Anode Interfacial Issues in Solid-State Li Batteries: Mechanistic Understanding and Mitigating Strategies

for further research. Innovative approaches such as developing magnetic Li and gradient composite Li anode demonstrate superior performances in reducing interfacial resistance and suppressing Li dendrites growth. A more revolutionary approach is to develop liquid solution anode that is composed of dissolved Li metal in organic solvents. Li dendrite-free plating can be achieved in these systems as the formed dendrites can be consumed by dissolving in the liquid solution. Ultra-long -cycle life can be enabled by this system. The application of this technology requires new battery design and development of protection layers to protect SE pellets from corrosive Li liquid solutions.

(PICEs) at Li anode side are presented. For effective mitigating strategies, various inorganic and organic materials that have been used as interfacial buffer layers are summarized. Their performances in suppressing Li dendrite growth and ameliorating interfacial contact/Li-ion transport are compared. SE modification methods that reduce porosity, increase relative density, and improve electrochemical stability of SE are also provided. Moreover, Li modification approaches that lead to increased Li diffusion coefficient, better wettability toward SEs, improved mechanical properties, and intrinsic elimination of Li dendrites are introduced. The overall summary of current research in Li metal/inorganic SE interfacial issues contributes to in-depth understanding of interfacial phenomena and better solutions for practical applications of ASSLBs.

Li Metal/SE Interfacial Issues
Electrode/electrolyte interfaces play important roles in determining the performance of Li-ion batteries. They show complex and dynamic characteristics due to the complicated electrochemical reactions and charge/ mass transfer occurring at the interface during charge/discharge. At Li metal/SE interface of ASSLBs, unstable Li dendrite growth, undesirable interfacial reactions, and electrochemo-mechanical degradations are main issues that limit battery performances. In this section, the characteristics and mechanisms of those issues are discussed comprehensively.

Li Dendrite Formation and Growth
Li dendrite growth is a complex electrochemical process affected by ion diffusion, overpotential, concentration gradient, mechanical stress, etc. Previous studies on Li deposition and dissolution in liquid electrolytes have ascribed the unstable Li dendrite growth to limited diffusion of Li ions to Li/electrolyte interface. [30][31][32][33][34][35] During changing of a battery, Li ions are reduced at Li anode side, decreasing the Li-ion concentration at electrolyte/Li interface, and anions are expelled from the anode under the electric field. [36] Ion diffusion in electrolytes near the electrode interface is governed by both concentration and potential gradient. [37] When the current density exceeds the diffusion limit of Li ions, Li ions at the interface cannot be replenished. As the Li-ion concentration decreases to zero at Sand's time, [38] dendrite starts to grow and propagates at a speed equal to that of the anions diffusing in the electric field. [39] Sand's time is inversely proportional to the transference number of anions, inferring that ceramic, solid electrolytes, which are single-ion conductors should lead to an infinite Sand's time, preventing the formation of Li dendrite. Furthermore, kinetic model for dendrite propagation by Monroe and Newman suggests that SEs with shear modulus two times higher than that of Li metal can suppress the protrusion of Li dendrite. [40] The measured shear modulus of Li metal is 3 GPa, [40,41] while those of lithium thiophosphate and garnet-type oxide SEs are~6-10 and 52-61 GPa, respectively, [42] implying that ceramic SEs should in theory suppress Li protrusions. However, Li dendrite growth in cells using ceramic SEs has still been widely observed, limiting the current density and charging rate of ASSLBs. [43][44][45][46] Understanding the mechanisms of its formation is crucial for designing strategies to mitigate this issue.
One of the triggering factors for Li dendrite growth is the amplification of electric field and current at heterogeneous microstructures such as current collector edges [47] or hot spots due to non-uniform interfacial contact between Li metal and SEs (Figure 1a). [48] Li 7 La 3 Zr 2 O 12 (LLZO) has poor wettability with Li metal due to the presence of lithiophobic Li 2 CO 3 on its surface, resulting in uneven interfacial contact and hence unstable Li plating and stripping. [52][53][54][55][56] Furthermore, microstructural defects such as cracks and voids are preferable sites for dendrite formation and growth in Li 3 PS 4 and 7Li 2 S-3P 2 S 5 glasses (Figure 1b,c). [49] The growth of Li filament further induces stresses at crack tips, causing crack propagation and Li infiltrations in SEs. [49] Study on the Na β-alumina system suggested that the critical current density (CCD) for dendrite formation due to microstructural defects follows an empirical relationship [57] i crit ¼ π 1Àυ 2 ð ÞFγ eff 2 8V m Eη c 2 l 3 (1) where F is Faraday's constant, ν stands for Poisson's ratio, γ eff is the surface energy, E represents Young's modulus, V m is the molar volume of alkali metal, η is the viscosity of alkali metal, and (c 2 =l 3 ) represents the aspect ratio of microstructural defects. Tian [58] In contrast, Krauskopf et al. [59] discovered fast charge transfer kinetics across clean and defect-free Li/Al-doped LLZO interfaces that lead to high Li-plating current density of 100 mA cm −2 . For polycrystalline SEs, Li dendrites preferably grow at grain boundaries due to their different atomic structure and lower elastic modulus compared to the grains. The propagation of Li dendrites along grain boundaries in Ga-doped Li 7 La 3 Zr 2 O 12 (LLZO) leads to intergranular fracturing of SEs (Figure 1d,e). [60] Liu et al. [50] discovered that the grain boundaries of LLZO tend to have low electronic bandgap, which is the reason for the preferential Li deposition along grain boundaries. Moreover, the shear modulus of grain boundaries is up to 50% smaller than that of bulk for LLZO, which also provokes Li penetration through grain boundaries. [61] Furthermore, the non-negligible electronic conductivity of SEs which facilitates the combination of Li + with e − allows Li electrodeposition inside SEs at Li-plating potential (Figure 1f,g). In situ neutron depth profiling revealed Li concentration profiles and Li deposition inside La-doped LLZO/Li 3 PS 4 SE due to their high electronic conductivity. [62] The electronic conductivity of SEs may be further increased due to the electrochemical reduction in SEs or components (like dopants or impurities) inside