Interfacial engineering for high‐performance garnet‐based solid‐state lithium batteries

Solid‐state batteries represent the future of energy storage technology, offering improved safety and energy density. Garnet‐type Li7La3Zr2O12 (LLZO) solid‐state electrolytes‐based solid‐state lithium batteries (SSLBs) stand out for their appealing material properties and chemical stability. Yet, their successful deployment depends on conquering interfacial challenges. This review article primarily focuses on the advancement of interfacial engineering for LLZO‐based SSLBs. We commence with a concise introduction to solid‐state electrolytes and a discussion of the challenges tied to interfacial properties in LLZO‐based SSLBs. We deeply explore the correlations between structure and properties and the design principles vital for achieving an ideal electrode/electrolyte interface. Subsequently, we delve into the latest advancements and strategies dedicated to overcoming these challenges, with designated sections on cathode and anode interface design. In the end, we share our insights into the advancements and opportunities for interface design in realizing the full potential of LLZO‐based SSLBs, ultimately contributing to the development of safe and high‐performance energy storage solutions.

2][3] In response to the ever-increasing energy requirements, there is a growing demand for batteries with higher energy density and higher safety. 4,57][8][9] Li metal offers the highest theoretical capacity (3860 mAh g −1 ) among all anode materials.However, significant practical challenges, such as dendrite formation, low Coulombic efficiency, and safety concerns, persist despite extensive efforts to integrate Li metal anodes into Li-ion batteries.1][12] These solid-state electrolytes are well-known for their mechanical stability, effectively mitigating the risk of lithium dendrite-induced short circuits.Additionally, their nonflammable properties enhance the overall safety of the battery system. 13,149][20][21] These SSEs also possess a Li-ion transference number approaching 1, which is notably higher compared to the often less than 0.5 values observed in liquid electrolytes. 22Among them, garnet-type Li 7 La 3 Zr 2 O 12 (LLZO) solid-state electrolytes stand out due to their exceptionally high ionic conductivity (ranging from 10 −3 to 10 −4 S cm −1 ) and impressive chemical stability when in contact with Li metal. 23,240][31] On one hand, the solid-to-solid contact at the interface between the SSE and the electrode introduces substantial impedance.This interfacial impedance can become the predominant factor contributing to internal resistance in a battery, impeding the effective transport of ions. 32,33Consequently, the rate performance of batteries is often limited to less than 2 C. On the other hand, chemical incompatibility between the reducing anode and oxidizing cathode and the solid electrolyte (SE) can give rise to side reactions at these interfaces, resulting in elevated internal resistance. 34hese undesirable reactions can occur both during battery cycling and in the manufacturing process.The latter is particularly common in oxide solid electrolytes, as achieving close interfacial contact typically necessitates high processing temperatures and co-sintering with the cathode material.In this context, the practice of interface engineering assumes critical importance in realizing high-performance SSLBs based on LLZO, ensuring exceptional energy density, superior rate capabilities, and long-term cycling stability.
Considering the swift evolution in this research domain and the pressing need for SSLBs, it is both timely and crucial to present a comprehensive review on this subject.In this review, we will commence by providing a foundational understanding of solid-state electrolytes, while also addressing the challenges associated with interfacial properties in SSLBs.Subsequently, we will offer a comprehensive overview of various interface engineering strategies aimed at enhancing interface compatibility within SSLBs.Finally, we will share our insights regarding the potential direction for interface design in the pursuit of practical applications for SSLBs.

CHALLENGES IN INTERFACIAL DESIGN FOR LLZO-BASED SSLBS
The primary challenges encountered in interfacial design for LLZO-based SSLBs are succinctly summarized in Figure 1.First, the inherent rigidity of garnet-type solidstate electrolytes results in suboptimal solid-solid contact between SSEs and electrodes, leading to elevated interfacial resistance.This elevated resistance constitutes the central impediment to the advancement of solid-state battery technology.Additionally, electrochemical and chemical incompatibilities between LLZO and the electrodes can give rise to undesirable side reactions at the electrode/electrolyte interfaces.These side reactions, in turn, contribute to increased impedance and interface instability, culminating in battery failure.Moreover, even though the high Young's modulus of the oxide ceramic electrolyte offers some resistance to dendrite formation, issues such as non-uniform ion transport at the interface, internal electronic conductivity, and the volume fluctuations accompanying lithium metal deposition and stripping processes pose challenges.These factors collectively create conditions that allow dendrites to proliferate and potentially breach the electrolyte, ultimately causing a short circuit in the battery.Detailed discussions of these challenges are presented in the subsequent sections.

Interfacial impedance
Interfacial impedance plays a crucial role in facilitating battery reactions as it governs the transport of ions and electrons within the entire battery.At the electrode/SSE solid to solid interface without any modification, the impedance can reach values exceeding thousands of ohm square centimeters. 35This high impedance poses significant challenges for the proper functioning of the battery.Typically, poor solid-to-solid contact, inadequate wetting of lithium on SSEs, and the formation of pores/voids during the manufacturing process or battery cycling are the primary factors contributing to the undesirable interfacial impedance.
A strong and consistent electrode/electrolyte interfacial connection is fundamental for achieving optimal battery performance.However, the inherent challenges arise from the high hardness of oxide ceramics and the surface roughness, which often results in streaks and other imperfections even after meticulous polishing.Simultaneously, lithium metal surfaces develop inherent defects during processing, and even when stored in an inert atmosphere, they are prone to passivation by Li 2 O formation.These issues collectively contribute to inadequate physical contact between the solid electrolyte and lithium metal, leading to point-to-point contact characterized by various pores and defects at the interface.This means that a tight and continuous interfacial connection cannot be maintained even under externally applied pressure conditions. 36,37hancing interfacial contact can be achieved by heating lithium metal to transform it into a liquid state.However, practical observations reveal that when molten lithium is applied to the surface of garnet-type LLZO electrolytes known for their stability and affinity for lithium metal, it fails to evenly spread across the surface.Instead, the liquid lithium metal tends to aggregate into spherical shapes, exhibiting non-wetting behavior. 38This phenomenon can be attributed to the LLZO electrolyte's instability in the presence of water and air.Contact with water results in the replacement of surface Li + ions by H + ions to form LiOH, which then reacts with CO 2 to generate Li 2 CO 3 impurities.][41] A fundamental study conducted by Wang et al. 37 reveals that the formation of Li 2 O on the surface of Li metal hinders the diffusion of liquid Li on the substrate, thus hindering the formation of a continuous and tight interfacial connection.The discontinuous interfacial contact between the electrolyte and electrode leads to an extended path for Li + transport and uneven stripping of lithium deposition.This, in turn, results in elevated interfacial impedance, increased polarization voltage, and reduced cycle life.
Even when a tight and continuous interface is initially established, the condition of the interface undergoes changes over time because lithium ions stripped during the cycling process may not fully return to their initial deposition positions.As cycling continues, some sites with inhomogeneities emerge on the interface.The nonuniform distribution of current density in the vicinity of these sites exacerbates their condition, eventually leading to their transformation into voids and, ultimately, resulting in the breakdown of interfacial contact.

Interfacial stability
For full battery application, the electrolytes have to be chemically stable with both anode and cathode materials or be able to form a stable SEI.Additionally, the electrolytes also need to possess excellent electrochemical stability to ensure a stable battery performance. 23Garnet SSEs possess a wide electrochemical window (>6 V vs. Li/Li + from CV studies and ∼3 V from computational analyses), making it possible to directly use Li metal anodes.Previous studies have shown that garnet SSE doped with elements such as Al and Ta can maintain stability toward solid lithium metal at room temperature and also maintain stability toward molten lithium at high temperatures up to 300 • C. [42][43][44][45][46][47] Therefore, it can be used to match metallic lithium to promote practical applications.9 In addition, the potential for elemental diffusion at the interfaces within and between electrodes and electrolytes can give rise to undesirable side reactions, ultimately compromising battery performance.elemental diffusion between/in electrode/electrolytes may cause undesirable side reactions, and lead to the deterioration of battery performance.These interfacial stability challenges need special attention for realizing high-performance SSLBs.

Lithium dendrites
The lithium dendrite problem has been one of the most difficult problems in liquid lithium metal batteries.Although the use of solid-state electrolyte can inhibit the penetration of dendrites, it cannot completely avoid the growth of dendrites as well as puncture.Earlier, Charles and John 50 considered that the shear modulus of electrolyte higher than 6 GPa could effectively inhibit the growth of dendrites through modeling calculations; however, the expected effect was not achieved in actual experiments. 51,52he growth of dendrites is related to several reasons.First, defects and voids at the interface as well as localized current concentration and uneven lithium-ion flux due to side reactions will directly lead to uneven deposition of lithium ions, which will result in the growth of dendrites.At the same time, it has also been shown that Li ions may be reduced to Li metal internally during migration in the electrolyte due to their internal electronic conductivity, which leads to the growth and spreading of Li metal from the inside. 53Han et al. 54 also pointed out that the electronic conductivity of the oxide electrolyte LLZO is several orders of magnitude higher than that of LiPON, and Li + may remain in the electrolyte during the migration process to generate metal Li, which directly forms dendrites in the interior.Tian et al. 53,55 investigated the growth behavior of lithium dendrites inside different electrolyte systems, and calculated the electronic properties at the interfaces by building a phase-field model in combination with DFT, which showed that various defects inside the electrolyte were able to trap electrons to induce the deposition of lithium metal.In addition, as the volume change of lithium metal as a frameless bulk electrode material during cycling can be considered to be nearly infinite, 56 repeated stripping/deposition will make it difficult to release the high interfacial stresses of lithium metal due to the volume change.
][62] Tippens et al. 63 utilizing in situ x-ray micro-computed tomography technology, revealed the mechanical damage induced by the growth of unstable intermediate phases in LAGP SSE.They observed that fracture initiation occurred at the edge of the Li/SSE contact region, and the fracture of SSE during the cycling was identified as the primary cause of cell impedance increase.Further analysis of the evolution of stress within SSE determined that cracks resulted from two distinct axial and radial stresses.With an increase in the thickness of the intermediate phase, the magnitude of axial stress gradually intensified throughout the entire SSE, leading to electrolyte fragmentation.The experiment provides valuable insights for the failure analysis of other non-stoichiometric LLZO-SSEs doped with unstable elements (Fe, Nb) of Li metal.Liu et al. 61,64 and Xiong et al. 65 established an electrochemical-mechanical model to visualize the stress distribution, relative damage, and crack formation at interface defects during the electrochemical lithium plating process.They discovered that the high-stress field generated by lithium plating at existing defects is a primary cause of mechanical failure in solid-state electrolytes.Because the stress field generated by the growth of Li metal in the interfacial defect space is concentrated at the tip of the defect, it is transmitted inward to the solid electrolyte, resulting in the continuous accumulation of damage in the electrolyte and local displacement.The internal void in the solid electrolyte can manage secondary damage pathways, resulting in the growth of lithium whiskers within the electrolyte.Ultimately, this leads to internal short circuits when lithium whiskers reach the opposing electrode.

