducing an artificial SEI layer and (2) incorporating a host material for Li. These methodologies aim to proactively tailor Li metal anodes, mitigate dendrite formation, and minimize their reactivity with LiPSs. By implementing these strategies, advancements can be made toward achieving enhanced the LSB performance. In addition, studies are currently being conducted to implement a scalable process while maintaining thin Li metal anodes for practical LSB.

4.1 Artificial SEI layer

Introducing an artificial solid electrolyte interface (SEI) layer into LSBs is important for enhancing their stability and performance. This SEI layer serves as a pivotal factor influencing the electrochemical kinetics of the Li metal anode, ensuring uniformity in the process of Li plating and stripping during cycling. Its versatile functions include preventing undesirable reactions between the anodes and electrolytes, facilitating uniform Li deposition, preventing dendrite formation, and improving the overall cycle life. An optimal artificial SEI layer requires attributes such as high Li⁺ conductivity, robust mechanical properties, resistance against corrosion by LiPSs, and scalability [92,93,94]. To meet these requirements, researchers have explored a spectrum of materials with organic, inorganic, and composite compositions. An organic SEI layer is generally flexible and can adapt to anode volume changes, whereas an inorganic SEI layer is mechanically rigid and can improve surface stability. By combining inorganic and organic components in an appropriate structure, the synergistic effect of different components can achieve highly stable and safe Li metal anodes [95, 96].

4.1.1 Organic-based artificial SEI layer

Research and development of organic-based artificial SEI layers are playing a crucial role in enhancing the performance and safety of LSBs [90, 91, 97,98,99,100,101]. Various studies have demonstrated that organic-based SEI layers can enhance the stability of Li metal anodes, minimize side reactions, and prevent dendritic growth, thereby extending the battery's cycle life. The outlook for organic-based artificial SEI layers focuses on several key aspects including high ionic selectivity and conductivity, superior mechanical properties, minimization of side reactions with the electrolyte, mass production, cost efficiency, and environmental friendliness. Chen et al. proposed an innovative approach for LSBs by employing a covalent organic framework (COF) in situ prepared as a 10 nm thin film, uniformly coating a Li metal anode (COF-Li) to serve as an artificial SEI layer [98]. This COF layer, characterized by its rigidity and abundant 1.5 nm-sized micropores, effectively mitigates side reactions between Li metal and electrolytes by selectively binding Li⁺ (Fig. 3a). Furthermore, it facilitated the uniform deposition of Li⁺ and acted as a robust protective shield. The notably high Young’s modulus (6.8 GPa) of the COF film reinforced the stability of the artificial SEI layer under the complex stress and strain dynamics inherent in Li-electrolyte interactions. Consequently, the COF-Li symmetrical cell achieves stable cycling for 400 h at a current density of 1 mA cm⁻², and the COF-LSB runs steadily for 300 cycles without short circuit. COF-Li coating improved the performance and safety of LSBs, showing stable operation for 300 cycles without short-circuiting, while polished Li/S cells failed after 111 cycles due to internal short circuits. Furthermore, the COF-Li/S battery maintained around 89% of its initial capacity even after 300 cycles. Liu et al. developed an ion-selective interface on the Li metal (IS-Li) by constructing artificial macromolecules of Li isopropyl-sulfide [99]. This was achieved through the straightforward electrochemical polymerization of alkyl disulfide (ADS) in an ether-based electrolyte (Fig. 3b). The resulting polymer network enhanced elasticity and toughness, accommodating volumetric changes in the Li metal anode. Additionally, the formed Li- organosulfides provide significant mechanical strength to resist the growth of Li dendrites. These Li- organosulfides also reduced the diffusion of LiPSs toward the metallic Li, thereby preventing the reduction of LiPSs to isolated Li₂S on the Li surface. This indicates that the interfacial layer may both limit the transport of LiPS anions toward metallic Li and effectively prevent Li corrosion from the organic electrolyte and LiPSs. As a result, the IS-Li symmetric cells demonstrate a low overpotential (~ 28.2 mV) for 700 cycles at 2 mA cm⁻², and the LSBs with IS-Li demonstrate a high capacity retention of ~ 78.8% even after 550 cycles at 1 C.

Fig. 3

a Schematic illustration of the effect of artificial COF SEI layer (Reprinted with permission from [98], 2019, Wiley–VCH). b Schematic illustration of ion-selective interfacial layer (Reprinted with permission from [99], 2020, American Chemical Society). c Illustration of the production of LiPON-Li by using roll-to-roll system. d Performance of a LiPON-Li anode in LSBs. e Galvanostatic cycling performance of Li-S pouch cells at 0.79 mA cm^-2 (Reprinted with permission from [104], 2019, Elsevier). f Schematic illustration of tuned Li metal as an anode during electrochemical process in Li-S cells. g Cycling performance of Li-S pouch cells with different Li metal anodes (Reprinted with permission from [105], 2020, American Chemical Society). h Schematic diagram of the morphology changes of PMCN-SEI protected Li during cycling. i Electrochemical performance of LSBs using PMCN-SEI protected Li and pristine Li (Reprinted with permission from [109], 2023, Elsevier)

4.1.2 Inorganic-based artificial SEI layer

With respect to inorganic-based artificial SEI layers, research is primarily focused on the ability to exploit their exceptional ionic conductivity and chemical stability to improve battery performance [101,102,103,104,105]. Current trends and advances focus on the use of nanoscale inorganic coating techniques to improve the uniformity and adhesion of the SEI layer, which in turn increases the ionic conductivity and reduces side reactions with electrolytes. In addition, there is a growing interest in the design of multilayer SEI structures composed of different materials, as opposed to single-material SEI layers. These multilayer structures aim to perform specific functions within each layer to collectively improve overall battery performance. Wang et al. presented a novel technique to create a dense and uniform lithium phosphorus oxynitride (LiPON) coating on Li metal surfaces [104]. This deposition process allows for the rapid coating of LiPON at an impressive rate of up to 660 nm min⁻¹, achieved through nitrogen plasma-assisted electron-beam reaction deposition (Fig. 3c). The resultant LiPON-coated Li establishes a highly ion-conductive (1.2 × 10^–7 S cm⁻¹), chemically stable, and mechanically resilient protective layer. This layer effectively suppresses corrosion reactions with soluble LiPSs or electrolytes, while facilitating consistent Li plating and stripping dynamics (Fig. 3d). The symmetric LiPON-coated Li metal cells demonstrated durable and non-dendritic cycling for over 900 cycles at a current density of 3 mA cm⁻². Furthermore, a Li–S pouch cell using LiPON-coated Li was obtained with a specific energy density of approximately 300 Wh kg⁻¹, maintaining a relatively stable CE of around 91%. It exhibited an extended cycle life, retaining over 1.0 Ah capacity of over 120 cycles (Fig. 3e). Luo et al. presented a scalable method for fabricating a tuned Li anode with an in-situ formed LiF-dominant coating, marking the first successful application of such a method in LSBs [105]. Ammonium fluoride (NH₄F) was used as a fluorine source for a LiF dominant artificial SEI layer. This layer acts as a mechanically and chemically stable, ionically conductive interface layer on the Li metal surfaces. In LSBs using tuned Li, it was confirmed through the scanning electron microscopy (SEM) images that corrosion and interfacial degradation of Li anode were significantly reduced. The coating serves as a chemically stable interfacial layer that prevents attack from the electrolyte/LiPSs and it also functions as an ionic conductor that facilitates smooth Li⁺ flux and regulates uniform Li deposition (Fig. 3f). Consequently, a Li–S pouch cell with a high-loading sulfur and tuned Li anode maintained a capacity of 680 mAh g⁻¹ after 45 cycles, with a high discharge capacity of 136 mAh (Fig. 3g).

4.1.3 Composite artificial SEI layer

Composite artificial SEI layers are considered a significant advancement in improving the performance and stability of Li-based battery technologies, especially for LSBs and Li metal batteries [106,107,108,109]. These composite SEI layers combine the strengths of both organic and inorganic materials to address several issues in batteries through an innovative approach. However, the scalability and massive production of composite artificial SEI layers still face technological and economic challenges. Research and development are crucial to overcome these challenges. Jin et al. proposed a new dual-layered artificial SEI on a Li metal anode [108]. The SEI configuration comprises an upper organic lithiated Nafion layer and a lower inorganic Li_xSiS_y layer, termed LNF (Li_xSiS_y/Nafion). The flexible Nafion layer impedes side reactions between soluble LiPSs and the Li metal anode while preserving the structural integrity of the SEI layer. Conversely, a rigid inorganic layer is beneficial for Li⁺ diffusion and effectively curtails the growth of Li dendrites. Consequently, LSBs with an LNF-Li anode exhibit enhanced cycling stability, retaining a capacity of 783 mAh g⁻¹ after 300 cycles at 0.5 C, and exhibiting improved rate performance with 789 mAh g⁻¹ at 2.0 C. Lu et al. developed a dual-function PMMA/PPC/LiNO₃ composite as an artificial SEI (PMCN-SEI) [109]. The SEI provides multiple C = O sites that anchor LiPSs, effectively preventing the corrosion of the Li metal anode, as illustrated in Fig. 3h. The presence of lithiated polymer groups and Li₃N within the PMCN-SEI contributed to the more uniform deposition of Li⁺, consequently preventing dendrite formation on the anode. As a result, the PMCN-SEI-shielded Li metal anode enabled Li||Li symmetric cells to maintain stability over 300 cycles at a capacity of 5 mAh cm⁻². Furthermore, LSBs employing the PMCN-SEI structure delivered an initial capacity of 1166 mAh g⁻¹ at 0.5 C and the capacity was maintained at 500 mAh g⁻¹ after 200 cycles (Fig. 3i).

4.2 Host for uniform Li deposition

Regarding the challenge of volume fluctuations within the Li metal anode, a recent approach involves the incorporation of host architectures to accommodate the Li metal, thus reducing thickness variations during cycling [110, 111]. These structures can manifest as layered or three-dimensional (3D) frameworks or as Li-containing alloys. Notably, in the context of LSBs, considerable attention has been directed toward dual-host systems that accommodate both the Li metal anode and the sulfur cathode. This approach aimed to mitigate the formation of Li dendrites, suppress the shuttle effect, and enhance the kinetics of the sulfur conversion reaction.

4.2.1 Layered or three-dimensional frame structure

Layered or 3D framework compounds have emerged as viable strategies for circumventing issues such as irregular Li deposition, substantial volume fluctuations, and safety concerns. Typically, the benefits associated with Li alloy hosts stem from their high specific surface areas and interconnected structural configurations. The highly conductive surface of the host facilitates numerous electrochemical reaction sites, thereby enhancing charge transfer and subsequently lowering the overpotential. By decreasing the areal current density, a more uniform Li⁺ flux was achieved, which extended the duration of Li⁺ depletion at the Li surface. This, in line with Sand's time theory, is expected to inhibit the formation of Li dendrites, as discussed in Sect. 3. 1 [112].

Yue et al. developed an ultralight hollow 3D carbon skeleton derived from soybean oil using a straightforward chemical vapor deposition (CVD) technique [113]. Figure 4a illustrates the fabrication process of the resulting Li composite electrode (OCCu-Li). CVD employs soybean oil as a carbon source to coat a commercially available Ni foam skeleton, followed by the removal of metallic Ni through etching to form a freestanding 3D carbon skeleton (OC). Subsequent Cu electroplating generated a coralloid-shaped Cu layer, and a CuO layer emerged on the OC surface after heat treatment. Ultimately, molten Li was infused into the OC skeleton to fabricate the OC-supported Li composite electrode. The infused Li enabled the OCCu-Li electrode to achieve an areal mass of up to 22.3 mg cm⁻², suggesting a theoretical specific capacity of up to 3628 mAh g⁻¹, which is significantly higher than that of previously reported Li-composite electrodes. These findings demonstrate the efficacy of the copper oxide layer in enhancing the Li wettability of the carbon host, which is attributed to the interactions between the copper oxide and liquid Li. The OCCu-Li configuration exhibited exceptional cycling performance, sustaining over 400 cycles at a current density of 8 C within the LSBs, and demonstrated minimal thickness changes after 100 cycles at 1 C.

Fig. 4

a Schematic illustration of the fabrication process of OCCu-Li electrode (Reprinted with permission from [113], 2020, Elsevier). b Synthesis diagram of C-1 (upper) and C-2 (lower) polymer. c Schematic diagram of Li deposition on Cu, C-1, C-2 Cu (Reprinted with permission from [114], 2022, Wiley–VCH)

Wang et al. reported the synthesis of two different polymer materials, C-1 (linear chain) and C-2 (cross-linked), based on diethylene glycol monomethyl ether methacrylate (DGMEMA) and poly (ethylene glycol) dimethacrylate (PEGDMA), respectively, which were subsequently grafted onto Cu foils [114]. These polymers, crafted using radical polymerization techniques (Fig. 4b), exhibited different solvent absorption behaviors, which were attributed to the stable crosslinked network structure of C-2, imparting substantially lower solvent absorption than that of C-1. Experimental analyses via Proton Nuclear Magnetic Resonance (¹H NMR), Fourier Transform Infrared (FT-IR) spectroscopy, and thermogravimetric analysis verified the distinct interactions between DOL and the two polymers. Computational assessments revealed that C-1 possesses a higher capacity for DOL distribution than C-2. Consequently, the SEI layer formed on the C-2-modified Cu foil contained fewer inorganic components and a blend of rigid and flexible characteristics (Fig. 4c). In the LSBs employing C-2 modified Cu foil, a retained capacity exceeding 600 mA g⁻¹ after 300 cycles was observed, which formed the SEI layer with excellent performance, reduced the loss of active Li, and reduced the corrosion of Li metal anode by LiPSs. In contrast, the LSBs utilizing C-1 modified Cu foil retained a capacity above 400 mAh g⁻¹.

