Recent advances and interfacial challenges in solid‐state electrolytes for rechargeable Li‐air batteries

Abstract Among the promising batteries for electric vehicles, rechargeable Li‐air (O2) batteries (LABs) have risen keen interest due to their high energy density. However, safety issues of conventional nonaqueous electrolytes remain the bottleneck of practical implementation of LABs. Solid‐state electrolytes (SSEs) with non‐flammable and eco‐friendly properties are expected to alleviate their safety concerns, which have become a research focus in the research field of LABs. Herein, we present a systematic review on the progress of SSEs for rechargeable LABs, mainly focusing on the interfacial issues existing between the SSEs and electrodes. The discussion highlights the challenges and feasible strategies for designing suitable SSEs for LABs.


 Introduction
As the global economy continuously develops with the dramatically improved people's lifestyle, the energy scarcity and environmental pollution resulting from fossil fuel overuse have drawn numerous concerns and urged people to seek myriad renewable energy power sources. Among the promising candidates for next-generation power supply, rechargeable LABs could deliver relatively high gravimetric energy density of 3500 Wh kg −1 , which is about an order of magnitude higher than that of the lithium-ion batteries (LIBs). [1][2][3][4] According to the reaction mechanisms, LABs can be divided into nonaqueous and aqueous systems. The reaction mechanism of the aqueous system involves reversibly forming/decomposing LiOH, while that of the non-aqueous system is the reversible generation/decomposition of lithium peroxide (Li 2 O 2 ). [5] Nevertheless, non-aqueous LABs have drawn more attention from researchers due to their higher theoretical capacity than the aqueous system. [6][7][8][9][10] Although numerous efforts have been dedicated into non-aqueous systems, aiming to enhance the cyclability and round-trip efficiency. Security volatile organic electrolytes and the blocked oxygen diffusion channels by the permeation of electrolytes. [3,4,[11][12][13][14][15][16][17][18][19][20][21] The conventional flammable organic electrolytes should be replaced by safer and more stable electrolytes, improving the practicality of LABs. [22][23][24] Constructing SSEs can be a promising strategy for eliminating the safety concerns of the LABs. Typically, a solid-state electrolyte (SSE)-based LAB (SSLAB) comprises a SSEs, a Li metal anode, and an oxygen-breathing cathode. [9] During discharge, a two-electron oxygen reduction reaction (ORR) process would dominantly occur on the cathode side, described as 2Li + +O 2 +2e − ↔Li 2 O 2 , accompanied by an anode oxidation reaction Li↔ Li + +e − . [25] While SSEs have been widely explored in LIBs, and many related review papers have been published to elaborate on the advantages and challenges of SSEs, [26][27][28][29][30] but many fundamental issues faced by SSLAB due to its open system configuration have not received sufficient attention. The available system brings more limitations in selecting suitable solid electrolytes with good compatibility of electrodes and increasing the interfacial resistance of SSLAB. [31,32] When operated in an entire air system, the contaminants such as O 2 , CO 2 , N 2 , and H 2 O in the air, could transfer from the air-breathing cathode to the Li foil, resulting in electrolytes Figure 1 for a timeline of developments), which was reported by Abraham and prepared by assembling the polyacrylonitrile (PAN)-based polymer electrolytes with a carbon composite cathode and a Li metal anode. [35] This work was the first step toward using SSEs in LABs, achieving milestone progress in the SSE research field. In 2008, Yahya and co-workers proposed a gel polymer electrolyte (GPE) based on natural polymer matrixes, plasticizer, and lithium triflate salt (LiCF 3 SO 3 ), achieving an ionic conductivity of 0.492 mS cm −1 . [36] Scrosati et al. proposed a modification strategy for poly(ethylene oxide) (PEO)-based solid polymer electrolytes (SPEs) by incorporating ZrO 2 in 2011, showing a low overpotential of about 400 mV during discharge and charge process without catalyst. [37] Afterward, Zhou et al. have exploited solid inorganic electrolytes (SIEs) based on Li 1+x Al y Ge 2-y (PO 4 ) 3 (LAGP) for SSLAB, which was investigated in the air atmosphere and presented a specific capacity of 1700 mAh g −1 when current density is limited at 0.5 A g −1 . [38,39] Subsequently, in the past 5 years, exploring solid composite electrolytes (SCEs) has become a hot pot, because SCE combines the good mechanical flexibility of polymer and high Li + ions transport properties of inorganic electrolytes, greatly enhancing the LABs electrochemical capability. Specifically, in 2018, Goodenough et al. have invented PEO/garnet composite electrolytes for LIBs. [39] Following this design strategy, Zhang and his coworkers have fabricated an adjustable-porosity plastic crystal electrolyte based on succinonitrile (SN) and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP). A high spe-cific capacity of 5963 mAh g −1 and a stable 130-cycle life is achieved in the as-fabricated SSLABs in 2020. [40] Furthermore, they also reported an SCE combing LAGP with PVDF-HFP, delivering a stable cycling stability upon 146 cycles. [40] In this review, the SSEs consist of four types: (a) GPE, (b) SPE, (c) SIE, and (d) SCE. We have discussed the progress in SSLABs and perspectives of its challenges and effective design strategy from different aspects, stability, interfacial chemistry, ionic conductivity, and functionalization. Finally, the prospects for SSLAB are outlined.

