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ISSN : 1225-0112(Print)
ISSN : 2288-4505(Online)
Applied Chemistry for Engineering Vol.34 No.4 pp.365-382
DOI : https://doi.org/10.14478/ace.2023.1049

Effective Approaches to Preventing Dendrite Growth in Lithium Metal Anodes: A Review

Jaeyun Ha, Jinhee Lee, Yong-Tae Kim†, Jinsub Choi†
Department of Chemistry and Chemical Engineering, Inha University, Incheon 22212, Republic of Korea

1 These authors contributed equally to this work.


† Corresponding Author: Inha University Department of Chemistry and Chemical Engineering, Incheon 22212, Republic of Korea Tel: +82-32-860-7476 e-mail: yongtaekim@inha.ac.kr
† Corresponding Author: Inha University Department of Chemistry and Chemical Engineering, Incheon 22212, Republic of Korea Tel: +82-32-860-7476 e-mail: jinsub@inha.ac.kr
May 15, 2023 ; July 5, 2023 ; July 5, 2023

Abstract


A lithium metal anode with high energy density has the potential to revolutionize the field of energy storage systems (ESS) and electric vehicles (EVs) that utilize rechargeable lithium-based batteries. However, the formation of lithium dendrites during cycling reduces the performance of the battery while posing a significant safety risk. In this review, we discuss various strategies for achieving dendrite-free lithium metal anodes, including electrode surface modification, the use of electrolyte additives, and the implementation of protective layers. We analyze the advantages and limitations of each strategy, and provide a critical evaluation of the current state of the art. We also highlight the challenges and opportunities for further research and development in this field. This review aims to provide a comprehensive overview of the different approaches to achieving dendrite-free lithium metal anodes, and to guide future research toward the development of safer and more efficient lithium metal anodes.


Lithium–metal anode , Dendrite-free , Lithiophilicity , Gradient structure , Electrolyte additive , Protective layer

초록

    1. Introduction

    Lithium (Li) has the lowest reduction potential (-3.04 V vs. SHE.) and smallest ionic radii, making Li–ion batteries the most promising battery system to achieve high power and energy densities[1,2]. Nowadays, graphite has been commercialized as a representative anode material, based on its low lithiation/delithiation potential and excellent stability[3]. However, the limited theoretical capacity of graphite (372 mAh·g-1) cannot meet the demand for high-capacity batteries fueled by the development of electric vehicles[4,5]. Thus, various attempts have been made to replace graphite-based anodes with high-capacity anode materials[4,5]. In the early stages of the lithium–ion battery development, lithium, which has high capacity (3,860 mAh·g−1), low gravimetric density (0.534 g·cm−3), and high energy density, was used as the anode material for lithium–ion batteries[6]. However, commercialization of lithium metal electrodes has significantly been constrained by the problem of rapid deterioration due to the growth of lithium dendrites during a reaction, which leads to internal short circuits and thermal runaway[7]. During the deposition of lithium, the large volume change of lithium and the uneven dissolution of dendrites can cause the formation of electrically isolated “dead lithium”, which consumes Li in the electrolyte and impedes the diffusion pathway of lithium ions, and results in low Coulombic efficiency[4,8]. In addition, the growth of dendritic lithium or volume expansion of lithium causes cracks in the SEI layer to expose a new interface, continuously forming a new SEI layer and depleting the electrolyte[4,8].

    In recent years, significant efforts have been made to overcome the challenges associated with using lithium metal as an anode material in rechargeable batteries[4,9]. One of the major problems with the use of lithium metal anode is the formation of dendritic lithium, which can cause short circuits that pose safety hazards and degrade battery performance. To address this issue, several strategies have been developed. The first strategy is to create lithiophilic site that has a low nucleation barrier for lithium, promoting uniform lithium nucleation[10, 11]. The second strategy involves forming lithiophilic or electrically conductive gradient host that induces bottom-up growth of lithium by controlling the distribution of the electric field[12,13]. The third strategy is to utilize electrolyte additives to form a mechanically and chemically stable SEI layer with enhanced ion conductivity and insulating properties[5,14]. The final strategy involves the introduction of an artificial protective layer that can perform a role like the SEI layer[8,15, 16]. These protective layers have an advantage over SEI layers in terms of controllable composition, thickness, and porosity. In this review, we offer a comprehensive perspective on the current research trends, limitations, and recommended future research directions and challenges associated with four strategies that are currently being pursued to address the issues faced by lithium metal batteries.

    2. Mechanism of dendritic lithium growth

    Lithium dendrites grow easily in areas where surface energy is low and the barrier to lithium–ion movement is high[17]. When lithium nucleation first occurs on a smooth electrode surface, lithium ions move to high-surface energy sites to stabilize the electrode surface. If the difference in surface energy between sites is negligible, or the movement of lithium ions is restricted, lithium ions do not move to other unstable sites. Instead, they stick preferentially to the previously formed lithium nucleus, initiating dendritic growth. The biggest problem with dendrite growth is that once dendrites are formed, the internal anode environment changes to a system that promotes dendrite growth. Typical dendrites have a tree branch-like crystal shape. At the end of the crystal, there is a ‘tip effect’, where the electric field is more concentrated than on a flat surface. As a result, more lithium ions are attracted to the tip, and dendrites grow preferentially again, falling into a vicious cycle of self-amplification[18]. Uneven lithium nucleation can also cause excessive electrolyte consumption. Reactive lithium metal forms a solid electrolyte interface (SEI) layer when it comes into contact with an electrolyte. As dendrite growth accelerates, new interfaces between needle-like lithium and electrolyte are created, along with the fracturing of existing SEI layers. This results in additional electrolyte consumption and the formation of new SEI layers[19]. Throughout the entire cycle of a lithium metal battery, lithium metal repeatedly undergoes this process. During the reaction, lithium dendrites steadily grow in the vertical direction, gradually narrowing the distance between the lithium electrode and the cathode electrode. These vertical grown lithium dendrites serve as minute electron transfer pathways, leading to local electron conduction with abnormally low overpotential and low internal resistance, which phenomenon is referred to as a soft short circuit. At these soft short sites, continuously grown lithium eventually penetrates the separator and directly connects to the cathode electrode, leading to a hard short circuit[20,21]. Figure 1 illustrates the development of lithium deposition and dissolution under low and high current density environments. At low current density, lithium plating and stripping occur at a relatively slow pace, allowing the SEI layers to sufficiently accommodate the resulting volume changes. However, at high current density, the SEI layers are unable to withstand rapid volume changes and may break. As a result, newly exposed lithium rapidly reacts with the electrolyte, leading to dendrite formation. Therefore, suppressing dendrite growth is essential in lithium metal anodes. It is important to prevent entry into a vicious cycle from the initial stage of lithium nucleation[22].