INTERFACIAL ENGINEERING FOR ANODE SIDE
7][68] The uneven deposition and stripping are inherent characteristics of metallic lithium due to its lower cohesive energy, making it prone to dendritic growth during the deposition process. 69To validate this viewpoint, Krauskopf et al. 70 employed solid-state half-cell setups to investigate the morphology of lithium deposition.They observed dendritic growth of lithium nuclei beneath the copper foil, while free lithium also exhibited a similar growth pattern on the surface of the solid-state electrolyte as shown in Figure 2B.Furthermore, the slow self-diffusion of metallic lithium also plays a significant role in promoting dendritic growth.The differential diffusivity between lithium metal (Li 0 ) self-diffusion and Li + diffusion within the solid-state electrolyte can lead to the formation of voids. 71,72Taking LLZO solid-state electrolyte as an example, according to the Nernst-Einstein relationship, the self-diffusion coefficient of lithium metal is 5.6 × 10 −11 cm 2 s −1 , whereas the diffusion coefficient of Li + within LLZO is 2.15 × 10 −9 cm 2 s −1 . 72Notably, the self-diffusion of lithium metal is slower than the diffusion of Li + in the LLZO electrolyte.Under low current densities, lithium can be uniformly stripped and deposited on the electrolyte surface.However, at higher current densities, the slower diffusion of bulk Li 0 prevents simultaneous uniform stripping at a planar position at the interface, resulting in the formation of vacancies and void.
During deposition, vacancies and voids are rapidly filled.Subsequent lithium deposition must occur at non-planar positions, leading to Li 0 stacking, which serves as the nucleus for lithium dendrite growth (Figure 2C). 73Therefore, the rapid migration of Li + and the sluggish diffusion of Li 0 contribute to local current density concentration and the formation of lithium dendrites. 74,757][78] For example, Jiang et al. 79 prepared LLZTO porous framework by a dry process, and then infiltrated the SN electrolyte into the voids, and the assembled lithium symmetric batteries showed low interfacial impedance, as well as long-term cycling stability.Zhao et al. 80 prepared an anionic domain-limited composite electrolyte to reduce the concentration polarization of the batteries, and the excellent interfacial bonding effect and homogenized lithium deposition enabled lithiumsymmetric batteries to be stably cycled for more than 400 h at a current density of 0.1 mA cm −1 .In addition, Xu et al. 81 constructed dual-phase single-ion pathway interfaces to realize the homogeneous deposition of lithium ions.As for solid-state LLZO ceramic electrolyte, it can leave behind scratches, pores, and other defects during the polishing process, which pose risks for the growth of lithium dendrites.Kim et al. 82 conducted in situ optical microscopy observations of the growth behavior of lithium dendrites at different interface states in LLZO.Upon applying a constant current, the deposition of island-like lithium began to occur and grew over time.These island-like deposits preferentially appeared at pre-existing defects, and during subsequent deposition processes, metallic lithium tended to deposit adjacent to these islands.This indicates that the deposition of metallic lithium at the Li/LLZO interface is uneven and exhibits a preference for pre-existing defects.Additionally, solid-state electrolytes exhibit a certain level of electronic conductivity, and residual electronic conduction can induce the nucleation and growth of lithium ions within the electrolyte, leading to the fracture of ceramic electrolyte.Tantratian et al. 83 revealed that the penetration mode of lithium dendrites is influenced by both the mechanical and electronic properties of grain boundaries through an electrochemical-mechanical phase field model.Lithium nucleates at the grain boundaries located at the interface between the lithium anode and the electrolyte, and propagates along these boundaries, consistent with the observations by Cheng et al. 84 Liu et al. 85 using in situ transmission electron microscopy, uncovered the correlation between lithium dendrites in LLZO and The interface evolution process at the Li/SSE interface.(B) The morphology of lithium dendrites deposited on the surface of LLZO.Reproduced with permission. 70Copyright 2019, Elsevier.(C) If the critical current density is exceeded during the process of constant current electrolytic dissolution, resulting in the growth of voids at the Li/solid-state electrolyte (SE) interface.Reproduced with permission. 73Copyright 2020, Wiley-VCH.(D) Mechanism of formation and growth of lithium dendrites in liquid electrolyte and solid-state batteries.Reproduced with permission. 85Copyright 2021, Nature Publishing Group.(E) The evolution process of voids at the Li/solid-state electrolyte (SSE) interface under different diffusion conditions.Reproduced with permission. 86Copyright 2021, Wiley-VCH.(F) Schematic of planar dendrite crack propagation and pores connected to the Li/SSE electrolyte interface by microcracks and filled by lithium deposits.Reproduced with permission. 68Copyright 2023, Springer Nature.(G) Schematic illustration and SEM images of the Li/SSE interface contact under low stacking pressure (5 MPa), moderate stacking pressure (50 MPa), and high stacking pressure (100 MPa) before cycling. 87Copyright 2023, Wiley-VCH.
the electronic band structure of grain boundaries.Due to almost half of the grain boundaries having lower band gaps than the bulk phase, Li + ions are prematurely reduced by the electronic charge of the boundaries during cycling, leading to the formation of localized lithium dendritic filaments, as shown in Figure 2D.The eventual interconnection of these filaments can result in short-circuiting of the battery.Han et al. compared the electronic conductivities of three common electrolytes 54 : LLZO (5.5 × 10 −8 S cm −1 ), Li 3 PS 4 (2.2 × 10 −9 S cm −1 ), and LiPON (10 −12 S cm −1 ).By utilizing neutron depth profiling, it was discovered that the ease of lithium dendrite growth is highly correlated with the electronic conductivity.The higher the electronic conductivity, the easier it is for lithium dendrites to penetrate through the solid-state electrolyte.Therefore, the impact of solid-state electrolytes on lithium dendrite growth is still a worthwhile area of investigation.
During the cycling process of batteries, the interface on the negative electrode side often undergoes continuous changes, which is particularly pronounced in solid-state lithium metal battery systems that employ inorganic solid electrolytes.Yan et al. 86 elucidates the correlation between the interfacial state during lithium metal delithiation and the creep stress experienced by lithium.In the ideal Li/SSE interface, stability can be achieved when J diffusion (the flux of lithium metal diffusion that induces vacancy migration from the interface to the interior of lithium metal) is greater than J migration (the flux of electrons migrating out of lithium metal and Li + migrating into the solidstate electrolyte, which represents the flux of lithium ions moving away from the interface).However, most of the actual Li/SSE interfaces have some defects in advance.In such cases, the diffusion of vacancies along the defect surface is much faster than the migration into the bulk phase (J migration > J diffusion ), interfacial voids form continuously while the creep of metallic lithium (J creep ) induced by external pressure emerges as the predominant factor driving the evolution of interface voids.When J creep > J diffusion , the interface is electromechanically stable and the original defects can be eliminated by creep behavior.And when J creep < J diffusion , the pores at the interface expand further, which leads to the formation of dendrites in the interstices during subsequent deposition (Figure 2E).The creep ability of lithium is enhanced by applied pressure.While increasing external pressure can promote lithium creep behavior, higher pressure is not always advantageous.Ning et al. 68 employing in situ CT technology, investigated dendritic growth behavior under different pressures, and delineated the dendrite penetration process into two stages: dendrite initiation and dendrite propagation (Figure 2F).The initiation of dendrites is challenging to avoid, but the propagation process is exceptionally sensitive to external pressure.Through the experiment and simulation comparison, it is found that the growth and propagation process of dendrites can be very slow when cycling under very low external pressure, even if the lithium dendrite has been triggered, it can be inhibited during the propagation process.Sang et al. 87 also investigated the impact of different pressures on the electrochemical performance of the electrolyte (Figure 2G).They found that at moderate pressures (30-50 MPa), the "plane-plane" contact could significantly reduce the current density over the actual unit area and mitigate the tip effect, thus achieving the highest critical current density (CCD), while at too high or too low pressures, the "point-plane" contact configuration leads to uneven lithium deposition and premature short-circuit.The study evaluated the optimal pressure range for suppressing lithium dendrite growth.
Considering the reported types and characteristics of modifying layers, we categorize interface modifications into three primary groups: interfacial modification of LLZO, modification of Li Anode, and three-dimensional (3D) structures design.Table 1 provides a comparative analysis of various modification strategies and corresponding electrochemical performance.

