Progress and perspective of interface design in garnet electrolyte ‐ based all ‐ solid ‐ state batteries

Inorganic solid ‐ state electrolytes (SSEs) are nonflammable alternatives to the commercial liquid ‐ phase electrolytes. This enables the use of lithium (Li) metal as an anode, providing high ‐ energy density and improved stability by avoiding un-wanted liquid ‐ phase chemical reactions. Among the different types of SSEs, the garnet ‐ type electrolytes witness a rapid development and are considered as one of the top candidates to pair with Li metal due to their high ionic conductivity, thermal, and electrochemical stability. However, the large resistances at the interface between garnet ‐ type electrolytes and cathode/anode are the major bottle-necks for delivering desirable electrochemical performances of all ‐ solid ‐ state batteries (SSBs). The electrolyte/anode interface also suffers from metallic dendrite formation, leading to rapid performance degradation. This is a fundamental material challenge due to the poor contact and wettability between garnet ‐ type electrolytes with electrode materials. Here, we summarize and analyze the recent contributions in mitigating such materials challenges at the interface. Strategies used to address these challenges are divided into different categories with regard to their working principles. On one hand,


| INTRODUCTION
The change from fossil fuels to renewable energy supply requires efficient and reliable energy storage systems. 1 Rechargeable Li-ion batteries (LIBs) are one of the greatest inventions that have been widely used in portable electronics and electric vehicles. 2,3 Safety becomes a major concern as high-energy devices pose the risk of failure and explosion. [4][5][6] For LIBs, the major risk stems from the ignition of liquid electrolytes in the case of short-circuit, causing explosion accidents such as the Samsung Galaxy Note 7 scandal. 7 To overcome the safety issue of liquid electrolytes, solid-state electrolytes (SSEs) are considered as the next-generation LIB technology. 8,9 SSEs are nonflammable and have good mechanical strength, enabling the direct pairing of Li metal as anode. 10 Compared to liquid electrolytes, their better thermal and electrochemical stability also enable the application of a higher working voltage. Therefore, all-solid-state Li-ion batteries (ASSLIBs) are expected to significantly exceed the durability, energy density, and reliability of current LIB technology.
Initially, a large number of efforts have been devoted to improving the ionic conductivity of the SSEs and it reaches 10 −2 -10 −3 S cm −1 which is comparable with the commercial level. 11 The next challenge is the interfacial issues between electrolyte and electrodes, including resistance, contact and wettability, and interface stability. 11,12 Those issues arise from the solid-solid interface, which is completely different from the liquid-solid interface in the commercial LIBs. 13 This is an area that new chemistry and materials properties are discovered, leading to a vivid field tacking solid-solid interface for and beyond energy storage systems. The wetting nature of liquid electrolyte enables itself to access all active electrode particles, allowing the rapid transfer of Li-ions at the electrode-liquid electrolyte interface. SSEs have much lower contact surface, due to their rigidity and roughness, thereby limiting the Li-ion diffusion. Therefore, electrode/SSE interfaces are important contributors to charge transfer resistance in ASSLIBs. 9,11,14,15 Interface interactions are also problematic. On the anode side, the poor contact potentially results in the uneven electrochemical deposition of metallic Li, such as the Li dendrites formation that causes short circuit. [16][17][18][19] On the cathode side, not all cathode particles have the access to Li + , leading to limited storage capacity. Interfacial chemical/electrochemical reactions between SSE and electrodes pose another problem. They usually decompose the SSE into a passivating layer that may block the Li + diffusion, degrade rate capability and reduce capacity. 20,21 Meanwhile, the fundamental study of the cathode/SSE interface has been restricted due to the complicated chemical environment at the interface and the limited measurement methods. 11 Various solid electrolytes, such as garnet, poly(ethylene oxide) (PEO) based, 22 NASICON, 23 Li phosphorous oxynitride (LiPON), 21 and sulfide 24 SSEs, have been developed in recent years. Sulfide based Li 10 GeP 2 S 12 (LGPS) shows a relatively high ionic conductivity of 10 −2 S cm −1 , which is comparable with liquid organic electrolyte. 25 However, it is highly sensitive to moisture and is easily reduced by Li metal to form insulating products (Li 3 P, Li 2 S, Li 15 Ge 4 ). 11,26 LiPON-typed electrolytes take advantage of higher stability than LGPS by forming a protecting layer. Nevertheless, they suffer from relatively limited Li-ion conductivity (10 −6 S cm −1 ). 11,21 In comparison, the garnet-typed electrolyte of Li 7 La 3 Zr 2 O 12 (LLZO) has a good ionic conductivity of 10 −3 S cm −1 . Both La 3+ and Zr 4+ are relatively stable cations, enabling a wide working voltage window. 11 Until now, significant efforts have been spent on solving the interface issues at garnet typed electrolytebased ALLSIBs. The focuses are as follows: (1) surface modification of garnet electrolyte; (2) application of artificial interlayer; and (3) addition of multifunctional additives in the electrode material. These methods improved the cycling stability of garnet electrolyte-based cells in some degrees. In this review, we begin with the development of garnet electrolytes and their intrinsic chemical and electrochemical properties (section 2). The different interfacial challenges at both anode/ and cathode/garnet interfaces are introduced and summarized in section 3. In particular, the fundamental studies of electrochemical and chemical stability of garnet electrolyte in contact with Li metal and cathode materials are analyzed. The corresponding strategies to the interfacial problems are introduced with regarding their working principles (section 4). On the basis of these analyses, a comprehensive understanding of interfacial properties in garnet electrolyte-based ASSLIBs is concluded in section 5, leading to the discussion of the remaining challenges and future perspectives of garnet electrolyte-based ASSLIBs (section 6).