    3. Lithiophilicity-based strategies

    Although lithiophilicity is not universally agreed upon, it generally refers to the affinity between lithium species and hosts[11,24]. One leading interpretation suggests that the difference in binding energy between lithium and other atoms determines lithium affinity. Gong et al. explain that the binding energy for lithium adsorption on the host surface must fall within a specific threshold range. Excessive binding energy can damage the host surface, while insufficient binding energy can hinder lithium nucleation, leading to the growth of lithium dendrites[ 25]. Using density functional theory calculations, Fan et al. reported that the binding energies required for lithium adsorption are -0.51 eV for Cu, -0.83 eV for Co, -0.26 eV for graphene, and 0.62 eV for N-doped graphene. Thermodynamically, a negative binding energy indicates a lithiophilic adsorption site, while a positive binding energy indicates a lithiophobic site[26]. Lithiophilicity is a crucial consideration in the design of electrodes for lithium metal batteries, because lithium deposition occurs preferentially at sites with high lithium affinity. Various strategies have been developed to enhance lithiophilicity and improve the Coulombic efficiency and lifespan of lithium metal batteries. These strategies include the formation of metals, metal oxides, nitrides, sulfides, fluorides, and other materials on the electrode surface. Such materials effectively inhibit dendritic lithium growth, increase the high lithiophilic materials on the electrode surface, and improve the Coulombic efficiency and lifespan characteristics of lithium metal batteries.

    3.1. Metals or their alloys

    The use of metals with high lithiophilicity can decrease the overpotential required for lithium nucleation during the plating process. Several metals, such as Mg, Al, Si, Zn, Sn, Ag, and Au, display lower overpotential than Cu, which is widely used as an anode current collector in the industry[27], and these metals easily form alloys with lithium metal. Kim et al. developed a 3D porous Li–Si alloy framework at room temperature by chemically etching an ultra-thin Si wafer and inducing a spontaneous alloying reaction with lithium metal(Figure. 2a)[28]. The fabricated electrode demonstrated stable long-term cycling performance, even at a rapid charge/discharge rate of 10 C, when combined with commercial cathodes, such as LiNi0.5Co0.2Mn0.3O2 (NCM523) or LiFePO4 (LFP). It exhibited 1,000 cycles for NCM523, and 2,000 cycles for LFP. Kim and Moon’s research group synthesized a 1D core–shell structure of Au nanoparticles dispersed in a porous carbon fiber shell using a dual-nozzle electrospinning method(Figure 2b)[29]. The authors could reduce the effective current density by creating a 3D conductive structure using this 1D fiber, and could induce lithium plating in the lithiophilic core space through facile lithium transport via the porous shell. The electrode exhibited persistent capacity retention of 88.0% (after 1,000 cycles) in LFP coin cell tests at a charge/discharge rate of 0.5 C, and 86.9% (after 160 cycles) in large-area NCM622 pouch cell tests. Choi et al. designed an electrode where Ag nanoseeds are placed inside a double-layer shell consisting of N-doped carbon hollow spheres (NC) and Li1.5Al0.5Zr1.5(PO4)3 (LAZP)(Figure 2c)[30]. The NC provides a lithium storage space, while Ag nanoseeds function as lithiophilic sites. LAZP plays a role in alleviating non-uniform lithium diffusion and volume expansion, resulting in stable cycling performance.

    Various strategies that utilize lithiophilic metals have also been reported, such as Sn-electroplated hierarchical 3D copper frameworks[7] and Zn-electroplated 3D Cu foam[8]. These approaches have commonalities: they all use lithiophilic metals as sites for lithium plating, and they provide space for lithium storage. To inhibit lithium dendritic lithium growth, it is crucial to prevent uncontrolled lithium nucleation and encourage spatially uniform nucleation using lithiophilic sites. In addition, there must be spatial flexibility to accommodate solid-state lithium. Strategies using alloys of copper and other metals are also employed to achieve both of these keys simultaneously. The Cu–M alloy (where M is a lithiophilic metal) is a strategy that uses M as a lithium nucleation site, and Cu, which has low lithiophilicity, as an inert substance that remains structurally unchanged during the charge/discharge process. Figure 3a demonstrates that the Cu–Zn alloy electrode effectively mitigates dendritic lithium growth and volume changes[31]. The authors suggest that the Cu-based matrix, in addition to the lithiophilic Zn site, aided in preventing volume changes, and contributed significantly to stable lithium plating/stripping behavior. Figure 3b shows the process of preparing a lithium pre-stored Cu-Mg alloy electrode, which involves immersing Cu foam in molten Mg–Li[32]. Li et al. reported that Mg was uniformly distributed throughout the entire structure during the formation of a Cu–Mg alloy, resulting in a homogeneous lithium–ion flux. Choi’s group used cyclic voltammetry to uniformly electroplate Sn onto the surface of Cu2O nanowires[33]. The resulting 3D hierarchical Sn/Cu6Sn5 electrode, which had high surface area and uniform distribution of lithiophilic sites, exhibited stable charge/discharge behavior in full-cell tests with a commercial LPF cathode, recording 80.1% capacity retention over 550 cycles.

    3.2. Metal oxides

    The fundamental basis of strategies that use metal oxides to inhibit dendrite formation is the reaction of metal oxide with lithium, which converts it into metal and lithium oxide:

    M x O y  + 2yLi +   →   xM  +  yLi 2 O
    (1)

    The formation of lithium oxide on the electrode surface improves the stability of the solid electrolyte interface (SEI) layer, and increases the lithiophilicity at the electrolyte/electrode interface. After conversion, the remaining metal can act as a lithiophilic site or form an alloy with lithium to induce uniform lithium plating. Yang et al. applied a copper foil with a vertically aligned copper oxide as an anode for lithium metal batteries[34]. Figure 4a depicts how vertically aligned copper oxide interacts with lithium. CuO itself has high lithiophilicity, reducing the nucleation overpotential. In addition, during half-cell discharge, some CuO reacts with lithium to convert to Cu and Li2O. Then, Li2O reduces lattice mismatch when new lithium is plated, and makes the surface more lithiophilic[35]. Chen et al. decorated MnO2 on 3D graphene foam, and immersed it in molten Li to produce a Li– Mn/graphene composite(Figure 4b)[36]. The high surface area and electrical conductivity of the graphene backbone, the electrolyte/electrode interface stabilization effect of Li2O, and the high lithiophilicity of MnO2 and Mn exhibited a synergistic effect. This resulted in stable symmetric cell cycling for over 400 h under harsh conditions of 5 mA·cm−2 / 1 mAh·cm−2.

    Similarly, Lai’s group manufactured an effective anode for lithium metal batteries by forming a SnO2 layer derived from SnC2O4 on the surface of graphite fiber(Figure 4c)[37]. Research has also been conducted on the practical use of various metal oxides. Zhou et al. demonstrated symmetric cell cycling for over 2,000 h at a current density of 2 mA·cm−2 / 4 mAh·cm−2 by growing Co3O4 nanowires on a nickel foam[38]. Liang et al. compared amorphous and crystalline ZnO when fabricating a 3D copper collector with ZnO nanosheets[39]. Isotropic ion channels were observed in amorphous ZnO, leading to isotropic lithium nucleation and growth.