4.2.2 Li-containing alloys to host Li

Numerous metals exhibit reactivity toward Li, where Li⁺ can infiltrate metal crystals during charging, forming a lithiated alloy phase that effectively suppresses the growth of Li dendrites. The benefits associated with Li-containing alloy hosts are as follows [115].

(1) High Li⁺ diffusion coefficients within Li-containing alloy hosts facilitate rapid Li diffusion during stripping and plating processes [116].

(2) The inactive component within the Li-containing alloy hosts, featuring an interconnected continuous 3D network, maintained compositional stability and structural integrity even under considerable volume changes, thereby reducing the local current density.

(3) Li-containing alloys exhibit lower reactivity than Li metal because other materials in the alloy structure limit the chemical reaction of Li metal, preventing side reactions of Li with the electrolytes. This characteristic also enhances the air stability of Li metal anodes against oxidation and moisture [117].

(4) The preparation of Li alloy hosts involves a straightforward and simple process under moderate conditions, which is conducive to the large-scale application of Li metal anodes.

Kong et al. proposed a Li-rich lithium-magnesium (Li-Mg) alloy as a promising anode for LSBs [118]. The alloy developed a porous, interconnected structure through electrochemical dealloying. The presence of Mg in the SEI layer enhances its stability, thereby preserving a smooth surface morphology. After Li stripping, a conductive Li-poor Li-Mg alloy matrix emerged, exhibiting high electric and ionic conductivities, effectively serving as both an excellent current collector and a host for subsequent Li plating (Fig. 5a). This conductive Li-Mg alloy matrix contributes to the maintenance of the microstructural and bulk integrity during cycling. In addition, the surface morphologies and X-ray photoelectron spectroscopies (XPS) of the cycled Li-Mg alloys were significantly different from Li metal, suggesting that the Li-Mg alloys have higher resistance to electrolyte corrosion. As a result, LSBs featuring Li-Mg alloy anodes retained a discharge capacity of 606.5 mAh g⁻¹ after 200 cycles, surpassing those employing Li metal anodes, which delivered 433.6 mAh g⁻¹ at 0.1 C (Fig. 5b). Xia et al. deposited a conformal thin layer of Sn on the surface of Li metal using thermal evaporation [119]. This Sn coating layer acts as a protective barrier, protecting the Li metal anode from parasitic reactions while forming Li₅Sn₂ alloy to accommodate substantial volume changes (Fig. 5c). The presence of the Sn protective layer significantly facilitated rapid Li⁺ transport at the electrode/electrolyte interface, resulting in a low electrode potential of approximately 0.1 V vs. Li/Li⁺ and exerting negligible impact on the LSB's output voltage. LSBs employing Sn-coated Li foil anodes demonstrated exceptional cycling stability, exhibited low capacity decay, and achieved high CE of around ~ 99.5% over 500 cycles (Fig. 5d). Li et al. employed an in situ electrochemical reduction approach to fabricate a remarkably thin layer of LiGa alloy on the surface of Li metal, with the aim of providing uniform Li deposition sites that would mitigate side reactions with LiPSs and dendritic growth [120]. After using a chemical infiltration method to induce Ga³⁺ on the surface of Li metal, treated with gallium acetylacetonate as a source of Ga³⁺, a thin LiGa alloy layer was obtained through a charge/discharge reaction (Fig. 5e). This layer is referred to as LiGa and is characterized by its low resistance, minimal reactivity, and homogeneous sites for Li deposition. The LiGa layer effectively directs Li⁺ towards a three-dimensional deposition pattern across the metal surface. As a result, the energy density of a Li–S pouch cell with the LiGa-based anode was significantly enhanced, reaching an initial value of 136 Wh kg⁻¹, compared to only 98 Wh kg⁻¹ for the unmodified counterpart (Fig. 5f).

Fig. 5

a Schematic structure illustration in the bulk and at the surface for Li (upper) and Li-Mg alloy (lower) anodes during cycles. b Cycling stability of Li metal and Li-Mg alloy anodes at 0.1 C (Reprinted with permission from [118], 2019, Wiley–VCH). c Schematic illustration of cycling of LSBs with Sn-coated Li foil. d Long-term cycling performance of Sn coated LSBs at 2 C (Reprinted with permission from [119], 2020, Elsevier). e Schematic diagram showing the fabrication of the in situ LiGa alloy layer. f Cycling performance of the LiGa-based pouch cell at 1 C (Reprinted with permission from [120], 2023, Elsevier)

4.2.3 Dual host for anode and cathode

Recently, significant research efforts have focused on dual-function hosts for use as both the anode and cathode of LSBs, with the aim of enhancing the overall cell performance [121]. These host materials play a dual role: they induce uniform Li deposition at the anode and serve as catalysts at the cathode. Their function involves promoting the sulfur conversion reaction and effectively suppressing shuttle effects.

Yao et al. devised a dual-functional, flexible, free-standing carbon nanofiber conductive framework integrated in situ with TiN-VN heterostructures (TiN-VN@CNFs) serving as a host for both the Li metal anode and sulfur cathode concurrently, as depicted in Fig. 6a [122]. As an anode host, lithiophilic TiN-VN heterostructures notably reduce the Li nucleation overpotential, facilitating uniform Li deposition and effectively restraining the growth of Li dendrites. When employed as a cathode host, it offers robust adsorption capabilities and high conductivity, effectively curbing the shuttle effect and promoting efficient LiPS conversion. This dual functionality leads to heightened sulfur utilization and notably reversible Li stripping and plating, culminating in impressive rate capability (650 mAh g⁻¹ at 5 C) and extended cycle longevity exceeding 600 cycles, displaying a minimal capacity decay of only 0.051% per cycle within LSBs. In a configuration comprising S/TiN-VN@CNFs||Li/TiN-VN@CNFs, the LSBs exhibited a notable areal capacity of 5.5 mAh cm⁻² even when accommodating a sulfur loading of 5.6 mg cm⁻². Furthermore, the good mechanical flexibility of the working electrodes is demonstrated by the pouch cell tested under LED illumination.

Fig. 6

a Schematic illustration of the S/TiN-VN@CNFs cathode || Li-TiN-VN@CNFs anode battery configuration (Reprinted with permission from [122], 2019, Wiley–VCH). b Schematic illustration of the synthesis process of the NbC/Co ⊂ PCFs (Reprinted with permission from [123], 2020, Wiely-VCH)

Wei et al. introduced a conductive composite architecture comprising bio-derived N-doped porous carbon fiber bundles (N-PCFs) hosting cobalt and niobium carbide nanoparticles (NbC/Co ⊂ N-PCFs) as an integrated multifunctional platform designed to address challenges in both Li metal anode and sulfur cathode (Fig. 6b) [123]. This composite, which features synergistically enhanced electrical conductivity and surface polarization, incorporates highly conductive NbC and Co within a hierarchical porous N-PCF matrix. When employed as a host for Li-metal anodes, the composite displayed robust lithiophilicity and facilitated favorable adsorption, diffusion, and uniform Li deposition, resulting in a nondendritic morphology. Utilized as the host for the sulfur cathode, this conductive, porous, and polar architecture endows the NbC/Co ⊂ N-PCFs with a combined functionality involving three-dimensional physical confinement, robust chemical trapping, and electrocatalytic effects, fostering a high sulfur content of 78% and expediting the kinetics of conversion reaction. The LSBs with S@NbC/Co ⊂ N-PCFs and NbC/Co ⊂ N-PCFs@Li demonstrate remarkable stability in cycling performance, exhibiting a capacity retention of approximately 82.3% after 500 cycles at 0.5 C, while accommodating a sulfur loading of 3.4 mg cm⁻².

5 Electrolyte engineering

Organic liquid electrolytes, particularly those based on ethers, are widely employed in LSBs owing to their notable attributes, such as high ionic conductivity, favorable interfacial contact with electrodes, and minimal side reactions with Li. Acceptable solvents for LSB electrolytes are predominantly restricted to linear and cyclic ethers such as DME and DOL because other common electrolyte solvents such as esters, carbonates, and phosphates tend to react with LiPSs. In this context, linear DME exhibits heightened solubility and quicker reaction kinetics for LiPSs, but with greater reactivity toward Li metal. Conversely, cyclic DOL creates a more robust solid-electrolyte interface on the Li surface. Hence, combining DME and DOL results in synergistic effects on both the specific capacity and retention of sulfur, surpassing the performance of either solvent used individually [52, 124, 125]. However, the dissolution of intermediate LiPSs poses significant challenges, necessitating the incorporation of appropriate additives to safeguard the Li metal anodes or modify the structure of Li⁺ ions. Electrolyte engineering has emerged as a crucial approach for regulating the SEI layer, curbing dendrite formation on Li-metal anodes, and managing the interactions between LiPSs and Li metal. This section describes electrolyte engineering employing two key strategies: (1) manipulation of the SEI layer constituents and dendrite suppression via additive integration, and (2) diminishing the solubility of LiPSs within the electrolyte to mitigate its interaction with the anode.

5.1 Additives

Additives, typically comprising less than 5 to 10% by weight or volume, are incorporated into the electrolyte to enhance the cycling performance of cells. These components play an active role in SEI formation, effectively curbing the reactivity of the Li metal anode and mitigating the LiPS shuttling effect. Among these additives, LiNO₃ has received considerable attention for LSBs because of its ability to safeguard Li metal anodes by fostering the creation of a robust nitrogen-based SEI layer [126, 127]. Many additives are actively under investigation beyond LiNO₃, investigated to facilitate SEI formation and dendrite inhibition.

Li et al. introduced a straightforward and efficient approach to hinder the growth of Li dendrites on Li metal anodes by employing thionyl chloride (SOCl₂) as an electrolyte additive, facilitating the in situ formation of a stable interfacial protective layer containing LiCl and Li₂SO₃ [128]. This dense SEI layer established on the Li metal anode effectively curtailed the shuttling of LiPSs during cycling and also inhibited Li corrosion. Additionally, when the Li metal anode is shielded from the electrolyte by the LiCl-enriched surface film, SOCl₂ decomposes to generate active sulfur. This sulfur acts as a 'redox additive,' providing additional capacity to the cathode in the LSBs (Fig. 7a). Consequently, employing 2% SOCl₂ yielded LSBs showcasing notably high discharge capacity (2202.3 mAh g⁻¹ at 400 mA g⁻¹) and exceptional rate performance (1348.6 mAh g⁻¹ at 3000 mA g⁻¹), exhibiting significant cycling stability (Fig. 7b). Lian et al. introduced a novel multifunctional electrolyte additive, 1,4-benzenedithiols (BDT), for LSBs [129]. The thiol group within 1,4-BDT underwent oligomerization with sulfur, forming S–S bonds that modified the original sulfur redox pathway and impeded the shuttling of LiPSs. The formation of S–S bonds between thiol groups and sulfur offers several benefits for LSBs. These include stabilizing sulfur sites, activating dead sulfur atoms, improving electrochemical kinetics, and facilitating homogeneous chemical reactions. Furthermore, 1,4-BDT interacted with Li metal, resulting in the creation of a smooth and enduring SEI layer through subsequent reactions. The resultant organic Li salt Li₂-1,4-BDT in the electrolyte was deposited alongside other Li compounds on the Li metal anode, contributing to the SEI layer formation (Fig. 7c). This layer facilitates ion transfer while impeding electron flow, safeguarding the Li metal anode and ensuring stable cycling. Consequently, LSBs with covalently stabilized sulfur maintained a substantial capacity retention of 67.5% over 500 cycles, exhibiting a reversible capacity of 909.3 mAh g⁻¹. Also, the Li–S pouch cell shows an initial specific energy of 349 Wh kg⁻¹ and remains 298 Wh kg⁻¹ after 26 cycles at a discharge current of 300 mA (Fig. 7d).