SEs. [63] Moreover, high potentials and temperature can also trigger the enhancement of electronic conductivity of garnet-type SE, limiting the charge/discharge rate and operating temperature of ASSLB. [64] Experimental studies revealed that LiPON has significantly smaller electronic conductivity (8 Â10 −14 S cm −1 ) [65] compared to that of LLZO (2 Â 10 −8 S cm −1 ) [66] and Li 2 S-P 2 S 5 (1.3 Â 10 −9 S cm −1 ). [67] Therefore, the CCD for LiPON has been found to be over 10 mA cm −2 , [68] much higher than that for Li 2 S-P 2 S 5 (0.4-1 mA cm −2 ) [69,70] and LLZO (0.05-0.9 mA cm −2 ). [71][72][73] As aforementioned, Li dendrite tends to penetrate inside SEs, leading to fracture of SEs and short circuiting. The mechanical properties of SEs and Li metal thus play important roles in governing the growth of Li dendrite and structural stability of SEs. Study has shown that fracture is prohibited in SE with fracture energy >4 J m −2 and microcracks are Figure 1. a) Non-uniform interfacial contact between SE and Li metal leading to amplification of electric field at hot spots, which triggers Li dendrite growth; Reproduced with permission. [48] Copyright 2019, American Chemical Society. b, c) SEM images of fractured LPS SE surface showing Li dendrite growth at cracks and pores between SE aggregates; Reproduced with permission. [49] Copyright 2017, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim d, e) more likely to form in compliant SEs with Young's modulus around 15 GPa. [74] Moreover, Li metal has a low yield strength of 0.8 MPa [41] and creep significantly at room temperature. The creep of Li metal is governed by the diffusion of Li atoms along grain boundaries at low stresses and dislocations climbing at high stresses. [75] The stack pressure applied during the operation of SSB is therefore critical for the stability of SE/Li metal interface. Study [51] has shown that a stack pressure of 5 MPa enables a low interfacial resistance of~110 Ω and stable Li plating and stripping at 75 μA cm −2 for 1000 h in a Li symmetric cell. While higher stack pressure of 10 and 25 MPa leads to smaller interfacial resistance of~50 and 32 Ω, the cell short circuits in 474 and 48 h, respectively (Figure 1h). At a high pressure of 75 MPa, the cell mechanically short circuits before applying any current. X-ray tomography indicates that Li metal creeps through the pores of SE pellets under high pressure, leading to dendrite penetration and cracking of SE.
Polymer/inorganic composite electrolytes (PICEs) take advantages of the deformability of polymer electrolytes and high ionic conductivity of inorganic electrolytes, allowing intimate contact at electrolyte/electrode interface while achieving decent ionic conductivity by controlling the interactions among Li ions, anions, fillers, and polymers. [76] Li dendrite growth in PICEs is influenced by the mechanical properties, ionic transport properties, and stability of polymer electrolytes. [77] Poly(ethylene oxide) has a low shear modulus of~26 MPa, [78] which is insufficient to hinder Li dendrite growth. [40] Moreover, the low Li + transference numbers (<1) of polymer electrolytes lead to ion depletion at polymer/Li interface and formation of space charge layer (SCL) which creates large electric field across the interface, promoting Li dendrites growth. [39] Furthermore, heterogeneous solid electrolyte interface (SEI) formed between polymer and Li metal leads to inhomogeneous Li flux which induces the nucleation and growth of Li dendrites. [79,80]

Interface Evolution
Electrochemical cycling involves mass and charge transfer at SE/Li interface and hence causes changes in interfacial morphology, which in turn affects the stability of Li plating/stripping. It is believed that Li diffusion in bulk Li metal is limited by the diffusion of Li vacancies. [48] If the Li stripping current density is higher than the diffusion limit of Li vacancies in Li metal, vacancies accumulate at Li/SE interfaces, causing voids generation and contact loss (Figure 2a). [48] The subsequent Li-plating process hence suffers from large polarization, and Li dendrite growth is promoted due to uneven interfacial contact. [48] Li vacancy diffusion j V in Li metal is driven by Li vacancy concentration gradient caused by Li dissolution (Figure 2b) and can be estimated by [82] where c V ξ ¼ 0 ð Þis the concentration of Li vacancy at the interface, c 0 V the equilibrium Li vacancy concentration in bulk Li metal, D V the diffusion coefficient of Li vacancy (D V ¼ D Li Á x V À1 ), and τ V the relaxation time for reaching equilibrium of vacancy concentration. The critical Li stripping current density is calculated as where z is the charge number and F is the Faraday constant. It is shown in Figure 2c that the current density for stable Li/Al-doped LLZO interface only ranges from 50 to 200 μA cm −2 at room temperature without external pressure. [48] Under external pressure, the creep of Li metal leads to diffusion of Li toward the interface. The plating and stripping of Li metal is therefore controlled by interfacial Li ion migration due to electrochemical reaction (j Li þ interface), diffusion of Li due to Li creep (j Li þ creep), and vacancyfacilitated Li diffusion within the bulk of Li metal (j Li þ diffusion, Figure 2d). At constant temperature and pressure, j Li þ diffusion is fixed. Li plating/stripping is governed by j Li þ interface and j Li þ creep that are determined by the applied current density and stack pressure, respectively. If the stack pressure is lower than the yield strength of Li metal, pores exist at the interfaces, leading to large interfacial resistance and creating hot spots for Li deposition. While pores can be filled during Li plating, voids may be re-generated during Li stripping as the migration of Li ions from Li metal to SE outbalances the selfdiffusion of Li ions within Li metal and Li diffusion induced by Li creep (j Li þ interface > j Li þ diffusion + j Li þ creep). As the stack pressure exceeds the yield strength of Li metal, contact at the interface can be improved due to Li creep, leading to more stable Li plating. However, if the cell is cycled under high current densities, j Li þ interface may still dominate over j Li þ diffusion and j Li þ creep (j Li þ interface > j Li þ diffusion + j Li þ creep) and voids are still generated during Li stripping. Only when j Li þ interface matches with j Li þ diffusion and j Li þ creep (j Li þ interface = j Li þ diffusion + j Li þ creep) can stable Li plating/ stripping be achieved. If j Li þ interface and j Li þ creep dominate over Li dendrite growth is facilitated. This mechanism explains the dynamic void generation and dendrite growth processes that occurred at Li/LLZO [48,83] and Li/Li 6 PS 5 Cl interfaces, [84] indicating the strong correlation between CCD and stack pressure.