    Conversely, bimetallic oxides can exhibit better electrical conductivity and higher electrochemical activity than monometallic oxides [40,41]. Wang’s group synthesized bimetallic oxides of ZnMn2O4, ZnCo2O4, NiCo2O4, and monometallic ZnO and MnO2 on carbon cloth using hydrothermal methods, and tested their reactivity with molten Li [42]. In the monometallic oxide of ZnO, lithium infusion occurred unevenly. In MnO2, the carbon cloth framework was damaged due to a violent lithium infusion reaction. In contrast, in bimetallic oxides, lithium infusion reactions occurred uniformly at a moderate rate, making them suitable as excellent anode materials for lithium metal batteries. Lou et al. developed an electrode with high lithiophilicity, large surface area, and lithium–ion pathways using metal acetate hydroxide precursors that upon heat treatment convert to hollow structures[ 43]. Lower lithium nucleation overpotential, uniform lithium plating, and long-life characteristics were reported when using Ni–Co bimetallic compounds, instead of monometallic compounds, as precursors.

    3.3. Other binaries (pnictide, chalcogenide, halide…)

    In addition to oxides, various compounds, such as pnictides (N, P, ...), chalcogenides (O, S, Se, ...), and halides (F, Cl, ...), can also be used as anode materials for lithium metal batteries. Similar to the mechanism of metal oxides, when Li binaries (Li3N, Li3P, Li2O, Li2S, Li2Se, LiF, LiCl, ...) are formed, the Li binary layer functions as an excellent electronic insulator and ionic conductor at the interface, and maintains a thermodynamically stable state against lithium metal, providing electrochemical stability to the anode[44,45]. Wang et al. proposed the use of in situ generated Li3N by manufacturing a 3D nickel foam architecture with lithiophilic cobalt nitride(Figure 5a)[46]. The Co3N nano brush increased the surface area of the nickel skeleton and maximized surface lithiophilicity by providing Li3N. When comparing CoO and Co3N with the same morphology, better electrical and ionic conductivity was confirmed in the Co3N electrode. In a 0.5 C LFP full-cell test using a Co3N electrode, a capacity of 168 mAh·g−1 and a capacity retention of 93% were shown at 600 cycles. Liu’s group also applied Li3P by modifying Ni2P on the surface of carbon nanofiber hosts(Figure 5b)[47]. Ni2P had lithiophilic properties and provided abundant lithium nucleation sites by exhibiting reversible conversion reactions with lithium. Uniform and compact lithium plating was observed in the Ni2P electrode. In a full cell test using a high-loading LFP cathode (10 mg·cm−2), stable battery cycling performance was demonstrated with 99.1% Coulombic efficiency over 1,000 cycles at a fast charge/discharge rate of 5 C. Figure 5c depicts the fabrication of the CoSe2–N-doped carbon@CFC host by decorating Co nano-flake on carbon fiber cloth (CFC) using an in situ growth method, followed by high-temperature carbonization and selenization[48]. Lu et al. reported that the Li2 Se generated during charge/discharge enhances electrode flexibility owing to its high ionic conductivity, chemical stability, and mechanical strength. The CoSe2–NC@CFC symmetric battery exhibited ultrastable cycling performance for more than 1,600 hours at 5 mA·cm−2 / 20 mAh·cm−2.

    Furthermore, it was operable in a pouch cell with an LPF cathode, even when folded, rolled, or bent at 90 or 180 degrees. Cao’s team developed a one-step process to fluorinate nickel foam using PTFE powder[49]. This method enables the synthesis of fluoride without the use of toxic chemicals. Li’s group also reported a one-step sulfide synthesis on a copper substrate[50]. In contrast to conventional solution immersion methods, the Cu7S4 nano-flake was formed vertically oriented to the substrate by conducting synthesis under a strong electric field. The Cu7S4 nano-flake significantly improved the wettability of the electrode surface. The high concentration of Li2S on the electrode surface effectively controlled lithium nucleation compared to LiF, and reduced dead lithium by maintaining lithium grains in a spherical shape. Yu et al. directly formed a protective layer composed of Li2S and Li2Se on lithium metal at a low temperature using a simple gas/solid reaction(Figure 5d)[51]. The Li2S and Li2Se composite protective layer improved lithiophilicity on the electrode surface, while reducing charge transfer resistance at the interface. The authors compared the migration barrier energy of lithium atoms on the [100], [110], and [111] planes of Li2S and Li2Se through DFT calculations, and demonstrated that lithium ions move more easily on all planes of Li2Se. The Li2S−Li2Se||LFP full-cell exhibited stable behavior for over 450 cycles at a current density of 1 C. These findings demonstrate that in situ or ex situ strategies that form lithium compounds on the electrode surface are effective approaches for achieving high lithiophilicity, lithium–ion conductivity, and uniform lithium plating.

    4. Lithium hosts with gradient structure

    In the process of lithium plating/stripping, the mass transfer of lithium ions, the charge transfer of electrons, and the interaction between lithium ions and the electrode interface are all involved. The shape of lithium plating can be intentionally controlled by manipulating the strength or weakness of three properties within the electrode structure.

    4.1. Lithiophilicity gradient

    During the lithium plating process, lithium ions are generated from the cathode and transported across the separator, resulting in a concentration gradient of lithium ions in the thickness direction of the electrode. The closer to the separator, the higher the concentration of lithium ions, while the further from the separator, the lower the concentration of lithium ions. As a result, lithium is highly likely to plate intensively at the site near the separator, leading to dendritic lithium growth. To solve this problem, a strategy is being studied to make the area close to the separator lithiophobic, and far from the separator lithiophilic, within the electrode structure. This strategy can suppress lithium nucleation at the top, and cause bottom-up plating, where lithium ions are first reduced at the bottom. Zhang et al. implemented a lithiophilic– lithiophobic gradient by dripping CNT solutions with different ZnO contents onto a lithium foil(Figure 6a)[13]. The lithiophilic ZnO/CNT bottom layer bonded tightly to the lithium foil, inducing uniform SEI formation, and preventing mossy Li corrosion. In addition, the porous CNT layer on top facilitated the diffusion of lithium ions, and physically blocked the penetration of lithium dendrites. The gradient ZnO/CNT structure effectively suppressed dendrite growth even at a high current density of 10 mA·cm−2, and demonstrated its ability to improve cycle performance when applied to a 10 cm × 10 cm pouch cell and a lithium–S battery. Wang et al. developed a strategy to decorate only part of a 3D wood-derived carbon (WDC) framework with Ag, ZnO, Au, etc., using a simple capillary-induced method(Figure 6b) [52]. A concentration gradient of lithiophilic particles was naturally formed as the precursor solution penetrated the framework with a uniform micro-pore structure through a capillary effect. The authors verified that spatially dispersed lithiophilic sites were closely related to the uniform distribution of lithium–ion flux and current density. In addition, Niu et al. manufactured a 3D lithiophilicity gradient framework by stacking electrospun PVDF with different Ag concentrations[53], while Yan et al. synthesized a flexible lithium metal host by vacuum filtration of a lithiophilic CNT/MXene/SnO2 layer and a lithiophobic CNT layer[54]. Cheng et al. proposed a method for manufacturing a lithiophilicity gradient structure using physical evaporation deposition [55]. The authors stated that they could form a thin double layer on the nanoscale by sequentially coating Ag and C60 onto the surface of a 3D copper foam, and could control the uniformity of lithium plating.

