1 Introduction

Towards peak carbon dioxide emissions, renewable and sustainable resources are highlighted on the research agenda. However, the uncertainty and instability of energy situation transformation poses an urgent test for energy storage devices. As emerging electrochemical energy storage alternatives, zinc ion energy storage devices (ZESDs) have received widespread attention due to their abundant resources, economic effectiveness, high safety, nontoxic, and environmental friendliness (Hu and Yang. 2018; Li et al. 2020; Wang et al. 2021b; Yin et al. 2021).

In ZESDs, both Faradic and non-Faradic charge storage processes can be involved. In non-Faradic reaction, Zn2+ ions or anions can be physically extracted from/embedded in the cathode material during charging/discharging, respectively. In parallel, Faradic process proceeds with redox reaction, promoting charge storage capability (Li et al. 2024a; Schoetz et al. 2022). Generally, ZESDs can be classified into zinc ion batteries (ZIBs) and zinc ion capacitors (ZICs) (Liu et al. 2021b; Wang et al. 2021a, 2022b; Yin et al. 2021). In ZIB, during discharging, Zn anode releases Zn2+ ions into zinc electrolyte, where Zn2+ ions in electrolyte pass through the separator and insert into the cathode, like manganese- or vanadium-based oxides. During charging, Zn2+ ions extract from the cathode and deposit as metallic Zn. Hence, ZIB can store Zn2+ ions based on the redox reactions, possessing the advantages of high capacity and energy density (Fang et al. 2018). On the other hand, ZIC typically adopts a carbon-based cathode coupled with Zn or intercalation-type anodes. As cathodes, carbons can store charges via the electric double-layer capacitance. For the electric double-layer capacitance, the charges can be stored electrostatically on the surface between electrode and electrolyte through a non-Faradic process, where the capability of adsorbing/desorbing charged ions can be depicted by the electric double-layer structure, where desolvation happens when solvated ions come across the outer Helmholtz plane and a dynamic equilibrium is achieved between charged species and metal plate in the inner Helmholtz plane. Carbon materials can be further endowed with pseudocapacitance, through incorporating redox-active sites or materials (Zhang and Pan. 2015). Consequently, ZICs present fast charging/discharging ability and high power density, long service life and good cycling stability (Ma et al. 2023; Zuo et al. 2017).

In ZESDs, carbon materials are widely applied, due to their promising chemical stability, high electronic conductivity, rich and tunable pore structure, multiple synthetic sources and environmental friendliness (Fig. 1; Wang et al. 2023). Sustainable research has been conducted and documented regarding novel carbon nanostructures for rechargeable batteries and supercapacitors with physical scales crossing zero-dimension (quantum dot, nanocage, sphere), one-dimension (carbon nanotube, carbon nanofiber), two-dimension (nanosheet, 2D graphene-based materials), and three-dimension (porous carbon, 3D framework) (Du et al. 2016; Shao et al. 2020; Wu and Dong. 2021; Zhu et al. 2011). The intrinsic structure characteristics of carbons determine their novel properties, e.g., hollow carbon spheres can accelerate mass transfer, CNTs with high aspect ratio provide direct electron transportation pathways, and graphene and its derivatives possess ultrahigh specific surface area (SSA). These unique features make carbon-based material one of the most promising cathode categories for ZESDs (Clancy et al. 2018; Li et al. 2024b; Wu and Dong. 2021; Xue et al. 2023b).

Fig. 1
figure 1

Carbon materials in zinc-ion energy storage devices

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However, ZESDs face challenges from materials design to device fabrication, which seriously hinder their development. Concerning cathodes in ZESDs, carbon materials still struggle against their limitations such as low energy density, dissatisfied specific capacity, and poor rate performance, due to the unsatisfactory design of pore structure, agglomeration and staking of layered structure, and insufficient electrochemical active sites (Yin et al. 2021). Various strategies have been developed accordingly, such as heteroatom doping into the carbon host, surface modification, defect engineering and advancing carbon-based composite. On the flip side, carbon materials are applied to address the problems of other types of cathode materials in ZIBs (like irreversible dissolution of metal oxides, vast volume variation during charging/discharging cycles and sluggish kinetics) (Mathew et al. 2020), owing to their electrical conductivity, ease of further modification or compositing, and excellent structural stability and accommodability. With rational design, carbon materials can serve as the coating or protective layer to provide optimized transmission path for electrons and ions, constraint structure change, or reduce direct contact between cathode materials and the electrolyte. In addition, they can be integrated as host or skeleton to facilitate the growth of active materials and accommodate the volume change of active materials during cycling.

On the anodic side, metallic Zn faces the entangled critical issues of dendrite growth, hydrogen evolution reaction and the growth of surface by-product, leading to deteriorated Coulombic efficiencies (CEs) and much-shortened lifespans (Fig. 2; Jia et al. 2020; Li et al. 2023b, 2023c; Liu et al. 2023; Yu et al. 2023; Yuan et al. 2023a, 2021; Zhu et al. 2024). Cabon materials, as surface coating or protective layer, can avoid direct contact between the Zn anode and the electrolyte, homogenize the interfacial electric field and eliminate the tip effects, regulate ion flux by pore structure and surface modification, and offer abundant active sites as nucleation sites for Zn deposition, thus favoring uniform Zn deposition and texture manipulation. When applied as host or skeleton for Zn anode, high SSA and porous structure of carbon materials with heteroatom doping or grafted functional groups can guide the conduction of zinc ions, promote interfacial kinetics, and further regulate Zn deposition, and in the meantime tolerating the vast volume change of Zn anode during cycling (Wang et al. 2024).

Fig. 2
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Current challenges for ZESDs: (a) issues for cathode materials, (b) issues on metallic Zn anode

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In light of the significance of carbon materials, a mini review focusing on the strategies and operational mechanism of carbon-based materials for advancing ZESDs is presented. Different types of carbons as cathodes in ZESDs are investigated first, with current methods of improving carbon-based cathode materials from the perspectives of pseudocapacitance engineering and composite engineering. Also, facile carbon coatings and skeletons are discussed to improve the performance of other types of cathode materials in ZESDs. In addition, carbon materials for advancing electrolyte design are introduced. Further, current strategies to achieve stable Zn anode by carbon materials are summarized from the angles of coating and skeleton/host with their operational mechanisms. Outlooks and perspectives on the future development of carbon materials for the applications of ZESDs are provided.

2 Carbon materials for cathodes in ZESDs

Carbons for electrochemical devices can be classified by their distinctive dimensions and corresponding structures, which will be briefly introduced with their merits and current limitations (Fig. 3). In order to enhance their performance, corresponding strategies are discussed from the aspects of pseudocapacitance engineering and composite engineering. Furthermore, carbon coatings or skeletons are assessed to address the issues of other types of cathode materials.

Fig. 3
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Carbon structures for charge storage with their merits and shortages

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2.1 Types of carbon nanostructures as cathode materials

2.1.1 One-dimension

Carbon nanotube is the representative one-dimensional carbon nanostructure, owning a hollow tubular structure formed by bending one or more layers of graphene with high length-to-diameter aspect ratio (Hu et al. 2023; Kinloch et al. 2018; Luo and Swager. 2023). Featuring unique fast transfer pathways, CNTs are widely used as cathodes in ZICs, due to its good electrical conductivity, outstanding ionic transportation, chemical stability and mechanical stability, and flexibility (Clancy et al. 2018; Shi et al. 2023; Zhang et al. 2022d), which can be integrated into micro-cathode for zinc-ion micro-supercapacitor (Sun et al. 2018).

However, the limited SSA of CNT with the lack of electrochemical active sites restrains its performance, e.g., the ZIC with cathode by pristine CNTs, an aqueous ZnSO4 electrolyte and Zn anode, showed a low capacitance of 53 F g−1 only, which was attributed to the low SSA of the pristine CNT (211 m2 g−1) (Tian et al. 2016). Modifications like introducing the pseudocapacitive active sites through heteroatom doping, have been applied to increase its capacitance. Further, the inner space of CNT can be exploited for extra charge storage behavior, e.g., nitrogen rich porous carbon nanotubes were incorporated with sulfur to endow the cathode with sulfur-based chemistry (Li et al. 2022).