During electrochemical cycling, the stack pressure of solid-state battery changes due to interfacial evolution, which in turn influences the electrochemical behaviors of cells. Lee et al. [85] demonstrate the fast decay of stack pressure for Li/Li 10 SnP 2 S 12 /Li symmetric cells due to interphase formation, causing interfacial contact loss and substantial rise of interfacial impedance, which is the main reason for cell failure. In contrast, the stack pressure drop of Li/Li 6 PS 5 Cl/Li cells is attributed to Li dendrite growth into pores of SE pellets, where a higher-density SE pellet hinders Li dendrite penetration to enable longer Li plating/stripping before short circuiting. [85] In addition, the microstructures of SE pellets also influence Li dendrite growth, where interconnected pores [86] and highly anisometric tortuosity [87] decrease the CCD for short circuiting.
Hydride SEs such as LiBH 4 , [88,89] Li halide-doped LiBH 4 (e.g., Li 4 (BH 4 ) 3 I), [90] and Li 2 B 12 H 12 [91] are known to possess high electrochemical stability against Li metal. Kisu et al. [92] demonstrated good maintenance of cell voltage and impedance for Li symmetric cells of LiBH 4 and Li 4 (BH 4 ) 3 I hydride SEs after cycling for 100 h under a current density of 0.2 mA cm −2 at 120°C. The invariant cell impedance even after cycling at a high temperature of 120°C suggests the high electrochemical stability of hydride SEs against Li metal, and good interfacial contact with only small crack formation after cycling infers that the deformability of hydride SEs keeps interfacial contact over cycling as claimed by the authors. [92] However, improved Li diffusion and creep of Li anode at such a high temperature should not be omitted, which may contribute to the maintenance of interfacial contact. Due to low ionic conductivity of Li 4 (BH 4 ) 3 I at 30°C (5 Â 10 −5 S cm −1 ), Li symmetric cells cycled at 30°C at 0.2 mA cm −2 display large polarization voltage of~200 mV and stable Energy Environ. Mater. 2023, 6, e12613 4 of 21 cycling within 10 h. [92] To increase the room-temperature Li-ionic conductivity of hydride SEs, Kim et al. developed a complex hydride 0.7Li (CB 9 H 10 )-0.3Li(CB 11 H 12 ) with ionic conductivity of 6.7 Â 10 −3 S cm −1 at 25°C. [93] The Li symmetric cell of 0.7Li(CB 9 H 10 )-0.3Li(CB 11 H 12 ) shows remarkably low interfacial resistance of 0.78 Ω cm −2 and cycling voltage of 6 mV and undergoes stable Li plating/stripping for 300 h at 0.2 mA cm −2 with unchanged interfacial resistance, indicating high electrochemical and mechanical stability at the interface. [93] For PICEs, while the soft polymer matrix can improve the solid-solid interfacial contact, large volume change of Li metal during plating/ stripping still leads to void formation at PICE/Li anode interface, which causes non-uniform Li-ion flux and increase in interfacial resistance. [94] Moreover, solid electrolyte interphases (SEIs) have significant impact on interfacial stability and cell performance. While some SEIs result in large interfacial resistance, SEIs composed of LiF are beneficial for suppressing Li dendrite penetration into polymer matrix and parasitic reactions between PICEs and Li metal. [95,96] Additionally, the breaking of Li dendrites and wrapping of Li by SEI cause inaccessibility to electrons, leading to the formation of electrochemically inert dead Li which reduces the coulombic efficiency. [97]

Electrochemical Instability of Solid Electrolyte
The phase stability of a material A in equilibrium with an element B with chemical potential μ B can be evaluated from the grand potential phase diagram (GPPD) constructed based on the energies of all compounds in the compositional space ( Figure 3a). [98] In Li-ion batteries, the electrochemical stabilities of SEs under potential ; with respect to the chemical potential of Li metal μ 0 Li can be evaluated from the phase equilibria at Li chemical potential The GPPD constructed with respect to μ Li reveals the potentials for electrochemical lithiation and delithiation, which are the cathodic and V ; a-c) Reproduced with permission. [48] Copyright 2019, American Chemical Society. d) Applied pressures and current densities affect the relationships between interfacial Li ion migration due to electrochemical reaction (j Li þ interface), diffusion of Li due to Li creep (j Li þ creep) and vacancy-facilitated Li diffusion in bulk Li metal (j Li þ diffusion), which determine the voids and Li dendrites formation at SE/Li interfaces. Reproduced with permission. [81] Copyright 2020, American Chemical Society.

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anodic limits of SEs ( Figure 3b). [98] The gap between the cathodic and anodic limits is known as the electrochemical stability window of SEs, beyond which the reduction and oxidation of SEs occur, accompanied by transfer of electrons from anode to SE and from SE to cathode, respectively ( Figure 3c). [99] The electrochemical reaction of SE creates interphases with low ionic conductivities, increasing the interfacial resistance. For interphases that are ionically conductive but electronically insulating, they will act as passivation layers that extend the electrochemical stability window of SEs ( Figure 3f). However, if the interphases are both ionically and electronically conductive, SE decomposition will continue, and the interfacial resistance grows significantly ( Figure 3e). In addition, the injection of electrons or holes into SE increases the concentration of electronic charge carriers in SE, which promotes self-discharge or short circuiting of batteries. [103] Theoretical calculations by Richards et al. display that most of the fast Li ionic conductors have reduction limits higher in potential than that of Li metal anode, indicating that they are thermodynamically unstable against Li metal (Figure 3g). [101] Only binary ionic lithium compounds are stable at 0 V versus Li/Li + since their anions are at the most reduced states. [101] Moreover, the stability of ternary Li ionic conductors with different metal cations is revealed by Wang et al. (Figure 3h). [102] Depending on the reduction limit and decomposition pathways, various SEs evolve differently at Li metal side, influencing the charge transfer and cycling stability of batteries significantly.