    4.2. Conductivity gradient

    For lithium ions to be reduced to metallic form, they must receive electrons from the cathode. Therefore, if the electrical conductivity is controlled at any part to suppress charge transfer reactions, lithium plating can be deliberately controlled. Lee et al. reported an electrically conductive gradient host consisting of three layers: Cu nanowire/Cu nanowire + CNF/CNF + SiO2(Figure 7)[56]. The bottom layer has the highest electrical conductivity with Cu nanowires, while the middle layer has weakened electrical conductivity, depending on the mixing ratio of Cu nanowires and CNF. The top layer has significantly increased electrical insulation, due to the mixture of CNF and SiO2. The authors confirmed that the starting point of lithium nucleation changes to the middle or bottom, depending on the electrical conductivity gradient and the thickness of the insulating layer. The conductivity gradient host showed that by forming an appropriate electrical conductivity gradient, stable operation for more than 250 cycles is possible in a symmetric cell of 1 mA·cm−2 / 1 mAh·cm−2 . Pan et al. implemented an electrical conductivity gradient in the thickness direction by electrospinning with different contents of carbon nanofiber in a polyacrylonitrile- based 3D matrix[57]. The top layer, composed of polyacrylonitrile and LiNO3, was designed to maximize insulative properties while forming a uniform SEI. Stacking structures by increasing the concentration of materials with high or low electrical conductivity is one of the easy ways to synthesize an electrical conductivity gradient structure. On the other hand, Yang et al. proposed a strategy to coat only one side of a melamine-based framework with low electrical conductivity with a Ni thin film using magnetron sputtering[12]. The amine-based insulating layer derived from melamine uniformly distributed lithium ions, and bottom-up lithium plating proceeded due to the electrical conductivity gradient formed by the thickness of the Ni coating layer. The full cell combined with an LFP cathode showed stable performance for more than 400 cycles at 1 C and excellent rate performance.

    4.3. Pore size gradient

    In addition to lithiophilicity and electrical conductivity characteristics, controlling the transport of lithium ions can induce reversible lithium deposition/stripping behavior. Choi et al. designed a 3D lithium host with a pore size gradient using a polymer template that could easily be removed by heat treatment(Figure 8)[58]. By casting a mixture of 15 μm polymethylmethacrylate (PMMA) and graphite material onto a copper foil in the first step, and then casting a slurry mixed with 50 μm PMMA in the second step, they synthesized a structure in which the pore size decreased from the top layer to the bottom layer. Lithium deposition was suppressed on the large-pore top surface layer, and reversible lithium plating/dissolution occurred from the bottom due to facilitated lithium–ion transport within the smooth upper passages. In a pouch full cell test combined with an NCM cathode, significantly improved results were shown with 87.2% capacity retention at 100 cycles, compared to when a lithium metal anode was used directly.

    5. Interface stabilization strategies

    Considering the high reducibility of lithium metal, a reaction with the electrolyte is inevitable, leading to the formation of a solid electrolyte interface (SEI) layer on the lithium surface due to electrolyte decomposition[ 59-61]. The generated SEI layer is essentially electrically insulating, but ion conductive[60,61]. Thus, the SEI layer prevents additional SEI formation and electrolyte depletion by obstructing direct contact between the lithium and electrolyte[62,63]. Typically, under a conventional electrolyte without additives, the SEI layer is composed of ROCO2Li, RCO2Li, ROLi, Li2CO3, Li2O, and trace amounts of LiF and LiP[64-66]. The inhomogeneous SEI layer containing ROCO2Li, RCO2Li, ROLi, and Li2CO3 has low ion conductivity and weak mechanical strength, causing local lithium deposition, and cannot withstand the volume expansion during lithium deposition, due to its low mechanical strength[64-66]. Continuous cracking and collapse of the SEI layer exposes a new interface and induces the formation of a new SEI layer, depleting the electrolyte and deteriorating the cell performance[ 5,14]. To overcome these problems, diverse efforts are underway to form a stable SEI layer with mechanical strength, excellent ion conductivity, and chemical stability, such as incorporating electrolyte additives and protective layers(Figure 9)[19,59,67-69].

    5.1. Electrolyte additives

    To use high-capacity anode materials, such as silicon-based anode or transition metal oxide, many strategies have been developed to form a stable SEI layer, to prevent cracks due to extreme volume expansion. It is well known that fluoroethylene carbonate (FEC) and vinylene carbonate (VC), which induce LiF in the carbonate electrolyte system, suppress the electrolyte side reactions by decomposing instead of ethylene carbonate (EC)[70,71]. Continuous cracking and collapse of the SEI layer exposes a new interface and induces the formation of a new SEI layer, depleting the electrolyte and deteriorating the cell performance[ 70,71]. In the lithium–metal anode system, LiNO3 decomposes during the reaction to form a SEI layer containing Li3N and LiNxOy, enabling interface stabilization, and greatly increasing stability[72-74]. Wu et al. investigated the effect of LiNO3 concentration on lithium plating in an ether-based electrolyte[75]. As the concentration of LiNO3 increased, the Coulomb efficiency increased, with the highest efficiency observed at 0.4 M[75]. However, at concentrations above 0.4 M, the Coulombic efficiency decreased[75]. Cui et al. investigated the effect of LiNO3 in ether and ester-based electrolyte[76]. In the presence of LiNO3 with both ether and ester electrolytes, it was observed that round lithium was plated instead of sharp dendrite lithium, which is generated in the absence of LiNO3(Figures 10a and b)[76]. In addition, when LiNO3 was present in the electrolyte, the Coulombic efficiency of Li plating/stripping increased dramatically(Figure 10c)[76]. Because of these excellent LiNO3 stabilization properties, efforts have been made to construct electrolytes containing high concentrations of LiNO3 additives[76-79]. Due to the low solubility of LiNO3 in ester solvent, ether such as DOL and DME-based solvent with high LiNO3 solubility are used as electrolytes instead[72,75,80]. Because most ether solvents show high HOMO energy, and therefore low oxidation stability (below 4 V vs. Li/Li+), they limit the use of high voltage cathode materials for high energy density in ether-based electrolyte system[ 78,79,81,82]. Thus, to overcome this problem, various methods of increasing the amount of LiNO3 in the ester electrolyte with excellent high voltage stability are being explored[78,79,81,82].

