High-Performance Anode Materials for Rechargeable Lithium-Ion Batteries
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Transformational changes in battery technologies are critically needed to enable the effective use of renewable energy sources, such as solar and wind, and to allow for the expansion of the electrification of vehicles. Developing high-performance batteries is critical to meet these requirements, which certainly relies on material breakthroughs. This review article presents the recent progresses and challenges in discovery of high-performance anode materials for Li-ion batteries related to their applications in future electrical vehicles and grid energy storage. The advantages and disadvantages of a series of anode materials are highlighted.
KeywordsLi-ion battery Anode materials Graphite Silicon Lithium metal Metal oxides TiO2
Due to climate changes, especially exhaust of fossil fuels, a significant worldwide interest has been driven into the development of electrochemical power devices (batteries), which could significantly help to effectively use the renewable energy sources such as solar and wind, as well as to intensively expand the electrification of vehicles [1, 2]. To achieve such goals, innovative technology and advanced materials development of the battery are urgently required, where the batteries must have high volumetric (Wh/L) and gravimetric energy density (Wh/kg), and they must have long cycle life and calendar life with sufficient safety. When such a situation arises, it is essential to develop high-performance batteries in order to meet various requirements, which certainly relies on further breakthroughs in battery materials [3, 4, 5, 6, 7].
Non-aqueous rechargeable Li-ion batteries are definitely one of the most successful energy storage devices of the modern materials electrochemistry in the last century [1, 8, 9, 10, 11, 12]. Since Sony Corporation commercialized the first Li-ion battery in 1991 for small portable electronic devices, the relatively low-voltage, water-based batteries including Ni–Cd  and Ni–MH  systems have been gradually replaced by Li-ion technology. Nowadays, the usage of Li-ion batteries has been expanded largely into various devices, including robots, various power tools, stationary power storage units, uninterrupted power supply (UPS) units, as well as electrical vehicles (hybrid, plug-in or pure EVs) . However, demand is still dramatically increasing for batteries with higher gravimetric and volumetric energy density to power EVs with long driving ranges (of approximately 300 miles per charge) and to store intermittent solar and wind energy for stationary power applications. Unfortunately, conventional Li-ion battery technologies fall short to meet the requirement for these applications [8, 16].
The reversible flow of lithium ions moving back and forth through the ionically conducting electrolyte between two electrodes allows the conversion of chemical energy and electrical energy repeatedly. During the initial battery cycling (charge/discharge), multicomponent (organic/inorganic) and multilayer passivation films form at each electrode known as solid electrolyte interphase (SEI) that is critical to protecting the electrodes from further reactions with electrolytes.
Although such lithium-ion batteries are commercially very successful, we must realize that we are reaching the limits in performance using the current electrode and electrolyte materials . The performance limitation together with the cost concern of the commercially available Li-ion batteries seriously limits the fast expansion of the electrical vehicle market and efficient usage of renewable energy sources. In addition, some other technical bottlenecks of the current Li-ion technology including slow recharge cycles, relatively short calendar life as well as the safety issues need to be fully addressed.
Significant research has been conducted to exploring higher energy density electrode materials in order to meet the requirement for EV’s application. Currently, LiCoO2 , layered metal oxides such as LiNi0.8Co0.15Al0.05O2 (NCA) [24, 25] and LiNi1/3Co1/3Mn1/3O2 (NCM) [26, 27], electrode derived from spinel-type LiMn2O4  and olivine-type LiFePO4  are the state of the art for cathode active materials. However, they deliver relatively low practical capacities in a lithium-ion cell, typically in the range of 100–180 mAh/g at moderate current rates. Clearly, it is important to develop new strategies to design alternative high-energy cathode materials that are superior to those achievable with standard LiCoO2-, LiMn2O4- and LiFePO4-type electrodes, yet they still have to maintain considerable structural stability, rate capability as well as long cycle life. For example, a family of high-energy manganese-based cathode materials have been developed recently by structurally integrating a Li2MnO3 stabilizing component into an electrochemically active LiMO2 (M=Mn, Ni, Co) electrode, which boosts specific capacity up to 250 mAh/g due to the excess lithium in the system and, thereby, significantly improves the energy density of cell (based on the active materials) to 900 Wh/kg [30, 31]. However, the decay of capacity and voltage of these materials during the long time cycling led to the severe loss of the cell energy density that prevents its practical application in EVs [32, 33]. Going beyond the horizon of Li-ion batteries could offer another great opportunity to increase the energy density of the cell, although it requires the exploration of new electrochemistry and materials. For example, sulfur- and oxygen-based cathodes have recently been intensely investigated due to their potentially much higher theoretical capacity than the conventional lithium metal oxide [34, 35, 36]. These systems, however, are still in the early stage and facing a formidable challenge. The science and technology of the development of high-performance cathode materials have been extensively reviewed in the previous reported papers, to which the reader is referred for more details [22, 37, 38, 39, 40, 41, 42, 43].