Fig. 7

a Schematic illustration of anode protection and capacity enhancements by adding SOCl₂. b Long term cycling curves using electrolyte with or without SOCl₂ additives at 1600 mA g.⁻¹ (Reprinted with permission from [128], 2019, Elsevier). c Reaction mechanism in the initial discharge and recharge process with 1,4-BDT. d Cycling performance of the Li-S pouch cell with 1,4-BDT at a current of 300 mA (Reprinted with permission from [129], 2021, American Chemical Society). e Schematics of the formation of SEI with and without BTB additive f schematics of SEI preformation in the LiPS electrolyte (left) and the optical images of LiPS electrolyte with a Li foil after 0 and 16 h in the visualized test (right) blue: organosulfur containing SEI, red: routine SEI. g cycling performance of LSB at 0.1 C with and without BTB (Reprinted with permission from [131], 2020, Wiley–VCH). h Molecular orbital energies of the different solutes and solvents. i CE of Li metal anodes in LiTFSI/KPF₆ and schematic illustration of Li plating process in LiTFSI/KPF₆ (inner) (Reprinted with permission from [132], 2020, American Chemical Society)

In addition to manipulating the SEI layer components, safeguarding the Li metal anode against LiPSs or ensuring uniform Li distribution through electrostatic shielding is of paramount importance [130]. Wei et al. explored the use of 3,5-bis(trifluoromethyl)thiophenol (BTB) as an electrolyte additive to establish an organosulfur-containing SEI for LiPS shielding [131]. This specific SEI-containing organosulfur shields the Li metal anodes from detrimental side reactions with LiPSs, significantly enhancing the uniformity of Li plating and diminishing the overpotential during both the plating and stripping processes (Fig. 7e). The effectiveness of LiPS shielding was verified by performing SEI and visualizing the test (Fig. 7f). In practical application under specified conditions (4.5 mg_s cm⁻², 5.0 μL mg_s⁻¹, 50 μm Li), LSBs exhibited improved cycling stability, completing 82 cycles with an electrolyte containing 80 mM BTB in 1 M LiTFSI in DOL/DME (1:1, by volume) with 2% LiNO₃, in contrast to 42 cycles with a blank electrolyte (Fig. 7g). Li et al. highlighted the enhancement in the cycling stability of LSBs upon the introduction of 0.01 M potassium hexafluorophosphate (KPF₆) into a 2 M LiTFSI/ether-based electrolyte [132]. This modified electrolyte demonstrated sustained cycling performance beyond 200 cycles, achieving high Li utilization of up to 33.3%. This enhancement resulted from the combined effect of the self-healing electrostatic shield effect attributed to K⁺ cations and the formation of a LiF-rich SEI derived from PF₆⁻ anions. Notably, Density Functional Theory (DFT) calculations revealed the lower LUMO energy of KPF₆ compared to other electrolyte components, leading to the preferential decomposition of PF₆⁻ anions, thereby promoting the formation of an underlying SEI enriched in LiF (Fig. 7h). Simultaneously, the electrostatic shield provided by the K⁺ ions facilitates regulated Li⁺ distribution, ensuring uniform Li electrodeposition. Consequently, the cell employing the modified LiTFSI/KPF₆ electrolyte exhibited a high CE, exceeding 98.8% after 20 cycles, maintained stability over 200 cycles (Fig. 7i).

5.2 Sparingly solvating electrolytes

The inevitable diffusion of LiPSs driven by concentration gradients leads to the corrosion of the Li metal anode. Reducing the solubility of LiPSs in the electrolyte can effectively reduce the high reactivity between the Li metal anode and LiPSs, ultimately enhancing the CE. The solubility of LiPSs in an electrolyte is intricately linked to the solvent properties, particularly its polarity and Lewis basicity [133]. Solvents with lower dielectric constants or Gutmann donor numbers exhibit reduced polarity and weaker Lewis basicity, resulting in diminished coordination with Li⁺ in LiPSs [134]. Consequently, when the interaction force between the solvent and the LiPSs was weaker than the binding force within the LiPSs, the solubility of the LiPSs in the electrolyte decreased. Employing a solvent with a lower solubility for LiPSs or adjusting the solvation structure can effectively limit the dissolution of LiPSs, leading to enhanced stability of the Li metal anode.

5.2.1 Solvent engineering

To decrease the solubility of LiPSs in solvents, various strategies such as increasing the carbon-to-oxygen ratio (C/O ratio) [135,136,137,138,139], introducing steric hindrance, or incorporating electron-withdrawing groups into the solvent can be employed [140, 141]. Sun et al. demonstrated the effectiveness of a high C/O ratio for reducing the dielectric constant of a solvent, thereby yielding a less polar solvent [137]. They investigated ethers with high C/O ratios, namely methyl tert-butyl ether (MTBE, C/O = 5, ε_r = 4.38) and diisopropyl ether (DIPE, C/O = 6, ε_r = 3.88), used as co-solvents in a DOL/DME (1:1, by volume). The solubility of Li₂S₈ in DOL/DME was approximately 0.5 M, whereas in MTBE and DIPE, when they constituted 50% of the electrolyte, it reduced drastically to 20 mM and 4 mM, respectively (Fig. 8a). Furthermore, the linear isomer of MTBE, methyl butyl ether (MBE), can dissolve up to 39 mM of Li₂S₈, suggesting that the steric hindrance introduced by the alkyl groups hinders Li⁺ solvation. Employing an electrolyte composed of DME/DOL/DIPE (25:25:50, by volume) resulted in a 15% enhancement in the CE of the LSB compared of the reference. Analysis of the anode after cycling indicated diminished damage to the Li metal anode and the presence of reduced sulfur in the SEI layer, signifying the inhibition of the shuttle effect. Weller et al. utilized hexyl methyl ether (HME) as a co-solvent with DOL in LSBs [139]. UV–vis measurements exhibited that only 50 mM of Li₂S₈ dissolved in a 2 M LiTFSI HME/DOL (9:1, by volume) (Fig. 8b). This HME-containing electrolyte effectively restrained the dissolution of LiPSs and successfully suppressed detrimental LiPS shuttling.

Fig. 8

a Molecular structure of MTBE and DIPE (upper) and room temperature Li₂S₈ solubility vs the volume percentage of high C/O ratio ether cosolvents mixed with DOL/DME (1:1, by volume) (lower) (Reprinted with permission from [137], 2018, American Chemical Society). b Addition of 5 mM, 7.5 mM, 10 mM, 20 mM Li₂S₈ and saturated Li₂S₈ in 2 M LiTFSI in HME/DOL (9:1, by volume) after stirring overnight (Reprinted with permission from [139], 2019, Wiley–VCH). c LiPS solubility test in fluorinated electrolyte solvents: 1.0 M Li₂S₈ dissolved in electrolytes (Reprinted with permission from [143], 2015, American Chemical Society)

The incorporation of electron-withdrawing groups into solvents diminishes their Lewis basicity. Hydrofluoroethers (HFEs), which possess strong electronegativity and an electron-withdrawing effect, exhibit decreased affinity toward Li⁺, effectively suppressing the dissolution of LiPSs. The extent of fluorination and arrangement of the fluoroalkyl groups in the HFEs significantly influenced their solvating capacity for LiPSs. The inclusion of highly fluorinated HFEs exhibits enhanced capacity retention in contrast to less fluorinated HFE cosolvents [142]. This phenomenon arises from the increased capacity of the fluoroalkyl group to attract electrons, correlating with the higher degree of fluorination within the HFE molecules. Azimi et al. observed that increasing the content of 1,1,2,2-tetrafluorethyl-2,2,3,3-tetrafluoropropyl ether (TTE) led to a notable reduction in the solubility of LiPSs [143]. The solubility of Li₂S₈ in 1 M LiTFSI in TTE/DOL (1:1, by volume) was approximately 2 mM (Fig. 8c). Furthermore, the interaction between TTE and Li metal generates a LiF-rich SEI layer, effectively curbing the undesired reactions of dissolved LiPSs with Li metal. The electrolyte formulation of TTE/DOL (1:1, by volume) displayed a notably higher CE of 97% that of the conventional DOL/DME (1:1, by volume) electrolyte in LSBs. Similarly, Talian et al. investigated an electrolyte composition containing 1,2-(1,1,2,2-tetrafluoroethoxy)ethane (TFEE), a fluorinated ether, in combination with DOL to reduce LiPS solubility [144]. For 1 M LiTFSI in the TFEE/DOL electrolyte, the solubility of Li₂S₈ was less than 2 mM. In addition, the Li–S pouch cell with 1 M LiTFSI in TFEE/DOL (1:1, by volume) achieved capacities exceeding 1200 mAh g_s⁻¹ and a CE just below 97%, indeed without the use of any LiNO_3. Su et al. additionally highlighted that the positioning of fluorine atoms has a more substantial impact on solvation than the number of fluorine atoms [145]. The presence of both α- and β- substituted fluoroalkyl groups in HFEs resulted in their exhibiting the least Li solvating capacity and the highest tendency to mitigate the dissolution of LiPSs.

5.2.2 Adjusting the concentration of electrolytes and solvation structure

Unbound solvent molecules that are not involved in the solvation of Li⁺, contribute to the dissolution of LiPSs. Increasing the concentration of Li salts reduces the quantity of free solvent, thereby decreasing the solubility of the LiPSs. In high-concentration electrolytes (HCEs), often described as solvent-in-salt systems, surplus salt anions form contact ion pairs (CIPs) or aggregates (AGGs) that bind to more than one Li⁺, resulting in a notable reduction in the amount of unbound solvents [146]. Moreover, CIPs and AGGs can shift the LUMO from the solvent to the salt. Consequently, an SEI layer was generated by the reduction products of the salt, primarily composed of inorganic constituents, which effectively hindered the formation of Li dendrites. This approach curtails the shuttle effect and Li dendrite formation owing to the decreased solubility of LiPSs and the inorganic-rich SEI layer, which consequently enhances the performance of LSBs.

Suo et al. introduced a highly concentrated “solvent-in-salt” electrolyte category designed for LSBs [147]. Employing 7 M LiTFSI in DOL/DME (1:1, by volume) at near-saturation levels, they achieved minimal dissolution of sulfur and lithium sulfide (Li₂S) over 18-day duration. As an increase in LiTFSI concentration from 2 to 7 M, the CE of LSBs significantly rose from 85% to nearly 100%. After 280 cycles, the 7 M LiTFSI-containing electrolyte displayed the least amount of damage to the Li metal compared with the electrolytes with lower concentrations. Additionally, the SEM images indicate that the solvent in the salt electrolyte system can effectively reduce corrosion and suppress the formation of Li dendrites. Zheng et al. used stable 12 M lithium bis(fluorosulfonyl)amide (LiFSI) in a DME electrolyte formulation for LSBs, ensuring enhanced safety and stability (Fig. 9a) [53]. This high concentration effectively curtailed Li dendrite growth on the anode and suppressed LiPS shuttle reactions on the cathode side, yielding impressive CE (99.7% for sulfur cathode and 99.2% for Li anode). Pang et al., on the other hand, observed that reducing the ratio of solvent to salt alters the sulfur reaction pathway, transitioning it from a dissolution–precipitation system to a quasi-solid-state conversion regime [138]. The diethylene glycol dimethyl ether system (G2)/LiTFSI notably differs from the triethylene glycol dimethyl ether (G3)/LiTFSI and tetraethylene glycol dimethyl ether (G4)/LiTFSI systems due to the chain length, enabling a complete wrapping around the TFSI- anion and forming an extended network structure. At room temperature, the solubility of Li₂S₆ in G2/LiTFSI is only 2 mM, which effectively mitigates the shuttle effect and supports uniform Li deposition. As a result, a Li–S pouch cell with a low E/S ratio of 5 µL mg⁻¹ in the G2/LiTFSI (0.8:1, by molar) showed a stable capacity of 720 mAh g⁻¹ over 100 cycles, whereas the cell in the G2/LiTFSI (7:1, by molar) showed a rapid capacity degradation after only 20 cycles. However, deploying such highly concentrated electrolytes leads to reduced Li⁺ conductivity because of their elevated viscosity, which affects the energy density of LSBs. Additionally, the increased salt volume significantly increases the electrolyte cost.

Fig. 9

a Schematic illustration of 12 M LiFSI/DME for LSB (Reprinted with permission from [53], 2018, Elsevier). b Effect mechanism of MDHCE for LSBs (Reprinted with permission from [149], 2021, Elsevier). c Scheme of electrolyte structure of LiPSs in EPSE (Reprinted with permission from [152], 2022, Elsevier). d Snapshots of the molecular distributions around S₄²⁻ in conventional electrolyte (1 M LiTFSI DOL/DME) (left) and EPSE (right) obtained from MD simulations (Reprinted with permission from [150], 2021, Wiley–VCH). e Cycling performance of a Li-S pouch cell using DIPS-EPSE containing LiNO₃ (Reprinted with permission from [152], 2022, Elsevier)

To overcome these challenges, solvents have been incorporated to reduce the viscosity and density of electrolytes. Low-viscosity HFEs significantly enhanced the ionic conductivity of the electrolyte without altering the solvation structure, thereby preserving the limited solubility of LiPSs. This electrolyte variant is referred to as a localized high-concentration electrolyte (LHCE) or a diluted high-concentration electrolyte (DHCE). Cuisinier et al. utilized a blend of acetonitrile (ACN)₂–LiTFSI and TTE to lower viscosity [148]. Their research revealed the formation of LiPSs in ACN-complexed electrolytes, despite their minimal solubility. However, the mobility of LiPSs is significantly restricted, affecting sulfur speciation within the cell and overall electrochemical performance. These findings suggest that incorporating diluents into high-concentration electrolytes not only diminishes the solubility of LiPSs arising from the solvation structure but also mitigates the diffusion resistance due to high viscosity. Jiang et al. proposed a modified dilute high-concentration electrolyte (MDHCE) for practical LSBs [149]. Considering LiFSI/LiTFSI/DME/DOL/TTE (0.66:0.33:1:0.2:3, by molar), MDHCE demonstrated enhanced performance. In this formulation, LiFSI and DOL contributed to the formation of LiF and organic polymers in the SEI and CEI (Cathode-Electrolyte Interphase) layers, whereas LiTFSI prevented DOL polymerization at higher concentrations, thus improving the sulfur utilization (Fig. 9b). Consequently, MDHCE achieves a high energy density of 325 Wh kg⁻¹ and exhibits stable cycling behavior in LSBs.