Lithiophilic layer design
When lithium foil is directly affixed to the solid electrolyte surface, incomplete solid-solid contact results in interface impedance that can exceed thousands of ohms, posing difficulties in battery operation.The construction of a lithium-compatible wetting layer to enhance wetting between the lithium negative electrode and the electrolyte interface represents a crucial approach for modifying the negative electrode in solid-state lithium metal batteries.In the early stage, researchers mainly used metal monolayers as the modifying layer.By incorporating a nanoscale buffer layer between the solid electrolyte and the lithium anode, not only can the interface transition from lithiophobic to lithiophilic, but it can also mitigate the side reactions between lithium and the solid-state electrolyte (such as LAGP, LLTO, etc.).As shown in Figure 3A, Li et al. 88 took advantage of the volatilization of I 2 , and deposited a thin layer of I 2 on the surface of LLZO to avoid the direct contact between LLZO and Li at high temperature and the reduction of Zr 4+ caused by the high-temperature melting process, and at the same time achieved good wettability to lithium.Similarly, metals such as Au, 89 Ag, 90 Sb, 91 In, 92 Ge, 93 and Sn 94 have also been used to construct lipophilic interfaces.The alloying reaction between lithium and alloys can be categorized into reconstructive and solid-solution reactions based on the resulting solid structure. 95While specific operational mechanisms may differ, these reactions facilitate the transfer of lithium ions at the interface.The process of alloying reaction between these materials and lithium, promotes the complete spreading of metallic lithium on the electrolyte surface, thereby achieving a tightly contacted negative electrode interface and leading to a substantial reduction in interface impedance.For example, Fu's team 96 deposited a thin amorphous Al layer on the surface of garnet electrolyte using plasma-enhanced chemical vapor deposition.Due to the high reactivity between Al and Li, a lithium-silicon alloyed nano-layer was formed at the interface, enabling rapid wetting by molten Li.Muller et al. 92 deposited 1 µm of In on the surface of LLZO electrolyte using magnetron sputtering (Figure 3B).The high reactivity between In and Li at the interface resulted in the formation of a lithium-indium alloy phase, facilitating rapid wetting by molten Li.Nevertheless, this approach typically relies on equipment such as chemical vapor deposition or magnetron sputtering, which incurs additional implementation costs.Furthermore, the alloyed negative electrode or interface resulting from lithium alloying undergoes significant volumetric changes during continuous alloying/dealloying processes, consequently resulting in an increase in interfacial impedance. 97n addition to single-element layers, in order to further enhance the multifunctionality of interfaces, some metal oxides or multi-element compounds such as Al 2 O 3 , 98 ZnO, 99 fluorinated graphite (CF x ), 100 titanium oxide nanoclusters, 101 In 2 O 3 , and SnO 2 , 102,103 Co 3 O 4 , 104 Li 2 O, 105 and AgSn 0.6 Bi 0.4 O x 106 have also been used as interfacemodifying layer materials.For example, Jiang et al. 105 proposed a dual-layer strategy, where the inner layer is composed of dense ALD-Li 2 O with good electronic insulation and high ionic conductivity, and the outer layer is made up of the chemical reaction byproduct Li 2 O.By combining these two layers effectively, the generation of dynamic voids are moved away from the Li/LLZTO surface, ensuring a smooth transfer of Li + at the interface.Bai et al. 101 brazed LLZTO with Li anode by using highly active titano-oxide clusters (TOC S ) as brazing filler (Figure 3C).
The brazed layer results in a significant decrease in the interface impedance of 8.32 Ω cm 2 .In addition, TOC S has the characteristics of isotropic amorphous ion-electron hybridization, which can significantly promote the Li + transport, but also regulate the electric field distribution and inhibit the growth of Li dendrites.It is worth noting that Gao et al. 102 further improved the performance by using ultrasonic spray coating to apply an ITO layer on the surface of LLZGO.The modified LLZGO achieved a high critical current density (CCD) of 12.05 mA cm −2 at room temperature and stable cycling for 2000 h at a current density of 2.0 mA cm −2 .Additionally, taking into account the advantages of alloying in promoting the transport of lithium ions at the interface, Cui et al. 106 successfully prepared different metal alloy modifying layers on the surface of LLZTO using rapid quenching techniques (Figure 3D).Through extensive comparisons, they found that the AgSn 0.6 Bi 0.4 O x modifying layer exhibited negligible interface impedance, with a high limit current density of 20.0 mA cm −2 at 60 • C, providing a new research direction for the preparation of high-entropy alloy interfaces.It is noteworthy that while the multi-element modification layer further enhances the electrochemical performance of oxide solid-state electrolytes in terms of CCD and sustained long-term cycling stability, the specific interaction mechanisms between individual elements still remain difficult to elucidate, especially for the interfaces with high-entropy compositions.Further fundamental and comprehensive research is still needed in this regard.
Many studies have shown that LiF, LiCl, and Li 3 N can significantly enhance the ability of electrolytes to suppress lithium dendrite formation in solid electrolytes or at the interface layer. 107Among them, LiF has a high interfacial energy, which can guide the planar deposition of lithium, and effectively inhibit the formation and growth of lithium dendrites. 108On the other hand, Li 3 N has a high room temperature lithium-ion conductivity, and can facilitate the rapid and uniform deposition of lithium ions at the interface. 109Therefore, introducing these functional additives into the interface has become one of the current research focuses.Ruan et al. 110 were the first to use phosphoric acid to treat the passivation layer on the surface of LLZO, forming Li 3 PO 4 .Li 3 PO 4 not only has good affinity for lithium, but also reacts with molten lithium to form Li 3 P, which is an effective inhibitor of lithium dendrites (Figure 3E).
Ideally, the interface should possess high ion conductivity while being electronically insulating.However, in practical situations, pure ion-conducting intermediate layers tend to be lithium-depleted.2][113] For instance, by constructing a lithium polystyrene sulfonate (LiPLSS) layer that exhibits ion conductivity but electron insulation, a successful Li-ion migration bridge was established at the LLZTO/PLSS interface. 114The excellent electron insulation of this layer effectively inhibits the growth of lithium dendrites.As a result, the resulting battery achieved a CCD of 1.1 mA cm −2 and cycled for over 400 h at a current density of 0.5 mA cm −2 .Nevertheless, an increased overpotential was observed, indicating the presence of higher interface impedance.Therefore, adopting an ion-electron mixed conducting layer (MCL) that reduces interface resistance, promotes uniform electric field distribution, and harnesses the advantages of functional products has emerged as a promising research strategy.
Currently, there are numerous studies focused on fluoride and chloride-based materials, such as AgF, 115 ZnF 2 , 112 MgF 2 , 116 AlF 3 , 117 SnF 2 , 118 BiOCl, 119 BiCl 3 , 120 and InCl. 121nder certain conditions, these materials can react with metallic lithium to form alloy phases and multifunctional products such as LiF, LiCl, or Li 3 N.This strategy serves to inhibit the growth of lithium dendrites while utilizing the alloy phases to achieve a more uniform distribution of electric fields at the interface.Lee et al. 118 introduced SnF 2 particles onto the electrolyte surface through wet coating, which resulted in the in situ formation of a multifunctional interface composed of LiF and Li-Sn alloy, promoting lithium wettability and suppressing lithium dendrite formation.Building on this, Feng et al. 122 introduced AlN at the interface and found that AlN underwent a transformation reaction with metallic lithium at high temperatures (Figure 3F), resulting in the formation of a mixed ionelectron layer composed of Li 3 N on the inner side and Li-Al alloy phase on the outer side at the interface.Physical field simulations were used to analyze the influence of the Li-Al alloy phase on the electric field, confirming the effectiveness of MCL's strategy.