| The development of garnet electrolyte
The first garnet Li-ion conductor was synthesized and reported by Thangadurai et al. 27 in 2003, with the composition of Li 5 La 3 M 2 O 12 (M = Nb, Ta) and the conductivity of 10 −6 S cm −1 . The Li + content can be varied, leading to Li 3 -, 28 Li 5 -, 27 Li 6 -, 29 and Li 7 -phases, 30 in which the conductivity of garnet electrolyte delivers a positive relationship with the Li contents. 1 In 2007, Murugan et al. 31 synthesized highly conductive cubic garnet-type Li 7 La 3 Zr 2 O 12 (LLZO) by substitution of Zr for M in Li 5 La 3 M 2 O 12 , showing a total conductivity of 3 × 10 −4 S cm −1 at 25°C. LLZO has two stable crystal phase, a cubic phase with a disordered Li-ion distribution and a tetragonal phase formed with an ordered distribution. The cubic phase shows much higher ionic conductivity than the tetragonal one. 1,11,32 Such cubic structure of LLZO belongs to the Ia¯3d space group ( Figure 1A), in which octahedral ZrO 6 and dodecahedral LaO 8 are connected with shared corners and edges. 33 In the Li-saturated structure, all Li-ions are located at tetrahedral 24d sites (Li 1, as marked by the yellow color in Figure 1A) and distorted octahedral 96h sites (Li 2, as marked by the pink color in Figure 1A), respectively. 34 During the Li-ions migration, the ions will transport from 24d sites to 96h sites and vice versa, suggesting an overall transport pathway of 24d-96h-24d-96h-24d ( Figure 1B). The calculated barrier of Li-ions transportation is 0.3 eV, which is the smallest one among all garnet materials. 31 The crystallization of the cubic phase is favored by the substitution of Li + and Zr 4+ with other metal cations. This can also improve Li-ion conductivity. For example, the substitutions of Li by Al, 35 Fe, 36 Ge, 37 and Ga, [38][39][40] and the substitution of Zr by Nb, 41 Ti, 42 Ta, 43,44 Sb, 45 Mo, 46 Cr, 47 Y, 48,49 W, 50 and Te 51 were reported. The garnet typed electrolyte, Li 6.55 Ga 0.15 -La 3 Zr 2 O 12 , has achieved an ionic conductivity of 2.06 × 10 −3 S cm −1 , which is comparable to conventional liquid organic electrolyte. 40 The garnet electrolyte offers the advantages in relatively highest stability with Li metal, wide electrochemical stability window, and high Li-ion conductivity. 52 The interface problems are the current limitation in the garnet type solid-state battery (SSB) performances. Recent years witness a rapid increase of publications in studying the interfacial chemistry of garnet electrolyte ( Figure 1C). It is clear that research

| Chemical and electrochemical stabilities of garnet electrolyte
The chemical and electrochemical stabilities of the garnet electrolyte are the main descriptors for determining the interface performances during charge and discharge processes.

| Electrochemical voltage window
On the basis of the first principle calculation prediction, the voltage profile of LLZO upon lithiation/de-lithiation and detailed phase equilibria are shown in Figure 1E. 25 The results show LLZO has a stable electrochemical window between 2.91 and 0.05 V versus Li + /Li, which avoids oxidative or reductive decomposition at high or low potentials, respectively.
The electrochemical window of the solid-state LIB system is shown in Figure 1D, where the color bars are electrochemical window versus Li + /Li, and the y axis is the Li chemical potential profile. 21 In equilibrium with an opening Li reservoir, the Li chemical potential μ Li is described as the function of electrostatic potential φ: where μ Li 0 is the chemical potential of Li metal and e is the elementary charge. 21 With the increase of voltage, the Li-ions transfer from Li metal anode to cathode, in the form of different Li-compounds. These compounds, such as Li metal, interphase (e.g., Li 2 O), LLZO, interfacial coating, and cathodes, follow the decrease of μ Li .
Despite the theoretical window between 0.05 and 2.91 V versus Li + /Li, a slight wider working voltage window of LLZO (~5 V vs. Li + /Li) is normally observed experimentally in many cyclic voltammetry studies, which can be partially attributed to the sluggish kinetics during the decomposition reactions. The driving force for the interphase's formation is the difference of μ Li between garnet and electrodes. The decomposition reaction energy E D with an applied voltage φ can be described as: where E(phase equilibria, φ) is the energy of the phase equilibria at the potential φ, E(solid electrolyte) is the energy of the solid electrolyte and Δn Li is the change of Li + transference number from electrolyte to the phase equilibria during the electrochemical cycling. 21 The reduction of LLZO at 0.05 V versus Li + /Li releases the energy of −0.021 eV, which is the lowest among most of the SSBs. 21

| Surface instability in atmosphere
The surface of garnet electrolyte is sensitive to atmosphere, especially to moisture. 53 The surface reaction with H 2 O leads to an intermediate layer containing LiOH, Li 2 CO 3 , and Li oxides. 53 Sharafi et al. 54 reported the mechanism of the intermediate layer formation based on the density functional theory calculations and photoelectron spectroscopy (XPS) measurements. Li + /H + exchanges occur between garnet electrolyte and moisture and form LiOH on the surface of LLZO. Afterward, the LiOH is gradually carbonatized into Li 2 CO 3 , which is further confirmed by Xia et al. 55 The existence of Li oxides is observed by Sharafi's research group. 54 The proton exchange process enables the substitution of surface-active Li sites by H + , changing the surface crystal structure and thus blocking the Li-ions conducting pathway. Furthermore, by-products such as LiOH and Li 2 CO 3 have low Li-ion conductivity and reduce the interfacial contact area between LLZO and electrodes, leading to high interfacial resistance.