Another effective approach of increasing the energy density of lithium-ion batteries is to search for high-capacity anode materials. As mentioned earlier, graphite is definitely the most widely used anode active material in the commercial Li-ion cells thanks to its excellent features including low working potential, low cost and good cycle life . However, graphite only delivers a relatively low capacity of 372 mAh/g since it allows the intercalation of only one lithium for six carbon atoms forming a stoichiometric LiC6, as shown in reaction 2. In addition, battery with graphite anode usually has moderate power density due to the relatively slow diffusion rate of lithium ion into carbon materials (between 10−12 and 10−6 cm2/s) . Therefore, there is an urgency to find alternative anode materials with high-capacity and high lithium-ion diffusion rate that could help to improve the energy and power densities of the cell. During the past decade, numerous efforts have been devoted to this blooming field with significant progress being achieved; yet there is still a sizable challenge facing those involved in the design and development of high-performance anode materials.
This review article will go through a brief description of the importance of high-performance anode materials for Li-ion batteries related to their applications in future electrical vehicles and grid energy storage. In the main part of this review, we will present the recent progresses and challenges of a new type of carbon materials, i.e., graphene. We will then discuss high-rate (power) anode material, since it is critical for the hybrid electrical vehicle (HEV) application. Moving toward the high-capacity anode, we will discuss the recent progresses on Si-based alloys and metal oxides as anode materials, although these materials are still facing significant challenges. Finally, the progresses and challenges faced by Li metal will be presented. Through this review, we intend to show that development of high-performance anode materials is one of the key factors toward high-energy and high-power battery research; and it also intends to familiarize the readers with the frontier research of different anodes for Li-ion battery and to evaluate and summarize the progress and challenges presently at hand.
2 Overview of Anode Materials
Currently, Si-based alloys are one of the most attractive anode materials and have been investigated intensively in the past years due to their significantly high capacity. However, the large volume changes of these materials associated with the lithium alloying processes have been the main impediment, which needs to be addressed before their implementation. In terms of safety and rate capability of the anode, lithium titanate (Li4Ti5O12, LTO) [65, 66, 67, 68] and β-TiO2 [69, 70, 71] are the best choice, since they offer a significant safety and power advantage over graphite anode. However, the potential of gassing issue of this type of material during the repeated charge–discharge cycles needs to be fully understood and addressed. Ultimately, Li metal is an ideal anode for rechargeable batteries, including Li-air, Li–S and other Li batteries using intercalation compounds or conversion compounds as cathode materials. However, Li dendrite growth and low coulombic efficiency during the charge/discharge process have largely prevented the use of Li metal for rechargeable batteries. To enable broad applications of Li anodes, more fundamental studies need to be conducted to simultaneously address these barriers. In the following sections, we will discuss the recent research to address the aforementioned issues and share our perspectives with readers on how to better understand the chemistries involved in these anode materials in order to improve their electrochemical performance.
3 Intercalation Anode: Graphene
Carbon-based materials with various morphologies have been long considered as potential anode materials due to their promising physical and chemical features. Indeed, graphite has very impressive electrical (~ 10−4 S/cm) and thermal (~ 3000 W/mK) conductivity, which makes it the most widely used anode material in the commercial Li-ion cells [72, 73, 74]. Equally impressive in terms of electrical conductivity and mechanical strength, graphene derived from exfoliated graphite provides a modular approach to study the lithium intercalation/deintercalation behavior in layered carbon as well as composite layered carbon materials . Since the pioneering work on lithium insertion in carbonaceous materials by Dahn et al. , great progress has been made in understanding lithium intercalation mechanism for ordered and disordered carbon-based anodes in lithium-ion batteries which has been summarized in several reviews [63, 77, 78]; hence, this area will not be discussed here. We will only focus on the graphene-based materials as anode in the following discussion.