In situ contaminants conversion
The chemical properties of the solid electrolyte surface play a crucial role in the anode.Zhu et al. 123 have calculated that almost no inorganic solid electrolyte is thermodynamically stable to lithium metal, and even some of their decomposition products will be further reduced and decomposed by lithium until the formation of elemental and lithium oxides.From the perspective of decomposition energy, LLZO is the most chemically stable for lithium metal among the reported oxide SSEs.Unfortunately, while LLZO exhibits thermodynamic stability to lithium metal, it is highly sensitive to air.Particularly in humid environments, the electrolyte surface tends to develop a passive layer rich in lithium carbonate due to the notorious proton exchange reaction.The process of proton exchange remains controversial, with computational analysis suggesting two plausible pathways for lithium carbonate formation. 40,124The first involves the preferential reaction of LLZO with moisture in the air, resulting in the formation of lithium hydroxide, which then combines with CO 2 to yield lithium carbonate.The second pathway involves direct interaction between LLZO surface and CO 2 .Li et al. 124 through firstprinciples calculations of the adsorption energy, found that the second reaction pathway possesses slightly lower adsorption energy than the first, indicating a higher likelihood of occurrence.Regardless of the specific process, Li 2 CO 3 emerges as the ultimate reaction product.Extensive research has demonstrated that the garnet-type electrolyte LLZO exhibits intrinsic lithium affinity.Sharafi et al., 125 using density functional theory (DFT) calculations, demonstrated the theoretical adhesion energies between Li and LLZO with contact angles ranging from 60 • , indicating favorable wetting behavior.However, the contact angle between Li and lithium carbonate reaches as high as 140 • .By employing methods such as polishing LLZO within a glovebox environment, Zheng et al. 126 managed to reduce the impedance to 6.25 Ω cm 2 .Therefore, Li 2 CO 3 is an important reason for the poor wettability between garnet solid electrolyte and lithium metal. 127part from the approach of eliminating Li 2 CO 3 , there has been an increasing emphasis on conversion strategies in recent years, aiming to converting the lithiophobic layer into a lithiophilic layer offers a way to turn Li 2 CO 3 into something valuable.The earliest proposed method for converting the passivation layer on the surface involved carbon thermal reduction.Taking advantage of carbon's strong reducing properties at high temperatures, Li et al. 128 heated LLZO containing Li 2 CO 3 together with carbon to 700 • C. By utilizing the principles of carbon thermal reduction, they successfully removed the surface lithium carbonate, resulting in a reduced interfacial impedance of 28 Ω cm 2 .Similarly, Feng et al. 129 achieved carbon thermal reduction of Li 2 CO 3 (Li 2 CO 3 + C → Li 2 O + 2CO) and high-temperature decomposition of LiOH through thermal decomposition vapor deposition.This method not only removed the Li 2 CO 3 but also provided an additional carbon layer to enhance wetting with lithium (Figure 4A).Furthermore, due to the amorphous structure of low-graphitized carbon, more Li + transport pathways were made available, enabling rapid lithiation at the electrolyte interface.The interfacial impedance was reduced to only 9 Ω cm 2 , and the ultimate current density reached 1.2 mA cm −2 .Apart from carbon, Li is prone to react with solid electrolytes due to its lower electrochemical potential.Even the relatively stable LLZO solid electrolyte exhibits a gray-black surface during the lithium melting process at 300 • C-350 • C. 44 Building upon this, Yang et al. 130 achieved the simultaneous formation of a middle layer rich in Li 2 O and oxygen vacancies at the interface by subjecting lithium and LLZO to prolonged heating (Figure 4B).Thanks to this reconstructed interface, stable cycling of over 1200 cycles was achieved at a 1 C rate when paired with a lithium iron phosphate cathode.
Compared to the energetically demanding and operationally intricate thermal reduction method, liquid-phase reduction appears decidedly simpler and more convenient.When LLZO is immersed in water, it undergoes a process of proton exchange. 131Due to the more extensive liquidsolid interface contact area, an H-LLZO structure forms on the surface of LLZO, which hampers ion conductivity.It is worth noting that this process is reversible. 132Inspired by this, Cai et al. 133 employed a metal aqueous solution to transform into metal hydroxides on the electrolyte surface, subsequently converting them into metallic elements or metal oxide layers through heat treatment as shown in Figure 4C.For instance, utilizing a Pb(CH 3 COO) 2 aqueous solution, an in situ lithium-affinitive layer of PbO nanoparticles was constructed on the LLZO surface, achieving an interface impedance of 10 Ω cm 2 and a maximum CCD of 1.1 mA cm −2 .As shown in Figure 4D Recently, solid-state conversion has also been proven to be an effective strategy for interface modification.However, unlike thermal reduction, carbon-thermal/lithiumthermal treatment, although capable of effectively removing the surface passivation layer of Li 2 CO 3 , inevitably leads to the formation of a lithium-deficient layer due to the consumption of lithium within LLZO during the formation of Li 2 CO 3 .Prolonged high-temperature conversion results in the presence of certain lithium-deficiency defects at the interface. 136,137Under high current density conditions, the lithium-deficient layer is prone to be penetrated by lithium dendrites.To address this issue, Zhang et al. 138 proposed a dual conversion strategy that involves a double displacement reaction between Li 2 CO 3 and SiO 2 .This chemical transformation of surface Li 2 CO 3 into lithium-affinitive Li x SiO y prevents the formation of lithium-deficient defects, which effectively inhibit the regeneration of Li 2 CO 3 on the surface when exposed to moisture (Figure 4E).At the proper temperature, molten NH 4 H 2 PO 4 can convert surface contaminants into ionic conductor Li 3 PO 4 (Figure 4F). 139Different conversion agents can introduce various interface structures during the conversion process.Liu et al. 140 proposed a simple strategy for gas molecule release and cleaning.The NaH 2 PO 2 salt undergoes a mild decomposition reaction (2NaH 2 PO 2 → Na 2 HPO 4 + PH 3 ) upon gentle heating, releasing PH 3 gas molecules.PH 3 gas exhibits strong reducing properties, facilitating the thorough reduction of surrounding Li 2 CO 3 .The in situ formation of Li 3 PO 4 serves as a lithiophilic layer on the surface of LLZTO, while the remaining Na 2 HPO 4 on LLZTO possesses preferential hydrating ability, aiding in alleviating the re-passivation effect of garnet due to moisture.
F I G U R E 4 (A) TVD carbon thermal reduction of LLZO surface Li 2 CO 3 .Reproduced with permission. 129Copyright 2020, Wiley-VCH.(B) Reconstruction of Li/LLZO interface by lithium thermal reduction.Reproduced with permission. 130Copyright 2022, Elsevier.(C) The mechanism underlying the role of stable conversion-type Li/LLZO interface in metal salt solutions.Reproduced with permission. 133Copyright 2021, Elsevier.(D) The mechanism of NH 4 F treatment on the surface of LLZO.Reproduced with permission. 134Copyright 2020, Wiley-VCH.(E) The process diagram of LLZO preparation through reaction with SiO 2 powder.Reproduced with permission. 138Copyright 2022, Elsevier.(F) The schematic diagram illustrating the process of molten phosphate driving the conversion of Li 2 CO 3 .Reproduced with permission. 139opyright 2022, Wiley-VCH.

Conformal interface design
Introducing a nanoscale metal buffer layer between the solid-state electrolyte and the lithium metal anode can transform the interface from lipophobic to lipophilic, thereby improving the wettability of the anode interface.For instance, lithium silver nanoparticles can fill smaller gaps at the interface, achieving a close contact between the lithium anode and the garnet solid electrolyte on a microscopic scale. 141This alleviates the volume expansion resulting from lithium-silver alloying.However, the mitigation of volumetric effects is temporary, and prolonged cycling inevitably leads to the formation of cracks and voids at the interface.
Employing appropriate amount of liquid additive allows for the construction of a closely contacting conformal lithium-ion exchange interface, which cannot only improve the contact condition of the negative electrode interface, but also alleviate the volume effect caused by interface circulation.Lu et al. 142 improved the interface between the lithium anode and the electrolyte by using liquid electrolyte (1 M LiTFSI in TEGDME).The assembled semi-solid lithium symmetric cell could withstand a high current density of 1.5 mA cm −2 .However, some commonly used lithium salts in electrolytes react with LLZO surface components, 143 such as LiBOB, LiPF 6 salts react with LLZO to form LiF, LaF 3 , and ZrF 4 , resulting in increased interface impedance.Additionally, there is also an issue of Li + /H + exchange between the liquid electrolyte and LLZO.Even though the application of the super base n-BuLi can slow down proton exchange, 144 suppress the decomposition of the interface layer between the electrolyte and LLZTO, and inhibit the formation of Li 2 CO 3 , ensuring the lithium-ion conductivity and interface stability, the flammable and volatile nature of liquid electrolytes increases safety risk.Huang et al. 145 introduced self-assembling acidic molecules on LLZO to deal with these proton ion exchange problems.The acidic molecules preferentially self-assemble on the surface of LLZO to form a stable and dense intermediate layer, preventing further Li + /H + exchange between LLZO and the protons in the electrolyte.An unprecedented impedance near 0 Ω cm 2 is achieved.
Polymers, compared to electrolytes, offer better safety, chemical stability, and flexibility, rendering them more promising for enhancing the anode interface in solid-state batteries. 146The use of polymers as a negative electrode interface buffer layer can significantly improve the contact The mechanism diagram of Li-PDMS composite anode with LLZO.Reproduced with permission. 147Copyright 2020, American Chemical Society.(B) Schematic illustrations of Li dendrite growth at different LLZTO/Li interface, LLZTO-Au/Li interface, and LLZTO-EBS/Li interface.Reproduced with permission. 148Copyright 2019, Springer Nature.(C) Diagram of the reaction of anhydrous PPA to convert Li 2 CO 3 on the surface of the LLZTO to Li-PPA.Reproduced with permission. 149Copyright 2022, Wiley-VCH.(D) Schematic comparison of interface chemistry and bulk chemistry of LLZTO and PPA-LLZTO.Reproduced with permission. 150Copyright 2023, Elsevier.(E) Schematic of scalable elastic and Li + -conducting interlayer for garnet electrolyte. 151Copyright 2023, Wiley-VCH.(F) Diagram of in situ solidified gel polymer electrolytes (GPEs) as interfacial modification layer for garnet electrolyte. 152Copyright 2021, Elsevier.performance of the interface.Zhang et al. 147 used superelastic polydimethylsiloxane (PDMS) as the substrate of lithium sheets, and constructed the LLZO/Li interface with stress self-adaptability to stabilize the volume change of the lithium anode during the cycle (Figure 5A).Interestingly, some polymers can react with Li or Li 2 CO 3 , in situ forming a stable interface layer with high elastic modulus.Huo et al. 148 reported a flexible electronic barrier layer (EBS) composed of polyacrylic acid (PAA).PAA polymer reacts with molten Li at 250 • C (Figure 5B), forming lithiated PAA (Li-PAA).The in situ formed EBS not only enhances lithium affinity but also mitigates the interface stress caused by lithium volume changes.The assembled battery can cycle stably for over 400 h at a current density of 1.0 mA cm −2 .Guo et al. 149 utilized anhydrous polyphosphoric acid (PPA) to convert Li 2 CO 3 impurities on the surface of garnet electrolyte (LLZTO) into an approximately 30 nm ultra-thin layer of lithiumpolyphosphate (Li-PPA) (Figure 5C).The formed interface layer also possesses characteristics such as "lithiophilicity" and "electron barrier."Experimental results indicate that the Li-PPA-modified interface exhibits lower impedance, approximately 4 Ω cm 2 .Similarly, Xiong et al. 150 immersed LLZTO in a dilute solution of PPA (0.25% PAA in DMSO), and then vacuum heated to remove the DMSO solvent, during which PPA penetrated into the interior of LLZTO and formed an electron-insulating but ion-conducting interface layer at the grain boundary.Thus, Li is prevented from nucleating and growing inside LLZTO (as shown in Figure 5D), and the purpose of interfacial grain boundary dual modification is realized.Zheng et al. 151 introduced the photocrosslinkable polymer in a scalable elastic skeleton, which promotes the migration and diffusion of Li + (Figure 5E).Moreover, adding perfluoropolyether in the interlayer benefits to regulating the formation of LiF-rich interface, sufficiently suppressing the growth of lithium dendrites.Benefitting from the elasticity, high Li + conductivity and the lithium dendrites suppression capability, the interlayer can significantly improve the interfacial performance of the solid electrolyte/lithium interface, thus leading to the greatly enhanced electrochemical performance of solid-state lithium metal batteries.Bi et al. 152 used the in situ solidified gel polymer electrolytes (GPEs) as interfacial modification layer, which are constructed as not only the interlayer to buffer electrode/garnet interfaces, but also the adhesive to join garnet blocks together, realizing the scale expansion and good flexibility of solid garnet batteries (Figure 5F).On anode sides, the Li/garnet interfacial resistance notably drops to 88 Ω cm 2 , accompanied by the symmetric cell stably cycling over 400 h with excellent solid electrolyte interphases.