A recent study investigates the relations between Li-ion conductivity and the thickness of the LiOH layer. 56 A H 2 O bath for the proton-free Li 6.55 Ga 0.15 La 3 Zr 2 O 12 (Ga 0.15 -LLZO) is performed for 5, 15, and 30 min, forming the LiOH layer with thickness of 0.85, 1.63, and 1.65 μm, respectively, as determined by focused ion beam secondary ion mass spectrometry (FIB-SIMS). The calculated bulk resistivity from electrochemical impedance analysis shows the calculated bulk resistivity of Ga 0.15 -LLZO is approximately 1 × 10 5 Ω cm in samples treated via 100°C H 2 O for both 15 and 30 min, which is over 100 times of that of pristine Ga 0.15 -LLZO (8.6 × 10 2 Ω cm). When coupling with metallic Li electrodes, the interfacial resistance was dramatically increased from 9.4 to 2.4 × 10 3 Ω cm 2 in the sample immersed in H 2 O for 15 min. These results reveal the importance of inhibiting the proton exchange on the garnet surface. Similarly, the formation of Li 2 CO 3 layer on garnet surface also leads to insufficient interfacial contact with Li metal. 53 Furthermore, the formation of LiOH and L 2 CO 3 generates inhomogeneous current distribution during Li plating/stripping process, favouring the growth of harmful Li dendrites. 57 Therefore, the removal of the by-product layer formed on LLZO before cell assembly is an efficient way to improve the electrochemical performance of garnet electrolyte based on ASSLIBs.

| Origins of the Li + transfer resistance at cathode/garnet and garnet/Li metal interfaces
In traditional LIBs, the liquid electrolyte can fully wet the electrode surface, allowing uniform and even Li + transfer at the solid-liquid interface. In comparison, solid-solid interface suffers from contact loss and wettability which can induce formation of high interfacial resistance and space charge layer (SCL) (Figure 2). SCL is formed because of a chemical potential difference between two contacting species, forming a charge-enriched region and a charge-depleted region that are separated from each other. 58 In general, there are three interfaces in the ASSLIBs. They are Li anode/SSE, cathode/SSE, and individual particle interface within the cathode. Those particles include electric conductive additive, binders, ionic conductive additive, and cathode active materials, where the Li transportation between different particles is poor. Different types of cathode will result in different interface properties. For example, Li transition metal oxides cathode will easily form interphases, whereas sulfur cathode has insulating problems at the particle interface. All three interfaces have point contact and suffer from contact loss, which will get even worse during charging and discharging cycles. At the Li anode/SSE interface, the stripping/plating can deposit metallic Li unevenly due to current constriction, forming Li dendrites. Moreover, the formation of interphases with different Li-ion conductivity can change the interfacial resistance. Side reaction occurs, consumes Li-ions, and changes the surface crystal structure of LLZO, affecting the Li-ions conducting pathway. SCL is another factor that can slow the Li + transfer. 59 Such SCL is formed via significant displacement polarization in the SSB, especially at the interface between electrode and electrolyte. [60][61][62] For the anode/garnet interface, the main challenges are the solid-state contact resistance and the short circuit caused by metallic Li dendrite growth. The cathode/ garnet and particle interfaces have complicated chemical environment. The study related to this side is rare and mostly focuses on improving the interfacial performance.
LLZO is lithiophobic and cannot be wetted by molten Li ( Figure 4A). 67 This is also visualized locally via scanning electron microscopy (SEM) ( Figure 4B), showing the poor physical contact between garnet electrolyte and the solid Li layer at μm scale. The surface compositions and structures of LLZO are also found to be responsible. Formation of Li 2 CO 3 increases the contact angle from 95°for pure LLZO to 146°( Figure 4C). 68 First principle calculation shows that Li-Li 2 CO 3 only has 0.1 J m −2 of interfacial work of adhesion, W ad , whereas that for Li-LLZO is 0.67 J m −2 . Interestingly, the reported contact angle of Li-LLZO varies from 90°to 122°, 64,67,69-71 possibly due to the difference and cleanliness of the surface. Therefore, a standardized measurement procedure, including the precise surface cleaning procedure is required to enable comparison in literature.