2.1.2 Two-dimension

Compared to one-dimensional nanostructure, two-dimensional carbon materials have much larger lateral size-to-stacking size aspect ratios, booting in-plane kinetics (Georgakilas et al. 2016; Sun et al. 2020).

With their superior SSA, 2D graphene and its derivatives, e.g., graphene oxide (GO), and reduced graphene oxide (rGO), can offer abundant active sites for chemical/electrochemical reactions, e.g., chemically active graphene as cathodes for supercapacitors (Zhu et al. 2011). Graphene-based materials can be further anchored with redox-active materials to enhance its electrochemical performance, like redox polymer polypyrrole grown on electrochemical graphene oxide for ZIC (Yang et al. 2021). On the other hand, graphite, as the natural source for graphene exfoliation, possesses iconic layered structure and chemical stability, can be used directly for ZESDs, where novel charge storage mechanisms can be explored, like anion intercalation (2017), as we discuss later in Section 3.

Recent works mainly focus on solving the prone problems of aggregation/agglomeration of two-dimensional layered materials with the sharply reduced charge storage capacity. Synthesis of graphene-based nanocomposites may serve as a facile way to stabilize the layer structure (Li et al. 2021b).

Besides, 2D carbon nanosheets can be readily synthesized by bottom-up approaches, where heteroatoms, such as N, O, S, and P, can be easily introduced by selecting the corresponding precursors. Note that biomass stock is an attractive resource from this perspective. Previous study successfully fabricated N, O co-doped 2D carbon nanosheets through one-step combustion conversion of wood with urea, achieving a delightful SSA of 1248 m2 g−1 and capacity of 110 mAh g−1 at 0.1 A g−1 for ZIC (Lou et al. 2021).

2.1.3 Three-dimension

Commercial products of activated carbon (AC) have been widely used in batteries and supercapacitors. The regulation of its porous structure is long pursued, where advancing synthetic approaches are being established, e.g., tetra-alkali metal pyromellitic acid salts was used as precursors for AC (Wang et al. 2022b). Meanwhile, conversion of renewable biomass resources into novel carbon structures is ongoing (Gong et al. 2023; Yu et al. 2024), e.g., an ice template-assisting activation method was developed for bio-oil to prepare porous carbon cage with open structure (Xue et al. 2023a).

Hierarchical porous carbon network might be considered as modern 3D caron structure (Yuan et al. 2023b), where the effects of the unique porous structure are complex. Large specific surface area and high porosity can provide abundant contact area for the electrolyte and then enhance the transmission capacity. However, improper pores size distribution (macropores, mesopores, micropores) might restrict the performance enhancement, influenced by micropore effects and poor compatibility with electrolyte. Thus, the regulation of porous structure is crucial, which could be achieved by adjusting the precursor and templates.

Additionaly, 3D carbon strucutre can be formed by assembling 0D, 1D, 2D structures, or their composites, whose indivisual merits can be inherited, e.g., flexiblity can be achieved when 2D graphene-based materials are incorporated into the framework.

In short, though carbon structures of various dimensions exhibit promising properties as cathodes for ZESDs, there is large room to improve their electrochemical performance. The key factors to concern may include SSA, active sites, pores (size, distribution, molecular/ionic affinity, arrangement), crystallographic structures, electronic/ionic conductivity and redox-active entities with their arrangement. Accordingly, current strategies to improve the performance of carbon-based cathodes in ZESDs are discussed in the following.

2.2 Pseudocapacitance engineering

Pseudocapacitance engineering facilitates more efficient charge transfer/storage process and increases the energy density and power density of energy storage devices (Gao et al. 2022), including surface redox pseudocapacitance and intercalation pseudocapacitance (Fleischmann et al. 2020). From the angle of crystallographic structure, replacing one atom in a host site by a heteroatom, forms a defect. Also, vacancy is another type of defects. In addition, carbon atom with extra energy or unsaturated bonds can be considered as defects, which could be generated by physical methods, like ball milling (Dong et al. 2019). Comparing to pristine carbon structures, heteroatom doping, such as B, O, N, S and P, is one of mostly efficient routes to gain additional capacity (Liu and Wu. 2023). Chemical methods are widely used to achieve heteroatom doping, where heteroatoms can be introduced via precursor entity, fuel for calcination, or fuel for post-synthesis.

Previous research used nano-MgO as template and melamine-phytic acid supramolecular aggregate as dopant coupled with KOH activation, successfully synthesizing the interconnected N/P co-doped carbon nanocages (NP-CNC) (Fig. 4a; Yang et al. 2022), where the nanocage framework provides space to absorb ions and the N/P co-dopants offer pseudocapacitance. As a result, NP-CNC had a much higher capacitance of 165.9 mAh g−1 than the pristine CNC (93.3 mAh g−1) or single atom-doped CNC at 0.5 A g−1. Previous study optimized the zinc ion storage capacity of onion-like carbon (OLC) by inserting N and P dopants (Wang et al. 2022a). The fabricated N,P-OLC presented a fully accessible external surface for ion adsorption and a high proportion of mesopores for fast ion migration, where N and P co-doping in the carbon matrix favors the chemical adsorption of Zn2+ ions. The constructed ZIC delivered a high specific capacitance of 420.3 F g−1 (184.5 mA h g−1) at 0.5 A g−1. N,P-OLC can be further integrated with a carbon cloth (CC) or carbon fiber (CF) to form a 3D freestanding and flexible structure. The unique hollow nanosphere structure of carbon can facilitate the transportation of electrolyte ion by offering accessible inner surface, by selective template or surface modification. Previous study used a dual-functional template-induced approach together with a subsequent carbonization process to create dual-doped carbon hollow nanospheres (Fig. 4b; Li et al. 2021a). The assembled ZIC provided a ultra-long cycling stability up to 12,000 cycles, an extraordinary energy density of 116.0 Wh kg−1 at a power density of 141 W kg−1, and an exceptionally high power density of 21660 W kg−1 under a respectable energy density of 36.1 Wh kg−1.

Fig. 4
figure 4

Pseudocapacitance engineering for carbon-based cathodes. a Schematic of the synthesis process of MPSA and NP-CNC (Yang et al. 2022). Copyright Springer-Verlag GmbH Germany. b Formation process of dual-doped carbon hollow nanospheres (Li et al. 2021a). Copyright 2021, Elsevier B.V. c Synthesis of carbon nanosheet of WC-6ZnN-12U and its charge storage mechanism (Lou et al. 2021). Copyright 2020, Elsevier B.V. d First-Principles Calculations for zinc storage behavior (He et al. 2021). Copyright 2021, Elsevier B.V. e Schematic illustrating the formation mechanism of the LNPC with high edge-nitrogen doping and large mesopores. f First-principles calculations of Zn adsorption (Zhang et al. 2022c). Copyright 2022, Elsevier Ltd. g Synthesis process of PC700 and NPFCs. h Schematic of the operation mechanism of ZHC during charge/discharge process and (i) Adsorption energy between zinc and various graphene (Wei et al. 2022). Copyright 2022, Elsevier B.V

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Considering 2D structure, previous research adopted a one-step combustion conversion of wood to achieve N, O co-doped 2D carbon nanosheets for ZIC (Fig. 4c; Lou et al. 2021). Using urea as fuel and Zn(NO3)2·6H2O as oxidant, this combustion conversion synthesis may readily accomplish carbonization, pore formation, and heteroatom doping in a single step. The ZICs based on N, O co-doped 2D carbon nanosheets showed an attractive energy density of 109.5 Wh kg−1 at 225 W kg−1, a high capacity reservation of 92.7% after 50,000 cycles.