Modification of Li anode
Metallic Li, due to its ultrahigh theoretical capacity (3860 mAh g −1 ) and low density (0.534 g cm −3 ), is considered the ultimate anode material to replace graphite in high-energy rechargeable batteries.However, its low surface energy and high self-diffusion barrier result in initial nucleation as island-like deposits on the substrate's surface, eventually forming dendritic structures. 153,154The preparation of Li composite anode as alternatives to lithium metal anode can address the issue of lithium dendrite growth.Modifying the lithium can influence the kinetics of lithium deposition, achieving improved interfacial wetting behavior without directly modifying the solid electrolyte interface.Lithium metal possesses a lower chemical potential, enabling the preparation of a range of composite alloy anodes with other metals.Such as Li-Sn, 155 Li-Ag, 156 Li-Na, 157 Li-Mg, 158 and Li-Cu. 159These composite anodes enhance the wetting behavior of the lithium metal anode toward LLZO electrolyte, promoting effective contact between the lithium anode and the electrolyte, thereby significantly reducing the Li/LLZO interface impedance.Besides, lithium-rich alloys can also address the drawback of slow lithium self-diffusion by facilitating the interfacial transport of lithium ions.As shown in Figure 6A, the molten alloy of Na and Li, known as Na-Li eutectic alloy, exhibits good wetting behavior toward LLZO electrolyte.In contrast to pure molten lithium or sodium, the addition of a certain amount of sodium in Li-Na composite anode leads to excellent wetting properties toward LLZO.The presence of trace amounts of sodium facilitates the transfer of Li 2 CO 3 from the interface to the surface of the anode, allowing the newly formed microstructure to extend into the freshly deposited lithium, thereby stabilizing its performance during repeated lithium deposition/stripping processes.It is worth noting that the alloying process of Li with metals such as Sn and Zn involves significant crystal structure changes and higher reaction activation energies, which can lead to higher polarization during later cycling processes.On the other hand, alloying processes with metals like Ag, which form solid solution phases, exhibit minimal structural changes during lithiation-de-lithiation processes.Instead of lithium atoms, the lithium-rich alloy phase Li x Ag primarily deposits on the metal surface, thereby avoiding the formation of lithium dendrites. 156n addition to commonly used alloy anodes, certain materials that can spontaneously react with lithium, such as graphite, 160 BN, 161 Si 3 N 4 , 162 AlF 3 , 163 and Ag-C 164,165 have also been employed for the preparation of composite anodes.Du et al. 162 achieved a tight Li/LLZO interface by using a trace amount of Si 3 N 4 (1 wt%) to modify molten lithium.In the formed Li-Si-N melt, Li 3 N and LiSi 2 N 3 reduce the formation energy of the Li/LLZO interface, leading to a transition from point-to-point contact to continuous face-to-face contact at the Li/LLZO interface.Wang et al. 163 introduced AlF 3 into lithium metal, forming a Li-Al alloy phase with a lithium-repellent inner side and lithium-affinitive outer side at the Li/SSE interface.The lithium-repellent inner layer of lithium fluoride acts as an electronic insulating phase, inhibiting electron transport at the interface.The outer alloy framework provides abundant nucleation sites, promoting uniform deposition of ions.The results demonstrated that lithium symmetric cells with this composite anode exhibited an increase in the limit of the current density (CCD) to 3.0 mA cm −2 (Figure 6B).The strategy of using a gradient structure is also applicable to solid-state sodium batteries. 166The composite anodes formed by these materials often have higher viscosities.As shown in Figure 6C, when tilted at a certain angle, conventional molten lithium quickly slides off the surface of the solid-state electrolyte, but the one doped with cubic BN adheres to the electrolyte surface, which indicates that these materials reduce the surface tension of molten lithium, and lower the interfacial formation energy between Li and the solid-state electrolyte, thereby forming a tight interface. 161 I G U R E 6 (A) The wettability of Li, Li-Na alloy, and Na to LLZO.Cross-section of Li-Na anode and LLZO.Reproduced with permission. 157Copyright 2020, Wiley-VCH.(B) Schematic diagram of the gradient distribution of Li-Al and LiF on LLZTO surface.Reproduced with permission. 163Copyright 2022, The Royal Society of Chemistry.(C) Difference of adhesion of Li and Li-BN to the surface of solid electrolyte.Reproduced with permission. 161Copyright 2019, American Chemical Society.(D) Mechanism diagram of Ag-C composite negative electrode.Reproduced with permission. 164Copyright 2023, Springer Nature.(E) The schematic diagram illustrating the manufacturing process of the semi-liquid composite lithium anode (SLMA).Reproduced with permission. 169Copyright 2019, Elsevier.
Recently, the Samsung Advanced Institute of Technology reported a high-performance all-solid-state lithium metal battery with a composite Ag-C anode and sulfide electrolyte (Figure 6D). Lee et al. 165 effectively regulated the deposition of lithium by using a thin layer of Ag-C and achieved effective bonding between the modified layer and Li 6 PS 5 Cl through isostatic pressing.Solid-state cells with a matching areal capacity higher than 6.8 mAh cm −2 for the ternary cathode demonstrated stable cycling for over 1000 cycles.Building upon this, an Ag-C composite lithium anode was introduced into an LLZO system with a surfacesputtered Ag layer. 164The inner Ag layer ensured effective contact between the Ag-C layer and LLZO, while the outer Ag-C layer regulated the deposition of lithium toward the anode side, preventing direct contact between lithium and the electrolyte.The material has garnered significant attention since its initial report.Spencer-Jolly et al. 167 utilized a spray deposition method to fabricate Ag-graphite anode materials and employed in situ XRD to track the evolu-tion of Ag and graphite.They observed that the working mode varied at different charging rates.Under low charging rates, lithium infiltrated the graphite, forming LiC x , which gradually reacted with Ag to form LiAg. Lithium metal deposition occurred at the current collector.On the other hand, at high charging rates, the electrochemical rate of lithium infiltration into graphite exceeded the rate of chemical reaction until the rate of lithium embedding into the graphite layer is insufficient, resulting in lithium deposition at the interface and the growth of dendrites.Similar conclusions were obtained by Wu et al. 168 In addition, Li et al. 169 has devised a method involving the incorporation of lithium foil into polyethylene glycol dimethyl ether (Figure 6E).By leveraging the interaction between hydroxyl groups and lithium, they have successfully achieved uniform dispersion of lithium particles within the polymer matrix, resulting in the creation of a liquid-state composite anode for lithium.Notably, the in situ generated lithium particles are evenly distributed throughout the blended conductive polymer framework.The symmetrical cells assembled by a fluid semi-liquid metal anode (SLMA) and Garnet ceramic electrolyte exhibit extremely stable Li deposition behavior.Taking inspiration from natural phenomena like quicksand, Zhang et al. 170 introduced lithium powder, carbon nanotubes, and polyepoxide into the electrolyte, carefully adjusting their ratios to create a unique semi-solid composite anode with a quicksand-like consistency.This anode exhibits shear thinning behavior, effectively mitigating the stress associated with dendrite growth and accommodating volume changes that occur during the cycling of the lithium anode.Impressively, it has demonstrated stable cycling for over 400 h even at high current density of 20 mA cm −2 .