The chemical stability problem of the LLZO against Li anode and oxide cathode is realized recently. 72 Ma et al. 65 applied the in-situ electron microscopy to investigate interface chemical change during the direct  Figure 5A). Combined with electron energy loss spectroscopy (EELS), it demonstrates that the direct contact between Li metal and LLZO can induce phase transition of cubic LLZO (c-LLZO) to tetragonal LLZO (t-LLZO), which shares the same chemical composition but different atomic structure and lower ionic conductivity ( Figure 5B-D). Fingerle et al. 20 applied CO 2 -laser chemical vapor deposition to synthesize thin garnet film Li 5 La 3 Ta 2 O 12 (LLTaO). Then Li metal is stepwise evaporated onto LLTaO. The as-formed interphase was then monitored by XPS and ultraviolet photoelectron spectroscopy. 20 As shown in the XPS O 1s ( Figure 5G) and Li 1s ( Figure 5H) spectra, Li 2 CO 3 on LLTaO surface is removed before the Li deposition. Formation of Li 2 O is observed along with the Li deposition ( Figure 5E), suggesting a passivating Li 2 O layer that prevents the further reduction and decomposition of LLTaO ( Figure 5F). There is no formation of Li peroxide.
Apart from the chemical reaction between Li and solid electrolyte, electrochemical cycling can also form low conducting interphases. 10,32 For example, a thick interphase layer enriched with Li is formed between Li metal and electrolyte, which covers the surface of the Li anode. This is visualized via time-of-flight secondary-ion mass spectrometry (TOF-SIMS) ( Figure 6A,B). 73 However, it is not clear what is the chemical structure/composition of this interphase and how it is formed from the original SSE lattice. The electrochemical interface properties were further studied by Krauskopf et al. 74 in the Li 6.25 Al 0.25 La 3 Zr 2 O 12 system. The galvanostatic potential profile of Li/LLZO/Li symmetry cell during stripping/ plating under a current of 100 μA cm −2 ( Figure 6E) shows an increasing overpotential. During the stripping process ( Figure 6C), the vacancy accumulated to contact spots and form pores near the interfaces. The behavior results in a contact loss, leading to an increasing of interfacial resistance and thus restricting the rate | 391 capability of Li metal anode. It is also demonstrated that the interface can remain stable only when the anodic load is smaller than that of a critical value around 100 μA cm −2 , which is not high enough for the practical application.
Uncontrollable Li dendrite growth is another major issue related to the electrochemical performance. Although garnet SSEs, such as Li 6.24 La 3 Zr 2 Al 0.24 O 11.98 , show a high-shear modulus of 60 GPa, 76 the Li dendrites cannot be avoided in the system. Recently, Kazyak et al. 75 studied the Li propagation and dendrite growth in LLZO SSE by operando optical microscopy. During the charging at 0.2 V with 0.5 mA cm −2 current density, small Li dendrites start to form on the Li/LLZO interface, as shown in (e) of Figure 6I. Those Li dendrites grow gradually during discharging (e to f). When Li dendrites are large enough to penetrate the whole electrolyte layer, a short circuit occurs (g). On one side, a range of imaging techniques, including optical microscopy, X-ray computed tomography (CT), and electron microscopy can visualize the Li dendrite formation. On the other hand, the study of Li dendrite growth in garnet SSEs has not reached a general agreement. It is widely believed that contact area loss, 77 grain boundaries, 67 defects, 18 and the electronic conductivity of SSEs 19 contribute to the growth of Li dendrites. For example, the contact area loss between garnet electrolyte and metallic Li will contribute to an inhomogeneous deposition of Li + ions on the interface, promoting the growth of Li dendrites. Therefore, a universal growth model is yet established. It is then even more difficult to prevent the formation of Li dendrites without an understanding of its chemistry and structure. 7 Li NMR (nuclear magnetic resonance) was used to detect the microstructural growth of Li dendrites at the early stage. However, it only confirmed the formation of Li dendrites before the short circuit. 16

| Garnet/cathode interface
A cathode/electrolyte interface, as well as an electrolyte/ interparticle interface, can be formed on the cathode side. In traditional liquid electrolyte-based LIBs, cathode active materials are mixed with electric conductive materials and fully wetted by liquid electrolyte to achieve good ionic transport at solid-liquid interface. In contrast, solid electrolyte cannot provide such wettability, so the electrolyte/ cathode interface is a big challenge ( Figure 7A,B). The complicated chemical composition of the cathode also leads to the formation of interphase layers ( Figure 7C). To address the challenges, several methods have been designed [e.g., nanostructured garnet host ( Figure 7D), low melting-point alloy additives (Figure 7E), and artificial interlayer ( Figure 7F)], but the most publications related to cathode/ garnet electrolyte interface are focused on improving performance instead of understanding the chemistry and mechanism at the interface.
Garnet SSEs are normally stable with most of the common cathode materials at room temperature. 11,81 However, to achieve a better wettability and interfacial Li-ion conductivity, high-temperature or co-sintering treatment is widely used. 82 The treatment could introduce chemical reactions on the cathode/garnet SSEs interfaces.