Garnet-type oxide SEs (e.g., LLZO) have been shown to be electrochemically stable at 0 V versus Li/Li + by cyclic voltammetry measurements. [104][105][106] However, small decomposition currents may be omitted due to the strong peaks associated with Li deposition/dissolution at~0 V. First principal calculations indicate that LLZO is reduced at an onset voltage of 0.05 V, decomposing to Li 2 O, Zr 3 O, and Figure 3. a) Grand potential phase diagram revealing phase stability between A and B; b) grand potential phase diagram revealing stability of A at different Li chemical potential, μ Li ; a, b) Reproduced with permission. [98] Copyright 2018, Elsevier Inc. c) Anode with Li chemical potential, e μ e À ,Anode , higher in energy than the reduction limit of electrolyte will cause lithiation and reduction of electrolyte at anode/electrolyte interface, while cathode with Li chemical potential, e μ e À ,Cathode , lower in energy than the oxidation limit of electrolyte will cause delithiation and oxidation of electrolyte at cathode/electrolyte interface; Reproduced with permission. [99] Copyright 2018, Royal Society of Chemistry. d) Fast Li ion transport across an ideal SE/Li interface where no SE decomposition occurs; e) mixed conductive interphases formed by SE decomposition cause electron transfer towards SE, leading to continuous SE decomposition and growth of interphases; f) purely Li-ion-conductive interphases with low Li-ion conductivity hinder Li ion transport across SE/Li metal interface but prevent SE from further decomposition; d-f) Reproduced with permission. [100] Copyright 2015, Elsevier B.V. g) Calculated electrochemical stability window of various SEs; Reproduced with permission. [101] Copyright 2015, American Chemical Society. h) Calculated electrochemical stability window of various SEs with different cations. Reproduced with permission. [102] [101,107,108] Given the small driving force and nature of decomposed products, LLZO may be passivated and reach kinetic stability against Li metal. Furthermore, Li 2 CO 3 impurities on LLZO surface may also serve as passivating layer to prevent reduction by Li metal. [56,109] XPS characterizations suggested oxygen deficiency and formation of La 2 Zr 2 O 7 and Zr 3 O at LLZO surface after being discharged to 0 V versus Li/Li + , indicating the reduction of Zr by Li metal. [107,110] In addition, in situ STEM also disclosed a five-unit cell thick layer of lithiated LLZO phase with a tetragonal-like structure at LLZO/Li interface, which passivates LLZO from further reduction. [111] As the reduction of LLZO is closely related to the reduction of Zr cation in the electrolyte, substitutional dopants for Zr to improve structural stability and Li ionic conductivity may influence the electrochemical stability of the electrolyte. Ta has better stability against Li metal and distributes homogeneously within the bulk of electrolyte, leading to less Zr reduction at the interface. [110] In contrast, Nb has higher reduction potential and concentrates at the surface of LLZO, causing expanded decomposition layer at the interface. [110] NASICON-type oxide SEs, Li 1+x-y Al 3+ x M 5+ y M 4+ 2-x-y (PO 4 ) 3 , where M represents Ti, Ge, Ta, are believed to react with Li metal to form LiGePO 4 , Li 3 PO 4 , TiPO 4 , and AlPO 4 according to density function theory calculations. [112] XPS study on NASICON-type SEs reveals the reduction of Ti 4+ and Ge 4+ after contact with Li metal (Figure 4a). [113] Li-Ti/Ge alloys formed at the interface are mixed conducting interphase (MCI) which conduct both electrons and Li ions, leading to continuous degradation of SEs. [113] Similarly, in situ XPS study revealed that Ti ions in perovskite-type Li 0.35 La 0.55 TiO 3 (LLTO) are also reduced to lower valence states and Ti metal by Li metal (Figure 4b), causing MCI formation and serous interfacial degradation. [100] Sulfide SEs generally have high reduction voltage of~1.7 V as shown by both calculations and experiments, [107,108,117] indicating that they will decompose at Li anode side. Depending on the electronic and ionic properties of decomposed products, the interfacial stability and impedance vary significantly. Li 10 GeP 2 S 12 (LGPS) was calculated to decompose into Li 2 S, Li 3 P, and Li-Ge alloys at Li anode side. [117] These decomposed products were confirmed by Wenzel et al. using in situ XPS. [100] Calculation considering the interfacial atomic structure, electronic structure, and Li migration proposed that the lower energy barrier for Li migration across GeS 4 -bridged LGPS/Li interface compared to that for PS 4 -bridged LGPS/Li interface is responsible for the formation of Ge-rich Li alloy phases. [118] Similar to the degradation mechanism of NASICON-type SEs, the MCI causes continuous interfacial degradation and drastic increase in interfacial resistance, which finally leads to cell failure. In contrast, Li 3 PS 4 , [119] Li 7 P 3 S 11 , [114] and Li 6 PS 5 Cl [120] react with Li metal to form Li 2 S and Li 3 P (Figure 4c) which are pure ionically conductive compounds and stable against Li metal. The interfacial layer, therefore, grows to limited thickness and the SEs reach kinetic stability against Li metal, which promotes long cycle life of cells. [121][122][123] Shin et al. [115] compared the interfacial stability of Li 3 PS 4 and LGPS against Li metal by conducting cyclic voltammetry (CV) in voltage range of 0.05-2.00 V versus Li/Li + to avoid the interference from Li deposition/dissolution. Results show that LGPS displays significant degradation at 0.05 V in both first and second cycles, whereas Li 3 PS 4 exhibits high stability throughout the voltage range (Figure 4d). [115] Halide SEs, Li 3 MX 6 (M is a metal cation, X = Cl, Br, I), exhibit high ionic conductivity and excellent stability at high voltages versus Li/Li + . However, they generally show poor stability against Li metal. [102] The reduction onset voltage for Li 3 YCl 6 and Li 3 YBr 6 is~0.6 V versus Li/Li + , while that for Li 3 InCl 6 increases to 2.38 V versus Li/Li + , showing the significant impact of metal cation on electrochemical stability window (Figure 3 h). [102] Riegger et al. observed the decomposition of Li 3 InCl 6 and Li 3 YCl 6 to LiCl and In/Y metal using in situ XPS, [116] consistent with the first principles calculations. [102] Furthermore, time-resolved impedance spectroscopy reveals an increase in interfacial