Experimentally, due to the presence of additional active sites for lithium-ion storage such as defects, graphene sheets normally deliver high gravimetric capacity in the range of 790–1050 mAh/g. A capacity as high as 1200 mAh/g was reported at the initial cycle with values around 850 mAh/g at 40th cycle for high-quality graphene sheets (~ 4 graphene layers) with a surface area over 490 m2/g . However, the disordered structure usually results in poor electrical conductivity of graphene and therefore low power density. Results also demonstrated that the graphene characteristics including defect density and surface area associated with the electrical conductivity strongly impact the capacity for lithium storage, which highly depends on the processing methods of graphene.
One approach that potentially can increase the lithium storage capacity of graphene is using spacers to enlarge the interlayer distance of graphene sheets by reassembling a so-called pseudo-graphite-type material. Yoo et al.  were able to demonstrate that the specific capacity of graphene sheets can be improved from 540 to over 700 mAh/g by incorporating nanocarbon macromolecules [such as carbon nanotubes (CNT) and fullerenes (C60)]. Their study suggested that the d-spacing between the reassembled graphene sheets, which needs to be carefully tuned and controlled, plays the key role in determining the reversible capacity of graphene-based materials. If the d-spacing of the graphitic carbon layer in the graphene sheets can be expanded to 0.40 nm compared to 0.34 nm in graphite, large reversible lithium storage capacities up to 784 mAh/g could be expected. Other alternative approaches, including preparing graphene papers by filtration and reduction of prefabricated graphene oxide paper [93, 94] or using graphene sheets functionalized by oxalic acid molecules , have been also reported to expand the d-spacing of the graphene layers. Although the capacity increase in graphene beneficial from the expansion of the d-spacing of graphitic carbon layer is obvious, the first cycle irreversible capacity loss and poor cycle performance of this type of materials are very problematic in terms of their practical application, which requires further research to identify the underline reasons and to search for potential solution to address these issues.
Heterogeneous atom doping is another effective approach to improve the electrochemical performance of graphene for lithium-ion storage through tuning the electronic structure of the graphene base plane [96, 97]. Nitrogen doping appears to be more feasible one, which forms pyridinic, pyrrolic and graphitic structures [96, 97, 98, 99]. Such structures are electronically deficient that could strengthen the binding energies of lithium to graphene layer and thus increase the lithium coverage. For example, doped hierarchically porous graphene (DHPG) electrodes exhibit high capacity with long cycling capability (~ 3000 cycles) at a current density of 5 A/g. Low- and high-magnification SEM and TEM images indicated that hollow graphene assemblies can contain small nanoscale pores . These promising results are attributed to the synergetic effects of the hierarchically porous structure, good conductive network and heteroatom doping, which facilitate the mass transport and speed up the electrochemical reactions. In terms of the fabrication of the doped graphene, several approaches have been reported, including thermal treatment, arc discharge, high-energy irradiation, chemical vapor deposition, bottom-up chemical synthesis, electrochemical synthesis [101, 102, 103].
Although graphene-based materials show some promises as the anode for next-generation lithium-ion battery, they are facing formidable challenges. The restacking of graphene driven by the van der Waals forces between the adjacent layers leads to low specific surface area of the material and therefore low specific capacity. Introducing porosity in graphene-based materials appears to be an effective approach to address this issue, which can increase the accessible area and accelerate the transport of lithium ions in graphene [52, 100, 104]. In general, pores can be introduced either as in-planes holes within individual sheets [52, 105, 106], or as interstitial space between neighboring sheets . In most cases, these types of pore can be integrated in graphene-based materials, which further enhance the electrochemical performance. Another issue of the graphene-based materials is the relatively high irreversible capacity loss during the initial discharge/charge cycle, which results in a low coulombic efficiency [108, 109]. Unfortunately, this problem is associated with the high surface area of graphene, since a solid electrolyte interphase (SEI) layer is required to form on the graphene anode (which consumes lithium from the cathode) during operation of the cell in order to protect the lithiated graphene from further reactions with electrolytes . At the moment, limited attempt has been conducted to address this issue and, therefore, still remains an open question.