The chemical compatibilities between garnet electrolyte and metal oxides cathode materials under high temperature were reported by Ren et al. 83 On the basis of the X-ray diffraction pattern of different types of cathode materials mixed with LLZTO at elevated temperature, the LiCoO 2 (LCO) delivered the best stability up to around 700°C, whereas LiMn 2 O 4 and LiFePO 4 (LFP) started to react with LLZTO at approximately 500°C. However, other scientists found the LCO could be reactive to garnet type electrolyte even at 500°C, resulting in cross-diffusion and decomposition. 84 The cross-section TEM image of LLZO/LCO interface ( Figure 8A, top) and the energy-dispersive spectrometer (EDS) line profile ( Figure 8A, bottom) show an intermediate layer. It is a reaction phase by mutual diffusion of elements between LLZO and LCO. Similar result was also reported by Vardar et al. 85 The O K-edge X-ray absorption spectroscopy (XAS) measured at total fluorescence yield ( Figure 8B) shows a decrease of LLZO features at a higher temperature, comparing to the peaks of Li 2 CO 3 and LCO. This shows the decomposition of the LLZO at the interface. TOF-SIMS is also a powerful tool to study the cathode/garnet electrolyte interface chemical compositions. According to Figure 8C, A cross diffusion at the interface between LCO/LLZO (Al contained) is formed, where Co diffuses into LLZO and Zr/La diffuses into LCO. 82 The change of LLZO itself is also observed with air exposure ( Figure 8D). Al element is commonly used to occupy the Li sites and stabilize the cubic phase of LLZO. It becomes inhomogeneously distributed after one year, indicating the transition of LLZO from cubic phase to tetrahedral phase, which will lower the ionic conductivity. 82 Garnet typed electrolyte is also widely used in Li/S battery. 3,71,[86][87][88] Solid electrolyte can block the "shutter effect" in Li/S battery by blocking the transfer of intermediate polysulfides to the Li anode. 89 As the insulating nature of sulfur, current publications focused on using nanostructured garnet to adopt sulfur to increase the contact area, which will be discussed in section 4. However, the chemical or electrochemical stability between garnet and sulfur in such a system is seldom discussed. carbonates (such as Li 2 CO 3 ) on their surfaces, thereby increasing the interfacial resistance. 90 Therefore, Li 2 CO 3 prevention and removal is the key point to improve the interface properties between garnet-typed SSEs with electrodes. By controlling the grain size of LLZO particles, the surface sensitivity towards moisture and CO 2 can be optimized. 91 Two LLZO samples with~150-and 20-µm grain sizes were exposed in air for 24 h. Soft X-ray absorption spectroscopy shows the lower content of Li 2 CO 3 on the LLZO with~20-µm grain sizes than that of the LLZO with~150-µm grain sizes, indicating less coverage of the Li 2 CO 3 on the surface. The results are also consistent with impedance spectroscopy ( Figure 9B).
The mechanical polish and thermal treatment are considered effective ways to remove the by-product formed on garnet SSEs. Li et al. 92 reported that heating Li 6.5 La 3 Zr 1.5 Ta 0.5 (LLZT) together with carbon at 700°C could reduce the Li 2 CO 3 layer and other impurities formed on the LLZT surface. Raman mapping ( Figure 9C) was applied to detect surface carbonates. The thermal aged LLZT still has very high carbonate contents, whereas the addition of carbon significantly reduced carbonate signals (LLZT-C in Figure 9C). The LLZO-C shows smaller interfacial resistance and better charge and discharge stability compared to the aged LLZT ( Figure 9D P_LLZO_L is pristine LLZO with a grain size of~150 µm, P_LLZO_S is pristine LLZO with a grain size of~20 µm, E_LLZO_S24h is 24-h air-exposed LLZO with a grain size of~20 µm, E_LLZO_L24h is 24-h air-exposed LLZO with a grain size of~150 µm. (B) Nyquist plots of impedance data of Li/LLZO/Li cells containing different materials: Left is P_LLZO_L and E_LLZO_L24h and mid is P_LLZO_S and E_LLZO_S24h. Right: area-specific interfacial resistances (ASRs) of pristine LLZO samples and those exposed to air for 24 h. Reproduced with permission: Copyright 2015, American Chemical Society. 91 Figure 9F). Moreover, the Li symmetric cells also deliver a stable Li plating/striping for over 700 h under 0.2 mA cm −2 at 30°C. Ruan et al. 93 used H 3 PO 4 to remove the Li 2 CO 3 and LiOH, and then, a uniform intermediate layer (Li 3 PO 4 ) was formed along with the H 3 PO 4 etching reactions. The Li 3 PO 4 intermediate fills the gap between electrolyte and Li metal, improving the contact between LLZO with metallic Li significantly, as shown in Figure 9G.
In summary, strategies based on a garnet electrolyte are mainly focusing on the removal of the contamination layer, to decrease the interfacial resistance and poor wettability resulted from the Li 2 CO 3 .

| Toward garnet/Li interface aspects
To address the interfacial problem arisen at the garnet/Li interface, molten Li has been widely used to solve the contact problem to some degree. 11 However, this does not solve the intrinsic lithiophobic nature of the LLZO. Introduction of an artificial interlayer at the Li/garnet interface is an effective way to improve the wettability between them. 12 The most representative example is atomic layer deposition (ALD) of Al 2 O 3 on LLZO electrolyte. 64 The ALD method maximizes the contact between Al 2 O 3 and LLZO. Al 2 O 3 is lithiophilic, showing a much smaller contact angle with molten Li ( Figure 10B). Therefore, the Al 2 O 3 layer not only physically occupies the voids formed at the garnet/Li interface ( Figure 10A), but also improves the wettability between LLZO with metallic Li. The coating of Al 2 O 3 on garnet electrolyte was proved by TEM cross-section image at the interface of ALD-Al 2 O 3 -coated garnet with Ti protection layer, where Al is found between Ti and Li ( Figure 10E, a-g). EELS spectroscopy ( Figure 10E, h) shows that there is no peak at around 285 eV for C 1s, suggesting the absence of Li 2 CO 3 in the system. Upon heating, Al 2 O 3 -LLZO/Li shows an extremely low interface resistance to 1 Ω cm 2 ( Figure 10C) and a much better cycling performance than that of LLZO/Li ( Figure 10D). In addition to the Al 2 O 3 deposited via ALD, various metal and lithiated materials such as Li 3 N 94 and Li y Sn 95 layer can also improve the wettability between LLZO and metallic Li.