resistance with time ( Figure 4e), indicating continuous interfacial reactions. [116] In contrast, another type of halide SEs, Li 3-2x M x HalO (Hal stands for halogen and M is Ba, Mg, and Ca), with anti-perovskite structure are thermodynamically stable against Li metal according to both calculations and experimental studies. [101,124] More importantly, anti-perovskitetype SEs achieve high room-temperature Li-ion conductivity of 10 −4 -10 −2 S cm −1 and can be synthesized at temperatures ranging from 220 to 360°C, making them promising SEs at Li anode side. [124][125][126] The electrochemical stability of PICEs against Li metal is highly related to that of their polymer components, as the polymer electrolytes generally surround the inorganic fillers and are in close contact with Li metal. [127] PEO, [128,129] polyethylene glycol diacrylate (PEGDA), [130] polystyrene-poly(ethylene glycol)-polystyrene (PS-PEG-PS), [131] poly (vinyl alcohol) (PVA), [132] poly(diethylene glycol carbonate) (PDEC), [133] poly(vinylene carbonate) (PVCA), [134] poly(methylmethacrylate) (PMMA), [135] etc. have been shown to form stable interfaces with Li metal. XPS analysis shows that lithium fluoride and lithium alkoxides species dominate at PEO-lithium bis(trifluoromethanesulfonyl) imide (LiTFSI)/Li metal interface. [136] In contrast, poly(acrylonitrile) (PAN), [137,138] poly(N-methyl-malonic amide) (PMA), [139] polyvinylidene fluoride hexafluoropropylene (PVdF-HFP), [140] etc. form continuously growing and resistive interfacial layers with Li metal. Adding inorganic fillers into PVdF-HFP matrix significantly reduces the interfacial impedance possibly because inorganic fillers block the contact between Li and PVdF-HFP and thus reduce the interfacial reaction. [140] In other cases, the interfacial resistance of PICEs can be ameliorated by the scavenging effect of inorganic fillers, [141] Lewis acid-base reactions between surface groups of inorganic particles and polymer segments, [142,143] and electrode passivation due to favorable reactions between anions of added Li salts and Li metal. [144]

Chemo-Mechanical Degradation
Chemo-mechanics describes the interplay between (electro)chemical reactions and mechanical responses in materials. The complex and dynamic electrode and interfacial reactions in ASSLBs lead to various chemo-mechanical phenomena which play outstanding roles in determining the stability and performance of ASSLBs. At Li anode side, the volume change of Li metal during electrochemical deposition/dissolution and void formation due to limited Li diffusion can lead to loss of contact or delamination between SE and Li metal, increasing the interfacial resistance and creating "hot spots" for Li dendrite growth. In situ SEM investigation on the failure mode of Li 3 PS 4 (LPS) and Li 6 PS 5 Cl (LPSCl) sulfide SEs indicates substantial increase in gaps between SE and Li metal over cycling. [145] Moreover, volume change in electrodes and Li nucleation produce mechanical stress which induces crack propagation within SE pellets. [145] Due to higher porosity and brittle nature of LPS pellet, cracks grow faster in LPS pellet than in LPSCl pellet, leading to faster short circuiting of cells with LPS than those with LPSCl. [145] Ning et al. discovered that cracks in LPSCl initiate at the high-stress field area near the edges of Li anode, and transverse cracks propagate through SE pellets due to Li penetration, moving ahead of Li dendrite growth. [146] Energy Environ. Mater. 2023, 6, e12613

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Additionally, as most SEs are unstable against Li metal, the formation of interphases which involves lithiation and volume expansion exerts mechanical stress on SE pellets. At Li 1.5 Al 0.5 Ge 1.5 P 3 O 12 (LAGP)/Li metal interface where the interphase growth is continuous and tends to be non-uniform at high current density, the protrusion of interphase creates large localized stress that fractures SE and causes cell failure. [147] Tippens et al. [148] monitored the fracture of LAGP pellets due to interphase growth using in situ X-ray tomography and found that the mechanical degradation of SE is mainly responsible for the drastic increase in impedance ( Figure 5). Lewis et al. used operando X-ray computed microtomography to study the linkage between chemo-mechanical behaviors and electrochemical performance of Li 10 SnP 2 S 12 /Li cells. [149] Formation of redox-active interphases and significant growth of contact loss at Li 10 SnP 2 S 12 /Li interface lead to increase in voltage, current constriction, and eventually cell failure. [149] Voids generation at Li 10 SnP 2 S 12 /Li interface is ascribed to fast removal of Li at the surface without sufficient replenishment from the bulk of Li anode and consumption of Li due to unwanted chemical reactions. [149] Moreover, the formation of interphase also causes overall volume shrinkage of Li symmetric cell during cycling as the partial molar volume of Li in interphase is smaller than that in Li metal, leading to decrease in stack pressure and aggravated overpotential rise. [149] In addition, the non-uniform shrinkage of SE creates non-uniform stress within the cell. [149] The mechanical stresses within SEs and at the interface may change the energy barrier for Li diffusion [150][151][152] and local electrochemical potential, [40] which influence the Li deposition/dissolution and dendrite growth.

Strategies to Mitigate Li/SE Interfacial Issues
As mentioned above, interfacial issues in ASSLBs mainly include unstable Li plating/stripping, interfacial electrochemical reactions, and electrochemo-mechanical degradation. Effective strategies to resolve these challenges can be categorized into interfacial protection layer, Li anode modification, and SE modification.

Interfacial Protection Layers
Large interfacial resistance and Li dendrite growth have been attributed to poor interfacial contact and interfacial reactions. Interposing a protection layer at Li/SE interface can facilitate interfacial Li-ion transport and mitigate undesirable side reactions. Such layers should be ionically conductive, electronically insulating, and electrochemically stable against both SE and Li metal, and highly wettable with Li metal/SE. Various inorganic, polymeric, or composite intermediate layers have been reported for Li/SE interface, leading to improvement in interfacial stability.