Note that acid treatment on AC has been well established to introduce the oxygen-containing functional groups. Abundant oxygen-containing functional groups can provide additional pseudocapacitance by introducing fast redox reactions on or near the carbon surface (Wang et al. 2023), by generating adsorption/desorption sites for ions. The oxygen-enriched hierarchical porous CF films with super-hydrophilic character were synthesized via a facile electrospinning and subsequent nitric acid-treatment process (He et al. 2021). The pseudocapacitance is attributed to the chemical adsorption of zinc ions by oxygen containing functional groups on the cathode surface (Eq. [1]). Three possible Zn2+ adsorption sites on OPCNF-20 (Zn1: between two carboxyl groups, Zn2: between two carbonyl groups, and Zn3: between carbonyl and carboxyl) were assessed by density functional theory (DFT) (Fig. 4d), which indicates that Zn2 sites with the lowest adsorption energy are most probable sites to capture Zn atoms. Metal–organic framework (MOF)-derived carbon composites are considered promising materials for high-performance ZESDs (Tan et al. 2021; Umezawa et al. 2023; Zhang et al. 2023b). MOF-based carbon composites with nanostructure were synthesized by adopting a bidirectional electrostatic generated self-assembly strategy. With the synthesized nanocarbon composite as the cathode, the ZIC exhibited a high specific capacitance of 236 F g−1 at 0.5 A g−1. Previous study developed a direct pyrolysis method using supermolecules to create highly heteroatom-doped 3D porous carbons with a large mesopore size from sustainable sodium lignosulfonate resources (Fig. 4e; Zhang et al. 2022c). Six configurations (G, pristine graphene, N-Q, graphene with graphitic nitrogen, N-6, graphene with pyridinic nitrogen, N-5, graphene with pyrrolic nitrogen, CO, graphene with carbonyl and COH, graphene with hydroxy) were compared for Zn adsorption (Fig. 4f). Compared with G and N-Q sites, configurations with edge-defect sites showed enhanced Ea. In addition, the N- and O-induced defects presented higher Ea values for H adsorption at edge sites and the sites near the dopant atom, than that for the pristine graphene. N-doped porous carbon then demonstrated a high gravimetric specific capacitance of 266 F g−1 with high rate capability. Previous research used osmanthus flowers as carbon source, prepared N, P co-doped foamy-like carbons (NPFC) with abundant defect sites (Fig. 4g; Wei et al. 2022). The spin-polarized DFT calculations (Fig. 4h) show that the adsorption energy of Zn on N/P dual-doped graphene was stronger than graphene, N-doped graphene and P-doped graphene. The introduction of defects meanwhile reduced the distance of ions transport and improved kinetics (Fig. 4i). Based on theoretical calculations on binding energies between a Zn atom and an O-/N-doped graphene modeled with all possible doping sites, high-loading carbon cathodes based on holey activated carbon sheets (HACS) were fabricated from potassium phthalimide. HACS-based high-loading cathodes (16.1 mg cm−2) delivered a large capacity of 208 mAh g−1 at 1 A g−1, resulting in the highest energy density and power density of 2.40 mWh cm−2 and 10.72 mW cm−2 (Xu et al. 2024). A novel 3D phosphorus-doped carbon nanotube/reduced graphene oxide (P-CNT/rGO) aerogel cathode was synthesized through a synergistic modification strategy of CNT insertion and P doping modification combined with 3D porous design. The as-obtained P-CNT/rGO aerogel cathode manifested significantly increased surface aera, expanded interlayer spacing, and enhanced pseudocapacitance behavior, thus leading to significantly enhanced specific capacitance and superb ions transport performance (Yao et al. 2024).

$$2C=O+{Zn}^{2+}+2{e}^{-}=C-O-Zn-O-C$$
(1)

Regarding pseudocapacitance engineering, the mechanism of heteroatom doping during the electrochemical process is attribute to chemical adsorption/desorption of Zn2+, H+ and other groups, as below:

The faradic reaction between zinc-ion and C–OH (Zhang et al. 2021a, 2019a):

$$C-OH+{Zn}^{2+}+{e}^{-}\leftrightarrow C\cdots O\cdots Zn+{H}^{+}$$
(2)

The electrochemical interaction between C⋯O / N⋯O and Zn2+/ H+ (Deng et al. 2020; Li et al. 2021a; Tian et al. 2016):

$$C\cdots O+{Zn}^{2+}+2{e}^{-}\leftrightarrow C\cdots O\cdots Zn$$
(3)
$$N\cdots O+{Zn}^{2+}+2{e}^{-}\leftrightarrow N\cdots O\cdots Zn$$
(4)
$$C\cdots O+{H}^{+}+{e}^{-}\leftrightarrow C\cdots O\cdots H$$
(5)
$$N\cdots O+{H}^{+}+{e}^{-}\leftrightarrow N\cdots O\cdots H$$
(6)

The reversible surface coordination reaction between Zn2+ and the carbonyl group (He et al. 2021; Wang et al. 2021b):

$$2C=O+{Zn}^{2+}+2{e}^{-}\leftrightarrow C-O-Zn-O-C$$
(7)

The electrochemical adsorption/desorption of H+ (Wang et al. 2021c):

$$C\cdots O+{H}^{+}+{e}^{-}\to C\cdots O\cdots H$$
(8)
$$C\cdots O+{H}^{+}+{OH}^{-}-{e}^{-}\to C\cdots O\cdots {H}_{2}O$$
(9)

The reversible surface redox reaction between the carbonyl group and hydroxyl group (Wang et al. 2021b; Yin et al. 2020):

$$C=O+{H}^{+}+{e}^{-}\leftrightarrow C-OH$$
(10)

2.3 Composite with redox-active materials

Synthesis of nanocomposite for carbon materials is effective to enhance their kinetics and capacities (Wang et al. 2022d; Yin et al. 2020). Composites consisting of two types of carbon materials are powerful candidates, e.g., rGO/CNT (Zhang et al. 2019b), whose structure integrity especially for interlayer spacing can be well retained during cycling. And both redox-active inorganic and organic materials can be integrated into the carbon structures by facial synthetic methods, where carbon materials can act as synthetic templates for the growth of redox-active materials, or as self-assembly platform.

Previous study incorporated pseudocapacitive layered niobium oxyphosphide (NbPO) into rGO (Fig. 5a) (Patil et al. 2022). By using the optimal zinc electrolyte of 0.2 M ZnSO4 with 5 M NaClO4, rGO-NbPO based ZIC presented over twice-enhanced capacitance of 123 F g−1 at 40 mV s−1, compared with NbPO. By comparative capacitance, the charge storage of rGO-NbPO is attributed to surface Zn2+ adsorption, where CV analysis further verified a dominant capacitance-controlled process (Fig. 5b). Previous research prepared an interlayer spacing-regulated MXene/rGO foam for zinc ion microcapacitor and realized a large area-specific capacitance of 83.96 mF cm−2 (Zhang et al. 2022a). The hydrazine vapor-induced reduction strategy can precisely regulate on the density (100–360 mg cm−3) and macropore size of the MXene foam (5.08–61.04 μm) by tuning the mass ratios of MXene/GO. EIS analysis revealed that higher conductivity and rapid kinetics by the MXene/GO composite (Fig. 5c). Mxene/GO in the microcapacitor further exhibited enhanced electrochemical properties, i.e., a large area-specific capacitance of 83.96 mF cm−2 at 0.5 mA cm−2, a high energy density 21.3 μWh cm−2 at 337.5 μW cm−2, and a high power density 6750.4 μW cm−2 at 2.4 μWh cm−2.

Fig. 5
figure 5

Composite engineering for carbon-based cathodes. a Preparation of hybrid architecture of rGO-boosted 2D layered niobium oxyphosphide. b Probable charging/discharging processes in rGO–NbPO ZIC (Patil et al. 2022). Copyright 2021, Elsevier B.V. c Schematic configuration of ZIMC (Zhang et al. 2022a). Copyright 2022, Elsevier B.V. d Schematic illustration of the preparation of RGO@PPD (Xu et al. 2021). Copyright 2021, Elsevier B.V

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Besides inorganic materials, redox-active organic molecules can provide extra pseudocapacitance, meanwhile preventing the agglomeration of layered structure. Previous study utilized p-phenylenediamine (PPD) as the interlayer spacer for the graphene film (Fig. 5d), which can effectively enlarge the interlayer spacing and expose additional accessible surface for ions (Xu et al. 2021). Further, PDD can undergo a reversible redox reaction with electrolyte ions, endowing graphene film with an incredible pseudocapacitance. The ZIC fabricated by RGO@PPD (6:7) as cathode and Zn foil as anode exhibited an exceptional areal capacitance of 3012.5 mF cm−2, and a high energy densities of 1.1 m Wh cm−2 at power densities of 0.8 mW cm−2.