3D structure design
In the context of solid-state lithium metal batteries, the volume variation of the lithium anode holds significant importance regarding its impact on the electrodeelectrolyte interface.The elevated hardness and Young's modulus of commonly used solid-state electrolytes were initially expected to inhibit dendritic growth of lithium.However, the observed outcomes have not met the anticipated results thus far.On one hand, the stress generated during the growth of lithium dendrites can reach as high as 130 MPa, which is sufficient to penetrate the majority of reported solid-state electrolytes, 171 on the other hand, inherent defects such as voids and pores within the solid-state electrolyte can act as reservoirs for lithium, exerting substantial pressure and leading to the formation of "hydraulic fracture" when compressed beyond a certain threshold. 172urthermore, the excessively rigid bulk structure of the solid-state electrolyte struggles to accommodate the volume changes of the lithium anode.Krauskopf et al. 173 investigated the pressure-induced electrode kinetics and discovered that under high external pressure of 400 MPa, the interfacial resistance of Li/LLZO reduced significantly, nearly converging to near-zero values of about 0 Ω cm 2 .However, due to vacancy accumulation and the formation of voids in the vicinity of the interface, the stability of the interface during cycling becomes challenging and eventually leads to its deterioration.Therefore, interface volume effects further contribute to the deterioration of interface contact, accelerating the transition from an ideal interface to a non-ideal interface, as shown in Figure 7A. 174Employing structural design for the interface or electrolyte itself is an effective means to suppress volume effects.For example, the use of a 3D porous structure or a 3D porous skeletal framework can increase the specific surface area, enhanc-ing the contact between the electrolyte and the electrode and improving their interfacial performance.
Acid treatment is a simple and effective approach for interface structural design.For instance, Ruan et al. 175 and Cai et al. 176 achieved surface modification of the LLZO electrolyte by co-treating it with two different acid solutions (Figure 7B).This resulted in the formation of a modified layer with a micro-porous structure.The capillary force enhanced the physical adsorption of metallic lithium, allowing the molten lithium to spread completely over the electrolyte surface and achieve a tightly continuous contact at the LLZO/Li interface.In addition to micro-pores, acid treatment can also etch out 3D frameworks on the surface of solid-state electrolytes.For example, Huo et al. 177 immersed ceramic sheets in 1 M hydrochloric acid for 1 h, successfully creating a 20 µm 3D framework on the surface.Subsequently, a thin layer of zinc oxide was deposited onto the framework surface using atomic layer deposition (ALD) to further enhance the wettability of the interface.Similarly, Chen et al. 178 achieved similar results by etching the LLZO surface with highly corrosive hydrofluoric acid.The exposed 3D framework contained abundant products such as lanthanum fluoride and lithium fluoride.The spontaneous reaction between lanthanum fluoride and metallic lithium facilitated the wettability behavior of lithium within the framework.With this modification, the limiting current density reached 2.7 mA cm −2 .However, prolonged acid treatment can lead to irreversible damage to the structure of solid-state electrolytes, resulting in capacity loss.
Considering the advantages of supporting framework, buffering volume changes, and mechanical stress in the design of the current collector structure for the lithium anode, introducing a current collector at the solid-state electrolyte interface can also be a viable option.Duan et al. 179 used thermally treated foam copper as a lithiumaffinitive pattern to achieve close contact with LLZO and guide the uniform deposition of lithium.The surface of the thermally treated foam copper was covered with a layer of copper oxide that exhibits good wettability toward lithium.It can be tailored into different shapes to meet various requirements.Additionally, Wan et al. 180 utilized the affinity of metallic zinc to metallic lithium, and deposited a 3D zinc skeleton on the surface of LLZTO through magnetron sputtering, as shown in Figure 7C, aiming to address issues related to wetting behavior and volume effects.As shown in Figure 7D, Xu et al. 181 have reported a novel 3D-micro-patterned solid-state electrolyte (3D-SSE) using laser ablation.This 3D-SSE can form a morphologically stable interface with metallic lithium even at relatively high current densities and limited compaction pressure.Compared to traditional planar SSE, from an electrochemical perspective, this 3D-SSE increases the effective contact F I G U R E 7 (A) Schematic diagram of volume change of lithium metal negative electrode during lithium stripping/deposition. Reproduced with permission. 174Copyright 2019, Wiley-VCH.(B) Schematic diagram of improving the wettability of lithium by using the capillary action of nanopores.Reproduced with permission. 176Copyright 2022, Elsevier.(C) Volume effect diagram of lithium anode improved by three-dimensional (3D) zinc skeleton.Reproduced with permission. 180Copyright 2021, Elsevier.(D) Schematic and SEM of laser-etched 3D micropatterned solid-state electrolyte.Reproduced with permission. 181Copyright 2021, Wiley-VCH.(E) Schematic of solid electrolyte with porous-dense-porous sandwich structure and its action mechanism.Reproduced with permission. 184Copyright 2023, Springer Nature.(F) Schematic diagram of battery cycle based on porous LLZO solid electrolyte membrane.Reproduced with permission. 185Copyright 2022, American Chemical Society.(G) Schematic diagram of a multiscale arrangement of mesoporous garnet membranes combined with polymer electrolyte.Reproduced with permission. 186Copyright 2019, American Chemical Society.area with lithium, reducing local current density and slowing down lithium detachment at the interface.From a mechanical perspective, it introduces stress amplification effects to promote lithium creep near the interface.
In addition to interface structural design, the overall structural design of the solid-state electrolyte can also effectively improve the performance of solid-state lithium metal batteries.Fu et al. 182 pioneered the use of a bilayer LLZO framework fabricated by casting process, consisting of a dense layer and a porous layer.The dense layer acts as a separator while maintaining good mechanical stability.The porous layer acts as a cathode support layer, loading the cathode material while providing a continuous pathway for ion transport.Building on this concept, Yang et al. 183 designed a porous-dense-porous "sandwich" structure for solid-state electrolytes, where the porous structure on the anode side serves as an ion-conducting framework for the lithium anode, enabling high-capacity cathodes and close contact with the lithium metal anode in a full-solid-state lithium-sulfur battery.The porous structure of the electrolyte requires a coating that can simultaneously conduct ions and electrons to achieve continuous lithium ion and electron transport.However, unstable coatings can lead to detrimental cracking during long-term cycling at high current densities, hindering lithium-ion transport.To address this issue, Alexander et al. 184 reported a garnet-type solid-state electrolyte with simultaneous lithium ion and electron conductivity, with electron conductivity ranging 1.16-10.2µS cm −1 and ion conductivity ranging 10 −4 to 10 −5 S cm −1 , as depicted in Figure 7E.This material was used as a porous framework on both sides.The porous structure helps alleviate stress on the solid-state electrolyte during cycling by evenly distributing the potential across the entire surface, achieving stable and continuous interface transport.This design achieved the highest reported current density (100 mA cm −2 ) and lithium metal plating capacity (60 mA cm −2 ) for garnettype solid-state electrolytes.Kravchyk et al. 185 assembled the NCM/LLZO lithium-free anode battery with a porous LLZO ceramic membrane and found that the porous LLZO scaffold could not only hold a large amount of lithium during plating, but also avoid the problem of battery volume change, as shown in Figure 7F.It is worth noting that although the porous ceramic membrane will produce additional volume energy density loss, the porous ceramic membrane volume density of 30-50 µm can still reach 930-1080 Wh L −1 , which still has a great advantage over traditional lithium-ion batteries (700 Wh L −1 ).
The structural design of the electrolyte mentioned above is also applicable to inorganic-polymer systems.Inorganic solid-state electrolytes are relatively rigid, and contact issues are difficult to avoid.Therefore, they can be used as supporting structures.Dai et al. 186 developed a highly conductive garnet framework with aligned mesostructure by utilizing basswood as template.By incorporating ionconductive polyethylene oxide (PEO) into the mesoporous, a flexible solid-state composite electrolyte was created with high ion conductivity (1.8 × 10 −4 S cm −1 ), as shown in Figure 7G.The above findings demonstrate the effectiveness of structural design of the negative electrode or solid-state electrolyte in mitigating the volume effect in lithium metal.However, the impact of different designs on overall capacity remains unknown at present.

INTERFACIAL ENGINEERING FOR CATHODE SIDE
At cathode side, we will discuss the interfacial engineering separately for all-solid-state batteries and quasi/half-solidstate batteries according to whether there are gels/liquids added at the interface.Specifically, all-solid-state batteries do not contain any liquid components, ensuring their safety features.However, achieving robust solid-solid contact represents a primary challenge within this context (Figure 8A,B).Additionally, the volume expansion of the cathode material during the cycling process can lead to fur-ther separation of cathode particles from the electrolyte in all-solid-state batteries (Figure 8C).While for quasi/halfsolid-state batteries, they possess enhanced contact performance by using gel or liquid components to wet the electrolyte/electrode interface.Nevertheless, the primary concerns in this realm pertain to the chemical stability (Figure 8D) of the intermediate layer/electrode/solid electrolyte triple-phase interface and the overall safety of the battery.Significant progress has been made in addressing the issues at the garnet/cathode interface.

All-solid-state batteries
In the context of solid-state batteries utilizing block garnet solid electrolytes, research efforts have primarily focused on exploring various approaches such as interface thermal treatment and interface structure design to enhance the performance of the garnet/cathode interface.