To improve interfacial contact and Li-ion transport, thin layers of Al 2 O 3 , [153] Ge, [154] Si, [155] Al, [156] Au, [157] Sn, [158] Zn(NO 3 ) 2 , [159] SnF 2 , [160] and gallium liquid metal [161] have been conformally coated on SE surface, which convert to lithiated compound or Li alloy interphases following ex situ or in situ reaction with Li metal. These interfacial layers have high Li diffusion coefficient and form homogeneous contact/strong bonding with Li metal, hence decreasing the interfacial resistance and preventing Li dendrite formation. Methods including plasma-enhanced chemical vapor deposition (PECVD), atomic layer deposition (ALD), electron beam deposition (EBD), and magnetron sputtering (MS) provide precise deposition of thin films with a few nanometers thickness but high cost. Another facile liquid-phase coating (LPC) methods like drop casting (DC) and brushing can also achieve uniform deposition of interlayers. Due to high Li diffusion coefficient and insignificant volume changes during de/ lithiation, Li-Sn alloy protective layer leads to stable cycling of Li symmetric cells at 0.5 mA cm −2 for 500 h, [158] exceeding the performance of other Li alloy interfacial layers shown in Table 1. Zn(NO 3 ) 2 -modified LLZO-type SE has been shown to suppress Li dendrites growth both at the interface and within SE, leading to extremely high Li deposition/dissolution current density of 4 mA cm −2 and outstanding cycling stability over 5500 and 2400 h at current densities of 1 and 2 mA cm −2 ( Figure 6). [159] Additionally, Li-conductive materials such as graphite, [162] Li 3 N, [163] and lithium naphthalene (Li-Naph) [172] have also been deposited at Li/SE interface to improve the interfacial contact and Li ionic conductivity, leading to superior cycling performance (Table 1). A pinhole-free Li 3 N film grown via reaction between molten Li and N 2 shows mechanical robustness due to strong bonding between textured Li 3 N grains, which is critical for maintaining morphological stability during Li stripping and plating. [186] Otherwise, H 3 PO 4 treatment has been used to convert Li 2 CO 3 impurities on LLZO surface to Li 3 PO 4 layers, which improves the wetting of SE by Li metal and promotes interfacial Li-ion transport. [53] For SEs that are electrochemically unstable against Li metal (such as LATP and LAGP), the degradation of SE can be inhibited through the application of appropriate interlayers. Cheng et al. demonstrated a 5-to 10-nm-thick Li-ion-permeable boron nitride nanosheets/PEO composite interlayer employed as a physical barrier to prevent electrochemical reactions between LATP and Li metal leads to stable cycling of Li symmetric cells for 500 h at 0.3 mA cm −2 . [187] Al 2 O 3 thin film was also deposited on LATP [173] and Li 7 P 3 S 11 [188] by ALD to suppress SE degradation, which significantly reduces interfacial resistance and improves cycling stability. The uniform deposition of ZnO on LATP by magnetron sputtering (MS) leads to the formation of Li-Zn alloy and Li 2 O at SE/Li interface, preventing Li dendrite growth and interfacial side reactions. [174] Zhang et al. [182] spin-coated H 3 PO 4 tetrahydrofuran (THF) solution on Li foil to grow LiH 2 PO 4 protective film which inhibits LGPS decomposition at Li anode side and leads to superior cycling stability of ASSLB with LiCoO 2 cathode (Figure 7a-c). To ameliorate the chemical compatibility between Li 3.833 Sn 0.833 As 0.166 S 4 (LSAS) and Li metal, Sahu  [183] Air-stable Li x SiS y protective film produced on Li metal surface by dipping Li metal into Li 2 S 8 and SiCl 4 THF solution mitigates electrochemical degradation of Li 3 PS 4 , achieving stable cycling of Li symmetric cell at 0.1 mA cm −2 for 2000 h. [184] Moreover, Li binary compounds such as LiN, LiF, and LiI with high stability against Li metal are also beneficial for preventing SE degradation as well as promoting interfacial Li-ion transport. [181,189] Furthermore, organic materials such as polymers and gels are also effective interfacial protection materials as they can adjust themselves to comply with the morphologies of electrode and SE surfaces and accommodate the volume change of electrodes during cycling. Polyvinylidene fluoride hexafluoropropylene (PVDF-HFP) + tetraethylene glycol dimethyl ether (TEGDME) coating has been applied on LATP surface, where TEGDME as plasticizer improves the ionic conductivity and electrochemical stability window of coating layer, leading to stable cycling 10 of 21 of Li symmetric cells at 0.1 mA cm −2 for 3000 h. [177] LAGP has been coated by thermally induced in situ polymerized methyl methacrylate (PMMA) and pentaerythritol tetraacrylate (PETEA) crosslinker with high mechanical stability, good compatibility with Li metal, and capability of suppressing Li dendrite growth. [178] Li-ion transport in interlayer is boosted by high dissolution of Li salt in polymer and movement of polymer segments rich in oxygen atoms. [178] PEO-LiTFSI interlayer has been shown effective in mitigating the morphological mismatch and diminishing side reaction at ceramic SE/Li metal interface, increasing CCD and Li cycling stability [170,176,190] (Figure 7d-h). Attachment of polyacrylic acid (PAA)-coated LLZO-type SE with molten Li at 250°C leads to Li insertion in PAA and formation of close contact with Li metal. [167] With good electron-blocking property, Li-PAA layer suppresses the dendrite growth in LLZO-type SE and enables stable cycling of Li symmetric cells at 1 mA cm −2 for 400 h. [167] LiTFSI-poly (ethylene glycol) methyl ether acrylate (PEGMEA) with high thermostability and wide electrochemical stability window has been interposed in between Li-and LLZO-type SE via in situ polymerization in the presence of benzoyl peroxide (BPO) heat initiator, forming a flexible interface in close contact with anode and SE. [169] The addition of diethylene glycol dimethyl ether (DGM) plasticizer in PVDF increases the lowest unoccupied molecular orbit (LUMO) of PVDF and facilitates Li salt dissociation, resulting in increased electrochemical stability against Li metal and improved Li diffusion. [191] Uniform poly(acrylic acid-comaleic acid) (P(AA-co-MA))-LiOH layer with high decomposition temperature of 370°C can be spay coated on molten Li-Sn anode, significantly improving the solid-solid interfacial contact. [179] The ionic conductivity of (P(AA-co-MA))-LiOH layer is enhanced by adding LiCl due to increased Li concentration. [179] PVDF-DGM-LLZO-LiTFSI composite layer demonstrates high ionic conductivity of 0.13 mS cm −1 and Li dendrite suppression capability. Polymerization of ally-ether-ramified PEO with pre-deposited propyl acrylate silane groups on Li metal surface has been achieved under ultraviolet (UV) irradiation, leading to strong linkage between Li metal and polymer coating layer and significant reduction of interfacial resistance (Figure 7i). [171] Additionally, ionic liquid electrolytes (ILE) with good thermal stability, low flammability, and wide electrolyte chemical potential window [192,193] can be incorporated to fill the voids at Li/SE interfaces, generating interfacial Li-ion conduction pathways. N-butyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide (Pyr 14 FSI)-LiTFSI mixed ILE has been used at Li/LLZO interface to reduce interfacial resistance and improve cycling stability due to its low viscosity and good stability with Li metal. [166] With high flowability, ILE conforms itself to morphological variations at Li/LLZO interface, enabling Li plating/stripping without the application of external pressure. [194] One of the drawbacks of