In short, promising performance has been achieved in carbon cathodes for ZICs (Table 1). High power densities and retention rates are general features for ZICs, however, introduction of heteroatoms or redox entities is critical to boost their energy densities.

Table 1 Comparison of electrochemical performance of recent carbon cathode materials for ZICs
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2.4 Carbon coatings/skeletons for cathode materials

There are critical issues for the representative cathode materials for ZIBs, such as manganese-based and vanadium-based oxides. The dissolution of active materials causes a rapid decline in capacity, related to either the Jahn–Teller effects of Mn2+ or solubility of vanadium oxide in aqueous environment (He et al. 2023; Liang et al. 2020; Liu et al. 2024a; Ren et al. 2019; Zhang et al. 2020; Zhu et al. 2024). Also, the structural distortion and volume change of oxide materials during repetitive insertion/desertion process of large size zinc ions can lead to fast decay of cathode capacity. The above problems seriously affect the lifespan of ZIBs. Carbon materials can be applied as coatings or hosts/skeletons to improve the dissolution problem and accommodate the volume change of cathode materials during cycling.

2.4.1 Carbon materials for MnO2 cathodes

With the advantages of high theoretical capacity, low cost, low toxicity and multiple valence states, manganese-based materials are widely used as cathode materials for aqueous ZIBs (Chao et al. 2020; Luo et al. 2024; Wang et al. 2022d; Wu et al. 2022; Zhong et al. 2020). However, inherent obstacles of low conductivity and poor cycle stability hinder their further application (Lv et al. 2023, 2022a). Strategies like coating by or compositing with carbon materials have been developed (Jia et al. 2020; Lv et al. 2024).

Previous study used graphene scroll-coated α-MnO2 as the cathode for ZIB (Wu et al. 2018). With an average width of 5 nm, the graphene scroll was evenly covered on MnO2 nanowires (Fig. 6a), increasing their electrical conductivity. ICP analysis showed that rGO was effective in inhibiting the dissolution of Mn2+ from Mn3+ disproportionation into the electrolyte during cycling, thereof improving the cathode stability. Also, the rate capacity was greatly improved, which was attributed to the improved conductivity and enriched adsorption sites by the rGO coating. A remarkable energy density of 406.6 Wh kg−1 (382.2 mA h g−1) at 0.3 A g−1 was achieved, with a good long-term cycling stability of 94% capacity retention after 3000 cycles at 3 A g−1 in 2M ZnSO4 and 0.2 M MnSO4. Previous study developed an onion-like N-doped carbon and amorphous carbon coated MnOx nanorods (MnOx@N–C) (Fig. 6b, c) (Fu et al. 2018). The porous structure and conductive network favored the reaction kinetics, leading to lower polarization in the electrochemical reaction process, as verified by both redox peaks in CV and charge/discharge plateaus. In particular, it presented a high capacity of 305 mAh g−1 after 600 cycles at 500 mA g−1 in 2 M ZnSO4 electrolyte with 0.1 M MnSO4, and an achievable capacity of 100 mAh g−1 at a relatively high rate of 2000 mA g−1 with long-term cycling of up to 1600 cycles.

Fig. 6
figure 6

Carbon materials for other types of cathodes in ZESDs. a Schematic illustration of the formation of the MGS (Wu et al. 2018). Copyright 2018, WILEY–VCH Verlag GmbH. b Schematic illustration of MnOx@N–C composite. c The interconnected part of three MnOx@N–C nanorods (Fu et al. 2018). Copyright 2018, WILEY–VCH Verlag GmbH. d Fabrication and characterization of the NCAVA electrode. e Schematic illustrating the structural evolution of the NCAVA cathode at different stages (Fei et al. 2023). Copyright 2023, Wiley–VCH GmbH

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2.4.2 Carbon materials for V2O5 cathodes

Though with high capacities, the performance of vanadium oxide-based cathodes is limited by their low electrical conductivities, inevitable dissolution in aqueous solutions, and slow reaction kinetics (Chernova et al. 2009; Kundu et al. 2016; Song et al. 2018). In addition, the layer structure of V2O5 polymorphs can be destructed during cycling, which acquires a host or skeleton.

Previous study proposed a core–shell N-doped carbon-encapsulated amorphous vanadium oxide arrays as a highly stable and effective cathode material for ZIB (Fig. 6d, e) (Fei et al. 2023). The ZIB demonstrated an impressive 0.92 mAh cm−2 discharge capacity at 0.5 mA cm−2, an exceptional 0.51 mAh cm−2 rate capability at 20 mA cm−2, and an exceptionally extended cycling stability with approximately 100% capacity retention after 500 cycles at 0.5 mA cm−2 and 97% capacity retention after 10,000 cycles at 20 mA cm−2. The strengthening mechanism is attributed to the incorporation of unobstructed ions diffusion routes and abundant active sites extended by vanadium oxide and electron transfer efficiency and stability provided by N-doped carbon shell.

Edge-rich vertical graphene (VG) nanosheets were designed as effective nanotemplates loading V2O5 nanosheet (Zhang et al. 2021c). VGs were firstly grown on CC substrates by microwave plasma chemical vapor deposition (MPCVD), and V2O5 nanosheets were decorated on the surface of VGs by a facile one-step hydrothermal approach. The edge-rich V2O5/VG/CC hybrid cathodes with the large SSA and porous nature contributed to the pseudocapacitive behavior of V2O5/VG/CC. V2O5/VG/CC exhibited a high discharge capacity of 370 mAh g−1 at 0.2 A g−1with a high CE of approximately 92% at the first charge/discharge process, owing to the suppression of by-product formation on surface, compared with fluctuated CEs for V2O5/CC. Moreover, V2O5/VG/CC favored the reaction kinetics by its lower charge-transfer resistance.

3 Electrolyte Regulation for carbon materials

The electrolytes for ZESDs are intensely reviewed (Li et al. 2023d; Liu et al. 2021a; Liu et al. n.d.; Lv et al. 2022b). The physicochemical properties of electrolytes, including solvation structure, pH, electrochemical window, ion conductivity, and viscosity, significantly influence the electrochemical process (Li et al. 2022; Liu et al. 2022). Novel electrolytes with widen EWs are desirable for ZESDs, which depress the water-induced side reactions. However, when working voltage is close to the limits of EW, different charge storage mechanisms can proceed, compared to the conventional ones in ZESDs. Caron materials, coupled with novel electrolytes, can demonstrate their superior stability to explore novel charge storage mechanisms; like H adsorption/desorption, anion intercalation/de-intercalation can operate (Fig. 7).