In line with the above concern, recent research effort has been focused on exploring graphene as a novel support for other anode materials such as silicon, tin or transition metal oxides by taking the advantage of graphene’s high surface area, mechanical strength and good electrical conductivity . In such graphene-based heterogeneous hybrid anode materials, a nanoporous 3-D structure with significant amount of void spaces is created by homogenously distributing the graphene nanosheets between the host particles such as SnO2 or Si. This structure together with its mechanical strength can confine the host particles by the surrounding graphene sheets, which effectively limits the volume expansion during lithium intercalation, while the voids can act as buffering spaces to absorb the volume change during discharge and charge cycles. In addition, the good electrical conductivity of graphene can provide conducting network to improve the power density of the electrodes [112, 113, 114, 115, 116, 117]. Although this compelling approach does help to improve the rate capability and capacity retention of the graphene-composited materials, long-term stability of these hybrids still needs further improvement to be applicable for commercial purpose.
4 Intercalation Anode: Li4Ti5O12 (LTO) and TiO2
In the above section, we mainly focused on the progresses and challenges for the potentially high-capacity anode based on the graphene materials that represent one of major research effort on development of next-generation lithium-ion batteries. It should be emphasized, however, similar to the graphite, a stable SEI layer is required to enable graphene-based materials as secondary Li-ion battery anode, despite other challenges facing these materials, as pointed out earlier. In general, because the SEI on the graphite surface will easily decompose at a temperature as low as 60 °C, the continuous heat flow from the exothermal reaction between the lithiated graphite and the electrolyte can quickly accumulate . When a large amount of heat is generated on the anode side during a short period of time, it would eventually trigger the major reaction between the cathode and the electrolyte, leading to a thermal runaway associated with fire or explosion of the battery. Although it is widely accepted that the thermal runaway is associated with the thermal instability of the delithiated cathode (Li1−xMO2), more attention should also be paid to the anode side. Apparently, the thermal stability of the SEI plays a critical role in the safety of lithium-ion batteries using graphite as the anode, which is another major barrier that hinders the deployment of lithium-ion batteries in automobiles.
From the safety perspective, titanium-based oxides including Li4Ti5O12 (LTO) and TiO2 hold the great potential to be a class of attractive alternatives to graphite anode, since they operate at a potential above 0.8 V versus Li+/Li where the formation of a SEI layer on the anode surface can be avoided by eliminating the reduction of electrolytes. In this sense, titanium-based anode materials showed great advantage over the graphitic carbon anode along with other features such as low cost, low toxicity, low volume change during discharge/charge process and outstanding power and excellent cycle life. Therefore, many research efforts have been devoted to develop large-scale Li-ion batteries for hybrid electrical vehicle (HEV) applications using Ti-based oxides anode materials, taking their advantages of the extremely safe lithium-ion chemistries, high power density and long cycle life. However, the main drawbacks of this type of materials are the low inherent theoretical capacities (in the range of 175–330 mAh/g) and low electronic conductivity in bulk materials with micrometer-sized particles. The structure, morphology and size of titanium-based oxides strongly determine their electrochemical performance in a cell. Nevertheless, spinel Li4Ti5O12 (LTO) and TiO2 with various allotropic forms have been extensively studied as one of the most promising anode materials.