The Encapsulating Polysulfide Electrolyte (EPSE) model, pioneered by the Zhang research group, aims to curb detrimental reactions by designing a nano-heterogeneous solvation structure, specifically for LiPSs (Fig. 9c). They explored DIPE, di-isopropyl sulfide (DIPS), and TTE as co-solvents for LSBs [150,151,152]. These co-solvents possess limited solvation capacity and high reduction stability, impacting the solvation configuration of LiPSs. In EPSE, LiPSs are enveloped by dual solvent shells: an inner shell with robust solvation properties and an outer shell from a weakly solvating co-solvent (Fig. 9d). The inner shell ensures efficient conversion kinetics at the sulfur cathode, whereas the outer shell hampers the kinetics of the parasitic reactions on the Li metal anode. In conventional electrolyte, Li⁺ lower the LUMO level of DOL and DME through strong ion–dipole interactions. This accelerates the decomposition of DOL and DME and exposes the reactive LiPSs to corrode the Li metal anode. However, the addition of co-solvents changes the solvation structure, which prevents corrosion of the Li metal anode. The outer solvent shell, with highly reduction-stable co-solvents, encapsulates the centered LiPSs and inner solvent shell to mitigate the parasitic reactions on the Li metal surfaces. Molecular dynamics (MD) simulations and NMR measurements corroborated the existence of these two concentric solvent shells. One of the EPSE models, a 1.2 Ah Li–S pouch cell with DIPS-EPSE containing LiNO₃, maintained a capacity retention of 50% after 103 cycles with an average CE of 93.0% (Fig. 9e).

5.2.3 Using other types of electrolytes

The ionic liquid electrolyte composed of weak Lewis acidic cations and basic anions notably diminishes the solubility of LiPSs [153]. The solvation of Li⁺ with the ionic liquid ions determines the dissolution of LiPS in the ionic liquid electrolyte (Fig. 10a). However, the high viscosity of ionic liquids leads to reduced ionic conductivity. A suitable solvent can be added in appropriate proportions to strike a balance between ionic conductivity and LiPS solubility. Maisner et al. investigated the influence of co-solvents and LiTFSI concentrations in ionic liquid-based electrolytes on LSBs [154]. They utilized DOL, DME, or glymes as co-solvent and found that DOL performed optimally in a binary solvent system with the ionic liquid Pyr₁₄TFSI. An appropriate level of co-solvent content is crucial because augmenting DOL content yielded reductions in solvation energy. Conversely, elevating LiTFSI concentration from 1 to 2 M notably augmented solvation energy from -1.25 to 1.38 eV, signifying a decline in LiPS solubility as visualized by the change in solution color (Fig. 10b, c).

Fig. 10

a Schematic of solvate ionic liquid (Reprinted with permission from [153], 2013, American Chemical Society). b Li₂S₆ (50 mM) dissolved in Py₁₄TFSI/DOL binary electrolytes with various volume ratios and LiTFSI concentration. c Summary of electrolyte properties and cell performance for formulations of Py₁₄TFSI/DOL (Reprinted with permission from [154], 2020, The Electrochemical Society). d Cycling performance of solid-state Li-S pouch cell with PEO/LiTFSI/In₂O₃ SPE at 0.5 C, 60 ℃ (Reprinted with permission from [168], 2022, Elsevier). e Schematics for the proposed mechanism of PVDF- and PEO- coated C/S cathodes in PEO-based electrolytes (Reprinted with permission from [169], 2020, Wiley VCH)

Solid electrolytes are an effective solution for addressing the LiPS shuttle effect by altering the form of the electrolyte [155,156,157,158]. There are two primary categories of solid electrolytes: ceramic- and polymer-based. Ceramic electrolytes, including oxides, sulfides, and hydrides, exhibit near-unity Li⁺ transfer numbers, which effectively impede the dissolution and diffusion of LiPSs. Due to the limited solubility of LiPSs within ceramic electrolytes, the active material undergoes a direct “solid–solid” conversion, producing Li₂S in a single step [159, 160]. This conversion fundamentally circumvents the damage caused by the shuttle effect, thereby promoting extended cycle stability by protecting Li metal anode from LiPSs. Moreover, the crystal structure of ceramic electrolytes confers robust mechanical strength and provides pathways for smooth Li⁺ transport, mitigating Li dendrite intrusion and bolstering ionic conductivity. Nonetheless, their loose interfacial contact often leads to poor compatibility with electrodes. To address this issue, oxide-based solid electrolytes are often blended with liquid electrolytes or polymers to have higher ionic conductivity and good adhesion, finally reducing the interfacial resistance [161,162,163,164].

Conversely, polymer-based solid electrolytes provide flexibility and establish closer contact with the electrodes. However, their room-temperature ionic conductivities are relatively low because they rely on an amorphous phase, which is stable at elevated temperatures. Consequently, conductive Li salts are usually incorporated into polymer electrolytes to improve their ionic conductivity. Polyethylene oxide (PEO), which is commonly used as a polymer electrolyte, exhibits an excellent dissolution capability for LiPSs owing to the high DN of its ethylene oxide unit, which also enhances its strong Li-salt solvation ability [165]. However, due to these characteristics, PEO-based electrolytes partially dissolve LiPSs, leading to LiPS shuttle effect. Consequently, researchers have addressed this issue by creating composite polymer solid electrolytes through the infusion of inorganic fillers into polymers or by combining different solid-state electrolytes to impede the shuttle effect (Fig. 10d) [166,167,168]. Fang et al. optimized the PEO-based electrolyte reaction mechanism by altering the polymer binder in a sulfur cathode [169]. The low solvent property of Polyvinylidene fluoride (PVDF) renders long-chain LiPSs insoluble, shifting the reaction pathway from “solid–liquid-solid” to a single-step “solid–solid” reaction (Fig. 10e). The insolubility and instability of polysulfides in PVDF promoted the direct conversion of elemental sulfur to solid Li₂S₂/Li₂S during cycling, bypassing the formation of highly soluble LiPSs, as indicated by DFT calculations.

6 Cathode engineering

In LSBs, the cathode plays a significant role in influencing key parameters, such as capacity, energy density, and lifespan, making the development of cathode materials critical for enhancing the performance of LSBs. Additionally, the dissolution of LiPSs, which are highly reactive with Li, begins at the cathode. To protect the Li metal anode, substantial research has been conducted over the years to develop effective sulfur hosts with adsorption and catalytic effects on LiPSs that can efficiently prevent the LiPS shuttle effect and enhance LiPS conversion reactions [170,171,172].

Initially, research on cathode materials for LSBs focused primarily on physically entrapping LiPSs within the cathode to mitigate the LiPS shuttle effect. Among these materials, carbon-based substances have attracted significant attention because of their large surface area for sulfur accommodation and excellent electrical conductivity. Nazar et al. introduced a cathode design using ordered mesoporous carbon (CMK-3) to improve LSB performance [173]. CMK-3 is distinguished by its structure of hollow carbon nanorods and 3 nm channel voids, which effectively accommodate sulfur through melt-diffusion at 155 ℃. This method ensures a uniform sulfur distribution and close contact with the carbon structure.

However, carbon-only sulfur hosts have limitations owing to their reliance on physical interactions with LiPSs, such as Van der Waals forces and confinement effects, which are not sufficient for effectively capturing dissolved LiPSs. Consequently, research has evolved toward incorporating other components with carbon that induce chemical adsorption effects on LiPSs or catalytic effects to accelerate the conversion reactions of LiPSs, thereby suppressing the LiPS shuttle effect [174,175,176,177]. Previous studies predominantly focused on using polar metal compounds to achieve chemical adsorption and catalytic effects [178]. Recent studies have also explored effective catalysts, including heterostructures and single-atom catalysts [179, 180]. Such cathode engineering strategies have proven to be effective for adsorbing LiPSs, mitigating the LiPS shuttle effect, and minimizing adverse reactions with Li metal anodes.

6.1 Metal compounds

6.1.1 Metal oxides

Owing to their pronounced polar nature and the abundance of hydrophilic oxygen groups on their surfaces, metal oxides exhibit strong affinity for LiPSs. Consequently, when metal oxides are incorporated with carbon as a sulfur host, the adsorption capability of LiPSs is enhanced compared to that of carbon-only hosts. [181]. Furthermore, metal oxides possess inherent electrocatalytic properties that enable them to substantially suppress the LiPS shuttle effect, thereby reducing detrimental side reactions between LiPSs and Li metal [182, 183]. However, metal oxides are characterized by low electrical conductivity, which leads to increased internal resistance and consequently diminishes redox reaction kinetics. To facilitate electron transfer, researchers have focused on enhancing the electrical conductivity of sulfur hosts through methods such as developing an effective carbon matrix or defect engineering within metal oxides [184, 185].

Wang et al. developed a unique structure using an oxygen-deficient niobium oxide (Nb₂O_5-x) framework with a 3D ordered microporous (3DOM) architecture and embedded carbon nanotubes (CNTs) [186]. This highly porous, open architecture ensures sufficient exposure of the active interfaces. Concurrently, defect engineering not only increases the electrical conductivity of Nb₂O₅ but also enhances its chemical interactions with LiPSs by reducing the bond length between Li in LiPSs and O in Nb₂O_5-x, thereby improving the reaction kinetics and LiPS adsorption capability. Additionally, the embedded CNTs form a highly conductive network, significantly enhancing the electrical conductivity of the sulfur host (2.46 × 10^–3 S cm⁻¹). This innovative design effectively suppressed the shuttle behavior of LiPSs and accelerated their conversion kinetics (Fig. 11a). As a result, LSBs based on the S-Nb₂O_5-x/CNTs cathode demonstrated exceptional cyclability, retaining a high capacity of 847 mAh g⁻¹ after 500 cycles, and a notable rate capability of 741 mAh g⁻¹ at 5 C.

Fig. 11

a Schematic illustration of Nb₂O_5-x/CNTs (Reprinted with permission from [186], 2020, Wiley–VCH). b Geometry configuration of Li₂S₆ binding to HEMO-1 (the oxygen, nickel, magnesium, zinc, copper, cobalt, Li and sulfur atoms are marked with red, white, orange, green, blue, purple, luminous yellow and light green, respectively.) (Reprinted with permission from [187], 2019, Elsevier). c Charge/discharge voltage profiles of CNF@V₅S₈/S, CNF@VS₂/S and CNF/S cathode at 0.2 C (Reprinted with permission from [193], 2021, Elsevier). d Illustration of the mechanisms during redox reaction for NC-S and NC/MoS₃-S NBs-based batteries (Reprinted with permission from [194], 2020, Wiley-VCH). e Interaction schematic and corresponding binding energies of S₈, Li₂S₈, Li₂S₆, Li₂S₄, Li₂S₂ and Li₂S on carbon and MoS₂ substrate. f Cycling performance of the Li-S pouch cells at 0.1 C (Reprinted with permission from [195], 2020, Elsevier)

Zheng et al. developed a high-entropy metal oxide (HEMO-1) as a chemical anchor for LiPSs, which enhanced LSB performance [187]. HEMO-1, produced using a facile mechanochemically assisted method, incorporates five metal components (Ni, Mg, Cu, Zn, and Co) into a single-crystalline structure. The uniformly dispersed metal species act as numerous active sites, effectively adsorbing LiPSs and promoting the conversion reactions. In particular, DFT calculations showed that the Ni sites in HEMO-1 served as effective adsorption sites, demonstrating a higher Li₂S₆ binding energy (− 6.916 eV) compared to Ketjen Black (− 0.742 eV) by forming Li–O and S-Ni bonds (Fig. 11b). As a result, cathode with HEMO-1 achieved a remarkably low capacity decay of only 0.077% per cycle after 600 cycles.

6.1.2 Metal sulfides

Metal sulfides have garnered attention for use in LSBs because of their pronounced affinity for sulfur and relatively low lithiation potential, making them suitable for the cathode environment and operating voltage of LSBs [188]. Similar to metal oxides, metal sulfides are polar, which endows them with significant LiPS adsorption capabilities and catalytic activities [189]. Consequently, the introduction of metal sulfides into the cathode effectively suppresses the LiPS shuttle effect [190]. Furthermore, metal sulfides typically possess enhanced electrical conductivity compared to metal oxides and some of which exhibit metallic or semi-metallic phases due to their covalent properties of soft basic S²⁻/S₂²⁻ ions rather than hard basic O²⁻ ions, making metal atoms within metal sulfides to have a higher density of valence electrons by soft acid-soft base interaction [191, 192].