To minimize the interfacial contact resistance between metallic Li and garnet SSEs, an additional organic interlayer has also been used in recent years. 11 The soft interlayer is normally consisting of polymer membrane and Li salts. It is deformable, which not only enables the intimate contact between Li metal and SSEs but also serves as a protective shield against Li dendrites, therefore, bridging the Li-ion transport between garnet and metallic Li ( Figure 11A). The common organic polymer film is PEO, which has been considered as suitable frame support and it also has provided a good flexibility to meet the interface stress-strain behavior. 96 Another representative sample is polyvinylidenefluoride-cohexafluoropropylene (PVDF-HFP) membrane soaked with 1 M LiPF 6 solution consisted of ethylene carbonate and diethyl carbonate with volume ratio 1:1. 97 With PVDF-HFP, the interfacial resistance between Li metal and Li 7 La 2.75 Ca 0.25 Zr 1.75 Nb 0.25 O 12 (LLCZNO) was decreased from 1.4 × 10 3 to 214 Ω cm 2 and a high capacity of 140 mAh g −1 is recorded when corporated with LFP cathode (Figure 11B,C). Similarly, PEO-ceramic particles-Li-salts composites electrolyte is another effective system to deal with the interfacial resistance. In this composite system, LLZO particles were added to influence the recrystallization kinetics of the PEO polymer chains to promote local amorphous regions, thereby increasing the Li salt/polymer system's ionic conductivity. 4 This composite system demonstrates a promising strategy to apply LLZO particles to reinforce PEO-based electrolytes in the aspect of mechanical property and ionic conductivity. The mechanical property and ionic conductivity were determined by the loading of LLZO filler in PEO polymer. 98 The highest ion conductivity (1.17 × 10 −4 S cm −1 ) can be achieved with 10 wt% LLZO fillers in the PEO electrolytes. 98 However, the brittle nature of LLZO particles will crack the membrane when the LLZO loading content is above 80 wt%. A comprehensive understanding of the Li-ion transport mechanism and the corresponding control parameters in this composite electrolyte are very important for achieving good performance. NMR is used to probe the Li-ion mobility and Li-ion transport pathways. 53 The sample preparation is shown in Figure 11E-H. By tracking the replacement of 7 Li in electrolyte by 6 Li in Li metal anode after cycling, the Li pathway was identified. As shown in Figures 11K and 11L, 6 Li ions replace 7 Li ions in the LLZO part of composite electrolyte (strong peaks at middle part) instead of LiClO 4 (polymer part, small peaks at right side) and LLZO/polymer interface; therefore, majority of the Li + transfer via LLZO particles. This leads to the discussion on how important it is for the LLZO/ polymer interface. Theoretical calculations show a space charge layer forms at PEO/LLZO interface, rendering a high resistance. 96 Its role in the Li + conductive pathway is unclear.
Li-based alloy anode also can be used to address the garnet/Li interface problem. This method modifies the surface tension and viscosity of molten metallic Li. Recently, Duan et al. 100 reported a Ligraphite (Li-C) anode for ASSLIBs. The Li-C anode was synthesized by mixing the commercially available graphite powder with molten metallic Li with  Figure 12C, where Li-C was easily wetted on the LLZO surface. The interface morphologies between anode and electrolyte were further characterized by SEM. And it shows the intimately contacted Li-C/garnet interface. The Li-C/LLZO/Li-C symmetric cell delivered a stable plating/striping curve at a high current density of 0.8 mA cm −2 for over 30 h, whereas pure Li-based symmetric cell failed to work in few hours at a current density of only 0.3 mA cm −2 . Later, the same group also reported using the graphitic carbon nitride as an additive for Li anode, which not only provides improved viscosity and decreased surface tension but also suppresses the growth of Li dendrites. The work shows a high critical current density of 1.5 mA cm −2 , which is around 30 times of that of pure Li metal anode. 65 Similarly, Li-Sn-, 101 Li-Al-, 102 and Li-Mg 103 -based anodes were reported. However, the composition change of anode can possibly result in the change of redox potential and the volume during cycling, 11 which is seldom discussed in current studies.
In summary, several strategies have been reviewed and proven to effectively reduce the interface resistance between garnet SSEs with metallic Li. Currently, the main barrier across the development of Limetal-based SSBs is the growth of Li dendrites. Although a lot of studies and methods have been applied to inhibit the Li dendrites growth, fundamentally preventing Li dendrites is still hard to achieve.