    The first method is to introduce additional additives that increase the solubility of LiNO3. In previous studies, various solvents with high solubility of LiNO3, such as dimethyl sulfoxide[83], pyridine[84], tris (pentafluorophenyl)borane[85], sulfolane[86], and Mg(TFSI)2[87], were added. Dimethyl sulfoxide and pyridine have higher solubility than solvents such as EC, DMC, and FEC, due to the number of donors. Xiong et al. introduced pyridine as a new carrier solvent that has a higher Gutmann donor number than NO3−[84]. Thus, LiNO3 can be dissolved in pyridine up to 4.5 M. In the standard carbonate electrolyte without pyridine, it was supersaturated upon the addition of 1 wt% LiNO3, but when pyridine was added, 1 wt% LiNO3 was completely dissolved. Uniform Li without dendritic or mossy structure was deposited on Li metal surfaces only in electrolyte with pyridine. Wang et al. investigated the effect of Mg(TFSI)2, Zn(TFSI)2, and Al(OTF)3 on increasing the solubility of LiNO3(Figure 11a)[87]. As shown in Figure 11b, LiNO3 was completely dissolved in all solvents containing the Mg(TFSI)2, Zn(TFSI)2, and Al(OTF)3, even though the amount was 5 times larger than that of the original electrolyte without additive. Multivalent cation acts as carrier to enhance the LiNO3 solubility by promoting NO3− adsorption. Consequently, high Coulombic efficiency and uniform Li deposition were achieved in an electrolyte containing high concentration LiNO3 by the addition of Mg(TFSI)2(Figures 11c–j). When tris(pentafluorophenyl)borane and sulfolane is added, solubility increases by constructing clusters with NO3− and by binding Li+ due to the strong affinity of sulfolane, respectively[85,86]. In addition to these examples, there are studies using some high-valent transition metal cations, such as Cu2+ and Sn2+, with Lewis acid sites as solubilizers to adjust and solvate NO3− on the surface of LiNO3 clusters to increase the solubility of LiNO3[79,88].

    The second method is to introduce additional solid state LiNO3 or LiNO3 embedded layer. This method has the advantage of being able to introduce an excessive amount of LiNO3 and continuously supply the consumed LiNO3. Sun et al. produced LiNO3 powder embedded lithium metal foil(Figure 12a)[89]. To make LiNO3 embedded Li foil, they mixed LiNO3 powder with the Li foil, and repeatedly folded and rolled it. During activation cycle, the as-prepared LiNO3 embedded Li foil induced Li3N/LiNxOy/Li2O rich-SEI layers, which were highly stable and highly ionic conductive. Ryou et al. and Cui developed LiNO3 embedded protective layer, such as Al2O3/LiNO3/PVDF−HFP/DMF/ plasticizer and PVDF−HFP/LiNO3(Figures 12f and g), respectively [76,90]. In this method, an excess of LiNO3 is coated with a polymer layer as a protective layer, and the excess LiNO3 plays a role in forming a stable Li3N SEI layer, and providing a continuous supply of LiNO3. As shown in Figure 12, excessive LiNO3-embedded PVDFHFP exhibits superior Coulombic efficiency, compared to DOL/DME + 2 wt% LiNO3 electrolyte with moderate amounts of LINO3 present[ 76]. Jiao et al. found that saturated LiNO3 powder in electrolyte remains in solid–liquid coexisting form without any container, and acts as a continuous supplier for LiNO3[91].

    Aside from LiNO3 additive, various electrolyte additives have been studied. These electrolyte additives are introduced to modify the composition of the SEI layer. In previous research, various additives, such as 2-Fluoropyridine[92], hexafluoroisopropyl trifluoromethanesulfonate (HFPTf)[93], Tris(2, 2, 2-trifluoroethyl)borate[94], and tris(hexa- fluoroisopropyl) phosphate[95], have been developed, starting with the FEC additive[96], to form the LiF-rich SEI layer with excellent stability. LiF has superior stability of the interface between lithium and electrolyte with its high mechanical strength, high chemical stability, and high interface energy(Figure 13a)[97,98]. However, LiF has relatively low ionic conductivity of 3 × 10−9 S·cm−1. Another stable compound of SEI layer is Li2S, which has high ionic conductivity (≈10−5 S·cm−1), and higher Yong’s modulus (82.6 GPa) than that of LiF (64.9 GPa)[93,99]. High ionic conductivity leads to easy lithium plating and larger nuclei size. Ma et al. investigated lithium oxysulfide and lithium fluoride composite SEI layer[93]. In this system, LiF with high interface energy located throughout the SEI layer with small amount Li2SOx (X = 0, 3, 4) with high ionic conductivity(Figures 13b-f). The HFPTf easily dissolved Li+ on the Li metal surface, increasing the Li+ flux, which resulted in high cycle stability and low charge transfer resistance due to uniform Li deposition(Figures 13g-j). In addition, electrolyte additives sometimes affect the formation of CEI on the cathode surface. The HFPTf causes a thin and uniform CEI layer on the surface of NMC811, which improves the performance of the full cell. In summary, various electrolyte additives have been developed to form SEI layers with high chemical and mechanical stability and excellent ionic conductivity. Various attempts in many studies have been made to include a large amount of LiNO3, due to the excellent performance of the Li3N rich layer. In addition to LI3N-inducing additives, electrolyte additives that form SEI layers rich in other compounds, such as LiF with excellent mechanical properties and Li2S with excellent ionic conductivity, have been extensively investigated. However, despite these excellent properties, electrolyte additives have disadvantages, in that they cannot be continuously intervened, and are vulnerable to deterioration due to irreversible reactions or side reactions.