Zhang et al. utilized self-intercalated two-dimensional vanadium sulfides (V₅S₈) as a cathode material to mitigate the LiPS shuttle effect [193]. The V₅S₈ nanoflakes synthesized via a direct vulcanization method served as advanced sulfur hosts in LSBs. This synthesis method enables the uniform dispersion of V₅S₈ nanoflakes with dimensions of 30–50 nm, onto carbon nanofibers (CNFs). The evenly distributed V₅S₈ acts as both a chemical barrier to restrain LiPS dissolution and a catalyst to accelerate the conversion of LiPSs. The CNF@V₅S₈ cathode led to remarkable improvements in battery performance. This is evidenced by the high specific capacity of 1260 mAh g⁻¹ at 0.2 C and extraordinary cycle stability, with a decay rate of just 0.0312% per cycle over 1500 cycles at 5 C (Fig. 11c).

Yu et al. presented an innovative strategy to suppress the LiPS shuttle effect in LSBs through the synthesis of amorphous N-doped carbon/MoS₃ nanoboxes (NC/MoS₃ NBs) [194]. These nanoboxes, characterized by their hollow porous architecture, offer both physical confinement and strong chemical anchoring to the LiPSs. The amorphous nature of MoS₃ provides a catalytic effect for LiPS conversion, whereas the integration of N-doped carbon enhances the electrical conductivity (Fig. 11d). This multifaceted approach results in NC/MoS₃-S cathodes delivering high areal capacity, superior rate capacity, and outstanding cycling stability, maintaining a specific capacity of 752 mAh g⁻¹ after 500 cycles at 0.5 C. Shao et al. investigated the use of MoS₂ to suppress the LiPS shuttle effect in LSBs [195]. Using DFT calculations, they demonstrated that MoS₂ provides a favorable environment for LiPS adsorption, as evidenced by its binding geometries and energies (Fig. 11e). This enhanced interaction is attributed to the stronger electrostatic affinities between the Li atoms in LiPSs and the sulfur atoms in MoS₂, which form robust Li–S bonds through Lewis acid–base interactions. Consequently, the Li–S pouch cells with mesoporous hollow carbon spheres (MHCS)@MoS₂-S cathode exhibit a high reversible capacity of 960.0 mAh g⁻¹ after 170 cycles at 0.1 C (Fig. 11f).

6.1.3 Metal carbides and nitrides

Metal carbides, such as metal oxides and sulfides, not only have a polar nature, but also exhibit metallic properties owing to the absence of a band gap. Due to their high electrical conductivity, metal carbides facilitate the LiPS conversion reaction and are widely used as a cathode material in LSBs [196, 197]. Similarly, Metal nitrides exhibit higher electrical conductivity than metal oxides and sulfides, and the N atoms in metal nitrides enhance the d-electron density and narrow the d-band in metals. These attributes render their properties akin to those of noble metals, leading to their utilization as catalytic materials in cathodes [198, 199].

Wang et al. developed a hierarchical porous N-doped carbon nanofiber architecture embedded with in situ generated iron carbide nanocatalysts (Fe₃C@PCNFs) [200]. Utilizing natural collagen fibers as both the precursor matrix and structure-directing agent, the sulfur host was delicately designed to address the notorious LiPS shuttle effect (Fig. 12a). The resulting Fe₃C@PCNFs framework, exhibited an impressive hierarchical porous architecture, and a high specific surface area of nearly 1300 m² g⁻¹. The embedded polar Fe₃C nanoparticles demonstrate excellent chemical adsorption and catalytic activity toward the immobilization and conversion of LiPSs. Figure 12b shows the strong adsorptive capability of the Fe₃C@PCNFs toward LiPSs, as evidenced by the rapid color dilution in the Li₂S₆ solution test. As a result, the S@Fe₃C@PCNFs cathode, with over 76 wt. % sulfur content, delivered outstanding rate capability (632 mAh g⁻¹ at 5 C) and excellent cycling performance, maintaining a capacity decay of only approximately 0.067% per cycle over 500 cycles.

Fig. 12

a Schematic illustration of the LiPSs adsorption and conversion during the operation of the S-Fe₃C@PCNFs cathode in LSBs. b Optical photographs of sealed vials of a Li₂S_n/DOL/DME solution contacted with Fe₃C@PCNFs and PCNFs after 0 min, 5 min, 1 h and 24 h (Reprinted with permission from [200], 2021, Elsevier). c Cycling performance of the Mo₂C/CC@S pouch cell (Reprinted with permission from [201], 2021, Elsevier). d UV-vis spectra and digital photographs of Li₂S₆ solution after absorption by Co₃Mo₃N, Co/Mo-N-C and Co-N-C (Reprinted with permission from [202], 2021, Elsevier). e Schematic illustrations of application of h-TiN as host material for sulfur. f The galvanostatic test of h-TiN and m-TiN at high current density of 5 C (Reprinted with permission from [203], 2018, Wiley-VCH)

Mao et al. introduced a novel, binder-free, and flexible 3D hierarchical sulfur host by directly growing Mo₂C nanoarrays on a carbon cloth (CC) substrate (Mo₂C/CC) [201]. Based on XPS results, Mo₂C acts not only as an active site for chemically adsorbing LiPSs through the formation of Mo-S bonds, but also exhibits desirable electrocatalytic activity. Furthermore, the 3D freestanding structure reduces the interfacial resistance between the current collector (carbon cloth) and sulfur hosts (Mo₂C/CC), as well as lowers the content of non-active components, thereby enhancing energy density and reducing “dead sites” formed by such materials. Consequently, Li–S pouch cells with the Mo₂C/CC@S cathode demonstrate an initial capacity of 2.03 mAh cm⁻² and maintain a reversible capacity of 1.88 mAh cm⁻² after 50 cycles at 0.1 C (Fig. 12c).

Yan et al. synthesized a novel sea-urchin-like Co–Mo bimetallic nitride (Co₃Mo₃N) to address the LiPS shuttle effect [202]. XPS analysis revealed that the introduction of Mo altered the electron density of Co/N, enhancing both the adsorption capability and catalytic activity toward LiPSs. Figure 12d illustrates the strong adsorption capability of Co₃Mo₃N toward LiPSs. In the static Li₂S₆ solution test, the LiPS solution containing Co₃Mo₃N became almost transparent after adsorption, as corroborated by the reduced intensity of the characteristic absorption peaks of S₆²⁻ at approximately 260 and 280 nm. The S@Co₃Mo₃N cathode exhibits a high rate performance of 705 mAh g⁻¹ at 3 C and maintains an excellent cycling stability with a low capacity fading rate of only 0.08% per cycle over 600 cycles. Lim et al. introduced novel hierarchically porous titanium nitride (h-TiN) as a multifunctional sulfur host without carbon, which integrates the advantages of multiscale porous architectures with the intrinsic properties of TiN [203]. The h-TiN materials were designed to effectively combine different pore scales. Macropores accommodate a large amount of sulfur while facilitating electrolyte penetration and Li⁺ transportation, whereas mesopores play a crucial role in effectively preventing LiPS dissolution (Fig. 12e). The strong adsorption of LiPSs by TiN, combined with its ability to mitigate the shuttle effect and promote redox kinetics, resulted in the h-TiN/S cathode demonstrating exceptional electrochemical performance. This is evidenced by superior reversible capacity of 557 mAh g⁻¹ even after 1000 cycles at 5 C (Fig. 12f).

6.2 Heterostructures

Recent developments have also been made toward the concept of heterostructures, which combine two distinct components. This approach offers several advantages: (1) Heterostructures can utilize the advantages of each component [204]. For instance, metal oxides have a high polarity for strong LiPS adsorption, whereas metal sulfides/carbides/nitrides offer high electrical conductivity to enhance reaction kinetics. By synergizing these materials, heterostructures can provide both the benefits. (2) The junction of the two compounds narrows the band gaps of the metal compounds, thereby improving their electrical conductivity [205]. (3) The junction also equalizes the Fermi levels of the two metal compounds, creating a “built-in” electric field at the interface, which in turn enhances Li⁺ diffusion kinetics [206]. (4) Charge redistribution at the heterostructure interface generated more active sites toward LiPSs. These characteristics provide new LiPS binding and conversion mechanism like “trap-diffusion-conversion,” significantly elevating the electrochemical performance of LSBs. [207, 208].

Zhang et al. introduced porous catalytic V₂O₃/V₈C₇ heterostructures derived from metal–organic frameworks (MOFs), to suppress the shuttle effect of LiPSs in LSBs [209]. These heterostructures were designed to strongly adsorb LiPSs onto the V₂O₃ surface, which were then rapidly transferred to the V₈C₇ surface for reversible conversion via an efficient catalytic process. After 200 cycles at 0.2 C, the separators and Li metal anodes in cells with different cathodes were examined (Fig. 13a). Cells with pure sulfur cathodes exhibited significant LiPS deposition and severe corrosion of the Li-foil anode, indicating a pronounced shuttle effect. In contrast, cells with the V₂O₃/V₈C₇ heterostructures showed remarkable reductions in both LiPS deposition and corrosion, demonstrating the ability of these heterostructures to alleviate the shuttle effect. As a result, the Li–S pouch cells with S-V₂O₃/V₈C₇@C@G cathode maintained an areal capacity of 4.3 mAh cm⁻² after 150 cycles at 0.2 C, with a high sulfur loading of 6.0 mg cm⁻² (Fig. 13b).

Fig. 13

a Characteristics of separators and Li foils in the cells after cycling; digital pictures of the separators, Li foils, and FESEM images of the cross-sectional Li foils in the cells with pure sulfur (a, e, and i), S-V₂O₃@C@G (b, f, and j), S-V₈C₇@C@G (c, g, and k), and S-V₂O₃/V₈C₇@C@G (d, h, and l) cathode after 200 cycles at 0.2 C. b Cycling performance of the pouch cell at 0.2 C (Reprinted with permission from [209], 2020, American Chemical Society). c Schematic energy band diagrams after forming heterojunction between MoS₂ and MnS. d The density of states profile of the MnS-MoS₂ heterojunction (Reprinted with permission from [210], 2023, Elsevier). e Schematic description of electron exchangeable binding (EEB) sites near Fe-N-C catalysts. f Galvanostatic test of the Cu-free 6 unit cell stacked Li-S pouch cell with an E/S ratio of 3.0 µL mg^-1 (Reprinted with permission from [214], 2023, Wiley–VCH). g High-resolution XPS spectra of Co of SACo@HC before and after adsorption of Li₂S₆ solution (Reprinted with permission from [215], 2022, Wiley-VCH). h AFM height images of Li metal anode from cells with (a) S@Nb-SAs@NC and (b) S@NC cathode (Reprinted with permission from [216], 2023, American Chemcial Society)

Xiong et al. constructed a MnS-MoS₂ heterojunction to suppress the shuttle effect [210]. The MnS-MoS₂ p-n heterojunction exhibits a unique structure of MoS₂ nanosheets decorated with uniformly dispersed MnS nanodots (7–10 nm in size), and both exhibit outstanding adsorption and catalytic capabilities toward LiPSs. Furthermore, a strong built-in electric field of the MnS-MoS₂ heterojunctions was generated at the interface owing to the equilibrium tendency of the Fermi level (Fig. 13c). The charge redistribution induced by this phenomenon significantly accelerates electron transfer. Moreover, the energy gap of the density of states (DOS) of the MnS-MoS₂ heterostructure (0.27 eV) is much lower than those of MnS (0.76 eV) and MoS₂ (1.62 eV) owing to the formation of a Mo-S-Mn bridge, which affects its electronic structure (Fig. 13d).

6.3 Single atom catalysts

Although metal compounds effectively adsorb LiPSs, their excessive use can reduce the energy density of LSBs. Consequently, reducing the catalyst content and size is an intuitive approach. In this context, single atom catalysts (SACs), which feature individual atoms dispersed in a matrix, have recently garnered significant attention in LSB research [211, 212]. SACs not only theoretically achieve 100% atom utilization but also offer the dual advantages of high intrinsic activity and cost-effectiveness. Additionally, the synergistic interaction between a single atom and its matrix endows the physicochemical properties and facilitates the fine-tuning of catalytic activity [213]. These characteristics enable SACs to effectively adsorb dissolved LiPSs and enhance catalytic conversion on the cathode side, thereby inhibiting the LiPS shuttle effect.