. The Li anodes
Li metal as the anode is beneficial for realizing high energy density because of its low redox potential (−3.040 V) and superior theoretical capacity (3860 mAh g −1 ). [41] However, Li metal anode in the LABs with organic liquid electrolytes inevitably faces serious safety concerns, including the shortcircuit fire resulting from separator being pierced by Li dendrites or electrolyte leakage. In addition, LABs face more challenges when contrasted to LIBs since the semi-open oxidizing environment would lead to complex side effects on the Li anode and electrolytes. For example, the degradation of the electrolyte caused by O 2 dissolution could generate H 2 O and further result in the Li metal corrosion, significantly deteriorating the LABs electrochemical performances. [42] Thus, it is urgent to explore a feasible strategy to avoid the pollution of Li metal efficiently.

. The O  -breathing cathodes
Numerous studies have focused on designing electrocatalysts as the O 2 -breathing cathodes for LABs, including noble metals, [43][44][45] transition metal oxides, [46][47][48] and carbides, [49] to lower the overpotential and enhance the reversibility of LABs. [50] The catalysts are related to the formation/decomposition process of Li 2 O 2 , promoting mass transport of Li x O 2 intermediates through decreasing the interfacial binding energy or adsorbing energy. [51] The O 2breathing cathodes make a significant difference in reducing the kinetic barrier for ORR and oxygen evolution reaction (OER), increasing O 2 recovery efficiency. Thus, the interaction with reaction intermediate dates at the surface of the O 2 -breathing cathodes should be modulated by designing a suitable catalyst. [52] .

Conventional electrolytes
The LAB electrolytes are divided into four types: aqueous electrolytes, aprotic electrolytes, SSEs, and ionic-liquid electrolytes. [13,15,19] The chemical reactions occurring in the oxygen electrode depend on electrolytes. The reactions in the aprotic systems and SSLAB are similar, while the aqueous LABs present different electrochemical reaction pathways owing to their cathodes both exposing to the aqueous electrolyte. [4] It is widely known that the commonly used electrolytes in aqueous LABs are alkaline solutions. [53] The ORR occurring in the discharge process is of high complexity and involves a series of electron-transfer processes containing O 2 -relating species, like HO 2 − , OH − , O 2 − , ascribing to the reversible generation and decomposition of LiOH. [54][55][56][57] In contrast, the non-aqueous LABs have a higher energy storage capability than the aqueous ones, which has attracted considerable attention in the past decades. [58][59][60] The prerequisite of the non-aqueous electrolytes for LABs includes a high O 2 solubility, high Li + transport property, strong solvation effect, and comprehensive and stable electrochemical window, which are usually acquired by dissolving Li salts in aprotic solvents. However, flammable aprotic electrolytes are volatile, causing rapid exhaustion due to the semi-open system of LABs, resulting in safety issues and short-circuit. Moreover, the pores in the air-breathing electrode are usually blocked by the permeation of aprotic electrolytes, causing severe polarization and irreversible electrochemical reactions. Thus, high power density and steady operation needs should be satisfied by exploring new electrolyte systems for LABs.