However, LTO is characterized by poor electronic conductivity (~ 10−13 S/cm) on the one hand and interfacial reactivity with the electrolyte on the other hand resulting in undesirable gas release that complicates the use of this important material in large-scale electrochemical energy storage applications . The first issue can be addressed by either surface treatment of LTO [67, 120] or downsizing LTO to nanoscale  to enhance lithium-ion diffusion rate. During the past years, a variety of nanostructured LTO has been successfully explored as the anode materials which demonstrated much improved electrochemical performance in terms of the rate capability and cycling stability [122, 123, 124, 125, 126]. However, the drawback associated with the nanostructured material is the relatively low loading density of the electrode due to the high porosity created by the nanomaterials which, consequently, significantly reduces the volumetric energy density of the battery . One potential approach to address this issue is to prepare micrometer-sized secondary particles with nanosized LTO primary particles (see Fig. 5b). In this type of particle design, the tap density and the loading at the electrode level can be effectively improved with all the benefits associated with nanostructure being maintained by the nanometer-sized primary particles. In particular, porous LTO with microspheres showed much improved electrochemical performance with high volumetric energy density because the spherical morphologies can efficiently minimize the Li-ion diffusion pathway [128, 129].
The intrinsic gassing issue of LTO is associated with the fact that the lithiated Li4+xTi5O12 has a tendency to react with the organic electrolytes such as ethylene carbonate when aged at high temperatures. Recent study by Lu et al. using electron energy loss spectroscopy (EELS) reveals that there is a reversible spontaneous charge transfer process of Ti3+ ↔ e− + Ti4+ occurring on the surface (ordered or disordered) of lithiated Li4Ti5O12 that potentially could be an essential step of the gas-releasing phenomenon, although more work needs to be conducted to fully confirm the mechanism . Nevertheless, it is urgently needed to address the gassing issue in order to enable the successful deployment of the LTO anode, especially in application that requires power such as HEVs. Surface modification on the LTO anode appears to show some promises in terms of avoiding the gas generation. For example, AlF3 coating acted as the buffer layer to reduce the activity of the lithiated LTO surface has been explored to suppress the electrolyte decomposition [130, 131].
TiO2 with various crystal structures, including anatase, rutile, brookites and especially TiO2(B) (monoclinic C2/m), is also an ideal host for lithium insertion/extraction . Similar to LTO, the electrochemical lithiation of TiO2 operates at high potential (1.5 V vs. Li+/Li) which provides excellent safety to the battery, but it delivers much higher capacity (330 mAh/g) than LTO since 1 mol of lithium can be inserted into TiO2 theoretically (corresponding to the composition LiTiO2). Experimentally, however, it has been proven that achieving the theoretical capacity is very challenging . In general, the electrochemical performance of TiO2 heavily relies on its crystal structure, morphology and particle size. Recent results demonstrated that controlling shape and size of TiO2 nanoparticles can offer great advantages . TiO2-B nanotubes or nanowires with diameter in the range of 40–60 nm and length up to several microns synthesized from a simple aqueous route demonstrated significantly enhanced rate capability with much higher capacity (305 mAh/g corresponding to Li0.91TiO2-B), as shown in Fig. 5c, d .
5 Alloying Anode: High-Capacity Si, Sn, P
There are two types of mechanisms attractive to anodes beyond the intercalation process: alloying and conversion mechanisms. The alloying mechanism has the general reaction of xLi+ + xe− + M → LixM, where typical examples of M are Si, Ge, Sn, P [55, 136, 137, 138]. In general, these materials can have multiple times of lithium-ion storage capacity of graphite, as shown in Fig. 2. For example, Si has 4200 mAh/g specific capacity which is about 11 times of graphite . In addition to the gravimetric capacity, the volumetric capacity at the expanded (lithiated) state is also an important consideration for portable and electric vehicle applications. Another important parameter is their delithiation potential, which should be low in order to maximize the discharge voltage in the full cells. (Note that the delithiation potential is more important, not the lithiation potential.) The delithiation potentials of Si, Sn and P are 0.45, 0.6 and 0.9 V, respectively, which are all in the reasonable potential ranges.
The alloying mechanism shares some of the most challenge issues. The first one is volume expansion and fracture . Large capacity of lithium storage in the materials unavoidably causes large volume expansion during lithiation, for example: by 4 times in Si, 3.7 times in Ge, 2.6 times in Sn, 3 times in P. The large volume expansion can cause mechanical fracture in individual particles, resulting in the loss of electrical contact and capacity fading. The second one is the instability of the solid-electrolyte interphase (SEI) [141, 142]. The volume expansion during lithiation and contraction during delithiation causes the movement of interface boundary between the particles and electrolyte and thus the challenge of forming stable SEI layer. The third one is the swelling at the electrode level. The volume expansion in the individual particle also results in the swelling at the whole electrode level and thus the challenges for the battery cell design.