Lim et al. designed highly active Fe-N-C single-atom catalysts by introducing electron-exchangeable binding (EEB) sites adjacent to Fe-N-C catalysts that manipulate their local environment for high-energy–density LSBs [214]. The EEB sites are composed of -S and -SO₂ which donate and withdraw electrons from the Fe-N-C site, respectively. Therefore, the Fe d-band center, which is a crucial parameter for catalytic activity, can be manipulated by changing the ratio of -S/-SO₂. Furthermore, the S and O atoms of the EEB sites provided additional binding sites to enhance the LiPS adsorption properties of the Fe-N-C catalyst (Fig. 13e). With an optimal ratio of -S/-SO₂, Li–S pouch cells with advanced Fe-N-C catalysts exhibited a high-energy–density of 320.2 Wh kg⁻¹ with a total capacity of ~ 1 Ah, low E/S (3.0), E/C (2.8), and N/P (2.3) ratios, and high sulfur loadings (8.4 mg cm⁻²) (Fig. 13f). Liu et al. introduced a single-atom cobalt catalyst embedded in a heteroatom-doped carbon matrix (SACo@HC) featuring a CoN₃S structure to enhance the performance of LSBs [215]. SACo@HC provides numerous sulfiphilic and lithiophilic active sites, forming Co-S, Li-S, Li-N, and Li-O bonding, which enhanced the adsorption capability of SACo@HC. This was evidenced by XPS analysis, which revealed significant changes in the binding energies of cobalt in SACo@HC before and after adsorbing Li₂S₆ (Fig. 13g). This shift supports the strong chemical adsorption between the LiPSs and SACo@HC. As a result, the SACo@HC composite, with 80 wt. % sulfur loading, demonstrated a high capacity of 1425.1 mAh g⁻¹ at 0.05 C, maintaining a capacity of 680.8 mAh g⁻¹ at 0.5 C even after 300 cycles.

Zhang et al. developed a highly efficient Nb single-atom catalyst (Nb-SAs@NC) to suppress the LiPS shuttle effect in LSBs [216]. The cornerstone of their strategy was the “trapping-coupling-conversion” mechanism, which effectively anchors and converts LiPSs, thus protecting Li metal anode and enhancing battery performance. The distorted Nb-N sites, featuring unfilled antibonding d orbitals, were coupled with the p orbitals of sulfur, facilitating strong interactions between the catalyst and LiPSs. Employing the Nb-SAs@NC catalyst suppressed surface deterioration of the Li metal anode. Atomic force microscopy (AFM) analysis revealed that the cells with the Nb-SAs@NC cathode had smoother and more compact Li metal anode surfaces than those with the S@NC cathode, indicating effective LiPS trapping (Fig. 13h). As a result, the S@Nb-SAs@NC cathode achieved a reversible capacity of 807.4 mAh g⁻¹ over 600 cycles, corresponding to a capacity retention of 87.1%.

7 Separator/interlayer engineering

7.1 Separator engineering

In LSBs, the separator is positioned between the cathode and anode and directly interacts with the active material of the cathode and Li at the anode. Therefore, the mitigation of the LiPS shuttle effects and Li metal degradation can be achieved simultaneously by modifying the separator, making this method efficient. Consequently, extensive research has been conducted on separator engineering to enhance the cycling stability through interactions with LiPSs and Li [217, 218]. Such studies can be categorized into (1) LiPS adsorption, (2) Li plating regulation, and (3) direct interactions with Li. However, modifying separators involves adding additional materials on the separator, which increase its thickness and mass, leading to a reduction in energy density, an increase in internal resistance, and the use of an electrolyte [219]. Therefore, recent studies have actively pursued thin and lightweight separators that retain these functionalities [220, 221]. The details of these studies are presented in the following sections.

7.1.1 LiPS adsorption

In typical LSBs, polypropylene (PP) separators with pore sizes of approximately 70 nm are commonly used [222]. However, the average size of the LiPSs is several nanometers, which allows them to permeate the separator and interact with Li, leading to undesirable reactions [223]. To mitigate such adverse interactions between the LiPSs and Li, researchers have focused on introducing additional materials with adsorptive and catalytic capabilities into the separator to inhibit the LiPS shuttle effect [224]. Moreover, the LiPSs adsorbed on the separator can be reactivated and contribute to capacity retention [225]. In parallel with cathode engineering, extensive research has been conducted on metal compounds, heterostructures, and single-atom catalysts [226, 227]. The introduction of these materials possessing both adsorption and catalytic capabilities significantly inhibit the LiPS shuttle effect, thereby preventing the diffusion of LiPSs to the Li metal anode and effectively protecting against Li corrosion caused by LiPS.

Zou et al. developed In₂O_3-x nanoparticles combined with carbon spheres (CS), resulting in defect-rich electrocatalysts that enhance the chemical adsorption and catalytic conversion of LiPSs [228]. DFT calculations confirmed that In₂O_3-x exhibits a higher adsorption energy for LiPSs than In₂O₃ which demonstrates the efficacy of defect engineering. In cells equipped with the In₂O_3-x@CS-0.6/rGO separator, a low-voltage hysteresis of approximately 150 mV was observed after cycling for 500 h at a current density of 5 mA cm⁻² and an areal capacity of 5 mA cm⁻² (Fig. 14a). Liu et al. developed a novel strategy to suppress the LiPS shuttle effect in LSBs by utilizing phosphorus-doped metal–organic framework-derived CoS₂ (P-CoS₂) nanoboxes [229]. Figure 14b shows the proposed adsorption schemes of Li₂S₆ on the CoS₂ and P-CoS₂ surfaces, highlighting the difference in adsorption behavior between CoS₂ and P-CoS₂ nanoparticles toward LiPSs. The P-CoS₂ surface, characterized by abundant Co − O − P-like species, can strongly capture LiPSs by forming P − S, Li − P, and strengthened Co − S bonds. This drives the conversion reaction and suppresses the shuttling. Consequently, the cell with the P-CoS₂/CNTs@Celgard separator exhibits a smoother and more stable voltage plateau during the rest period and experiences less capacity loss (Fig. 14c). This indicates that the P-CoS₂/CNTs layer effectively prevented LiPSs from shuttling during storage, thus mitigating the severe self-discharge behavior commonly observed in LSBs.

Fig. 14

a Comparison of the Li plating/stripping stability with and without In₂O_3-x@CS-0.6/rGO@PP separator (Reprinted with permission from [228], 2022, Elsevier). b Proposed adsorption schemes of Li₂S₆ on CoS₂ and P-CoS₂ surfaces. c Voltage curves under the self-discharge test of the cells with P-CoS₂/CNTs@Celgard and CoS₂/CNTs@Celgard at 0.1 C (Reprinted with permission from [229], 2021, American Chemical Society). d Photographs of H-type cell using f-NbN@PP and PP as separator (Reprinted with permission from [190, 230], 2020, American Chemical Society). e Schematic diagram of the optimal immobilization effect of VSe₂/V₂C-CNTs-PP on LiPSs (Reprinted with permission from [232], 2022, Elsevier). f SEM images of top and cross-sectional views of Li metal anode after 100 cycles for (a) PP and (b) Fe-N₄/DCS separators (Reprinted with permission from [233], 2022, Wiley-VCH). g Cycling performance of the Li-S pouch cell at 0.1 C with VN_1-x@V-NC modified separator (inset, the open circuit voltage of the fresh pouch cell) (Reprinted with permission from [234], 2023, Elsevier)

Kim et al. reported a bifunctional flower-like mesoporous NbN (f-NbN)-modified PP separator for LSBs [230]. The large surface area (67 m² g⁻¹) of f-NbN derived from the flower-like structure and the high LiPS adsorption capability of the NbN (200) plane effectively captured soluble LiPSs. Furthermore, NbN has a high intrinsic electric conductivity (1.6 × 10⁶ S m⁻¹), which assists the reactivation of LiPS captured on NbN to contribute to its capacity. The H-cell test demonstrated that when the PP separator was used, Li₂S₆ easily diffused through the separator 6 h after the start of the experiment, indicating its ineffectiveness in blocking LiPS migration. In contrast, the f-NbN@PP separator effectively restricts Li₂S₆ diffusion, highlighting the superior barrier properties of the NbN layer against soluble LiPSs (Fig. 14d). As a result, the addition of the f-NbN modification layer led to a significant improvement in the battery cycling performance, showing a capacity decay per cycle of only 0.061% during 300 cycles at 1 C. Wang et al. addressed the LiPS shuttle effect by developing a Zn²⁺-modulated bimetallic carbide electrocatalyst anchored on N-doped carbon, Co₃ZnC@NC [231]. Created through a straightforward one-pot synthesis process, this electrocatalyst consists of Co₃ZnC-embedded carbon submicrospheres anchored on 3D macromesoporous N-doped carbon. The inclusion of Zn cations effectively modulated the active Co²⁺/Co⁰ pair, thereby inhibiting the shuttle effect and accelerating the catalytic conversion kinetics of LiPSs. When used as an interlayer to modify commercial separators, Co₃ZnC@NC significantly enhanced the cycling stability and discharge capacity of LSBs demonstrating a substantial specific capacity of 805.5 mAh g⁻¹ after 50 cycles at 0.2 C under a high sulfur loading of 4 mg cm⁻². Lv et al. introduced a unique VSe₂/V₂C heterostructure, created from a facile thermal selenization process from V₂C [232]. This structure combines the strong affinity of V₂C for LiPSs with the electrocatalytic activity of VSe₂, and the local built-in electric field at the heterointerface effectively enhances electron/ion transport and boosts the conversion kinetics of LiPSs (Fig. 14e). LSBs equipped with a VSe₂/V₂C-CNTs-PP separator exhibited remarkable performance, achieving an initial specific capacity of 1439.1 mAh g⁻¹ and maintaining a capacity of 571.6 mAh g⁻¹ after 600 cycles.

Jing et al. introduced a defect-rich single-atom catalytic material (Fe-N₄/DCS) to enhance the LSBs by addressing the LiPS shuttle effect [233]. Abundant local defects in substrates not only facilitate the dispersion of Fe atoms but also provide stable sites for single atoms. Additionally, DFT calculations demonstrated that Fe atoms in the defect environment strongly adsorbed LiPSs and possessed a lower activation energy for the redox reactions of LiPSs. By incorporating Fe-N₄/DCS into a PP separator (Fe-N₄/DCS/PP), this study demonstrated that this approach can significantly inhibit LiPS shuttling and accelerate redox reactions. Figure 14f visually demonstrates the superiority of the Fe-N₄/DCS separator in preventing LiPS accumulation and corrosion on the Li metal anode compared with conventional PP separators. The LSBs with the modified separator exhibited a remarkable performance improvement, with a capacity decay rate of only 0.03% per cycle at 2 C after 800 cycles. Zhang et al. have developed a novel catalyst (VN_1-x@V-NC) featuring single vanadium (V) atoms with V-N₄ coordination and vanadium nitride nanodots characterized by substantial nitrogen vacancies (VN_1-x) [234]. Both theoretical calculations and experimental results have demonstrated that the V-N₄ and VN_1-x sites have catalytic effects of Li₂S formation and decomposition, respectively. Moreover, VN_1-x exhibits a strong affinity for soluble polysulfides through V-S and Li-N bonding interactions, with LiPSs being preferentially adsorbed near the nitrogen vacancies. This configuration effectively mitigates the LiPS shuttle effect. Consequently, when Li–S pouch cells are equipped with the VN_1-x@V-NC modified separator, they demonstrate an impressive initial discharge capacity of 1086.5 mAh g⁻¹ and maintain stable cycling performance, retaining 86.35% of their initial capacity after 50 cycles at 0.1 C (Fig. 14g).

7.1.2 Li plating regulation

In LSBs, the separator serves as a pathway for Li⁺ to move between the anode and the cathode. If the mobility within the separator is low, a cation concentration gradient is formed at the anode, facilitating the formation of dendrites [235]. Conversely, a high Li⁺ mobility can create a homogeneous Li⁺ flux and induce uniform Li plating, thereby suppressing dendrite formation [236]. Thus, enhancing the Li⁺ transfer within the separator is an efficient strategy for regulating Li plating and inhibiting dendrite growth. In addition, recent studies have reported that the presence of functional groups such as polar and negatively charged groups can further mitigate the LiPS shuttle effect. Bifunctional separators exhibiting these two effects are discussed below.

Wang et al. introduced a novel anionic metal–organic framework (MOF)-based bifunctional separator, named MMMS, which addresses the dendrite issue in Li metal anodes for LSBs [237]. The MMMS, fabricated using UiO-66-SO₃Li and poly(vinylidene fluoride) (PVDF) through a mixed-matrix membrane approach, featured well-defined anionic Li⁺ transport tunnels composed of abundant sulfonate (-SO₃⁻) groups (Fig. 15a). These tunnels facilitate homogeneous Li deposition, which is crucial for stabilizing Li-metal anodes. Meanwhile, the sulfonate groups exhibit electrostatic repulsion forces against the LiPS anions, thereby playing a role in suppressing the shuttle effect of LiPS. Li et al. enhanced the performance of LSBs by engineering a stable electrode-separator interface [238]. This was achieved by coating both surfaces of a commercial Celgard separator with an ultrathin conductive polymer nanolayer of polypyrrole (PPy) using a simple and scalable in situ vapor-phase polymerization process (Fig. 15b). This modification results in only a minor increase in the overall mass and volume of the separator (~ 0.13 mg cm⁻² of mass loading). The hydrophilic nature of PPy enhances the electrolyte uptake, which facilitates uniform Li⁺ flux across the separator, leading to uniform Li metal stripping and plating. Furthermore, the conducting properties of PPy facilitated electron transfer, enhancing the redox reaction of LiPSs on the cathode side. Consequently, the modified separator demonstrated superior performance, as evidenced by a low and stable overpotential of less than 30 mV for over 250 h at a current density of 1 mA cm⁻² with an areal capacity of 3 mAh cm⁻² during the Li stripping and plating tests (Fig. 15c).