 Recent progress of SSLAB
SSEs have appealed to many researchers' interests due to their open systems stability, which eliminates volatilizing or leakage issues compared with aprotic liquid electrolytes. SSEs are also more advantageous than ionic-liquid electrolytes (ILs) due to their simple design, convenient packaging, stable chemical stability toward moisture, and excellent mechanical strength.
What's more, the development of ILs has also been limited by their relatively high viscosity and corrosion. Benefiting from the high intrinsic modulus of elasticity for SSEs, the Li dendrites also can be avoided, technically suppressing internal shorting. Furthermore, the corrosion of Li foil is also alleviated with SSEs, which avoids the Li foil contacting with air and reacting with moisture, reactive O 2 species, and CO 2 . Hence, SSEs would be a superior choice for future LABs compared to the organic liquid electrolytes. Figure 2 shows the properties of typical solid electrolytes for LABs, with an air stability order of sodium/lithium superionic conductor (NA/LISICON) > polymer > garnet. Polymer electrolytes have become a research hotspot due to their flexibility, security, scalability, and low interfacial resistance with electrodes. SPEs incorporate polymer matrix with lithium salts, ion transport of which depends on the segmental motion of the polymer chains, limiting achievable ambient ion conductivity of 10 −7 to 10 −6 S cm −1 at room temperature. The common polymer matrix contains PEO, poly(carbonate) (PC), poly(siloxane), PAN, poly(vinylidene fluoride) (PVDF), [61] and PVDF-HFP. [62][63][64] Due to a low crosslinking density, a plasticizer can be introduced into SPEs to a large extent, forming a GPE that integrates a polymer matrix's good flexibility and security with the ideal Li + ion conductivity of liquid electrolytes. NA/LISICON and garnet are inorganic materials developed as SIEs because of their high ionic conductivity of > 10 −3 S cm −1 at room temperature. Garnet-type including Li 7 La 3 Zr 2 O 12 (LLZO) and Li 6.5 La 3 Zr 1.5 Ta 0.5 O 12 (LLZTO) and NASICON-type SSEs such as LAGP have been widely employed in LABs. However, the poor adhesion between SIEs and electrodes leads to high interfacial resistance, inspiring the development of an intermediate layer between SIEs and electrodes and the design of SCEs using the advantages of SPEs to compensate for the drawbacks of SIEs.

. GPEs
GPEs are synthesized by swelling the polymer matrix with plasticizers, combining the excellent mechanical property of the polymer framework with the high ionic conductivity of aprotic electrolytes. The combination also alleviates serious safety problems intrigued by the inevitable decomposition of common organic electrolytes, enhancing both electrochemical and safety performances of LABs. For example, to relieve the safety risks of organic electrolytes, Zhang et al. have exploited a GPE by plasticizing the PVDF-HFP matrix by introducing tetraethylene glycol dimethyl ether (TEGDME), presenting a transparent and uniform self-supporting film in Figure 3A, finally achieving a 50-cycle life, more stable than liquid TEGDME electrolyte. [65] In response to the demand for wearable devices, a fiber-shaped LAB assembled by a GPE and aligned carbon nanotube (CNT) sheet O 2 -breathing electrode has been prepared by Zhang et al., achieving a high discharge F I G U R E  Radar plots of the performance comparison between various solid-state electrolytes (SSEs) including polymer, garnet, and sodium/lithium superionic conductor (NA/LISICON) SSEs.
capacity of 12,470 mAh g −1 and a stable 100-cycle life in the air. [66] As exhibited in Figure 3B-D, this GPE-based LAB possesses high flexibility and stable electrochemical performance even being twisted, bent, and immersed in water, showing its potential to serve as the energy supply system for wearable and flexible electronic devices in our daily life. However, the Li passivation and deteriorative cathodes blocked by insoluble species, including Li 2 O 2 /LiOH/Li 2 CO 3, always lead to the poor reversibility and cyclability of LABs. [67][68][69][70] Focus on this issue, Xia et al. have introduced LiI into GPE, in which the I − /I 2 conversion can serve as a redox mediator and accelerate the reversible process of Li 2 O 2 generation/decomposition, finally achieving a 400-cycle lifespan in ambient air and offering the potential to develop practical LABs. [71] What's more, a quasi-solid state electrolyte (QSSE) was developed by Zhao's group in 2018 to improve the stability of the interfacial contact between GPE and metallic Li anode, comprising an inorganic ceramic electrolyte, a polymer, and a hybrid plasticizer with ether and ionic liquid in Figure 3E,F. [72] In Figure 3G,H, the LAB with 1-propyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (PMIMTFSI)-plasticized GPE shows a longer cycle life than that of the cell with TEGDME-plasticized GPE, which is ascribed to the matched surface energy between Li anode and electrolytes. The LAB based on the hybrid GPE of TEGDME and PMIMTFSI has the most stable cycle life of 200 cycles, providing a good design of GPE to eliminate the safety risks that occurred on the Li anode. According to the above discussions and recent advances related to GPEs, progress in GPEs for SSLABs in recent 5 years F I G U R E  (A) Photos of the self-supporting membrane composed of tetraethylene glycol dimethyl ether (TEGDME) and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) polymer framework; Reproduced with permission. [65] Copyright 2015, Elsevier. (B) A red light-emitting diode is powered by a flexible fiber-shaped Li-air battery (LAB); (C) charge-discharge curves of the LAB; (D) a light is powered by the LAB even immersed in water. Reproduced with permission. [66] Copyright 2016, Wiley. (E,F) Optical photograph of QSSE; selected voltage profiles of LABs based on (G) PMIMTFSI, and (H) the TEGDME/PMIMTFSI hybrid during cycle tests when the capacity is limited at 1000 mAh g −1 under a current density of 400 mA g −1 . Reproduced with permission. [72] Copyright 2018, The Royal Society of Chemistry. TA B L E  Performance overview of recent solid-state electrolyte-based Li-air battery (SSLAB) with GPEs in recent 5 years.