Fig. 7
figure 7

Functions of carbon materials in electrolyte. a Schematic illustrating the (Zn/PC)/PC ZIC involves reversible hydrogen adsorption and oxygen redox reactions during the charge and discharge process (Yin et al. 2020). Copyright 2020, Wiley–VCH. b Schematic representation of the cell structure and working principle of a zinc-dual-halogen battery using a molten hydrate electrolyte (Liu et al. 2020). Copyright 2020, Wiley–VCH. Cyclic voltammograms of highly oriented pyrolytic graphite (HOPG) electrodes in aqueous solutions of (c) 1 mol kg−1 NaFSA and (d) 19 mol kg−1 NaFSA. Scan rate: 1 mV s−1 (Kondo et al. 2019). Copyright 2019, Elsevier. e Schematic illustration of ZGB configuration and electrode reactions (Wang et al. 2020b). Copyright 2019, Wiley–VCH. f Nucleation overpotentials test in Zn symmetric cells at 1 mA cm−2. g Chronoamperograms at an overpotential of -150 mV versus the open circuit potential using three-electrode system (Zhang et al. 2023a). Copyright 2023, Elsevier B.V. h Numerical simulations of electric field distributions (Han et al. 2023). Copyright 2022, Elsevier B.V. i The electric field distribution on the surface (Abdulla et al. 2021). Copyright 2021, American Chemical Society

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3.1 Reversible H adsorption/desorption

A high-performance aqueous ZIC was fabricated by employing an oxygen-rich porous carbon (PC) as the cathode and 3 M Zn (ClO4)2 electrolyte offering a potential window of 0–1.9 V (vs Zn/Zn2+) (Yin et al. 2020). Reversible H adsorption–desorption was proposed to be the charge storage mechanism. Hydrogen ions can be electrochemically adsorbed and reduced on PC in 0.25–0 V (vs Zn/Zn2+) during discharging (Eq. [11]; Fig. 7a). During charging, the adsorbed hydrogen atoms are oxidized into hydrogen ions (Eq. [12]), rather than released as hydrogen gas (Eq. [13]) This highly reversible hydrogen adsorption pseudocapacitance is further supported by the accompanied formation and dissolution of zinc hydroxide species during discharging/charging. When discharging below 0.25 V (vs Zn/Zn2+), hydrogen adsorption leads to the pH of the electrolyte increasing to above 5.47, which induces the formation of zinc hydroxide on PC that gradually dissolves during charging.

$${H}^{+}\left(aq\right)+{e}^{-}\to {H}_{ads}$$
(11)
$${H}_{ads}-{e}^{-}\to {H}^{+} (aq)$$
(12)
$${H}_{ads}+{H}_{ads}\to {H}_{2} (g)$$
(13)

3.2 Anion intercalation/de-intercalation

A new prototype zinc-based battery was designed and fabricated with the graphite cathode and a molten hydrate electrolyte of ZnCl2·(xKBr)·2H2O, with optimized x to be 0.03 (Liu et al. 2020). The designed cell exploits both Br0/Br− bromine and Cl0/Cl− chlorine as the dual redox couples for cathodes, as halozinc complexes (Fig. 7b). The charge storage study implies that the Br0/Br− redox reaction (Eq. [14]) involves surface adsorption/desorption of Br0 at the graphite-electrolyte interface and (de)intercalation and diffusion of Br0 within the bulk of graphite. And the Cl0/Cl− redox reaction (Eq. [15]) is mainly dominated by the Cl0 diffusion inside the graphite.

$$\begin{array}{ll}Zn[{Br}_{m}{Cl}_{4-m}{]}^{2-}+C\to & \\ C\left[{Br}_{m}\right]+Zn[{Cl}_{4-m}{]}^{m-2}+m{e}^{-}& 1.7-1.8\text{V vs Zn}/{\text{Zn}}^{2+}\\ Zn[{Cl}_{4-m}{]}^{m-2}+C\left[{Br}_{m}\right]\to & \end{array}$$
(14)
$$\begin{array}{cc}C\left[{Br}_{m}{Cl}_{n}\right]+Zn[{Cl}_{4-m-n}{]}^{m+n-2}+n{e}^{-}& 1.8-2.0\text{V vs Zn}/{\text{Zn}}^{2+}\end{array}$$
(15)

Graphite intercalation compounds of bis (fluorosulfonyl)amide (FSA-GICs) were electrochemically synthesized in a highly concentrated 19 m sodium bis (fluorosulfonyl)amide (NaFSA) (Kondo et al. 2019). In CVs (Fig. 7c and d), only oxidative currents were observed in a dilute aqueous electrolyte, but large redox peaks at high potential appeared in a highly concentrated aqueous electrolyte, indicating the intercalation/de-intercalation of FSA anions. The formation of stage 3 FSA-GICs was confirmed by XRD. A high-voltage and durable Zn-graphite battery (ZGB) was enabled by a graphite cathode, a Zn anode, and a hybrid electrolyte comprising of 0.5 M Zn (TFSI)2 + 2 M LiPF6. During the charge process, both of PF6− and TFSI− are intercalated into graphite cathode (Wang et al. 2020b). And simultaneously, Zn2+ is electrochemically reduced and deposited as metallic Zn due to high redox potential of Zn (-0.76 V vs standard hydrogen electrode [SHE]) compared to Li (-3.04 V vs SHE). In the discharge process, reverse reactions take place, with PF6−/TFSI− and Zn2+ diffusing back into the electrolyte (Fig. 7e). Pulsed-field-gradient (PFG) diffusion NMR shows that PF6− can efficiently suppress the anodic oxidation of Zn (TFSI)2 electrolyte and result in a 4.0 V (vs Zn/Zn2+) electrochemical stability window. DFT study further supports that both PF6− and TFSI− can intercalate into the graphite cathode, leading to dual-anion intercalated graphite.

3.3 Electrolyte additives

Besides, carbons can be regarded as media in zinc electrolyte to homogenize electric field distribution, thereby avoiding serious "tip effects", promoting Zn2+ transport process, and achieving a uniform nucleation process.

Hybrid electrolyte based on DMF/H2O solution containing carbon dots (CDs) additive was proposed to construct robust electrode/electrolyte interface (Zhang et al. 2023a). When adopting CDs as functional additive, the stronger zinc affinity of the modified -SO3− group on CDs is conductive to reducing the nucleation barrier and promoting nucleation sites (Fig. 7f and g). Functionalized graphene quantum dots (F-GQDs) modified with multiple functional groups were introduced as additives into electrolytes (Han et al. 2023). Numerical simulations of electric field distributions for Zn anodes (Fig. 7h) indicate that the Zn anode coated by the F-GQDs benefits from an optimized Zn nucleation at nanoscale, contributing to a more uniform electric field distribution than the case without F-GQDs. A small amount of GO powder was dispersed into ZnSO4 electrolyte as an electrolyte modifier for stabilizing Zn anodes (Abdulla et al. 2021). The interaction between GO particles and Zn metal could not only inhibit the formation of differentiated electric fields and promote the even distribution of the electric field but also ensure the positively charged Zn2+ to quickly conduct to the anode surface, thus leading to a uniform Zn deposition layer (Fig. 7i).

4 Carbon materials for Zn anode

Various types of carbon materials have been investigated as surface layers for Zn anodes, possessing high electronic conductivity, high chemical stability, and large SSA. The important roles of carbon-based surface layers on regulating electrolyte-Zn interface include acting as physical barrier, homogenizing interfacial electric field, offering abundant active sites or nucleation sites, regulating the molecular interactions with functional groups and ion transport, and manipulating crystal orientation (Fig. 8; Yin et al. 2021).

Fig. 8
figure 8

Functions of carbon materials for Zn anode. a Schematic diagram of the Zn deposition evolution (Sun et al. 2022). Copyright 2021, American Chemical Society. b Simulated electric filed distribution on carbon film coated Zn@CF anode (Zhai et al. et al. 2021). Copyright 2021, Wiley–VCH. c Schematic diagram illustrating the Zn plating behavior of the bare Zn and Zn/rGO anodes (Xia et al. 2019). Copyright 2019, Elsevier. d Schematic illustration for morphology evolution of the bare Zn and ZIF-8@Zn anodes during stripping/plating processes (Pu et al. 2020). Copyright 2020, Springer Nature. e Schematic illustration of the sub-ångström ion tunnel of HsGDY (Yang et al. 2020). Copyright 2020, Wiley–VCH. f In situ optical microscopy visualization of Zn plating on bare Zn (left) and NGO@Zn electrodes (right) at 5 mA cm−2 at various times, respectively. The red dashed line represents the surface shape of the deposition. g Optical surface-profilometry image of the NGO@Zn electrode (Zhou et al. 2021). Copyright 2021, Wiley–VCH. h Scheme illustrating the design principle of epitaxial metal electrodeposition (Zheng et al. 2019). Copyright 2019, The American Association for the Advancement of Science. Zn atom adsorption structures and the corresponding binding energies in case of (i) Pristine Gr (E = -0.02 eV), (j) Gr with single defect (E = -1.26 eV) and (k) Gr with four defects (E = -4.41 eV) (Foroozan et al. 2019). Copyright 2019, American Chemical Society

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4.1 Functions of carbon materials for Zn anode

Carbon-based coatings can act as physical barriers to inhibit the side reactions. An artificial composite layer consisting of copper nanosheets and graphene oxide (GO) coating was constructed on Zn anode (denoted as MZn) (Fig. 8a; Sun et al. 2022). With smooth surface morphology by MZn after cycling compared with bare Zn, the XRD diffraction peaks for the byproduct of Zn4SO4(OH)60.5H2O on MZn was highly reduced compared with that for the bare Zn, indicating the depression of parasitic reaction.