    5.2. Protective layers

    Unlike SEI layers obtained through the decomposition of electrolyte, the artificially formed protective layer can easily change its properties by changing the materials used[8,15,16]. Protective layers, similar to the SEI layer, also require ion conductivity, electrolyte permeability, chemical stability, and mechanical strength[8,15,16]. Protective layers are classified into inorganic, organic, and organic–inorganic composites, depending on the material. Typically, inorganic protective materials have high shear modulus[15,101]. As mentioned before, because LiF has high Young’s modulus and high chemical stability, it is one of the promising candidates for the inorganic protective layer[97,98, 102,103]. Cui et al. introduced artificial LiF layer on the Li metal surface[ 102]. They investigated facile LiF coating methods under low temperature using fluoropolymers, such as CYTOP, Teflon, and PVDF. Only CYTOP releases F2 gas at low temperature (250 °C), and the released F2 gas reacts on the surface of lithium to form LiF. The as-prepared LiF-coated Li metal indicated superior Coulombic efficiency and cycle stability at symmetric cell test, compared to the bare Li metal. Al2O3 protective layer has high anticorrosive property with chemical stability and insulating nature under various atmosphere and electrolyte [104]. The Al2O3 thin film has Young’s modulus of (6.4~14.3) GPa, which value is sufficient to suppress dendritic lithium growth[105, 106]. Thin film Al2O3, which has low ionic conductivity, partially converts to high ionic conductive Li–Al–O or LiAl5O8 during lithiation, improving the stability of lithium metal anode[105,107]. Among various protective layers, the graphene-based protective layer showed high Young’s modulus and high deformability. Polymeric or organic protective layers have high flexibility and elasticity, which allow them to effectively withstand the volume expansion during lithium plating/stripping[ 108,109]. Polymeric protective layers are classified into polar polymer and non-polar polymers[15]. These layers are only ionically conductive in solvents with a similar solubility parameter. Thus, non-polar protective layers are used with less polar electrolytes to maximize conductivity. Poly(dimethyl siloxane) (PDMS) is one of the widely- used non-polar polymers[110]. PDMS can effectively suppress volume expansion and side reactions due to its high chemical stability and high elasticity. Zhu et al. developed nanoporous PDMS coated Cu foil host as lithium metal container to enhance the ionic conductivity of PDMS layer[111]. They formed nanopore in the PDMS layer via an etching process using HF. The non-porous PDMS acted as non-ionic conductive layer, and Li plating did not occur. The as-prepared nanoporous PDMS layer successfully suppresses the growth of lithium dendrites, and shows high Coulombic efficiency. One of the promising polar polymeric protective layers is PVDF. The β-phase PVDF has the highest dipolar moment per unit cell, while the α phase PVDFs are non-polar, due to antiparallel dipoles within the unit cell[112-114]. Therefore, high-polarity β-phase PVDF has the highest dielectric constant compared to other phases. The high dielectric constant of β -phase PVDF induces the deposition of small-sized Li particles, promoting more uniform lithium plating[115,116]. In addition, PVDF was used with hexafluoropropylene as poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF−HFP)[117,118]. In this co-polymer protective layer, HFP with crystalline phase exhibits good mechanical properties and induces partial amorphization, which improves ionic conductivity. These PVDF-based protective layers serve to suppress dendrite growth based on their excellent insulation properties and flexibility, and provide additional stability by constructing an in situ LiF-rich SEI layer[15,117,118]. Wu et al. investigated the dendrite growth inhibitory effect of a PMMA/PVDF blended protective layer[ 119]. PMMA has high electrochemical stability and acts as electrolyte container, due to its superior affinity with electrolyte. Thus, PMMA reduces the interface resistance and heterogeneous nucleation of lithium. PVDF serves to stabilize interface with its high chemical and mechanical stability. As shown in Figures 14a–i, dendritic and mossy lithium was deposited on the bare Cu foil, but dense lithium layer was deposited under the PMMA/PVDF blended protective layer, which prevents the growth of dendritic lithium. Consequently, the bare Cu foil exhibits much lower stability than that of Cu foil with PMMA/PVDF blended protective layer in both symmetric cell test and full cell test (Figures 14j and k).

    Polymer/inorganic protective layer has been researched to use the advantages of polymer and inorganic. As previously mentioned, inorganic compounds, such as Al2O3[105,107], ZrO2[120], and LiF, have high mechanical strength, while polymeric compounds, such as PDMS and PVDF, have high flexibility and elasticity. In addition, inorganic materials sometimes lead to constructing the highly lithiophilic and highly stable artificial SEI layer[121,122]. He et al. developed a high modulus AlPO4-embedded elastic PVDF/HFP protective layer[121]. They homogenously dispersed AlPO4 in PVDF/HFP via facile mixing AlPO4 dispersion with PVDF/HFP solution. The bare PVDF/HFP layer exhibited lower Young’s modulus (0.8 GPa) than that of the AlPO4-embedded PVDF/HFP layer (1.6 GPa). The AlPO4-embedded PVDF/HFP layer showed excellent electrochemical stability by suppressing dendritic lithium growth and side reactions. Zhong et al. investigated the multi-functional composite protective layer using PVDF and ZnCl2[122]. They synthesized the PVDF/ZnCl2 composite polymer layer via an electrospinning method. PVDF acts as an elastic and flexible layer, and ZnCl2 converts to LiCl and metallic Zn. LiCl has as high a surface energy as LiF and low Li+ diffusion energy barrier, thus preventing side reaction, and providing high Li transport ability[123]. In addition, metallic Zn is one of the high lithiophilic materials that lead to homogenous Li growth[124]. As shown in Figure 15, during lithiation, the PVDF/ZnCl layer converted to LiZn/LiCl–PVDF layer, which has various advantages of the LiZn, LiCl, and PVDF. In CE test, the LiZn/LiCl-PVDF layer showed high Coulombic efficiency for long cycles with unchanged voltage hysteresis and low lithium plating overpotential(Figures15 c–e). This stability of the LiZn/LiCl-PVDF layer was also confirmed in the symmetric test under low and high current density(Figures15 f–i). Based on the above discussion, the protective layer requires high chemical and mechanical stability, high ionic conductivity, and insulating nature to inhibit dendritic lithium growth. Nevertheless, there are still limitations to using protective layers, including issues with increasing thickness, and difficulties with the penetration of electrolytes, and their application in 3D structures. To overcome these problems, more research and development efforts are needed.

    6. Conclusions and perspectives

    Research on lithium metal batteries is rapidly increasing due to the need for high energy density storage systems. However, lithium dendrite growth presents a major obstacle to their commercialization. To address this issue, researchers have developed various strategies to improve the stability of lithium metal anodes. This review summarizes these strategies, and highlights the key elements that must be considered for effective anode design. During the lithium plating process, lithium ions are transported from the cathode to the easiest site for lithium nucleation on the anode surface. Once they reach an appropriate site, they can be reduced to their metallic form by receiving electrons from the anode. This process is influenced by properties such as the lithiophilicity between lithium ions and the electrode surface, charge transfer resistance of the electrode, and lithium mass transport within the electrode pathway. To design a lithium metal anode that effectively suppresses lithium dendrite growth, (1) lithiophilic sites must be evenly distributed on the electrode surface, (2) transport of lithium ions must be smooth, and (3) there must be enough space to accommodate volume changes that occur during lithium plating and stripping. Strategies such as forming metals, metal oxides, pnictides, chalcogenides, and halides on porous structural surfaces can be used to achieve this. Additionally, creating a gradient structure by lowering lithiophilicity and electrical conductivity near the separator, and facilitating lithium transport deep into the electrode, can also be an effective strategy for suppressing lithium dendrites and improving lifespan characteristics. After designing an effective anode host, attention should be paid to the SEI layer formed at the interface between the anode and the electrolyte. For stable operation of lithium metal batteries, (1) the lithium– ion flux within the SEI layer must be uniform, and (2) the mechanical and chemical stability of the SEI layer must be excellent. An SEI layer with these properties can be formed by adding a high concentration of LiNO3 to the electrolyte. However, its low solubility in electrolytes, decreased lithium–ion conductivity from using high concentration additives, performance degradation from additive consumption, and deterioration from irreversible side reactions are significant considerations. As a result, it is crucial to choose an appropriate organic solvent that can compensate for these shortcomings. In addition to using additives, a protective effect can be achieved by pre-synthesizing an artificial SEI layer on the anode surface. A good protective layer should have (1) low electrical conductivity to block electrolyte side reactions, (2) high lithium–ion conductivity, and (3) be able to provide sufficient physical and chemical protection, even at a thin thickness. Inorganic ceramic materials, such as Al2O3, or organic polymer materials, such as PVDF and PDMS, can be used to synthesize such protective layers. For the commercial use of lithium metal batteries, it is necessary to effectively design lithium metal anodes by combining the strategies discussed here, and thoroughly reviewing their compatibility with various cathode, electrolyte, and separator materials. Lithium–metal batteries, with their high power and energy density, are expected to become the mainstream of energy storage devices in the future. If critical issues related to lithium dendrite growth at the anode are intensively explored, it may be possible to advance the commercialization of high-potential lithium–metal batteries.