Type Component
Ionic conductivity (mS cm − )

Operating atmosphere
Cycle life/cycle capacity (mAh g − ) P u b l i c a t i o n y e a r R e f .
is concluded in

. SPEs
There is none analysis on the quantity of the aprotic electrolytes added in the polymer frameworks to fabricate GPEs. Thus, it is still confusing about the necessity of introducing extra liquid electrolytes into swollen polymer frameworks in order to extend the cycle life of LABs based on polymer matrix. It is an inevitable trend to develop novel polymers with intrinsically high ionic conductivity to fabricate SPEs, avoiding the introduction of liquid electrolytes. A representative work was proposed by Byon et al. in 2014, in which a three-dimensional (3D) SPE incorporating a CNT electrode was constructed that avoids the O 2 gas diffusion to the anode in the LABs ( Figure 4A-C). [84] The developed PEO/lithium bis(trifluoromethanesulphonyl)imide (LiTFSI) based SPEs here exhibited a high ionic conductivity (3.2 × 10 −4 S cm −1 at 55 • C), higher than that of typical SPEs with PEO. The discharge capacity of the 3D CNT/SPE LAB nearly reaches 500 mAh g −1 , exceeding other LABs based on conventional CNT and SPE sandwiched structures. In 2020, to alleviate the severe safety issues in standard liquid electrolytes-based LABs, Nan and his coworkers have constructed an ultra-dry polymer electrolyte (UDPE) for SSLABs, with a CNT O 2breathing electrode and a Li foil electrode ( Figure 4D). [42] The free-standing polymer electrolyte membranes are about 80 µm-thickness, exhibiting transparent and flexible features, as shown in Figure 4E,F. The electrochemical performances of these PVDF-HFP based UDPEs are more stable than the PVDF-based UDPEs during cycling, which can operate stably over 60 cycles. This significant performance enhancement implied the probability of substituting conventional liquid electrolytes with UDPEs in LABs. Table 2 presented recent progress made in SPEs for SSLABs, contrasting their component, ionic conductivity, operating atmosphere, and cycle life. Although SPEs have the capacity to accommodate volume changes occurred for Li 2 O 2 formation/decomposition and scalability for wearable electronics, some challenges still need to be solved for further application. First, the low Li + transport abilities with an ionic conductivity of lower than 10 −4 S cm −1 caused by the sluggish dynamics of the polymer chain needs to be improved by suppressing polymer crystallization through preparation strategies such as copolymerization cross-linking. Second, the insufficient active reaction zones and diffusion channels for O 2 should be promoted to accelerate the interaction between O 2 and Li + , such as constructing a 3D structure with numerous void spaces mentioned in Byon's work. [84] Third, the degradation of SPEs under the harsh scenarios needs to be avoided, which is intrigued by reactive O 2 species generated when high voltage triggers the polymer decomposition. For example, PEO is confirmed to be oxidated under the O 2 atmosphere. [85] It would be helpful to suppress the degradation of SPEs by adding antioxidants.
oxynitride (LiPON) electrolyte, and the transportation of Li + depends on the defects of these materials. Li + in SIEs usually migrates based on the vacancy mechanism or the diffusion mechanism, contributing to a high ionic conductivity, good thermal stability, and excellent electrochemical performance. However, their high interfacial resistance toward cathode and Li anodes is the most crucial factor affecting the LABs performances. Among all the SIEs, NASICON and garnet-type SIEs are the two most suitable for LABs because of their superior Li + conductivity and stability toward O 2 and Li anodes. [34] However, NASICON SIEs exhibit a higher Li + conductivity than that of the garnet-type SIEs under room temperature, enabling them to be the best choice for LABs. Thus, Zhou et al. have proposed a solar photothermal SSLAB assembled with a plasmonic air cathode, a LAGP SSE, and a Li metal anode. [90] It achieved a stable 50-cycle lifespan at room temperature under a 1000 mAh g −1 capacity limitation for 400 mA g −1 , illustrating the feasible operation of LAGP for LABs. In addition, garnet-type oxides have a high Li + conductivity and also show good compatibility toward Li anodes. Li et al. reported an SSLAB with a garnet-type LLZTO SIE. [91] They have also blended garnet powder and Li salt (LiTFSI) in the cathode, which is an effective strategy to lower the interfacial impedance, realizing a high energy density of 20,300 mAh g −1 and a 50-cycle lifespan at 80 • C when the specific capacity is limited at 1000 mAh g −1 and the current density is 20 µA cm −2 .