Also, carbon coatings on Zn anode can effectively homogenize interfacial electric field. The high conductivity of carbon coatings can effectively reduce charge accumulation and lead to more uniform distribution of surface electric field by their fast and effective electron transfer channels (Shen et al. 2018). A 10 nm thick conductive carbon film was sputtered on a carbon fiber (CF)@Zn, which was fabricated by electrodepositing Zn on CFs (Zhai et al. 2021). The modified fiber anode can effectively regulate the surface electric field to mitigate tip effect, which was simulated by Ansoft Maxwell software (Fig. 8b).

In addition, carbon-based materials can also offer abundant active sites or nucleation sites (Cai et al. 2024; Liu et al. 2022; Zhou et al. 2024). A simple and effective method was developed to spontaneously reduce GO by Zn metal, where rGO self-assembles on Zn surface to form a protective layer (Xia et al. 2019). After Zn stripping/plating, rGO coated Zn anode remained a fairly dense and smooth surface, which is attributed to the layered structure of rGO, providing a larger surface area and nucleation site (Fig. 8c). The ZIF-8 derived carbon layer contains lots of N sites that can enhance the transport kinetics of Zn2+ (Yuksel et al. 2020). Meanwhile, the porous carbon layer retained a significant amount of OH or COOH, which along with N could provide active sites for Zn2+ to reside (Fig. 8d).

Ion tunnels can be formed by carbon layer, which can regulate the molecular interactions and ion transport with zincophility groups. Hydrogen-substituted graphdiyne (HsGDY) with sub-ångström-level ion tunnels was coupled on Zn anode, consisting of abundant nanosheets (Fig. 8e; Yang et al. 2020). The upper surface of the HsGDY layer was composed of brushy texture, while the lower part was a compact layer with a thickness of ~ 490 nm. Elemental mapping reveals the cross-layer diffusion process that Zn2+ migrated through in the HsGDY layer and was then plated on the Zn electrode beneath HsGDY. The constructed sub-ångström-level ion tunnels tend to homogenize the Zn2+ concentration flux on the surface, deviating from the non-uniformly distributed Zn2+ from its original migratory direction affected by the electric field.

A composite Zn-metal anode with an ultrathin nitrogen (N)-doped graphene oxide (NGO) layer was designed by the one-step Langmuir–Blodgett method. Equipped with an in situ optical microscope and accelerated video, the deposition of Zn on NGO@Zn electrode was flat and only occurred on the NGO layer (Fig. 8f and g; Zhou et al. 2021), which indicates the strong zincophilic nature and the uniform Zn2+ regulating ability of parallel NGO layers.

Moreover, carbon coating can manipulate the crystal orientation of the deposited Zn. According to current research, Zn (002) is the preferred exposed surface for reversible Zn stripping/plating (Liu et al. 2024a; Wang et al. 2022c; Wu et al. 2023; Yuan et al. 2021), with the lowest surface energy and highest resistance to side reactions. Therefore, a candidate substrate for epitaxial electrodeposition of Zn should exhibit a small lattice mismatch below 25%, with similar atomic arrangement to the Zn (002) plane (Zheng et al. 2019). A process to electrodeposit Zn with preferential orientation parallel to the electrode on a graphene-coated stainless-steel electrode was developed (Fig. 8h). The lattice match between G (002) and Zn (002) was ~ 7%, minimizing the lattice strain of material growth. Atomic force microscopy and X-ray diffraction results confirmed the epitaxial regulation of Zn deposition on G-coating. As a comparison, no epitaxial growth was observed on Ketjen black–coated (an isotropic carbon) stainless steel. Monolayer graphene (Gr) as the electrodeposition substrate for Zn deposition was ellabroated (Foroozan et al. 2019). Atomistic calculations indicate that monolayer graphene has strong affinity to Zn, leading to uniform distribution of Zinc adatoms all over Gr surface (Fig. 8i). However, the perfect G lattice showed a binding energy of only -0.02 eV with Zn. In sharp contrast, the binding energy of for G with four defect sites was as high as -4.41 eV. It indicates the defect sites in G or its analogues may play the key role in inducing the Zn (002) growth. This synergistic compatibility between Gr and Zn promotes subsequent homogeneous and planar Zn deposits with low interfacial energy (0.212 J m−2) conformal with the current collector surface.

4.2 Carbon materials as ex situ coatings on Zn anode

Following above discussion on their operational mechanisms, carbon materials for Zn anode are then discussed according to their classifications (Fig. 9).

Fig. 9
figure 9

Carbon materials coating on Zn anode. a Schematic illustrations of morphology evolution/antidendrite mechanism for bare and CNTs coated Zn foils during cycling process (Li et al. 2019). Copyright 2019, Wiley–VCH. The SEM and photo images of (b) Zn electrode and (c) Zn/rGO electrode before cycles (Shen et al. 2018). Copyright 2018, American Chemical Society. d Schematic illustration of Zn plating on bare Zn foil (upper) and NGO@Zn electrode (lower) (Zhou et al. 2021). Copyright 2021, Wiley–VCH. e Operando optical microscopy images of pure Zn and rGOSnCu/Zn in aqueous 2 M ZnSO4 electrolyte (Zhao et al. 2023a). Copyright 2022, Elsevier B.V. f Dual-field simulations uncover the redistribution of Zn2+ ion concentration field achieved by modified layer (Yang et al. 2020). Copyright 2020, Wiley–VCH. g Mechanism of Zn-graphite anode formation and the stability in 2 mol L−1 ZnSO4 electrolyte (Li et al. 2021c). Copyright 2020, Wiley–VCH. h Schematic illustration of bare Zn foil (ZF) and carbon black coating to zinc foil (ZF@CB) during Zn plating (Wang et al. 2020a). Copyright 2020, Elsevier. i Schematic diagrams of Zn plating behavior on Zn-3D@600 electrodes (Chen et al. 2021a). Copyright 2021, American Chemical Society. j Deionized water contact angles (Zhang et al. 2023c). Copyright 2023, Elsevier B.V. k Electric field simulations (Qin et al. 2022). Copyright 2022, The Royal Society of Chemistry. l Chronoamperograms of coated and bare Zn (Deng et al. 2022). Copyright 2021, Elsevier B.V. m Schematic illustration of Zn plating processes on different anodes (Fan et al. 2022). Copyright 2022, The Royal Society of Chemistry

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4.2.1 CNT

CNTs was coated on Zn anode to inhibit Zn dendrite (Li et al. 2019). The dendrite growth on the surface of CNTs coated Zn electrode was significantly inhibited (Fig. 9a). Through DFT calculations, the CNTs protective layer possessed strong oxygenophilic and sulfurophilic properties. The binding energies of oxygen and sulfur on CNT were 3.41/3.40 eV, higher than those of 2.08/1.37 eV on Zn. As a result, the adsorption effect of CNTs protective layer can guide the side reaction to occur only on the CNTs protective layer, ensuring that the faster kinetics of Zn deposition/dissolution on the Zn foil is not affected by the side reaction.

