Solid‐state Li–air batteries: Fundamentals, challenges, and strategies

The landmark Net Zero Emissions by 2050 Scenario requires the revolution of today's energy system for realizing nonenergy‐related global economy. Advanced batteries with high energy density and safety are expected to realize the shift of end‐use sectors toward renewable and clean sources of electricity. Present Li‐ion technologies have dominated the modern energy market but face with looming challenges of limited theoretical specific capacity and high cost. Li–air(O2) battery, characterized by energy‐rich redox chemistry of Li stripping/plating and oxygen conversion, emerges as a promising “beyond Li‐ion” strategy. In view of the superior stability and inherent safety, a solid‐state Li–air battery is regarded as a more practical choice compared to the liquid‐state counterpart. However, there remain many challenges that retard the development of solid‐state Li–air batteries. In this review, we provide an in‐depth understanding of fundamental science from both thermodynamics and kinetics of solid‐state Li–air batteries and give a comprehensive assessment of the main challenges. The discussion of effective strategies along with authoritative demonstrations for achieving high‐performance solid‐state Li–air batteries is presented, including the improvement of cathode kinetics and durability, solid electrolyte design, Li anode optimization and protection, as well as interfacial engineering.


| INTRODUCTION
Energy is the essential material basis for human survival and development. The constant revolution in energy technology perpetually promotes socioeconomic development and the progress of human civilization. Currently, fossil fuels contribute to over 80% of the world's energy supply, propping up the functioning of modern society while breeding a world energy structure that lacks renewability and sustainability. Severe environmental problems such as global warming inevitably emerge and deteriorate, pushing global average temperature 1.1°C higher since the preindustrial age, with a distinct impact on climate change and the ecological system. Given that modern energy is indispensable to the livelihoods and the developing economies on their navigation of urbanization and industrialization, the revolution of today's energy system for realizing nonenergy-related greenhouse gas emissions becomes a historical mission for all human beings. The announced climate pledges from participating states at the 26th Conference of the Parties effectively move the needle on the International Energy Agency's (IEA's) landmark Net Zero Emissions (NZE) by 2050 Scenario. For electricity has taken the prominent sector of the energy system, cleaning up the electricity mix and extending the electrification of end-uses through the shift toward renewable and low-carbon-emission sources of electricity, is an acknowledged tactic for rebuilding global energy supply and realizing NZE. According to IEA, the electricity sector has been responsible for 36% of all energy-related CO 2 emissions in 2020 already, while electricity demand is projected to reach 42,000 TWh by 2050 (almost 80% above today's level), which urges secure electrical energy storage technologies to connect end-use sectors and intermittent supply from clean energy (solar power, wind, etc.). 1 With high flexibility and compatibility with different service scenarios, electrochemical energy storage technologies become a favored choice for the energy market. Li-ion batteries (LIBs) have established supremacy in consumer electronics and power batteries such as electric vehicle cells, figuring as the central pillar of modern energy storage systems. However, LIBs manifest an upper limit of the energy density of about 400 Wh/kg even with the predictable advancement of both electrode materials and concomitant technologies 2 that can meet the demand from light-duty vehicles, while cannot support heavy-duty vehicles requiring long driving range and/or heavy load, with a targeted energy density of more than 500 Wh/kg. 3 Hence, "beyond Li-ion" batteries based on energy-rich redox chemistry such as a couple of metal dissolution/deposition and oxygen/sulfur conversion, emerge as promising candidates for next-generation energy storage strategies 2,4 in which, nonaqueous Li-air batteries (LABs) take the preponderance of ultrahigh theoretical energy density of 3600 Wh/kg (based on Li 2 O 2 discharge product), outstripping other choices notably.
However, like LIBs, the utilization of flammable and volatile organic electrolytes brings safety concerns of nonaqueous liquid-state LABs, accompanied by a narrow electrochemical window which unmatched for Li metal anode and limited operating temperature range. The electrolyte volatilization also incessantly damages the integrity of the triple-phase interface at the cathode, which accommodates the key reactions of LABs, then renders irreversible performance decline and even battery explosion. Meanwhile, the strong oxidizing reactive oxygen species ( 1 O 2 , O 2 − , O 2 2− , etc.) can readily decompose nonaqueous electrolytes and then form undesirable byproducts such as CO 2 , Li 2 CO 3 , HCOOLi, and CH 3 COOLi. These contaminants would inevitably deteriorate the long-term stability of LABs by blocking the gas diffusion channel and escalating overpotentials. The oxygen environment also pulls the Li anode of LABs into a more intractable condition, as the reactive oxygen species would parallelly proceed with the accumulation of insulating discharge products (Li 2 O 2 /Li 2 O, Li 2 CO 3 , etc.) on the Li anode surface. Characterized by a halfopen cell structure, the electrochemical environment of LABs is further challenged by complicated atmospheric species (CO 2 , H 2 O, etc.), and their shuttle behaviors in nonaqueous electrolytes arise severe threats such as parasitic reactions, complicated discharge routes, and anode corrosion. These species along with redox mediators (RMs; usually used as homogeneous catalysts) would greatly affect the composition and morphology of the solid electrolyte interphase (SEI) at the Li anode/ electrolyte solution interface, leading to undesirable electrolyte consumption, anode passivation, and formidable dendrite growth. Our group has devoted intensive efforts to mechanism study, 5-8 materials design, [9][10][11][12][13][14][15] and building sealed system with Li 2 O/Li 2 O 2 chemistry for nonaqueous liquid-state LABs, 16 and provided thorough discussions on this field in previous reviews. [17][18][19][20][21][22] Apparently, the nonaqueous electrolyte is the Achilles heel of LABs that dominates both cathode and anode electrochemistry, as well as battery stability and safety. However, an ideal combination of aprotic solvents and lithium salts that fits all criteria relevant to LABs is yet to be found. 23 The existing electrolyte solutions are only metastable for reactive oxygen species and limit the working potentials of standard LABs, 23 thus can hardly serve in practical LABs. Solid-state electrolyte (SSE) emerges as a promising substitute for liquid electrolyte, possessing superior stability (in thermal, chemical, and electrochemical aspects), wide electrochemical window, favorable mechanical strength, and cost efficiency. 24 These merits enable solid-state LABs to get rid of electrolyte evaporation and thus guarantee intact reaction interface, restrain the dendrite penetration and anode corrosion, eliminate the risks of battery combustion and explosion, and effectively enlarge working potentials and temperature range, then effectuate enhanced safety and life span as practical LABs expected. [25][26][27] However, the development of solid-state LABs is still in its infancy, facing several challenges, such as poor interfacial contact and/or low ionic conductivities of SSEs, limited triple-phase boundaries in the cathode, and questionable durability for the openair circumstance. [27][28][29] Intensive work has identified the fundamental underpinnings and effective strategies to build up high-performance solid-state LABs. Some progress on materials has been overviewed in our previous reviews. 26,30,31 But the comprehensive report covering basic mechanisms, challenges, recent progress, and possible solutions of solid-state LABs is in demand.
In this review, the discussion will start with the fundamental science and the main challenges of solidstate LABs, then emphasize the strategies apropos of improving air cathode kinetics, solid electrolyte design, Li metal anode optimization and protection, and interfacial engineering. It is expected that this review would provide the interested reader with a systematic understanding and instrumental guidance in constructing safe, stable, and practical solid-state LABs.