Fig. 15

a Schematic illustration of the MMMS for regulating Li deposition and blocking LiPS shuttle in LSBs (Reprinted with permission from [237], 2020, Wiley–VCH). b Schematic illustration of coating a PPy ultrathin nanolayer on both surfaces of the separator using a facile vapor-phase polymerization process. c Voltage profile of Li||Li symmetric cells with regular Celgard separator and PPy modified separator at the current density of 1 mA cm^-2 with the areal capacity of 3 mAh cm^-2 (Reprinted with permission from [238], 2019, Elsevier). d Schematic illustration of cycled Li behaviors with PP and COF-CN-S@PP separator. e In situ optical microscopy observations of Li/PP/Li and Li/COF-CN-S@PP/Li batteries during the Li plating/stripping process at 1 mA cm^-2 with 0.5 mAh cm^-2 (Reprinted with permission from [239], 2023, Wiley-VCH). f Calculated adsorption energies (E_ads) of PP and pVIDZ with soluble LiPS species (Li₂S₄, Li₂S₆, and Li₂S₈) (Reprinted with permission from [240], 2021, American Chemical Society). g Schematic representation and cycling performance of the flexible Li-S pouch cell (Reprinted with permission from [243], 2022, John Wiley and Sons)

An et al. designed a bifunctional separator (COF-CN-S) using an asymmetric covalent organic framework (COF) with strong cyanide groups (-CN) and polysulfide chains (-S) for LSBs. [239]. Polar functional groups (nitrile) and electronegative sulfur groups suppress LiPS shuttle effects through adsorption and electrostatic repulsion, respectively. Meanwhile, the lone pairs of cyano groups are inclined to coordinate with Li⁺, which enhances the Li⁺ transport capability via reversible coordination bonds. These characteristics regulated the dynamic behavior of LiPSs and Li⁺, thus inhibiting the shuttle effects of LiPSs and dendrite growth (Fig. 15d). In situ optical microscopy images provided a clear contrast between the two types of separators, COF-CN-S@PP and PP. The COF-CN-S@PP separator exhibited superior control over the Li deposition, leading to more uniform and dendrite-free Li layers (Fig. 15e). Lim et al. employed initiated chemical vapor deposition (iCVD) to uniformly coat polyvinylimidazole (pVIDZ) onto a separator, creating a ultrathin (70–100 nm) and ultralight-weight (0.055 mg cm⁻²) bifunctional separator for LSBs [240]. The imidazole functional groups in pVIDZ interacted with the anions of LiTFSI, enhancing Li⁺ mobility and effectively inhibiting dendrite formation. Additionally, DFT calculations confirmed that pVIDZ offers a high LiPS adsorption energy compared to the PP separator, with significant LiPS shuttle effects (Fig. 15f). Consequently, LSBs employing a separator with pVIDZ delivered a specific capacity of 881 mAh g⁻¹ at 0.1 C.

Fan et al. presented a synergistic functional separator composed of graphene quantum dots (GQDs) and polyacrylonitrile (PAN) on a polypropylene (PP) base (GQDs-PAN@PP separator) [241]. The quantum confinement effect of GODs and polar functional groups acts as a lithiophilic mediator that induces uniform Li⁺ nucleation and deposition. Additionally, the 3D network formed by the electrospun nanofibers combined with the polar functional groups of the GQDs effectively inhibited the shuttling of LiPSs and enhanced sulfur utilization. As a result, Li||Li symmetric cell with the GQDs-PAN@PP separator demonstrates stable cycling for over 600 h, and LSBs with this separator achieve high stability and desirable sulfur electrochemistry, including a high reversibility of 558 mAh g⁻¹ for 200 cycles and a low fading rate of 0.075% per cycle after 500 cycles at 0.5 C. Yan et al. implemented a versatile asymmetric separator composed of lithiated-sulfonated porous organic polymer (SPOP-Li) and Li_6.75La₃Zr_1.75Nb_0.25O₁₂ (LLZNO) layers to LSBs [242]. The asymmetric separator consisted of lithiated sulfonated porous organic polymer (SPOP-Li) and Li_6.75La₃Zr_1.75Nb_0.25O₁₂ (LLZNO) layers. The numerous negative charged sulfonates of SPOP-Li functions as a LiPS barrier and Li⁺ conductor, while the LLZNO layer acts as an “ion redistributor” to regulate Li⁺ flux. This design effectively facilitates homogeneous Li⁺ distribution and prevents dendrite formation. Consequently, the Li||Li symmetrical cell with this asymmetric separator achieved 5300 h of Li stripping/plating, and the modified LSBs exhibited an ultralow fading rate of 0.03% per cycle for over 1000 cycles at 5 C.

Zhou et al. have developed a novel and facile ultrasound-assisted photochemical reduction strategy to synthesize single atom (SA)-regulated heterostructures of binary nanosheets [243]. This method facilitates the electronic interactions at the heterointerfaces between Pt SAs/In₂S₃ and Ti₃C₂, which induce charge distribution at these interfaces. The Pt SAs/In₂S₃/Ti₃C₂@PP separator enhances the homogeneous distribution of Li⁺ flux, attributed to the hetero-interfacial electronic enhancement effects between Pt SAs/In₂S₃ and Ti₃C₂, thereby inhibiting Li dendrite growth even at a high current density of 5 mA cm⁻². Consequently, Li–S pouch cells equipped with the Pt SAs/In₂S₃/Ti₃C₂@PP separator have achieved an areal capacity of 5.54 mAh cm⁻² at a discharge rate of 0.2 C (Fig. 15g).

7.1.3 Direct interaction with anode

As discussed previously, the separator is in contact with the Li metal anode and can interact directly with Li. Therefore, functionalization of the separator can induce beneficial interactions between the separators and Li. Various mechanisms of interaction with the anode, such as regulating Li growth, reconstructing the SEI layer, and rejuvenating inactive or dead Li, have been suggested to protect the anodes of LSBs [244]. Research related to these mechanisms and their implementation in protecting the anode is discussed in the following section. These methods significantly increase the cycling stability of LSBs.

Song et al. introduced an innovative strategy using Mo-containing polyoxometalate (POMs)-modified separators to inhibit Li dendrite growth in LSBs [245]. Specifically, the optimized Dawson-type POM, (NH₄)₆[P₂Mo₁₈O₆₂]·11H₂O (P₂Mo₁₈), is employed for its strong oxidizability. When Li dendrites form and come into contact with the separator, P₂Mo₁₈ acts as a “dendrite-killer,” oxidizing Li⁰ into Li⁺, thereby mitigating the risks posed by dendrites. This prevents the formation of dead Li, which degrades the CE and cycling performance. Moreover, the reduced state of P₂Mo₁₈ can be re-oxidized, allowing its reusability. This mechanism effectively prevents short circuits and potential spontaneous combustion owing to uncontrolled dendrite growth. The Li||Li symmetrical cell with the P₂Mo₁₈ modified separator demonstrates exceptional cyclic stability for over 1000 h at 3 mA cm⁻² and 5 mAh cm⁻² (Fig. 16a). Furthermore, the assembled LSBs maintain a superior reversible capacity of 600 mAh g⁻¹ after 200 cycles at 2 C.

Fig. 16

a Galvanostatic cycling of Li||Li symmetric cells with POMs modified separators at 3 mA cm⁻² with areal capacity of 5 mAh cm⁻² (inset: enlarged diagrams at different periods of the cycling) (Reprinted with permission from [245], 2023, Wiley-VCH). b Scheme portraying the systematic evolution of SEI in the GQDs-modified Li symmetrical cell from the fresh cell to continuous plating-stripping cycles, where GQDs and Li and its salts undergo synergistic interaction to realize smooth and near F-rich SEI (Reprinted with permission from [246], 2022, Wiley-VCH). c Schematic illustration of Li deposition processes of PP@H-PBA, PP@S-PBA, and PP (Reprinted with permission from [247], 2023, Wiley-VCH)

Senthil et al. reported in situ restructured artificial SEI layer characterized by ultrasmooth, thin, and F-rich properties using a site-specific hydroxyl-functionalized graphene quantum dot (GQDs)-coated PP separator [246]. This artificial SEI layer was formed by the synergistic interaction of the plating-borne Li and its species, guided by the hydroxyl-functionalized GQDs (Fig. 16b). Li stripping triggers the dissolution of Li and the decomposition of the electrolyte, whereas Li plating facilitates its redeposition on the exposed Li surfaces. At this juncture, the synergistic interaction between the electron-rich GODs and electron-deficient Li promotes uniform redeposition across the Li surface derived from controlled growth, realizing a smooth and near F-rich SEI layer that affords uniform Li⁺ flux over the Li metal anode surface.

Liu et al. improved LSB performance by utilizing an Fe-Co-based Prussian blue analog (H-PBA) with a hollow and open framework to modify a commercial PP separator (PP@H-PBA) [247]. The macroporous structure of H-PBA and the open framework guided the growth of Li dendrites through space confinement. Moreover, the positive Fe/Co sites in the H-PBA, which are influenced by polar cyanide (− CN) groups, help in reactivating inactive and dead Li (Fig. 16c). As a result, Li|PP@H-PBA|Li symmetric cells exhibit long-term stability at 1 mA cm⁻² for 1 mAh cm⁻² over 500 h and LSBs with PP@H-PBA delivered stable cycling performance at 500 mA g⁻¹ for 200 cycles.

7.2 Interlayer engineering

Separator engineering typically involves designing new types of functional separators or modifying existing separators through physicochemical hybridization or functionalization with other materials such as carbons or inorganic materials, ensuring intimate contact between the functional layer and the separator. In contrast, an interlayer refers to a freestanding film inserted between the separator and the electrode [248]. Interlayers are generally divided into two categories: cathodic interlayers, which are placed between the cathode and separator, and anodic interlayers, positioned between the anode and separator. The methods used to protect the Li metal anode of these two types of interlayers differ significantly. Detailed explanations and relevant research are discussed in the following section.

7.2.1 Cathodic interlayer

The primary function of the cathodic interlayer, positioned between the cathode and the separator, is to mitigate the LiPS shuttle effect. Carbon-based materials have been considered promising candidates for the cathodic interlayer owing to their lightweight, high electrical conductivity, ease of fabrication, and stability [249,250,251]. However, the efficacy of interlayers composed solely of carbon, which primarily function by physically interacting with LiPSs, is limited in fully addressing LiPS diffusion due to their low chemical affinity towards LiPSs. As discussed in the section on cathode engineering, the integration of polar inorganic materials with carbon-based materials has proven effective in addressing the LiPS shuttle effect. This synergy promotes the dispersion of inorganic materials within the matrix, thereby increasing the number of active sites for adsorption and catalytic conversion of LiPSs [252]. Consequently, several studies adopting this approach have been reported, demonstrating substantial suppression of the LiPS shuttle effect and effectively protecting the Li metal anodes [253,254,255].

Mo et al. developed ultrafine Mo₂C catalysts embedded within a conductive vertical graphene framework as a cathodic interlayer for LSBs, utilizing a scalable and facile synthesis method that combines thermal CVD with slurry coating techniques [256]. This process results in the ultra-dispersion of Mo₂C nanocatalysts within the framework, which are capable of effectively adsorbing LiPSs, thereby mitigating the shuttle effect. As a result, the loss of active material and corrosion of the lithium anode are significantly reduced. Consequently, LSBs incorporating this cathodic interlayer have achieved a capacity of 4.8 mAh cm⁻² under practical conditions, characterized by a low E/S ratio of 6 µL mg⁻¹ and a high sulfur loading of 4.4 mg cm⁻², demonstrating the effectiveness of this approach in enhancing the performance of LSBs.

Zhao et al. developed a novel free-standing, multifunctional interlayer composed of V₂O₃/VN nanowires and carbonized bacterial cellulose (CBC) nanofibers to enhance the performance of LSBs [257]. This interlayer combines the advantages of its components, integrating the strong adsorption capabilities of V₂O₃ and the high catalytic activity of VN to mitigate the shuttle effect and improve redox kinetics. The CBC nanofibers provide a flexible, conductive matrix that supports the dispersion of inorganic nanowires, thereby promoting the exposure of active surfaces. Illustrated in Fig. 17a, digital and corresponding SEM images reveal the surface conditions of Li foils and the interlayer after 200 cycles. Li foils equipped with the V₂O₃/VN/C interlayer exhibit minimal roughness, in contrast to those with V₂O₃ or CBC interlayers which show significant cracking due to LiPS erosion. The deposition of sulfur on the V₂O₃/VN/C interlayer confirms its robust adsorption capability and superior catalytic activity on LiPSs. This combination of adsorption and catalysis effectively suppress the LiPS shuttle effect. Consequently, LSBs integrated with the V₂O₃/VN/C interlayer maintain a discharge capacity of 768 mAh g⁻¹, with a minimal decay rate of 0.037% per cycle and nearly 100% Coulombic efficiency (Fig. 17b).