4.2.2 Graphene

Graphene-based materials have high mechanical strength that can be used to suppress Zn dendrite growth (Clancy et al. 2018; Shen et al. 2018). The use of reductivity of Zn towards the GO precursor can easily produce a rGO-coated Zn anode (Fig. 9b and c). Electrochemical performances of the symmetric cell assembled by the rGO coated Zn anode showed greatly reduced overpotential and voltage hysteresis, which can cycle stably for 200 cycles at different current densities ranging from 1 to 10 mA cm−2 at the areal capacity of 2 mAh cm−2. An ultrathin nitrogen (N)-doped graphene oxide (NGO) layer can greatly enhance the stability of Zn anodes (Zhou et al. 2021). The directional deposition of Zn (002) planes was achieved thanks to the parallel graphene layer and beneficial zincophilic-traits of N-doped groups (Fig. 9d). Sn and Cu dual metals/ rGO coating layer (SnCu/rGO) was constructed on the Zn through the reduction reaction between metallic Zn and GO followed by the replacement reaction between Sn4+/Cu2+ and Zn (Zhao et al. 2023a). Sn and Cu metals with high electric conductivity can provide a balanced surface electrical field, which further decreases the Zn nucleation energy and guides the uniform Zn deposition (Fig. 9e). The Zn electroplating behavior in symmetric cells indicates that the rGO-SnCu layer has excellent corrosion resistance for Zn anode that can further slow down the side reactions and regulate Zn stripping/plating.

4.2.3 Graphite

Graphite has a low Young's modulus, high conductivity and high porosity, which can decrease the local current density and achieve a more uniform electric distribution of Zn anodes. Pencil drawing provides a novel strategy to construct graphite layer on the Zn surface (Li et al. 2021c). The electrode with graphite coating exhibited a stable surface after 15 days of immersion in the electrolyte (Fig. 9g). This artificial layer can act as not only an ionic buffer but also a porous nuclei inducer to guide even nucleation at voids. By conducting in situ optical microscope monitoring, the coated Zn displayed a smooth surface without obvious dendrites or corrosions at a current density of 1 mA cm−2 after 40 min.

4.2.4 Porous/Mesoporous carbon

AC was pressed on Zn to create a modified Zn anode (Zn@C) (Li et al. 2018). Due to porous structure, the porous carbon layer can serve as nucleation sites and reservoirs to capture Zn ions from the electrolyte, and meanwhile alter the prioritization of Zn2+ ions transfer toward the “hot spots” on Zn foil. A carbon black coating was applied to zinc foil (ZF@CB) by the drop casting method (Fig. 9h; Wang et al. 2020a). The effectiveness of carbon black coating was established by the ZF@CB|ZF@CB symmetric cells, with a lower overpotential and longer lifespan than pristine ZF at the current density of 0.5 mA cm−2. It can be attributed to the large surface area and high conductivity of carbon black. Zn-3D@600 anode was prepared with a MOF-derived Zn@C protective layer assembled on the 3D Zn skeleton (Chen et al. 2021a). As depicted in Fig. 9i, the trace amount of Zn0 anchored on the N-doped carbon framework can act as nuclei for the Zn deposition and help smoothen the average concentration gradients near the electrode surface. Meanwhile, the uniformly distributed electric field by Zn@C protective layer ensures a homogeneous Zn plating process. Zn metal with ultrathin carbon coatings (C@Zn) was prepared by magnetron sputtering (Zhang et al. 2023c). Raman spectra indicated that carbon coating was highly graphitized. The contact angles were then compared (Fig. 9j). The bare Zn showed a large contact angle of 99.1°, implying poor hydrophilicity and surface wettability. In contrast, C@Zn-1, C@Zn-2, and C@Zn-3 exhibited smaller contact angles of 54.5°, 51.8° and 47.1°, respectively, for higher hydrophilicity and surface wettability, which facilitate interfacial charge transfer on Zn anode. 3D porous carbon layer was proposed on Zn anode (3D-NC@Zn) to induce uniform and fast Zn deposition via a scalable doctor-blade method (Qin et al. 2022). The field simulations of electric field distribution were selected to simulate the variation of the electric fields during Zn deposition (Fig. 9k). The electric field distribution of a 3D-NC layer became more uniform, which could be ascribed to its unique 3D conductive network with an enlarged specific area.

Carbon materials can be further integrated with inorganic and organic/polymeric materials to form composite surface layers for Zn anode. A ZnO/C hybrid was designed via simple complex calcination method, serving as a multifunctional artificial protection layer on Zn to alter the deposition kinetics (Deng et al. 2022). Compared to continuous and rampant 2D diffusion for the bare Zn, Zn anode coated with ZnO/C layers exhibited initial short-term 2D diffusion, as shown by chronoamperometry study (Fig. 9l). Following 3D diffusion, there was an increase in Zn nucleation sites, resulting in a localized reduction of Zn2+ instead of its diffusion to other locations. An artificial electronic-ionic mixed conductive protective layer (Alg-Zn + AB@Zn) was prepared on Zn using zinc alginate gel (Alg-Zn) and acidified conductive acetylene black (AB) as ionic and electronic regulators (Fan et al. 2022). The establishment of Alg-Zn can guide the migration of Zn2+ by forming Zn ion channels. For the Alg-Zn@Zn anode, the carboxylate group of AlgZn interacts with Zn2+ ions through electrostatic force and guides their diffusion, thereby guiding the deposition of zinc ions under the coating towards a uniform dendrite-free Zn layer (Fig. 9m).

4.3 Carbon hosts/skeletons for Zn anodes

Hostless feature of metal plating/stripping is another inherent drawback when applying metallic anode. During continuous charge/discharge cycles, particularly at deep discharges with high current densities, pulverization of bulky Zn can happen. This pulverization inevitably hampers electrical contact, ultimately resulting in battery failure (Zhao et al. 2023b). Hence, it becomes imperative to build host or skeleton for Zn anode (Zhang et al. 2022b). Carbon materials can be candidature hosts for constructing 3D structured Zn anodes, owing to their multi-level porous structure, exceptional conductivity and flexibility. By employing carbonaceous hosts like CC, CNTs, and graphene derivatives, the resulted skeletons can provide more accessible active sites for Zn deposition compared with planar Zn foils/plates, which helps to restrict the dendrite growth and accommodate the volume changes (Fig. 10).

Fig. 10
figure 10

Carbon materials act as host/skeleton for Zn anode. a Simulation of the electric field distribution of MDC–Cu and a planar bare Cu foil. b SEM images of MDC–Cu under different plating/stripping conditions (Li et al. 2023a) Copyright 2022, Wiley–VCH. The charge density differences for Zn2+ adsorbed on (c) G and (d) N-VG (yellow and light blue areas represent positive and negative charge differences, respectively) (Cao et al. 2021). Copyright 2021, Wiley–VCH GmbH. e The SEM image of Bulk-ZA and 3DP-ZA anode (Zeng et al. 2023). Copyright 2022, Elsevier B.V. COMSOL simulation of the electric field distributions for (f) CC/Zn and (g) AgNPs@CC/Zn electrodes during Zn nucleation process. h Temperature distribution images of bare CC and AgNPs@CC scaffolds (Chen et al. 2021b). Copyright 2021, Wiley–VCH GmbH

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The hierarchical Zn anode structure was realized by simultaneously carbonizing the representative metal–organic framework (MOF-5) and reducing the trace amount of Zn2+ in MOF-5 into metallic Zn (Li et al. 2023a). Simulation of the electric field distribution (Fig. 10a) shows that the resultant MOF-5-derived carbon (denoted as MDC) MDC with a continuous conductive skeleton has a relatively more uniform electric field distribution and Zn0 nucleation sites, which is mainly due to the charge redistribution effect of the 3D conductive skeleton. As shown in SEM after Zn0 plating/stripping (Fig. 10b), the Zn0 deposits on the MDC-Cu electrode were well-arranged nano-flake arrays. In the subsequent stripping process, the plated Zn0 was reversibly removed from MDC–Cu and the original dense arrangement of particles was maintained. A self-supporting, highly flexible, and conductive CNT/paper scaffolds was built to stabilize Zn anodes (Dong et al. 2020). On the Zn surface, the porous skeleton of the scaffold mechanically regulated the deposition sites of Zn2+, and the conductive CNT network maintained a homogenized electric field.

Nitrogen doped vertical graphene (VG) nanosheets in situ grown on carbon cloth (N-VG@CC) were prepared as a 3D Zn framework (Cao et al. 2021). The presence of N doping in N-VG with higher zincophilicity, as demonstrated by DFT (Fig. 10c and d), can effectively enhance the interaction between Zn2+ ion and carbon substrate, thereby achieving more uniform Zn nucleation.