| Toward to cathode/garnet electrolyte interface
Researches on the cathode/garnet SSEs interfaces are much less than that of the anode side. The solid-solid contact, stability, side reaction, and space-charge layer are major challenges. As discussed in section 3, the common cathode materials require to be mixed with electronic conductive material (conductive carbon) and ionic conductive materials (SSE particles). However, the mixture brings a series of  6 Li foil/composite electrolyte/ 6 Li foil for probing Li-ion pathways within the composite electrolyte. In the illustration, the blue area represents the PEO polymer matrix, red filled circles are LLZO particles, and the white regions represent the interface between PEO and LLZO; (I) illustration of the symmetric 6 Li foil/composite electrolyte/ 6 Li foil battery and possible Li + transport pathways within the composite electrolyte upon cycling the symmetric battery; (J) electrochemical profile of the symmetric Li-battery cycled with a constant current that changes signs every 300 s; (K) comparison of the 6 Li NMR spectra of the LLZO-PEO (LiClO 4 ) composite electrolytes before (pristine) and after (cycled) cycling; (L) quantitative analysis of 6 Li amount in LiClO 4 , interface, and LLZO of the LLZO-PEO (LiClO 4 ) before and after cycling. Reproduced with permission: Copyright 2016, Wiley. 53 EIS, electrochemical impedance spectra; LFP, LiFePO 4 ; LLZO, Li 7 La 3 Zr 2 O 12 ; NMR, nuclear magnetic resonance; PEO, poly(ethylene oxide); SSE, solid-state electrolyte cathode particle/garnet electrolyte interface, rendering a complicated interfacial chemical process.
A range of deposition methods are used to obtain intimate contact between the LCO cathode and LLZO. 10 They are usually based on coating technology. For example, Asaoka et al. 104 use pulsed-laser deposition (PLD) to coat LCO on the Li 6.75 La 3 Zr 1.75 Nb 0.25 O 12 interface. 104 The cathode-solid electrolyte composite was fabricated with Li and delivered good cycling stability for 100 cycles with a reversible capacity over 120 mAh g −1 . Various buffer layers,  97 were used to block their direct contact, which can enhance the stability of oxide SSEs against cathodes. 10 Kotobuki et al. 106 reported another effective way by using a sol-gel method to prepare LCO on the LLZO surface. Followed by high-temperature treatment, the fabricated Li/LLZO/LCO cell cycled three times under a current density of 10 µA cm −2 but a relatively low capacity of 8.4 mAh cm −2 was achieved.
Co-sintering additives is another effective way to improve contact between the cathode and garnet electrolyte. The additives normally have a low melting point  Figure 13A). 107 After sintering, the LLZO and LCO particles are imbedded together by the chemical reaction between LCBO matric and Li 2 CO 3 , which is formed on the LLZO and LCO particle surface ( Figure 13D-I). The reaction contributes to a dense and compact cathode (Figures 13C and 13G). The interfacial contact between LLZO and LCO particles shows an improved wettability on both the cathode/garnet interface ( Figure 13C) and particle interface ( Figure 13D). The all-ceramic Li/LLZO/LCO cell has good initial capacity with different current rates and a high cycling stability at room temperature ( Figure 13J,K). Similarly, other additives, such as Li 3 BO 3 , 42 are used in recent years. Although the method is a good way to make an intimate contact between electrolyte and cathode materials, the resulted structure will have no much space to accommodate the volume change of electrode materials. Therefore, the design of the interface needs to consider electronic and ionic contact, as well as the volume change, during battery cycling.
In addition to the additives, nanostructured garnet electrolyte is widely used as a cathode active materials host to improve the contact between SSE with cathode. 3,5,78,[108][109][110] The high surface area provided by nanostructures can adopt the volume change of cathode materials during the cycling and increase the cathode/LLZO particle contact area. This is particularly important for Li-sulfur batteries. Fu et al. 78 prepared a bilayer structure with a dense layer and the porous layer, as shown in Figure 14A (scaffold structure in Figure 14A-C). The structure was revealed by SEM ( Figure 14E) and contributed to a high sulfur loading over 7 mg cm −2 . A good initial capacity of over 600 mAh g −1 was achieved with a current density of 0.2 mA cm −2 . From the long-term cycling performance ( Figure 14K), the high value of Coulombic efficiency (>99%) and gently decreased curve indicate no polysulfides lost and shutter effect occurred during the cycling.
Apart from the 3D bilayer, another nanostructured LLZO was synthesized via template methods, 87 3D-printer, 108 and freeze-casting. [111][112][113] The scaffold structure also shows promising potential for a corporation with metal oxides. On the basis of screen printing, an LCO/garnet composite cathode delivered a high utilization of active material (81%) at 0.1 C. 114 However, in most of these designs, an ionic liquid electrolyte was added to wet the cathode side interface. How to avoid the utilization of liquid electrolyte is required to be investigated in the future.

| SSE/anode interface
A summary of typical methods towards SSE and interface optimization is shown in Table 1. At the anode/ SSE interface, the removal of Li 2 CO 3 significantly reduces the interfacial resistance to 7-28 Ω cm 2 . However, Li dendrite formation is yet solved, as shown in the limited critical current density (CCD). CCD is defined as the current density at and above which Li metal propagates through an SSE. The best-reported values are in the range of 1.1-2.3 mA cm −2 , suggesting the very limited safety current window. Such CCD is lower than the commercial level (>3 mA cm −2 ). Direct contact between anode and electrolyte (e.g., Li 2 CO 3 removal, application of some Li-alloy anode) normally contributes to a low CCD. Some Li-alloy anodes show a high CCD, which attributes to their stable alloy products with Li metal (e.g., Li-Al). Similarly, artificial interlayers also show high CCD when stable interphases are generated. Nanostructured garnet electrolyte improves the working current density. However, most publications are focusing on the synthesis and characterization of nanoparticles instead of full cell testing. Hybrid electrolyte systems also show promising potential because they normally have stable cycling performance, but their Li-ion conductivity at room temperature is the bottleneck.