 of 
TA B L E  Performance overview of recent solid-state electrolyte-based Li-air batteries (SSLABs) with solid polymer electrolytes (SPEs) in recent 5 years.
used in SSLABs as a result of its stability toward oxygen and Li foil. Still, some methods to solve interfacial problems between LAGP and electrodes need to be adopted, such as sputtering an amorphous Ge 0 thin membrane on LAGP. [92] As for the garnet-type SIEs. They may react with CO 2 and moisture, accompanied by poor adhesion toward electrodes. Thus, the Li 2 CO 3 should be removed by polishing treatment and heat processes to eliminate the reaction between LAGP and CO 2 , and lithiophilic interlayers should be demonstrated to enhance the adhesion between LAGP and electrodes.
coworkers have added CNTs in polymer electrolytes to expedite Li + transport. The Li + conductivity of PEO-based SPEs was increased to 2 orders of magnitude than the undoped one, and its mechanical property was also improved, especially the tensile strength of the polymer was increased to 160%. [98] Introducing the SIEs into the SSE system is also a common way to improve the ionic conductivity of the SPEs. Cui's group blended 15 wt% LLTO ceramic with PAN polymer matrix, finally achieving an excellent ionic conductivity of 0.24 mS cm −1 at room temperature. [99] Zhou et al. also developed an ion-conducting SCE through combing SPEs with SIEs at a weight ratio of 1:1, displaying a high Li + transference number of 0.75 and ionic conductivity of 0.32 mS cm −1 . [100] The as-prepared SCE exhibits a flexible thin film with 30 µm thickness. They have fabricated a flexible SSLAB (as illustrated in Figure 5A), which can supply enough power to light a LED lamp even if the LAB is under bending position ( Figure 5B). Furthermore, the flexible SSLAB shows a stable and high discharge capacity and can maintain 90 cycles at 50 • C under 200 mA g −1 . High-performance SSLABs have also been limited by the triple-phase boundaries (TPBs) and interfacial resistance. Zhang and co-workers developed a plastic crystal electrolyte (denoted as PCE) targeting on solving this problem. They constructed in situ porous PCE to facilitate the ionic exchange process and ensure the fast adsorption of O 2 , generating stable TPBs. The PCE is composed of SN, PVDF-HFP, LiTFSI, and 2,6-di-tert-butyl-4-methyl phenol (BHT), the porosity of which can be adjusted by tuning the amounts of the solvents, dimethyl sulfoxide (DMSO). [61] The PCE-based SSLAB has shown excellent capacity, good rate performance, and stable cyclability, attributed to the existence of ample active sites in continuous TPBs. Zhang's group also developed another SCE based on rigid LAGP core@ultrathin flexible PVDF-HFP shell interface (as indicated by Figure 5C), exhibiting higher thermal safety when compared with the PVDF-HFP-based GPE ( Figure 5D,E). [63] The SCE owns a high stiffness and Li + diffusion property due to the introduction of nano-sized LAGP and low interfacial resistance of flexible PVDF-HFP shell. The cycling stability of SCEbased LAB was enhanced to 146 cycles after introducing LAGP in previous GPE-based LAB cells in Figure 5F. As presented in Table 4, we have concluded the characteristics and properties of efficient SCEs. Thus, the introduction of SCEs effectively alleviates the interfacial issues and improves the ionic conductivity, which is a promising candidate for developing SSLABs.