    Figures

    ACE-34-4-365_F1.gif
    An illustration of the morphological phenomena developed on Li electrodes during Li deposition and dissolution. Reproduced with permission[23]. Copyright 2000, American Chemical Society.
    ACE-34-4-365_F2.gif
    (a) Schematics of the spontaneous pre-lithiation strategy based on a self-discharge mechanism for fabricating the 3D porous Li–Si alloy-type interfacial framework (LSIF). Reproduced with permission [28]. Copyright 2021, Wiley–VCH. (b-1) Schematics of the experiment for the electrospinning of Au@porous hollow carbon fiber (PHCF). (b-2) Digital photograph, and (b-3) SEM images of the as-prepared polymer mat. (b-4-7) SEM and TEM images of the Au@PHCF. (b-8) STEM image and corresponding EDS mapping of carbon, gold, and oxygen. Reproduced with permission[29]. Copyright 2022, American Chemical Society. (c-1) Schematic for the preparation of Ag@NC and Ag@NC@LAZP. TEM images of (c-2) SiO2, (c-3) SiO2@Ag, (c-4) Ag@NC, and (c-5) Ag@NC@LAZP. Reproduced with permission[30]. Copyright 2023, American Chemical Society.
    ACE-34-4-365_F3.gif
    Operando investigation of the electrochemical behavior of the bare Li and Li/CuZn composite by optical microscopy. Cross-sectional images of (a-1) the bare Li, and (a-2) Li/CuZn composite electrodes during the Li striping/plating process. The stripping/plating current density was maintained at 2.0 mA·cm−2, and the stripping/plating time was set as 30 min. The red arrows represent the Li stripping/plating direction. Reproduced with permission[31]. Copyright 2020, American Chemical Society. (b) Schematics of the Cu–Cu2Mg–Li formation process. Scale bars, 20 μm. Reproduced with permission[32]. Copyright 2021, American Chemical Society.
    ACE-34-4-365_F4.gif
    Schematics of different Li plating behavior on a CuO-free Cu substrate and a VA−CuO-modified Cu substrate. (a-1) For the CuO-free planar Cu electrode, the Li+ flux tends to concentrate at the tips of the rough Li surface, which enhances dendritic and mossy Li formation. (a-2) In a VA−CuO NSs-modified electrode, Li was homogeneously nucleated on the electrode surface, and thus uniform Li plating could be realized. Reproduced with permission[34]. Copyright 2018, Wiley–VCH. (b) Schematic of chemical reactions between the birnessite-MnO2 and the molten Li. Reproduced with permission[36]. Copyright 2018, Wiley–VCH. (c-1) Schematics of metal oxide@GF synthesis and the Li plating processes. SEM image of (c-2) SnO2@GF−1 from cross-section view, and EDS mappings of (c-3) C and (c-4) Sn. Reproduced with permission[37]. Copyright 2022, Wiley–VCH.
    ACE-34-4-365_F5.gif
    (a-1) Schematic of the preparation of Co3N/NF nanobrush. (a-2, a-3) TEM, and (a-4) HRTEM, images of Co3N/NF. The SAED pattern is given in the inset. (a-5) EDX elemental mappings of Co, N, and O. Reproduced with permission[46]. Copyright 2019, American Chemical Society. (b-1) Schematics of Li deposition behavior on Ni2 P@CNF compared with CNF. Characterizations of Ni2P@CNF. (b-2) Scanning transmission electron microscopy images of Ni2P@CNF. Reproduced with permission[47]. Copyright 2022, American Chemical Society. (c-1) Schematic of preparation route for CoSe2–NC@CFC. (c-2) HAADF–STEM, and (c-3) elemental mapping images of CoSe2–NC nanoflake. Reproduced with permission [48]. Copyright 2022, Wiley–VCH. (d-1) Schematic of the fabrication process of LSSe@Li anode. (d-2) Surface morphology of the pristine lithium. (d-3) Top-view, and (d-4) cross-sectional, morphologies of LSSe@Li anode. Reproduced with permission[51]. Copyright 2020, Wiley–VCH.
    ACE-34-4-365_F6.gif
    (a-1) Schematic for Li deposition of Li foils coated with a GZCNT interfacial layer. (a-2) Schematic of interfacial layer fabrication in a glove box. Graphene, ZnO/CNT, or CNT were mixed with DOL, and the suspension was dripped onto the Li foil at 100 μL cm−2, and dried at 80 °C for 1 h. (a-3) SEM images of the Li cells coated with GZCNT interfacial layer and cycled at a current density of 1 mA·cm−2 with a stripping/plating capacity of 1 mAh·cm−2 after 520 cycles. Reproduced with permission[13]. Copyright 2018, The Authors. (b-1) Schematic of the preparation of the 3D WDC−GDM (M = Ag, ZnO, Au) scaffold. (b-2) Cross-sectional SEM image of the WDC−GDAg. High-magnification SEM images of the (b-3) top-side, (b-4) middle-side, and (b-5) bottom-side of WDC−GDAg scaffold. Schematic and cross-sectional SEM images of WDC−GDAg at 1 mA·cm−2 with the capacities of (b-6) 0, (b-7) 3, and (b-8-10) 12 mAh·cm−2. Reproduced with permission[52]. Copyright 2022, Wiley–VCH.
    ACE-34-4-365_F7.gif
    Fabrication process and morphology of a CG host. (a-1) Schemes and photographs (insets) demonstrating the fabrication process and the structural robustness of a CG host. (a-2) Cross-sectional SEM image of a CG host. (a-3) Schematic of Li–ion reaction flux modeled using COMSOL Multiphysics for the plating of Li during the lithiation process on the CG host. The cross-sectional side-view ex situ SEM images (a-4-8) demonstrate the stable Li plating behavior according to the images representing DOCs of (a-4) 0% (0 mAh·cm−2), (a-5) 42% (3 mAh·cm−2), and (a-6) 100% (7 mAh·cm−2, fully lithiated state). After complete plating, the stripping process was also confirmed by ex situ SEM images when the DOC was (a-7) 71% (2 mAh·cm−2) and (a-8) 0% (7 mAh·cm−2, fully delithiated state). Reproduced with permission[56]. Copyright 2020, Wiley–VCH.
    ACE-34-4-365_F8.gif
    Schematic of the 3D carbon-based porous anode (3D-CPA) with a pore-size gradient (a-1), and cycling behaviors of the as-prepared 3D-CPA (a-2). (a-3) SEM images: cross-section of 3D-CPA (15/50), (a-4) Li plated for 4 mAh·cm−2 into 3D-CPA (15/50). Reproduced with permission[58]. Copyright 2021, American Chemical Society.
    ACE-34-4-365_F9.gif