Conformal interface
For garnet-based all-solid-state batteries, conformal cathode-garnet interface is a common strategy to improve interface contact.Certain cathode materials, such as LiCoO 2 with high electronic conductivity (10 −3 -10 −4 S cm −1 ), can be used to construct a thin-film cathode on solid electrolyte without conducting carbon materials.Techniques like pulsed laser deposition (PLD) 187 or radio frequency sputtering (RF sputtering), 188 combined with interface thermal treatment, can achieve conformal interface between the thin-film cathode and garnet electrolyte surface, enabling cycling capability under specific conditions.However, there is a chemical instability issue between the cathode and garnet at high temperature.For instance, LiCoO 2 undergoes element diffusion with garnet at high temperature, resulting in the formation of an intermediate layer of La 2 CoO 4 , which hinders ion transport at the interface. 187To address this, low-melting point co-sintering agents (<800 • C) can be introduced to reduce the sintering temperature and minimize interface side reactions.The liquid co-sintering agent at high temperature can penetrate the gap between the cathode and garnet.After cooling and solidification, it improves the interface contact.Ohta et al. 189 first introduced a low-melting point Li 3 BO 3 additive into the cathode, and co-sintered it with bulk garnet to assemble an all-solid-state lithium metal battery.Liu et al. 190,191 subsequently introduced Li 3 BO 3 and In 2(1-x) Sn 2x O 3 (ITO) as additives into LiCoO 2 and NCM532 cathodes, enabling effective operation of the battery (Figure 9A).Han et al. 192 achieved an in situ reaction between the Challenges at the cathode/LLZTO interface in all-solid-state batteries.(B) Poor contact of particles in the electrode and the poor cathode/LLZTO contact.Reproduced with permission. 216Copyright 2018, Elsevier.(C) Volume effect.Reproduced with permission. 217Copyright 2019, American Chemical Society.(D) Chemical instability.Reproduced with permission. 218Copyright 2011, Elsevier.
Li 2 CO 3 layer on the garnet surface and the added Li 3 BO 3 (Figure 9B).The resulting Li 2.3 C 0.7 B 0.3 O 3 (LCBO) exhibits good electronic-ion conductivity.Additionally, LCBO can effectively suppress element diffusion between garnet and LiCoO 2 .All-solid-state batteries assembled based on this method demonstrate good cycling stability.
To mitigate the issues of element diffusion and interface side reactions between the cathode and garnet solid electrolyte during high-temperature sintering, the encapsulation of the electrolyte has emerged as an effective strategy, in addition to adding co-sintering agent.Zhu et al. 123 investigated the electrochemical stability of the electrolyte interface using first-principles calculations.The results demonstrated that the exceptional stability of solid electrolyte materials is not thermodynamically intrinsic, but rather originates from kinetic stabilization.Encapsulation of the solid electrolyte with artificial layers can passivate the interface and maintain interface stability.Kato et al. 187 introduced a thin layer (∼10 nm) of Nb as an interface modifier on the surface of garnet (Figure 9C).This modifier layer can in situ form an amorphous Li-Nb-O layer with ion conductivity, effectively reducing the interface impedance to less than 500 Ω.Additionally, it can isolate the cathode from the garnet solid electrolyte.
Furthermore, the thermal treatment atmosphere also significantly affects the performance of solid-state batteries.Kim et al. 193 studied the relationship between sintering atmosphere conditions and the chemical stability of the all-solid-state interface using NCM622 thin films as cathode (Figure 9D).Sintering in O 2 or N 2 atmosphere did not result in spontaneous reactions between the NCM cathode and garnet solid electrolyte, while CO 2 or H 2 O atmosphere led to interface side reactions and high interface impedance.

Interface structural design
Due to the limited contact area between the garnet solid electrolyte and cathode particles, the ion transport capability at the interface is restricted.Therefore, researchers have explored structural design at the garnet/cathode interface to enhance lithium-ion transport.In 2016, Van den Broek et al. 194 utilized starch sacrificial template method to fabricate a surface porous Li 6.25 Al 0.25 La 3 Zr 2 O 12 electrolyte (Figure 10A-E).Nanosized Li 4 Ti 5 O 12 particles were then cast as a slurry into the porous layer.The resulting 3D porous interface structure increased the contact area between the electrode material and solid electrolyte, facilitating ion conduction.Compared to simple coating, this approach significantly improved the electrochemical performance.Another strategy to enhance the effective contact area is by matching the sizes of the solid electrolyte and cathode particles in composite cathode.Park et al. 195 achieved this by adapting different-sized cathode and electrolyte particles (Figure 10F-H).The small electrolyte particles filled the gaps, effectively increasing the contact area.Similarly, Shi et al. 196 improved the specific capacity of all-solid-state batteries by matching large-sized cathode particles with small-sized electrolyte particles, F I G U R E 9 (A) Assisted sintering with Li 3 CO 3 and ITO.Reproduced with permission. 190Copyright 2018, Royal Society of Chemistry.(B) Schematic of composite cathode with LCBO solid-state electrolyte.Reproduced with permission. 192Copyright 2018, Elsevier.(C) Schematic of Nb-modified LiCoO2/LLZO interface.Reproduced with permission. 187Copyright 2014, Elsevier.(D) The influence of sintering atmosphere on the impedance of all-solid-state batteries.Reproduced with permission. 193Copyright 2022, Wiley-VCH.
thereby enhancing the active contact area and the specific capacity.
In summary, methods such as interface thermal treatment and structural design can achieve the assembly and cycling of all-solid-state batteries.In the course of solid-state battery research, there has been a deeper understanding of charge conduction and ion transport processes at solid interfaces.Although all-solid-state batteries possess high safety, they face challenges including low active material loading (<2 mg cm −2 ), low cycling rate (<0.1 C), stringent operating conditions (>50 • C), and limited cycle life (<100 cycles).Moreover, due to the reducibility of carbon, heat treatment methods cannot introduce conductive carbon materials into the cathode or interface.Consequently, achieving compatibility with low-conductivity cathode materials, such as LiFePO 4 , remains a challenge in the realm of all-solid-state batteries.The practical appli-cation of oxide all-solid-state batteries still faces significant challenges and lags.

4.2
Quasi/semi-solid-state battery Unlike solid-state batteries, quasi/semi-solid-state batteries utilize liquid/gel/polymer to wet the interfaces, ensuring excellent contact.At present, there are plenty of researches on liquid/gel/polymer interlayer, commonly referred to as the second-phase catholytes (Figure 11A).The most common second-phase electrolyte is carbonate-based commercial liquid electrolyte.Lots of researches on solid electrolytes or garnet/Li-anode interface use commercial electrolytes for wetting on the cathode side. 77,110,121,133It shows advantages of high ionic conductivity and low cost.However, commercial Reproduced with permission. 195Copyright 2019, Electrochemical Society.
electrolytes are flammable and have poor thermal stability, which contradicts the high safety goals of solid-state batteries.Additionally, the chemical/electrochemical stability of the second-phase electrolyte and garnet electrolyte greatly affects the overall battery performance.
Previous studies have shown that common inorganic lithium salts (such as LiPF 6 , LiBOB, LiDFOB) tend to produce interface impurities such as LaF 3 , ZrF 4 , Li 2 CO 3 , Li 2 O during cycling with garnet solid-state batteries, leading to increased interface impedance. 143,197,198Therefore, the current second-phase electrolytes mainly use organic lithium salts such as LiTFSI, LiFSI, which need high safety, stability, and ionic conductivity.The most extensively studied second-phase electrolytes include polymer catholytes represented by poly-ethylene oxide (PEO), deep eutectic catholytes represented by succinonitrile (SN), and ionic liquid (IL) catholytes.Figure 11B-E shows the radar maps of different catholytes' performance.
The incorporation of PEO improved the contact between the cathode and ceramic electrolyte, enabling stable cycling of LiFePO 4 full cells at 65 Polymer second-phase electrolytes represented by PEO exhibit low room temperature ionic conductivity, limiting the cycling of solid-state batteries to higher temperatures.Additionally, PEO is unstable at high voltages, posing challenges in matching with high-voltage NCM cathodes and hindering the improvement of energy density in solid-state batteries.Therefore, the development of novel secondphase polymer electrolytes with high ionic conductivity and wide electrochemical window at room temperature is necessary.Reproduced with permission. 215Copyright 2021, Wiley-VCH.

4.2.2
ILs second-phase catholytes Ionic liquids are liquid salts composed of ions, with a melting point typically below 100 • C.They often exhibit high ionic conductivity, wide electrochemical windows, and thermal stability, while being nonflammable.4][205] Guo et al. 206 employed N-methyl-Npropylpiperidinium bis(trifluoromethanesulfonyl)imide (PP13-TFSI) ionic liquid as second-phase electrolyte, filling the voids between cathode and garnet solid electrolyte, to achieve stable contact.LiCoO 2 solid-state batteries assembled using this method demonstrated stable cycling performance for over 400 cycles at 60 However, ionic liquids exhibit high viscosity at room temperature, which hinders the diffusion of lithium ions.Additionally, the cost of ionic liquids is relatively high.Therefore, there is a pressing need to investigate and develop more economical and effective second-phase catholyte.

Deep eutectic second-phase catholytes
In recent years, deep eutectic solvent (DES)-based electrolytes have garnered attention as novel electrolytes in lithium-ion batteries.Deep eutectic electrolytes are formed by mixing hydrogen bond donor and acceptor materials, and the intermolecular hydrogen bonding interactions lead to a decrease in the melting point of the system, enabling it to remain liquid over a wide temperature range. 209Among wide variety of deep eutectic solvents, succinonitrile (SN) exhibits exceptional thermal stability and superior lithium salt solvation capabilities.Owing to the two -CN groups in its molecular structure, electrolytes employing SN as a solvent demonstrate high ionic conductivity, low flammability, and extensive electrochemical window akin to ionic liquids.Furthermore, it is more costeffective compared to ionic liquids.][212][213] Lu et al. 214 introduced second-phase plastic crystal electrolytes containing 5 vol.%FEC and 4 mol% LiTFSI in SN, sandwiched between Ta-doped garnet solid electrolytes.This second-phase electrolyte achieved an impressive ionic conductivity of 0.92 mS cm −1 at room temperature.When paired with a LiFePO 4 solid-state full battery, stable cycling performance was observed for over 50 cycles at room temperature.Yang et al. 215 discovered that due to the coordination between -CN groups and La, Ta, and so forth in LLZTO electrolytes, self-polymerization reactions occurred at the SN/LLZTO interface, leading to a decrease in ionic conductivity.To mitigate this issue, they incorporated PAN as a polymer matrix and composite SN as a second-phase interlayer (Figure 12H-J).The competitive coordination between -CN groups in PAN effectively alleviated the interface side reactions between SN and LLZTO.As a result, the assembled LiFePO 4 solid-state battery exhibited stable cycling performance for over 300 cycles at room temperature.Quasi/half-solid-state batteries offer simpler assembly and the ability to operate at room temperature for extended cycles, making them highly practical and an important direction for the future development of garnet solidstate batteries.The semi(quasi)-solidification of batteries is not only a modification strategy but also holds significant research value in its own right.It encompasses the composition and stability studies at the solid-liquid interfacial crossover.However, the current electrolyte/cathode quasi(half)-solid-state strategies mainly focus on improving interface contact and wetting properties.Further research is still lacking in exploring the second-phase electrolyte and its interaction with the cathode/garnet.What is more, multifunctional second-phase electrolytes with capability of stabilizing nickel-rich cathodes and a wide operating temperature range are required for further development.Table 2 provides a comparative analysis of various interface modification strategies and corresponding electrochemical performance of full cells.