| Cathode and full cell test
Liquid electrolyte is usually used to improve the wettability between cathode particle/garnet interface. In other words, it is hard to realize all-solid-state batteries at the current state. Polymer membranes are applied with Li salts between the cathode and garnet electrolyte. This is still not liquid-free as a solvent is used to dissolve those Li salts. They will be squashed to wet the whole cathode during the cell fabrication, which is then not different from the common LIB with liquid electrolyte. The presence of liquid electrolyte and Li salts leverages the overall full cell performance. Thus, the true performance from the garnet electrolyte and any improvement via structure/interface modification are difficult to define. The only successful example is the application of Li 2.3 C 0.7 B 0.3 O 3 as a sinter additive. As discussed in section 4.3, the method enables an in-situ connection with LCO and LLZO particles via chemical reaction, which is very meaningful for the future development.

| Understanding of the interface
With the help of several in-situ characterization methods (e.g., XPS, TOF-SIMS, EELS), some progress has been mode in the interfacial understanding in garnet electrolyte-based ASSLIBs (shown in section 3). They are focused on the investigation of chemical interphases formed during the contact between electrolyte and electrode materials. However, most systems cannot avoid the possibility of contamination (e.g., Li 2 CO 3 , LiOH, air atmosphere). On the basis of the current study, it seems the interphase is consisting of various chemicals and could be varied by not only the composition of the material but also the assembly, external environment, and operation condition (e.g., working current, voltage window) of the batteries.

| SUMMARY AND PROSPECTIVE
Garnet typed SSEs were considered as promising candidates in ASSLIBs, attributing to their high ionic conductivity, wide working voltage window, good chemical and electrochemical stability. Despite the recent progress highlighted in this review, the fundamental understanding and effective control of solid-state interfaces between garnet-typed SSEs with electrodes are still challenging. The poor solid-solid contact and metallic Li dendrite growth are the major bottlenecks. The formation of interphases such as Li 2 O, via (electro)chemical reactions, can also lead to the poor change transfer behavior across electrode/electrolyte interface. Additionally, mechanical instabilities at garnet-typed SSE/ cathode interfaces can also arise because of the deformation or breaking of rigid SSE particles to accommodate the electrochemically induced volume expansion of cathode particles during electrochemical cycling. To address the interface issues, surface treatments of the garnet-typed SSEs, the introduction of the artificial interlayer, and Li-alloy/composite anodes have been used to control chemical and electrochemical processes at garnet-typed SSE/electrodes interfaces. Moreover, imaging and spectroscopic study are used to probe the change of the interface reaction before and after cycling. Few in-situ and operando studies are reported, mainly on the formation of the Li dendrites. X-ray based techniques, including X-ray CT, interface XPS experiments, and X-ray absorption spectroscopy are useful tools to probe the physical and chemical states of the elements in the interface. On the basis of these understandings, three perspectives are discussed below: 1. How to permanently prevent the formation of Li dendrites inside the garnet electrolyte, especially when the high current density is applied? In addition to the method of artificial interlayer applied to suppress Li dendrites on the Li/garnet interface, introduction of active nanoadditives in the grain boundary and defective sites of garnet electrolyte could be a direction. It can eliminate the defects in SSE so that the formation of Li dendrites in the electrolyte is possibly prohibited. Eventually, the target is to achieve uniform distribution of current density at the interface, preventing the hotspot formation. This also calls for in situ imaging technique to reveal the current density distribution. 2. Cathode side interface requires more attention. As discussed in section 5, most publications use liquid electrolytes to solve the interfacial challenge at this side, which is not "fully solid." Sintering in the presence of additives is an effective method to deal with the problem, but the number of publications using this method is very limited. In addition, hybrid solid electrolyte system with inorganic SSEs and organic filler also shows its potential when applied in ASSLIBs. However, how the fillers work is still questionable. The particle interface is not well defined and difficult to understand, calling for a combination of theory and experimental studies. 3. Advanced characterization methods for operando observation of electrode/garnet electrolyte interface are required, especially at the full cell level. Growing activity in this area is expected along with the development of fourgeneration synchrotron X-ray sources worldwide. The major challenges are as follows: (1) extract the interface information out of the bulk information; (2) identify the difference between chemical and engineering problems; (3) probe light element, such as Li and O in the full cell.
Operando study can also reveal the relationship between the decay in cycling performance and the evolution of garnet electrolyte. To achieve this, the battery needs to be charged and discharged at relevant conditions in commercial applications. Ideally, the chemical environment of Li can be determined along with the decay of the cell. This can be coupled with theoretical calculations to determine the change of Li chemical potential, and thus map the structure/potential diagram in the battery.
In conclusion, garnet typed electrolytes have shown their potential in the fabrication of realistic ASSLIBs. Although the interfacial problem is still difficult to overcome, garnet electrolyte-based solid batteries have a great future with continuous efforts.