polymeric interlayer is the large anion transference number which leads to anion depletion at Li/polymer interface, creating a strong electric field across the interface that drives Li dendrite growth. [36,[195][196][197] Coating of lithium poly (acrylamide-2-methyl-1-propane-sulfonate) (PAS) + PEO with high Li + transfer number (0.9) on LLZO-type SE surface can effectively suppress Li dendrite growth due to inhibited anion movement and decreased overpotential gradients. [168] The high glass transition temperature of PAS also leads to rigid matrix, preventing Li penetration. [168] Moreover, the tert-butyl group bonded with the amide unit has large steric hindrance, enhancing the electrochemical stability against Li metal. [168] Since the nearly single-ion-conduction nature of the polymer leads to low ionic conductivity, a thin layer ( ≤ 5 μm) of coating is preferable, which is also beneficial for achieving high energy density and improving the safety of the cell given the flammability of polymer materials. [168] Alternatively, Zhou et al. demonstrated that cross-linked poly(ethylene glycol) methyl ether acrylate (CPMEA) mixed with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) with a wide electrochemical stability window of 4.8 V versus Li/Li + can stabilize SE/Li interface. This is because the ceramic SE prevents the diffusion of anions and lowers the electric field across polymer/Li metal interface, which suppresses both dendrite growth and interfacial reaction caused by electron tunneling. [198] However, polymer electrolytes still suffer from low Li ionic conductivity at room temperature, leading to large interfacial resistance at ceramic/polymer interface. Additionally, differences in chemical or electrical potential between ceramic SE and polymer electrolyte induce diffusion of Li ions and formation of space-charge layer at the interface, impeding Li-ion transport. [199]

Modification of Solid Electrolyte
Since interfacial resistance and Li dendrite growth are strongly influenced by voids and grain boundaries in SE, engineering the microstructures of SE plays an important role in improving interfacial conductivity and cycling stability of Li metal anode (Table 2). Cheng et al. modified the densities and grain size of sintered LLZO-type SE pellets by controlling particle sizes of SE powders. Sintering small particles of 1-2 μm leads to SE pellets with large grains of 100-200 μm and high density of 92%. [200] The mixture of 10 wt% 10-μm-sized large particles with 1-μm-sized small particles significantly reduces the grain size of sintered pellets to mainly 20-40 μm while maintaining a high density of 90% (Figure 8a). [71] Comparison of SE pellets with different grain sizes at the interfacial region demonstrated that small grain size results in reduced interfacial resistance and increased CCD due to increased fraction of grain boundaries with high Li conductivity [202] at the surface of SE pellet. [71] Suzuki et al. [46] used hot isostatic pressing (HIP) to prevent void formation in Al 2 O 3 -doped LLZO at high sintering temperatures, creating highly dense SE pellets (with relative density of 99.1%) that suppress Li dendrite growth at 0.5 mA cm −2 . Alternatively, Basappa mixed Ta-doped LLZO with LiOH to fill the voids in SE pellets after calcination, achieving a relative density of 96% and CCD of 0.6 mA cm −2 . [203] Xu et al. [204] demonstrated that glassy Li 3 PO 4 distributed around LLZO-type SE grain boundaries reacts in situ with Li to form Li 3 P, which is conducive to Li dendrite suppression and interfacial stabilization.
Alternatively, SEs have been incorporated with other chemical compounds to improve interfacial electrochemical stability and suppress Li dendrite formation (Table 2). Li binary compounds that are ionically conductive and electrochemically stable are promising additives for constructing stable interfaces at Li anode side. Incorporations of LiI, [70,[205][206][207] LiF, [208] and Li 2 S [209] have been shown to improve the electrochemical stability of sulfide SEs against Li metal and enhance ionic mobility across the interface, leading to homogeneous interfacial  (Figure 8d). [201] 3.

Modification of Li Metal
Modification of Li metal anode is another strategy to tackle the SE/Li metal interfacial challenges. Li can be mixed with In, Al, Mg, Zn, Si, Ag, Bi, etc. to form binary Li alloys. [211,212] They possess several advantages including high potential, high Li diffusion coefficient, and improved wettability toward SE, which are beneficial for improving interfacial stability (Table 3). For example, Mg with low density of 1.7 g cm −3 can alloy with Li at a wide range of concentrations and decently high temperatures (Figure 9a). [216] Yang et al. demonstrated that Li 0.93 Mg 0.07 alloy anode forms intimate contact with Ta-doped LLZO, ensuring homogeneous current flux at the interface (Figure 9b, c). [213] More importantly, Li-Mg alloy has similar crystal structure and lattice size to that of Li metal, which prevents phase transformation and fracture of electrode during Li plating/stripping, resulting in stable cycling of Li-Mg symmetric cells without dendrite penetration (Figure 9d,e). [213] Wang et al. [214] showed that alloying Li metal with Sn improved its wettability toward LLZO-type garnet SE, allowing the formation of conformal interface between Li-Sn alloy and SE (Figure 9f-h). However, Li alloys are constrained by several drawbacks depending on the alloy element and composition, such as large volume change during phase transformation, [217] limited diffusion kinetics, [218] compromised capacity and energy density, and increased cost. Alternatively, Si 3 N 4 can be added into molten Li to form Li-Si-N melt with low surface tension, achieving uniform face-to-face contact with Ta-doped LLZO, and hence, stable Li plating/stripping. [219] Moreover, Li-graphite (Li-C) composite can form close contact with Ta-doped LLZO due to both improved wettability and minor interfacial reactions between Li-C and SE (Figure 9i-n). [215] The reaction between AlF 3 and molten Li produces self-regulated gradient Li-Li 9 Al 4 -LiF composite anode due to the large interfacial energy difference between Li/Li 9 Al 4 and Li/LiF. [220] The LiF layer at the surface inhibits Li dendrites growth while the Li 9 Al 4 underneath improved interfacial Li transport, leading to greatly reduced interfacial resistance of 1 Ω cm −2 and high CCD of 3 mA cm −2 for all solid-state cells using Ta-doped LLZO. [220] Li metal incorporated with ferromagnetic Fe particles displays magnetic response, allowing programmable deposition of Li anode film in desired patterns on various substrates. [221] Thin-film Li anode coated on Ta-doped LLZO shows low interfacial resistance of 11 Ω cm 2 and dendrites suppression capability. [221] Additionally, this approach demonstrates great potential in industrial fabrication of Li metal batteries. [221] The Li-plating/stripping stability can also be improved by modifying the microstructures of Li metal. Singh et al. fabricated fineand coarsegrained Li foils by quenching (in liquid N 2 ) and natural cooling molten Li. They also analyzed the effects of microstructures (grain size, grain boundaries, and dislocation density) and stack pressure on interfacial resistance and evolution in Li|Al-doped LLZO|Li symmetric cells. [222] It is shown in their study that fine-grained Li anode has more intimate contact with SE pellet compared to coarse-grained Li anode during Li striping at 2 MPa, which is attributed to the faster deformation (creep) of fine-grained Li as a result of shorter