PERSPECTIVES
Garnet-type LLZO solid-state electrolytes stand out among numerous solid-state electrolytes due to their attractive material properties and chemical stability.Therefore, we discussed the interface characteristics and challenges faced by LLZO-based lithium metal solid-state batteries, and summarized the latest progress and research strategies committed to overcoming these challenges from the perspective of the cathode and anode interfaces.However, some fundamental challenges remain unresolved, including the stability of solid-state electrolyte preparation, theoretical guidance on anode interface modification methods, challenging in situ characterization, and cathode interface compatibility.Therefore, in order to further meet the practical application needs of LLZO-based solidstate lithium metal batteries with high energy density and high safety, the following issues still need further in-depth research.
(i) For electrolytes, we need stable production of LLZO solid electrolytes.The continuous growth of dendrites in electrochemical cycles can lead to electrolyte puncture, short-circuit failure of the battery system, and high density and low defect electrolytes can effectively block the infiltration of dendrites.The preparation and production of LLZO are strictly limited by various conditions, such as raw materials, temperature, humidity, atmosphere, and production process.At present, the performance of LLZO cannot be stabilized through standard production processes, for Li 6.5 La 3 Z 1.5 Ta 0.5 O 12 , the preparation processes in different research papers are also different.Therefore, a scientific, efficient, and stable LLZO production process is a necessary guarantee for studying its interface.(ii) For the anode interface, the current modification strategy is still difficult to meet the practical needs of high load and high magnification.The current anode modification strategies usually meet the needs of longterm cycling at a certain current.However, when faced with high current density and irregular charging and discharging applications, anode interface modification strategies often encounter structural failure, interface peeling, and other problems, resulting in the inability to prevent lithium dendrites from piercing the electrolyte.In order to design an anode modified layer with excellent electrochemical performance, the following characteristics must be comprehensively considered: electronic insulation, or strong mechanical properties to prevent the infiltration of lithium dendrites at the interface.
Besides, there is currently a lack of scientific theoretical guidance on the thickness of the structural design of solid electrolyte.When the structural layer was very thin, the weaknesses of LLZO's brittleness and low fracture toughness were exposed, which required special solutions to prepare LLZO films of large size.When the structural layer is quite thick, the problem of interface and lithium dendrite comes one after another.Moreover, an excessively thick structural layer will also reduce the energy density of the entire battery system.Therefore, a scientific and unified theory is needed to guide the range of interface thickness that can balance the relationship between modification effect, interface impedance, and energy density.
It is worth noting that there is a lack of effective in situ characterization of the anode.The original method of characterizing the interface has been subject to numerous limitations, and the in situ characterization of the interface of lithium metal negative electrodes is even more difficult.If we want to obtain real and effective lithium metal interface information, inert atmosphere is necessary for in situ characterization.Therefore, convenient and agile in situ characterization methods are needed to obtain lithium metal interface information.
(iii) For the cathode interface, methods such as interface heat treatment and structural design can achieve the assembly and cycling of all-solid-state batteries.Due to the reducibility of carbon, heat treatment methods cannot introduce conductive carbon materials into the cathode or interface.Therefore, achieving compatibility with low conductivity cathode materials such as LiFePO 4 remains a challenge in the field of all-solid-state batteries.Increasing the cathode active material mass loading and achieving stable cathode-electrolyte interface are important issues for oxide-based all-solid-state batteries to address.The practical application of oxide allsolid-state batteries still faces significant challenges and lags behind.Quasi/semi solid-state batteries provide simpler assembly and the ability to operate for long periods at room temperature, making them highly practical and an important direction for the future development of garnet-SSE.However, the current electrolyte/cathode quasi(semi)-solid-state strategy mainly focuses on improving interface contact and wetting performance.Further research is still lacking in exploring the second-phase electrolyte and its interaction with cathode/garnet.In addition, a multifunctional second-phase electrolyte with stable nickel-rich cathode capacity and a wide operating temperature range is also needed for further development.(iv) LLZO-based solid electrolytes currently have various types of element-doped derivatives, such as Ga and Al at Li sites, Ta and Nb at Zr sites, and Yb at La sites, and so forth.However, based on current human capabilities, we can only explore the impact of doping with one or two elements on the performance of LLZO.Therefore, we need to combine theoretical calculation methods such as AI and DFT to continuously explore the physical and chemical properties of different single-element-doped LLZO, and more importantly, we can use this research paradigm to explore the synergistic effects between different doped elements, high-throughput screening of the coupling effects of multiple elements on LLZO, and even make theoretical predictions for the recently popular high-entropy LLZTO, in order to achieve the continuous discovery of LLZO-based SSLBs.
The garnet-type solid electrolyte LLZO has advantages such as good thermal stability, high safety, high Li + conductivity, and wide electrochemical window.It has broad theoretical application prospects.In order to better utilize the role of garnet solid electrolytes, we need to pay more attention to the theoretical establishment and in situ characterization of their cathode and anode interfaces, as well as combine AI and theoretical calculations to continuously expand the family of garnet solid electrolytes.Therefore, more critical interface information can be obtained to promote the overall development of lithium metal solid-state batteries.

C O N F L I C T O F I N T E R E S T S TAT E M E N T
The authors declare no conflict of interest.

F I G U R E 1
Schematic of challenges in interface design for garnet-based solid-state lithium batteries (SSLBs).

F I G U R E 3
(A) Schematic diagram of I 2 modification layer on the surface of LLZO.Reproduced with permission.88Copyright 2023, Elsevier Ltd. (B) Schematic diagram illustrating the operation of a magnetron-sputtered indium modification layer.Reproduced with permission.92Copyright 2022, American Chemical Society.(C) The mechanism diagram of titanium oxide clusters (TOCs) improving interface wetting.Reproduced with permission.101Copyright 2023, American Chemical Society.(D) Rapid sintering method for fabricating high-entropy alloy interfaces.Reproduced with permission.106Copyright 2023, Wiley-VCH.(E) The process of converting Li 2 CO 3 to form Li 3 PO 4 .Reproduced with permission.110Copyright 2019, The Royal Society of Chemistry.(F) Schematic diagram depicting the operational principle of an ion-electron modification layer of AlN.Reproduced with permission.122Copyright 2023, American Association for the Advancement of Science.
, Duan et al.134 converted contaminants into a fluorinated interface at intermediate temperatures below 180• C. The modified LLZO exhibited high chemical stability when exposed to air, preventing the regeneration of contaminants.The maximum current density also reached 1.1 mA cm −2 .Yang et al.135 achieved the same outcome by utilizing lithium salts such as LiPO 2 F 2 , which transformed surface contaminants into a lithium-affinitive interface rich in LiF and Li 2 PO 3 F.The newly formed LiF-Li 2 PO 3 F interfacial layer not only enhances the interfacial wettability between Li and LLZTO, but also helps to combat surface corrosion caused by moisture in the air, enabling stable cycling for over 70 h at a high current density of 1.0 mA cm −2 .

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I G U R E 1 0 (A and B) SEM images of pure and modified electrolyte surface.(C and D) SEM images of cathode/electrolyte interface for non-modified and modified electrolytes.(E) Nyquist plots of all-solid-state battery for non-modified and modified electrolytes.Reproduced with permission. 194Copyright 2016, Wiley-VCH.(F-H) Schematic of composite cathode with different-sized solid-state electrolytes.

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I G U R E 1 1 (A) Schematic diagram of cathodic interface.(B-E) Radar map of different catholytes performance.

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I G U R E 1 2 (A and B) Illustration of the electric potential profile in the charge process.(C and D) Charge and discharge voltage profiles of Li/LLZO/LiFePO 4 and cycling performance of Li/LiFePO 4 with LLZO containing polymer interlayer.Reproduced with permission. 146Copyright 2016, American Chemical Society.(E) Schematic diagram of the chemical interface engineering of solid garnet batteries on both cathode and anode sides.(F and G) Cycle performance and EIS of cells with ionic liquid catholyte and ether anolyte.Reproduced with permission. 207Copyright 2021, Elsevier.(H) SEM images of SN/LLZTO complexes before and after partial polymerization.(I) The function mechanism of PAN-SN interphase on the surface of LLZTO electrolyte.(J) Cycle performance of solid-state cell with PAN-SN interlayer.
This work was financially supported by the National Key R&D Program of China (grant number 2022YFB3807700), the National Natural Science Foundation of China (No. U20A20248, No. 52372247), Key-Area Research and Development Program of Guangdong Province (No. 2020B090919001), Shanghai Pujiang Programme (23PJD110), the China Academy of Engineering Physics (No. U1930208), Natural Science Foundation of Shandong Province (ZR2021QB007), and the Science and Technology Commission of Shanghai Municipality (No. 18DZ2280800).
Performance comparison of garnet-based SSLBs with different modification strategies.
• C.Meanwhile, Chi et al. 78 designed the Li anode and utilized PEO polymer electrolyte to wet the interfaces on both sides of garnet electrolyte.The assembled LiFePO 4 solidstate battery achieved stable long cycling of 200 cycles at 90 • C.