The design of nano-Si anodes can be categorized into four areas: The first one is solid Si nanostructures [146, 147, 148, 149, 150, 151, 152, 153]. There has been a rich morphology of solid Si nanostructures including nanowires, nanoparticles and carbon-Si composite particles. The key of all these solid nanostructures is their small sizes, less than the critical breaking size of Si (Fig. 6a). An interesting further advancement in the solid nanostructure concept is the core-shell nanostructures (Fig. 6b), where the core materials provide stable mechanical support and efficient electron transport while the Si shell stores lithium ions . Examples for the core materials include carbon [155, 156], NiSi , TiC . The second design is hollow Si nanostructures since it provides facile strain relaxation which is important to enable Si anodes without fracture . Compared to solid Si particles, hollow Si structures would provide the interior hollow space for strain relaxation (Fig. 6c, d) [160, 161]. Typical, hollow structures include Si nanotubes  and hollow Si nanospheres . The third one is constraint hollow Si . Nanostructured Si does not have a serious fracture problem; however, establishing stable SEI is much more challenging due to continuous volume expansion and contraction during charge/discharge. In order to solve this problem, a breakthrough materials design concept is to have a mechanically constraining layer on hollow Si structure. It was first realized by having a double-walled nanotube: Si as inner tube and SiO2 as outer mechanical constraining layer (Fig. 6e) . The SiO2 layer allows the diffusion of lithium ions for reaction with Si but is mechanically strong to force volume expansion toward the interior space during lithiation. During delithiation, the interior Si interface moves back. Therefore, the outer SiO2 surface remains static during charge/discharge. Such a concept enables the formation of stable and thin SEI, and a long life of 6000 was demonstrated (Fig. 6f). This powerful concept of constraint hollow structure was further confirmed with Si–C, S–TiO2 and Al–TiO2 yolk–shell structures (Fig. 6g, h)  and permanganate-like Si–C structures (Fig. 6i, j) . The constrained hollow structures have additional benefit of avoiding the swelling at electrode level since there is empty space pre-reserved for interior volume expansion. For example, permanganate-like structures show negligible volume change at the electrode level. The final one is exploring polymer binder that represents another approach to bring the cohesions of Si particles and to maintain the electrical connection during the large volume change. One set of polymers (CMC, carboxymethylcellulose, alginate) have a strong binding with Si particle surface has been shown to have certain improvement over traditional polyvinylidene difluoride (PVDF) binder to maintain electrode integrity. Another set (conducting PANi hydrogel) is conducting polymers with engineered binding sites as well as electronic conducting function . Recently, self-healing polymer has been shown as an exciting set of binder, which has dynamic hydrogen bonding to self-heal if the volume expansion causes the breaking of polymer (Fig. 6k) .
6 Conversion Anode: Transition Metal Oxide
In order to be able to cycle the conversion oxides, nanoscale materials design is needed for interconversion of multiple solid phases [5, 11, 63, 174]. The small dimension of nanostructures reduces the strain of solid transformation and the distance for atomic diffusion. The pioneering work by Tarascon has shown that MOx would be divided into very small particles of M and Li2O after the first conversion reaction [62, 175]. The starting morphology and structure of MOx play an important role on the cyclability of electrodes. An attractive approach is to chemically attach small nanoparticles of MOx to the reduced graphene oxide (RGO) . The RGO offers good electronic conductivity and strong chemically ankled point for MOx (Fig. 7c, d). Excellent cycling performance and rate capability have been demonstrated (Fig. 7e, f) in this case. Another set of materials design is to have void space design (similar as Si anodes) in MOx due to the large volume changes, which also show excellent performance .
7 Lithium Metal Anode
Li metal is a holy-grail anode with very high specific capacity of 3860 mAh/g for rechargeable batteries. Unfortunately, Li dendrite growth and low coulombic efficiency during the charge/discharge process have largely prevented the use of Li metal for rechargeable batteries [177, 178]. To enable broad applications of Li anodes, more fundamental studies need to be conducted to simultaneously address these barriers. In this part, techniques utilized to characterize the morphology of Li deposition and the results obtained by modeling of Li dendrite growth will be reviewed. Recent development on interfacial materials design, electrolyte additives and solid electrolyte approaches in limiting the dendrite formation will be also discussed.