diffusion length (smaller grain size) for intragranular atomic transport and more grain boundaryassisted (higher grain boundary density) intergranular atomic transport (Figure 10a,b). [222] However, under low stack pressure of 0.2 MPa, the lower yield strength of coarse-grained Li due to lower densities of grain boundaries and dislocations (less barrier to dislocation motion) causes more Li deformation at the interface, resulting in less interfacial contact loss than fine-grained Li. [222] To intrinsically eliminate Li dendrite formation, Peng et al. [223] exploited Li-biphenyl-DME (Li-Bp-DME) liquid solution anode which dissolves Li dendrites and forms stable interface with PEO-coated Li 6 PS 5 Cl SE (Figure 10f). By tuning the concentrations of Li and Bp in solution anode, a high-room-temperature conductivity of 12.2 mS cm −1 was achieved, allowing Li plating/stripping at an ultrahigh current density of 17.78 mA cm −2 and stable cycling at 0.127 mA cm −2 for nearly 3000 h (Figure 10d). [223] Peng et al. further applied the Li-BP-ether liquid solution system to full battery and proposed a new battery configuration to achieve high-safety dendrite-free Li battery operable in a wide temperature range (−20~50°C) and under negligible pressure (3 kPa). [224] Full cells using LiCoO 2 cathode display reversible capacity of 140.4 mAh g −1 with high coulombic efficiency of 99.7% and >96% capacity retention after 100 cycles. [224]

Summary and Perspective
This review covers the key interfacial problems at the anode side of ASSLBs, which limit the power performance, cycling stability, and coulombic efficiency of the cell. Detrimental interfacial challenges include Li dendrite growth, interfacial evolution, interfacial electrochemical reactions, and (electro)chemo-mechanical degradation. The mechanisms of these complex and dynamic interfacial phenomena and their mitigating strategies are discussed in detail. Perspectives related to future research are also provided.
The wettability of Li anode toward SE pellet surface is important for achieving uniform and intimate contact between two components, which significantly affects the homogeneity of electric field and current density at Li/SE interface, influencing the stability of Li deposition/dissolution. Defects in SEs (such as pores and cracks) should be monitored and minimized when fabricating SE pellets. Grain boundaries in polycrystalline SEs are the vulnerable sites for Li dendrite growth due to their low band gap, different atomic structure, and inferior shear modulus. For PICEs, low mechanical strength of polymer matrix, SCL effect, and heterogeneous SEI are main reasons for causing Li dendrite growth. Possible methods to strengthen the grain boundaries of SEs is necessary to suppress intergranular fracture of SE pellets. Electronic conductivity should be controlled at low level and impurities that are electronically conductive should be avoided. The operation conditions including pressure, temperature, and current density should be controlled to achieve stable Li plating/string and avoid Li dendrite/void formation at the interface. The mechanical properties, electrochemical stability, and Li transference number of polymer electrolytes should be improved to prevent Li penetration in PICEs.
SEs with high stability against Li metal are scarce due to the high reactivity of Li metal. Doping that leads to higher Li-ion conductivity and structural stability may deteriorate the electrochemical stability of SE against Li metal due to high reduction limit and inhomogeneous distribution of dopants. Doping strategies that improve all the aforementioned properties of SEs are needed and worth investigation. Another approach such as SE coating has been proved to be effective but is rarely investigated, possibly because that coating materials that are stable against Li metal (e.g., binary Li compounds) normally have lower Li Physical contact loss at Li metal/SE interface and within SE due to large volume change or induced stress is a critical challenge for ASSLBs, which is worsened during long-term cycling. Developing SEs that can accommodate the volume change of Li metal and have high Young's modulus/fracture toughness to prevent cracking is essential for maintaining intimate contact and efficient Li-ion transport during cycling. Possible strategies include incorporating inorganic SEs with polymers to form composite electrolytes, but interfacial reaction/space charge effect between inorganic SEs and polymeric electrolytes should be suppressed to lower interfacial resistance.
Constructing interfacial protective layers has also been widely investigated. A variety of inorganic and organic materials have been proposed. Either Li-containing or non-Li-containing compounds can be used for interfacial protection. The latter normally convert to Licontaining compounds with good Li-ion conductivity after posttreatment or in situ reaction with Li metal. The criteria for desirable interfacial layers include the following three, namely high Li ion conductivity, good wettability with SE /Li metal, and electrochemical stability against them. The future research in this field should be focused on developing interlayers that have strong bonding with Li metal and SE pellets. This can be realized by careful design of interfacial chemical reactions and polymerization. The products should have high Li-ion conductivity or provide fast Li conduction pathways. Materials that experience large volume change and phase transformations should be avoided to maintain close contact of interlayer with SE and Li metal.
SE modifications have been focused on reducing internal porosity, increasing relative density, and improving electrochemical stability of SEs. As mentioned above, more attention should be paid to develop deformable SEs that can accommodate the large volume change of Li metal during cycling.
Li metal modification is another promising approach for achieving stable Li plating/stripping and interface. Alloying approaches have been widely exploited due to their benefits of lifting electrochemical potential, enhancing Li diffusion, and improving wettability of Li metal toward SEs. However, most of Li alloys experience large volume change and phase transformation, which may induce fracture and crack of electrodes. Moreover, some Li alloys show hindered Li diffusion. The capacity of Li metal is sacrificed and the cost is increased through alloying. In addition, the microstructural properties such as grain size and grain boundary density of Li metal have recently been modified to improve creep diffusion of Li metal. The crystal facets of Li metal that lead to fast Li diffusion and more uniform Li plating may be of interest  [223] Copyright 2021, Wiley-VCH GmbH. f) schematic showing the effect of Li-Bp-DME in dissolving Li dendrites. Reproduced with permission. [224] Copyright 2022, Elsevier B.V.
Energy Environ. Mater. 2023, 6, e12613 for further research. Innovative approaches such as developing magnetic Li and gradient composite Li anode demonstrate superior performances in reducing interfacial resistance and suppressing Li dendrites growth. A more revolutionary approach is to develop liquid solution anode that is composed of dissolved Li metal in organic solvents. Li dendrite-free plating can be achieved in these systems as the formed dendrites can be consumed by dissolving in the liquid solution. Ultra-long -cycle life can be enabled by this system. The application of this technology requires new battery design and development of protection layers to protect SE pellets from corrosive Li liquid solutions.