Fig. 17

a (a–d) Digital photographs of the Li foils; (e–f) Top-view SEM images of the Li foils; (i–l) Digital photographs of the interlayers; (m–p) Top-view SEM images of the interlayers. b Long-term cyclic test of cell with V₂O₃/VN/C interlayer at 1 C (Reprinted with permission from [257], 2022, Elsevier). c Schematic illustration of LSBs with SnO₂ NWs@CP interlayer (Reprinted with permission from [258], 2020, Elsevier)

Ahn et al. synthesized SnO₂ nanowires (NWs) integrated with carbon paper (SnO₂ NWs@CP) for use as an efficient multifunctional interlayer, effectively suppressing the LiPS shuttle effect [258]. The SnO₂ NWs act as adsorption sites for LiPSs through chemical interactions, while the carbon paper enhances electron transport across the SnO₂ NWs@CP composite, thereby providing a conductive pathway. This configuration enables rapid adsorption-conversion of LiPSs at the SnO₂ NWs, coupled with efficient electron transport (Fig. 17c). Owing to the synergistic interaction between SnO₂ NWs and carbon paper, LSBs equipped with this interlayer achieve a specific capacity of 815 mAh g⁻¹ after 100 cycles at 0.2 C, under a high sulfur loading (4.0 mg cm⁻²).

7.2.2 Anodic interlayer

The anodic interlayer, positioned between the Li metal anode and the separator, primarily focuses on ensuring uniform Li plating/stripping. As discussed in the introduction section, the use of elemental sulfur as the active cathode matessrial, which has a high theoretical capacity, dramatically increases the current density applied to the Li metal anode. Under these conditions, Li dendrite growth becomes noticeably pronounced, ultimately leading to rapid cell failure. This failure is due to instability caused by a thin SEI layer and continuous reactions with the electrolyte and LiPSs, exacerbated by the mossy Li metal anode. Therefore, maintaining uniform Li plating/stripping, even under high current density conditions, is crucial for cell stability. Recent research is primarily focused on designing channels that guide Li growth in desired directions or regulating nucleation to protect the Li metal anode in LSBs [259,260,261].

Xie et al. employed a CNT film as an anodic interlayer to inhibit Li dendrite growth in LSBs [262]. Characterized by small nanospaces between the CNTs, the film acts as an effective physical barrier against the formation of Li dendrites when positioned between the separator and the Li metal anode. Furthermore, the integration of conductive CNTs with high surface area contributes to a reduction in local current density. This reduction promotes more uniform deposition across the anode surface. By utilizing this anodic interlayer, they have developed designs for dendrite-free LSBs that enhance both cycling stability and performance.

Zhao et al. investigated the utilization of carbon paper (CP) as an interlayer to enhance the stability of Li metal anode [263]. This strategy involves the facile and scalable fabrication of a Li-CP composite electrode. The CP interlayer induces uniform Li deposition and inhibits mossy and dendritic growth, attributed to the uniform distribution of local current throughout the CP. Moreover, the porous structure of CP enhances electrolyte transport and acts as a buffer for Li plating and stripping processes. These benefits are evidenced by scanning SEM images of Li metal anodes after 10 cycles with a current limit of 3 mAh cm⁻² (Fig. 18a). Consequently, LSBs equipped with the CP interlayer demonstrate exceptional cycling stability and reversibility, achieving an initial capacity of over 1200 mAh g⁻¹ and maintaining 600 mAh g⁻¹ after 150 cycles (Fig. 18b).

Fig. 18

a Top view and cross-sectional SEM images of bare Li foil and Li-CP after 10 cycles with a 3 mAh cm⁻² capacity limit. b Cycling performance of full cells using bare Li foil and Li-CP (Reprinted with permission from [263], 2018, Elsevier). c Contact angle images of O-Ti₃C₂@CNF. d Optical microscopy study of in situ Li plating behavior of O-Ti₃C₂@CNF and bare Li metal. e Cycling performance under a sulfur loading of 6.7 mg cm^-2 and an E/S ratio of 6 µL mg^-1 (Reprinted with permission from [264]. 2022, Elsevier)

He et al. developed a porous carbon nanofiber integrated with an oxygen-rich Ti₃C₂ MXene nanosheet (O-Ti₃C₂@CNF) based on first-principles calculations [264]. This O-Ti₃C₂@CNF interlayer effectively reduces the over-potential for Li nucleation and growth by mitigating the tip effect and lithiophilic properties of the O-Ti₃C₂ MXene, resulting in a dendrite-free Li metal anode. Additionally, the improved electrolyte affinity of the O-Ti₃C₂@CNF enhances compatibility at the electrode/separator interface, which is particularly crucial under conditions of a low E/S ratio (Fig. 18c). In situ visual monitoring through an optical microscope confirmed the inhibition of Li dendrite formation on the O-Ti₃C₂@CNF. It was observed that Bare Li began to deposit on the lithium anode at a constant current density of 2 mA cm⁻², with surface bulging initially visible, followed by gradual dendrite formation after 30 min. In contrast, the O-Ti₃C₂@CNF displayed a homogeneous and dendrite-free Li structure across its entire 3D configuration (Fig. 18d). Consequently, when employed as the anodic interlayer in LSBs, the O-Ti₃C₂@CNF enabled the batteries to maintain a reversible areal capacity of 5.2 mAh cm⁻² across 30 cycles, with a high sulfur loading of 6.7 mg cm⁻² and a low E/S ratio of 6 µL mg⁻¹ (Fig. 18e).

8 Conclusion and perspectives

In this paper, we review recent studies on the protection of Li metal anodes in LSBs, categorizing them as anode, electrolyte, cathode, and separator/interlayer engineering. In LSBs, Li metal anode faces several challenges: the formation of Li dendrites & dead Li, high reactivity with electrolytes & LiPSs. Numerous studies have been conducted to protect Li metal anodes because these problems are aggravated under the practical conditions of high sulfur load, low E/S ratio and limited Li anodes. We summarized the performance of the practical pouch cells mentioned in our review article according to the component (Table 1).

Table 1 Electrochemical performance of recent reported Li–S pouch cells

However, research on anode protection under the practical condition of high sulfur loading (> 6 mg cm⁻²), low E/S ratio (< 3 µL/mg), and N/P ratio (< 2) remains insufficient. Therefore, to achieve LSBs with high energy density, considerations beyond the current scope must be addressed for each component of the battery. Therefore, we present the limitations of this study and suggest research directions for each component to achieve practical LSBs.

• Anode

  Two predominant approaches have been explored for modifying Li-metal anodes: the integration of artificial SEI layers and the use of host materials for uniform Li deposition. For practical LSBs, Li metal anode must be modified by a scalable process and the thickness of Li metal must be kept below 50 µm. For application in pouch cells, the modification must be applied equally on both sides of the Li metal. Additionally, there are still areas in which basic research needs to be conducted. For example, forming an artificial SEI layer on lithium alters the surface work function of Li [265]. This change fundamentally affects reactivity of Li with the electrolyte and LiPSs; however, research into these energy levels is not well-developed. Therefore, anode engineering must consider the work function changes and other fundamental factors. Currently, most strategies applied to LSBs are those that have been used in Li metal batteries, but since LSBs involve a system where LiPSs are dissolved in the electrolyte, different strategies must be established. For instance, a dual-layer approach that includes an elastic layer capable of enduring volumetric expansion/contraction on the Li anode side and a functional layer on the electrolyte side to block LiPSs could be effective. Although many studies have been conducted on Li deposition to suppress dendrite formation and dead Li, the stripping process has not been studied extensively. Therefore, a fundamental study of the stripping process should be conducted, especially considering the characteristics of LSB starting from the discharge. Additionally, research on self-discharge, which occurs even when the cell is not in operation, should be conducted.

• Electrolyte

  Electrolyte engineering is crucial for improving the performance and stability of LSBs. To protect the Li metal anode, many sparingly solvating electrolytes with low LiPS solubility have been studied; however, there are challenges associated with high viscosity and increased overpotential. Conversely, an electrolyte with high LiPS solubility can address these issues but may lead to the degradation of the Li metal anode owing to the severe shuttle effect. Furthermore, the electrolyte may be gelled by a super-high concentration of solubilized polysulfides under lean electrolyte conditions. To achieve practical LSBs, a next-generation of electrolytes must be developed to overcome the aforementioned trade-off while maintaining a low E/S ratio. Additionally, researchers are investigating various additives to form a desirable SEI layer. As discussed in Sect. 3.2, LiPSs exhibit an oxidative nature, which leads to their more rapid reduction compared to other electrolyte components, resulting in the formation of a Li₂S₂/Li₂S-rich SEI layer. Therefore, it is necessary to develop effective additives that not only possess lower LUMO energy level to ensure more rapid reduction than LiPSs but also form desirable SEI layer which has high mechanical properties and Li⁺ conductivity. Another approach of additives involves direct chemical interaction with LiPSs to elevate their LUMO energy level that can modify the reactivity towards the Li metal. These additives should significantly raise the LUMO energy level of LiPSs to reduce their oxidative nature while maintaining original LiPS conversion reaction. In this way, the components of the electrolyte actively interact with LiPSs, significantly influencing their characteristics. Therefore, rather than solely focusing on the Li metal conducted by previous studies, a comprehensive consideration of the impacts on both LiPSs and the Li metal anode is essential for electrolyte design.

• Cathode

  As previously discussed, most cathode studies have been conducted under conditions of high E/S ratios. Consequently, porous materials have been predominantly used as sulfur hosts to increase the contact area with the insulating sulfur and ensure electrical conductivity. However, the use of porous materials with low E/S ratios can lead to the incomplete wetting of the electrode, resulting in Li⁺ transfer issues. Additionally, a high proportion (~ 30%) of conductive materials in the cathode still reduces the energy density of LSBs. Therefore, it is essential to develop effective sulfur hosts that can ensure electrical conductivity with minimal sulfur content and suitable surface area. Although numerous experimental and computational studies have demonstrated the effects of catalysts on the adsorption and conversion of LiPSs, the fundamental and specific mechanisms of adsorption and conversion remain unclear. Hence, it is important to gain an understanding of the adsorption and conversion reactions of LiPSs using methods such as in situ analyses. Based on this understanding, the design of elaborate catalysts can effectively address the LiPS shuttle phenomenon and Li metal corrosion to a minimal extent. Additionally, for practical application in pouch cells, it is necessary to employ facile synthesis methods and enable large-scale production.

• Separator/interlayer

  In LSBs, separators and interlayers interact with both the cathode and the anode, thereby offering a strategy to simultaneously address issues related to the LiPS shuttle effect and Li metal. However, modification of separators and introduction of interlayers can increase the energy density of LSBs. To achieve LSBs with high energy densities, it is necessary to develop thin and lightweight separators and interlayers that retain these beneficial effects. However, thin separators exhibit reduced capability to suppress the LiPS shuttle effect due to decreased surface area and thickness, and they are vulnerable to internal short circuits caused by Li dendrite formation. Similarly, thin interlayer is likely to fail in suppressing LiPS shuttle effect and Li dendrite formation. Therefore, to overcome this trade-off, there is a need to develop an effective separator and interlayer that exhibits excellent LiPS shuttle effect suppression capability with robust mechanical strength while maintaining thin and lightweight properties. Furthermore, the surface area, pore volume, and surface functional groups of separators and interlayers significantly impact electrolyte wettability, Li⁺ migration capability, and the required amount of electrolyte. Therefore, these factors must be considered in the design of separators and interlayers. Such effects are more pronounced in practical pouch cells operating at low E/S ratios (< 3 µL/mg) with limited electrolyte amount and in high viscosity environments, thus requiring thorough consideration to effectively protect Li metal anode. Finally, for practical application in pouch cells, it is essential to ensure large-scale production and uniform properties across large areas.

  Compared to traditional LIBs, LSBs offer a significantly higher energy density and cost efficiency. However, the realization of these advantages requires practical conditions. Under such conditions, the operational environment of LSBs changes drastically, and issues such as Li metal challenges become more pronounced. Furthermore, to satisfy the practical conditions, the required properties of each battery component are changed. However, many studies on LSB have only been conducted at the coin cell stage, neglecting various problems that may occur in the Li metal anode. The Li metal anode not only corrodes when reacting with the electrolyte, but also experiences extreme capacity decay when reacting with high concentration LiPSs in the pouch cell of practical condition. Unfortunately, this issue of Li metal anode corrosion is not treated as importantly in LSB studies as it is in lithium metal battery studies. Existing research lacks direct investigation into the corrosion of Li metal by LiPSs, with most studies focusing on characterizing the morphology of Li via SEM or assessing the degradation of Li metal through XPS. Consequently, a fundamental understanding of lithium metal surface corrosion by LiPS is required, paving the way for the development of strategies to protect Li metal. Furthermore, Research on dead sulfur formation at the anode is not extensively addressed. Dead sulfur at the anode refers to species that are reduced from LiPSs through the shuttle effect, subsequently deposited, and unable to function as active cathode material, leading to capacity decay [266,267,268]. Although it is important to suppress the reactivity between LiPSs and Li metal anode, as is currently conducted, reactivating the already-formed dead sulfur at the anode using the methods such as additives to re-engage it in electrochemical reactions is also crucial for enhancing the cycling performance of LSBs. Based on these considerations, a comprehensive LSB design that considers the interactions between each component could pave the way for the commercialization of LSBs.