A novel 3D printed Ag-anchored three-dimensional hierarchical porous flexible Zn anode (3DP-ZA) was fabricated (Zeng et al. 2023). 3DP-ZA anode possessed regular square holes constructed by layer-by-layer stacked cylindrical strips (Fig. 10e). The macro-size square holes were beneficial for accommodating sufficient electrolyte and homogenous ion distribution, which can induce dendrite-free Zn deposition and avoid stress concentration. The magnified SEM images showed the distinctive entangled fiber network as the template for Zn plating. A flexible and dendrite-free Zn metal anode (AgNPs@CC/Zn) was prepared by inkjet printing silver nanoparticles on a 3D carbon matrix (Chen et al. 2021b). As shown in COMSOL simulation (Fig. 10f), uniformly dispersed Ag nanoparticles on the surface of CC as abundant nucleation sites can guarantee a uniform electric field distribution. Moreover, the superior thermal conductivity of AgNPs@CC scaffold can avoid non-uniform temperature distribution, ensuring much improved high-temperature endurance. An infrared thermal camera was used to monitor the temperature distribution on CC and AgNPs@CC scaffolds. The samples were heated for 5 min on the hot plate at 50 °C and then cooled naturally. AgNPs@CC scaffold had a lower surface temperature and more uniform temperature distribution than bare CC scaffold due to the enhanced thermal conductivity of AgNPs@CC scaffold (Fig. 10g).

5 Outlook and perspectives

Based on the above discussion, we have summarized the perspectives of carbon materials on the development of cathode, electrolyte, and anode for ZESDs, with their distinctive characteristics, operational mechanism and upgrading strategies. Accordingly, outlooks and perspectives are provided for advancing carbon materials in future ZESDs.

5.1 Cathodes

Various types of carbon materials applied in cathodes of ZESDs are discussed with their pros and cons. In general, the superiorities of resource abundance and preparation diversity, rich pore structure, chemical stability and favorable conductivity, as well as high compatibility with other types of active materials, make carbon a key to promote the development of ZESDs (Huang et al. 2023; Zhang et al. 2024). Despite the thrilling progress, crucial problems remain for the carbon-based cathodes in future research.

Pore structure and accessible surface are crucial for carbon nanostructures. Templating a proper pore size distribution is beneficial for improving the compatibility of electrolyte and cathode, which depends on the progressing novel synthesis methods (Zhao et al. 2022). Design of hierarchical porous carbon structure may favor the storage of Zn2+ ions, by providing actives sites and eliminating the micropore confinement effect (Zhang et al. 2021b). Heteroatom doping provides controllable redox-active centers and delivers extra pseudocapacitance. Meanwhile, novel nanocomposites for carbon materials coupled with active materials may be reliable solutions to high-performance ZESDs. However, the complex reaction paths require fundamental insight by in situ study and computational tools to decouple the specific roles of carbons. From this concern, model carbon materials are ideal platforms. Differentiable interfacial chemistry is recently scrutinized on different categories of surface with selected pore sizes (~ 2–4 nm), which regulates the adsorption and solvation structure of anions on CNTs (Fan et al. 2023). Based on the local structure ordering parameters by nuclear magnetic resonance (NMR), a strong correlation between capacitance and electrode structural disorder for porous carbons is revealed. Carbons with smaller ordered domains have higher capacitances, which is attributed to their more efficient storage of ions in the nanopores with high concentration of topological defects (Liu et al. 2024b).

5.2 Electrolyte

The imperative role of electrolyte in the development of aqueous ZESDs should be key-noted. It is essential to regulate the water activity in the aqueous zinc electrolyte to extend the electrochemical window and meanwhile depress the water-induced side reactions. On the other hand, ion transport mechanism depends on the solvated water molecules with cations. Water-in-salt electrolytes or deep eutectic solvents (DESs) with widen EWs usually present high viscosities and unsatisfactory ion conductivities, owing to their strong intermolecular interactions. Input of water turns to be an efficient way to circumvent this issue, which is also the case for aqueous/non-aqueous hybrid electrolyte. Thus, the balance between EW and ion conductivity serves as a design rule for an electrolyte system.

On the other hand, the stability of carbon materials is desirable for the emerging electrolytes. A wide class of DESs is based on ZnCl2, where the highly active and corrosive chlorine ions could restrict the cathode choices. However, carbon-based cathodes are currently the most suitable anti-corrosive candidates to demonstrate the potential of ZnCl2-based DESs. Also, the chemical/electrochemical stability of graphite materials is desirable for exploring new charge storage mechanisms that operate at high redox potentials, e.g., anion intercalation. However, parasitic reactions might contribute to poor cycling performance, which is attributed to the randomization of layer structure by repeated intercalation of ions. This might arise new challenges.

5.3 Anode

The reversibility of metallic Zn anode is catching intense attention. Towards stable Zn anode, the important functions as protective layers or hosts can be accomplished across the dimensions of carbons, from 0 to 3D. With the attractive performance, a nontrivial question may remain as where Zn0 deposits on carbon surface, taking into account the high conductivity and large SSA of carbon. The scenario might be straightforward for graphene that works as a substrate for Zn deposition. The situation could be more complex when heteroatom-doped carbon is applied. The active sites can be zincophilic and induce adatom attachment (Yang et al. 2024), however, the scarified conductivity might indicate an interfacial path between surface layer and Zn. So far, the compromise between active sites and conductivity of carbon materials are still unclear for Zn deposition.

In terms of performance matrices for Zn anode, it is important to specify the depth of charge/discharge (DOC/DOD) when claiming the lifespan of Zn anode under certain current density/capacity density. The values of DOC/DOD reflect the usage of Zn in a specific cell design. Achieving high DOC/DOC at high current density/capacity above 10 mA cm−2 and 10 mAh cm−2 is still challenging. Integrating the strength of inorganic or polymeric materials with carbon materials as artificial surface layer may benefits the Zn stripping/plating. On the other hand, in situ formation of solid-electrolyte interface (SEI) can proceed with additives or organic solvents prone to decomposed on Zn surface. It might be novel for the active sites in carbon materials to manipulate the surface reaction for forming a robust SEI for Zn.

Towards implementation of aqueous ZESDs, the vital factors further include cost effectiveness, scale-up and production upgrade. AC may present the lowest market price (~ $57–114 kg−1) among the commercial carbon products (Yin et al. 2021). Though novel carbon candidates exhibit outstanding performance, their higher price is intimately associated with the scale-up of production. Thus, the costs of reactants/precursors and synthetic routs need to be evaluated, where biomass resource can be highly desirable. Note that the price of these novel carbons depends on the specific category, which can be largely reduced by technology development, e.g., the price of wafer-scale graphene is down to ~ $600 kg−1. Besides, green chemistry/synthesis is critical to produce carbon materials towards carbon neutralization and eco-friendliness. Meanwhile, the issues of side reactions on metallic Zn anode can not be overlooked when scaling up ZESDs. Especially, HER could be the critical point, where the generated gas can lead to the swelling of pouch cell, leading to battery failure or safety issues.

5.4 Endowed functionalities

Beyond fundamental charge storage capabilities, carbon materials can endow ZESDs with multi-functionalities. The use of 2D graphene or rGO or their composites can realize the flexibility of the device. Together with electrolyte synthesis, especially novel gel electrolytes, stretchability, self-healing, and biodegradability can also be achieved with proper characteristics of carbon materials, even for working under extreme conditions, e.g. low temperature. ZESDs can bend at different angles, revealing a good flexibility and excellent performance retaining ability under deformation states (Wang et al. 2022a). By designing carbon materials electrodes on a self-healing polymer substrate, ZESDs can repair after external injury, ascribed to dynamic hydrogen bond networks (Cai et al. 2024). Moreover, the biocompatibility of ZESDs is being assessed recently, further driving the green device design.

Therefore, we envision that the present review can motive exploring novel carbon materials in emerging energy storage devices.