These challenges of lithium metal anode are rooted to the following two fundamental reasons. The first reason is mechanical in nature. During battery charge and discharge, Li metal is plated and stripped without a host material. Compared with the finite volume expansion of lithium-ion battery anodes such as graphite and Si hosts, the “hostless” Li metal has virtually infinite relative volumetric change. The spatial control of Li deposition in Li metal anodes is absent, resulting in various possible morphologies including dendrites. Such an uncontrolled deposition creates significant mechanical instability and cracks in electrodes and the SEI. The second reason is chemical in nature. Li metal reacts with nearly all the chemical species in gas, liquid and solid phases. In liquid electrolyte, Li metal decomposes solvent and salts to form SEI. There is very little control over the SEI thickness, grain size, chemical composition or spatial distribution of the reaction products. Such an SEI layer is weak against the unavoidable mechanical deformation during Li plating/striping, and it continuously breaks and repairs by reacting with more electrolytes.
The challenges of Li metal anodes calls for strategies to form a stable interfacial layer with the dual functions of suppressing the dendrite formation and stop the side chemical reaction. Liquid electrolyte and additives are an important area of research to stabilize Li metal anodes since the formed SEI can have significantly different morphology, composition and property. For example, Zhang et al. recently discovered that highly concentrated electrolytes composed of ether solvents and the lithium bis(fluorosulfonyl)imide salt enable the high-rate cycling of a lithium metal anode at high coulombic efficiency (up to 99.1%) without dendrite growth and demonstrated long cycling in symmetric Li/Li metal cells . The authors attributed the excellent performance to the increased solvent coordination and increased availability of lithium-ion concentration in the electrolyte. Zhang et al. invented a self-healing electrostatic shielding mechanism to prevent dendrite growth (Fig. 8g). Certain cations (such as cesium) as additive at low concentrations have reduction potential below the standard reduction potential of lithium ions . The additive cations form a positively charged electrostatic shield around the initial growth tip of Li metal protrusion and prevent further deposition of lithium onto it, effectively suppressing the dendrite growth. The cooperation effects of LiNO3 and polysulfides in ether solvent are recently shown to work well on forming stable SEI layer on Li metal and suppressing the Li dendrite growth .
Solid electrolytes including polymers and ceramics have strong mechanical property for dendrite suppression, which is another interesting direction to go. The Young’s modulus of Li metal is only 4.9 GPa. Indeed, Li metal is a soft metal and it would be easy to find materials with larger mechanical strength. However, most solid electrolytes have low ionic conductivity, resulting in low power output. Ceramic solid electrolytes with framework structure such as Li10GeP2S12  and garnet-type Li7La3Zr2O12  are promising due to their high Li-ion conductivity (~ 10−2–10−4 S/cm). Polymer–inorganic nanostructure composites recently show promising results in improving ionic conductivity. Another issue of solid electrolytes is the instability of interfacial layer and increased impedance with cycling, which result from the high chemical reactivity of Li metal with solid electrolyte as well.
In the past few years, nanoscale interfacial materials design emerges as a new approach to address the Li metal anode problems [187, 188]. The key design criteria of both mechanical and chemical stability need to be satisfied in order to have a stable interfacial layer. Such an interfacial layer might not be formed by electrolyte additive alone but needs to be pre-deposited before battery fabrication. Several exciting examples were recently demonstrated. One example is interconnected hollow carbon spheres as interfacial layer to promote Li metal deposition between carbon and Cu current collector (Fig. 8h–j) . The morphology of deposited Li exhibits nice large column-like structures free of dendrites. The efficiency is higher than the bare Cu collector control. Another example is 2D layered materials such as h-BN and graphene, which are both chemically stable against Li metal and mechanically strong . Li metal was found to be sandwiched between Cu metal collector and the 2D layers (Fig. 8k). Most recently, an interfacial layer of Al2O3 synthesized by atomic layer deposition is also shown to improve the Li metal cycling .