 STRATEGIES TOWARD SOLVING INTERFACIAL CHALLENGES
In practice, SSLABs usually exhibit rapidly degraded performance, poor cycle stability, and low power density, originating from the SSE|electrode interface. Li + ions suffer prominent obstacles arising from parasitic reactions to transport across the interface, performing more sluggish transport speed when compared with that of the bulk SSE. [105] Some solid electrolytes are not stable against Li metal, for example, tetravalent Ti 4+ of Li 1+x+y Al x (Ti,Ge) 2−x Si y P 3−y PO 12 (LATGP) ceramic electrolyte would trigger unrecoverable decomposition of LATGP due to the redox reaction between Ti 4+ and Li anode . The protection layers would be useful to avoid the direct contact between unstable solid electrolytes and Li metal, including introducing a polymer layer and a thin film ionic conductor with low resistance. On the cathode side, matching the interfacial energy of SSE with cathode is a major task for achieving a high-performance SSLAB. It is proved to be an effective method to integrate the solid electrolyte particles into cathodes. To conclude, it is significant to facilitate the Li + transport across the interface for achieving high-performance SSLABs.

. Physical contact modification
The Li + transference across the interface mainly depends on the physical contact between the electrodes and SSE. However, when SIEs are employed, the Li|SSE interface shows point-to-point contacts and does not wet the Li anode. To achieve an ideal physical contact, the surface energies of SSEs and Li metal need to be matched through various methods, such as the modification of the SSE surface and the introduction of Li alloy layers. [106,107] In addition, the cathode|SSE interfaces also show the discontinuous distribution and pointto-point contacts, enabling the large interfacial resistance and quick failure of SSLABs. To alleviate the difficulties, strategies must be adopted to ensure the contact of flowing air and Li 2 O 2 discharge species for reversible electrochemical cycling, (H) Schematic illustration of the interfacial changes occurred during discharge and charge process for the LABs based on hybrid ionic and electronic conductors. Reproduced with permission. [110] Copyright 2021, Wiley.
optimizing air cathode, designing even contact between ionic and electronic conductors, and reducing the quantity of interfaces. To match the interfacial energy at the cathode|SSE interface, Zhou's group has integrated the solid electrolyte particles into cathodes, but the performance of SSLABs is unsatisfying. [108] As presented by Figure 6A,B, to reduce the charge overpotential caused by the large interfacial resistance, Zhou et al. proposed a solar-driven strategy by utilizing ZnS photocatalyst because the required electric energy can be compensated by solar energy. What's more, the replacement of Li metal with Li x Si anode in this work enhances the safety of SSLABs. [109] Finally, the charge voltage of SSLAB is reduced to 2.08 V, resulting in the high energy efficiency of 113%.
Targeting modifying the point-to-point interface generated from a mixed ionic and electronic conductor ( Figure 6C-G), Sun's group utilized mixed ionic and electronic conductors to achieve a facet-to-facet interface in Figure 6H, which decreases the interfacial resistance and enhances Coulombic efficiency from 38.6% to 80.8%. [95] .

Kinetics enhancement
The interface kinetics of SSEs is non-uniform and hinges on the microstructure of electrode|SSE interface, including the grain size, crystal orientation, and grain boundaries at the surface of both sides. It is effective for promoting the transport of lithium ions across the interface by engineering an interface with redox mediators doped SSE or ionic conductors embedded cathode. The significant resistance at the electrode|SSE interface is attributed to the sluggish kinetics, [37] It is effective for promoting the transport of lithium ions across the interface by engineering an interface with redox mediators doped SSE or ionic conductors embedded cathode, which boosts the ORR/OER kinetics at discharging/charging states for LABs. Integrating an RM with a polymer electrolyte also helps solve significant resistance problems. [2,110,111] Kim et al. have combined redox mediator (p-benzoquinone, pBQ) with PVDF-based electrolyte, achieving an enhanced cycling performance because pBQ can decrease the reaction barrier for ORR and OER. [112] Meanwhile, developing high-efficiency cathodes is also inevitable to improve the ORR/OER kinetics in solid-state LABs, which dramatically influences the overpotential, cycle life, and round-trip efficiency. [113] For instance, Zhou et al. have engineered an interface between an RM doped SPE and a RuO 2 -based cathode catalyst, dramatically decreasing the overpotential of SSLABs. [114] .