    Conceptual Schemes of Design Principles for ASEIs (A and B) Scheme of the Li-metal morphological evolution with cycling under natural SEI (A), and ASEI (B). (C) Conceptual scheme showing 3 key properties of ASEIs and the corresponding approaches for each point. Principle 1: mechanically strong ASEIs suppress dendrites with high modulus, while conformally adapt the surface fluctuation during cycling. Principle 2: uniform and fast Li+ flux across the ASEIs enables non-dendritic growth. Principle 3: completely blocking the electrolyte from contacting Li reduces the side reactions, while controllable reactions between coatings and Li generate a favorable interfacial layer to mitigate further Li corrosion. Reproduced with permission[59]. Copyright 2020, Elsevier
    ACE-34-4-365_F10.gif
    The effect of nitrate on Li deposition morphology and CE in ether electrolyte. SEM images of Li deposition morphology using 1.0 M LiTFSI in 1:1 v/v 1,3- dioxolane/dimethoxyethane (DOL/DME) electrolyte (a) without, and (b) with, 2 wt% LiNO3 additive. The deposition was carried out at a current density of 1 mA·cm−2 and a capacity of 0.5 mAh·cm−2. Scale bars, 2 μm. (c) Li metal cycling CE with and without LiNO3 in DOL/DME electrolyte at a current density of 1 mA·cm−2 and a capacity of 1 mAh·cm−2 Reproduced with permission[76]. Copyright 2018, Springer Nature.
    ACE-34-4-365_F11.gif
    (a) Schematic of the role of multivalence ion in the formation of anion-derived SEI on Li metal anode surface in the designed BE MgLN electrolyte. (b) The digital images of 1.0 M LiPF6 in FEC−EMC electrolytes with only 25 mmol LiNO3 or with 25 mmol X−125 mmol LiNO3 (X = Zn(TFSI)2, Al(OTf)3, Mg(TFSI)2) additive. The Li Coulombic efficiency and the electrodeposited Li morphology in different electrolytes. (c) and (d) The typical Li plating/stripping profile and Li CE in BE and BE MgLN electrolytes at 0.5 mA·cm−2 with different areal capacity under 50 % Li utilization. (e)–(g) Typical SEM images of the plated Li in BE (e) and (f) and BE MgLN (h) and (i) electrolytes. Metallic Li is electrochemically deposited on the bare Cu substrate at 0.5 mA·cm−2 with 4.5 mAh·cm−2. (g) and (j) Cross-sectional density comparison of the plated Li metal in BE (g), and BE MgLN (j), electrolytes, of which the crater was sputtered by the Ga+ ion beam. Reproduced with permission[87]. Copyright 2022, Wiley–VCH.
    ACE-34-4-365_F12.gif
    (a) Schematic of the fabrication and working principle of LNO−SRF. (b) High-magnification SEM image, and (c)–(e) the corresponding EDX elemental mappings of LNO−SRF. Scale bar, 2 μm. Long-term Li cycling CE on Cu electrode in 0.5 M LiPF6 EC/DEC electrolyte without additives (Bare Cu), and with VC (2% VC) or nitrate additives (LNO−SRF), or in 1.0 M LiTFSI DOL/DME electrolyte with 2 wt% LiNO3 (DOL/DME + 2% LiNO3). (f) 0.5 mA·cm−2 current density and 0.5 mAh·cm−2 capacity, (g) 0.5 mA·cm−2 current density and 1 mAh·cm−2 capacity. Reproduced with permission [76]. Copyright 2018, Springer Nature.
    ACE-34-4-365_F13.gif
    (a) Plot of the relationship between the interfacial energy for possible SEI components and the number of Li metal formula units. Reproduced with permission[100]. Copyright 2018, Association for the Advancement of Science. Mechanism diagram of the electrolytes (b) with and (c) without HFPTf to optimize Li metal anode and improve the performance of Li||NCM811 cells. (d) XPS spectra of F 1s of Li||Li symmetric cells after 10 cycles with blank electrolyte, XPS spectra of (e) F 1s and (f) S 2p of Li||Li symmetric cells after 10 cycles with HFPTf-contained electrolyte. (g and h) Cycling stability of Li||Li symmetric cells in the blank and HFPTf-contained electrolytes, and MSIPE- and HFPTf-contained electrolytes at 1 mA cm−2 with a capacity of 0.5 mAh cm−2. (i and j) EIS spectra of the Li||Li symmetric cells with blank and HFPTf-contained electrolytes before cycling and after 100 h cycles. Reproduced with permission[93]. Copyright 2021, Wiley–VCH.
    ACE-34-4-365_F14.gif
    Morphology and composition analysis of lithium deposition on the modified/unmodified substrates. (a) Schematic of lithium deposition on electrode with/without modified coating. Top-view and cross-sectional view SEM images of lithium deposition with 1 mAh·cm−2 capacity on the (b) and (c) bare Cu, and (d) and (e) modified Cu, after 1 cycle, and on the (f) and (g) bare Cu, and (h) and (i) modified Cu, after 40 cycles. Electrochemical performance of the Li/Li symmetric cells and Li/LiFePO4 full cells. (j) Cycle stability comparison of the bare Cu@Li and modified Cu@Li at a deposition capacity and current density of 1.0 mA·cm−2 and 1.0 mAh·cm−2, respectively; (k) cycling performance of the bare Cu@Li/LFP and modified Cu@Li/LFP. Reproduced with permission[119]. Copyright 2021, RSC.
    ACE-34-4-365_F15.gif
    Schematics of Li deposition with and without PZEM: (a) A bare Li metal anode enables the continuous growth of Li dendrites in the absence of PZEM. (b) PZEM endows Li metal anode with the “digestion” ability for the Li dendrite sprouts, leading to dense Li deposition without the formation of Li dendrites. Electrochemical performances of the Li||Cu cells: (c) and (d) CE of Li||Cu cells using the bare Cu, PEM@Cu, and PZEM@Cu electrodes at (c) 1 mA·cm−2, and (d) 3 mA·cm−2, with a plating capacity of 1 mAh·cm−2. (e) Comparison of the voltage hysteresis of the Li plating/stripping at 1 mA·cm−2 for the bare Cu, PEM@Cu, and PZEM@Cu electrodes. (f)–(h) Voltage profiles of the symmetric cells with the bare Li, PEM@Li, and PZEM@Li electrodes at (f) 1 mA·cm−2, and (g) 5 mA·cm−2, for 1 mAh·cm−2, and (h) different current densities from (1 to 2) mA·cm−2 for 5 mAh·cm−2. (i) Rate capability under various current densities from (1 to 8) mA·cm−2 using the bare Li, PEM@Li, and PZEM@Li electrodes. The results indicate the superlithiophilic PZEM significantly improves the CE and cycling stability of Li metal. Reproduced with permission[122]. Copyright 2023, Wiley–VCH.

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