8 Concluding Remarks
The need for storage technologies with much greater energy density than the current Li-ion systems for applications such as electrical vehicles and grid storage calls for the search of higher-capacity electrode materials, both cathode and anode, which certainly relies on material breakthroughs. Although notable progress has been achieved in the development of high-performance anode materials for Li-ion batteries, further investigation of the underpinning mechanisms that limit their performance is required in order to aid the rational design and development of kinetically facile and chemically stable anode materials. As detailed in this review article, we mainly discussed the research activities and achievements of three different types of high-performance anode materials, i.e., intercalation anodes (graphene and Li4Ti5O12); alloy anodes (Si, Sn, P); conversion oxide anodes; as well as the holy-grail lithium metal anode. Without a doubt, substantial challenges exist for each component, which requires significant research efforts in a variety of fields to unlock their full potentials as the next-generation anode material.
Graphene-based materials show some promises; their restacking issue and high irreversible capacity loss during the initial discharge/charge cycle seriously limit their application. Its high surface area leads to low CE and significant consumption of electrolyte, also making it not a practical anode material. However, graphene could be used as a novel support for other anode materials such as silicon, tin or transition metal oxides by taking the advantage of graphene’s high surface area, mechanical strength and electrical conductivity. In this sense, further research is needed to find the best combination between the graphene and other materials that will give satisfactory capacity and capacity retention.
Alloy-type materials represented by Si and Sn are the most attractive anode due to their high capacity, but the large volume changes during cycling have been the main impediment to their implementation. To address this issue, one approach that demonstrates great success is using nanoscale design; however, future research is necessary in the following areas. First, quantitative understanding of the nanoscale design, such as size-dependent of the nanostructure properties, is still needed. Second, the nature of solid electrolyte interphase (SEI) layer that has the significant impact on the coulombic efficiency need to be fully investigated. Third, developing of advanced characterization techniques (both in situ and ex situ) together with multiscale modeling and simulation is vital to unravel the detail microscopic processes that occur during lithiation/delithiation, especially at both the atomic level and the electrode surface. Fourth, it is necessary to develop effective method to pack the nanostructured materials into the electrode and understand the deformation mechanism at the whole electrode level. Finally, in terms of the practical application, it is vital to develop large-scale and low-cost fabrication strategies for nanomaterials with desirable performance.
Similar to alloy anodes, the conversion anodes also have the issues of material pulverization at the individual particle level, unstable SEI layer, and the morphology and volume change at the whole electrode level. To enable good cycling performance for the conversion oxides, nanoscale materials design is needed for interconversion of multiple solid phases. Another challenging aspect of conversion anode is the large voltage hysteresis (~ 1 V) between charge/discharge, which needs to be addressed.
Titanium-based materials as the high-power anode showed much improved tolerance to thermal abuse compared with the conventional graphite anode. However, the main drawbacks of this type of materials are low inherent theoretical capacities and low electronic conductivity in bulk materials with micrometer-sized particles. Nanocrystallization, doping or surface doping could potentially solve the issue of low electronic conductivity. The intrinsic gassing issue of LTO is associated with that the lithiated Li4+xTi5O12 has a tendency to react with non-aqueous electrolyte when aged at elevated temperatures. Resolving the gassing issue would enable the successful deployment of the LTO as high-power anode for HEV applications.
Eventually, enabling the lithium metal electrode would be the ultimate goal for the high-performance anode of the next-generation Li-ion batteries. The challenges of Li metal anodes call for strategies to form a stable interfacial layer with the dual functions of suppressing the dendrite formation and stop the side chemical reaction. Liquid electrolyte and additives are an important area of research to stabilize Li metal anodes since the formed SEI can have significantly different morphology, composition and properties. Solid electrolytes including polymers and ceramics with strong mechanical property for dendrite suppression would be another interesting direction to go. Finally nanoscale interfacial materials design could be a promising approach to address the Li metal anode problems.
This work was supported by the U.S. Department of Energy under Contract DE-AC0206CH11357 with the main support provided by the Vehicle Technologies Office, Department of Energy (DOE) Office of Energy Efficiency and Renewable Energy (EERE).
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