Mechanical properties control
Another unignorable factor that affects the interfacial chemistry is the mechanical properties of the SSEs. [115,116] At the discharge and charge process, the active electrodes generally undergo some volume changes and structural fragmentation, leading to capacity fade. [69] The area of fragmentation of the active materials on the electrode can be reduced by employing solid electrolytes with a low elastic modulus. The contact condition between the electrode|SSE can be improved using the solid electrolyte and the electrodes with good elasticity. For example, the LiPON Li|SSE anode interface possesses inadequate Li + transportability but good resistance toward the incursion of lithium dendrites due to the high elastic modulus and hardness of LiPON. [117] Thus, the solid electrolyte should be tough enough at the Li|SSE to resist lithium dendrites. In practical application, the Li + diffusion property of SSE should also take the complex solid-electrolyte interphase formed on the Li metal surface into consideration. [118] Therefore, an efficient Li anode protection strategy is expected to enhance the electrochemical performances of SSLABs.
We have compared the advantages and disadvantages of the aforementioned interfacial strategies toward physical contact loss, sluggish kinetics, and mechanical weakness in Figure 7, which exhibits four standard methods, including improving ionic conductivity of cathode active materials (CAMs), coating a protective layer on SEs, using nano-sized efficient CAMs and introducing RMs. As demonstrated by Figure 7, building functional interlayers at electrode|SSE interface and tailoring SSE components are cost-efficient ways to significantly decrease the interfacial impedance by improving physical contact loss and mechanical property.

 CONCLUSION AND PERSPECTIVE
In this review, we put emphasis on recent works of SSLABs and interface challenges faced with SSEs. SSEs play significant roles in alleviating the safety concerns of conventional LABs with flammable organic electrolytes and realizing a stable open system. SPEs including PVDF-HFP, PEO, PAN, and PMMA and SIEs including garnet, NA/LISICON are promising candidates for SSLABs. To enhance the Li + conductivity and mechanical property, SCE has been developed to take the essence of solid matrixes such as SPE, SIE, and passive fillers. However, due to the large polarization and rapid failure of SSLABs, interfacial challenges remain not well addressed for further practical applications of SSEs or SCEs. The strategies for improving interfacial resistance are composed of three approaches: (a) ensure even physical contact: optimizing air cathode, designing even contact between ionic and electronic conductors, engineering of the functional interlayer, and minimizing the number of interfaces; (b) boost sluggish kinetics: enhancing the ionic conductivity of SSE and introducing RMs to decrease polarization; (c) improve mechanical strength: regulating the compositions of SCEs by incorporating Li + active or inactive fillers with SPE, and exploring new copolymer matrixes and ceramic electrolytes. Delightful progress has been made with the strengthened mechanical property and improved electrochemical performance. However, advanced characterization requires further exploration of interfacial reaction and evolution during cycling. Taking these factors into consideration, the future direction for developing SSLABs is highly recommended as follows: 1. Multiple strategies should be combined to achieve costeffective SSLABs with high mass production feasibility. For example, utilizing functional layers to eliminate physical contact loss and highly efficient catalysts to enhance ORR and OER ensures stable cycle performance of SSLABs. 2. Mechanism investigation. The interfacial reaction, process, and evolution occurring on both sides of electrode|SSE interface need to be monitored through advanced measurements, including in situ X-ray diffraction for crystallography orientation investigation, and cryogenic electron microscopy for morphology evolution. 3. Further optimization should be emphasized on realizing the stable operation of SSLABs under an ambient air environment. To be specific, functional interlayers or SSEs with good hydrophobic ability and high blockage of contaminants like O 2 , CO 2 , N 2 , and H 2 O, need to be well designed for a long-life SSLAB. In order to reduce the negative effects of CO 2 , a CO 2 -adsorbents filter can be introduced in SSLABs. Furthermore, designing hydrophobic SPEs should be designed for eliminating the effects of moisture by increasing the number of hydrophobic groups (─CH 2 ─).

A C K N O W L E D G M E N T S
This research was supported by the National Key R&D Program of China under project 2019YFA0705104.

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 conflicts of interest.