| Cell configurations and chemistry
A typical solid-state LAB (Figure 1) is composed of a Li metal anode, a Li-ion conductive SSE, and a porous cathode with desirable gas diffusion channels, as similar to the liquid-state LABs except for the use of condensed electrolyte. Thus, the solid-state LABs also primarily operate with the stripping/plating of Li metal at the anode during discharge/charge processes (Equation 1), and corresponding oxygen reduction reaction (ORR)/ oxygen evolution reaction (OER) at the cathode (Equation 2). The electrochemical reaction pathway of ORR is considered to be similar to the acknowledged mechanism for aprotic LAB proposed by Abraham et al. 32 but presents some differences. It should start with the reduction of O 2 into LiO 2 via one-electron transfer first (Equation 3) as liquid-state LABs, but the following chance of chemical disproportionation of LiO 2 (Equation 4) in electrolyte would be not expected in solid-state LABs for the absence of LiO 2 ion pair generated by strong Li + solvation. The electrochemical step for receipt of Li + and the other electron (Equation 5) to form Li 2 O 2 via the surface pathway is more reasonable. [33][34][35] But the solid evidence is hard to obtain in this condensed solid system, while the evolution of intermediates cannot be detected due to limits of in situ characterizations. Meanwhile, the recharge reaction route and relevant intermediates are still not ascertained and need to be explored carefully. Possible mechanisms have been proposed in liquid-state LABs, namely the direct two-electron transfer route, 32,36-39 the formation of Li-deficient Li 2 O 2 with delithiation, [40][41][42][43] and LiO 2 intermediated mechanism 42,44,45 which has also been verified in solid-state LAB. 46 The overall battery reaction is depicted in Equation (6). The E 0 a , E 0 c , and E 0 OCV are the thermodynamic anode potential, cathode potential, and open circuit voltage (OCV) under the standard condition of LABs based on Li 2 O 2 discharge product, respectively.  [50][51][52] They proposed that the electron-transport efficiency in Li 2 O 2 should limit the oxidation kinetics of Li 2 O 2 at high overpotentials, based on its insulating nature with a wide bandgap of >2 eV. But Zheng et al. 53 observed that the decomposition preferentially initiated at the surface of discharge products instead of their contact region with the CNT cathode or electrolyte, and continued along a certain direction of the bulk structure via an in situ environmental scanning electron microscope, in accordance with some TEM and SEM studies in liquid LABs. 54,55 In these surface-started observations, the higher electronic conductivity on the surface of Li 2 O 2 than that in the bulk was considered as a trigger, while the favorable O 2 release occurred on the surface area under a vacuum condition stimulated Li 2 O 2 decomposition.
F I G U R E 2 (A) Schematic illustration of the Li-O 2 nanobattery in an environmental TEM chamber. The time-resolved TEM images for showing the evolution of (B) discharge products and (C) charge products. The corresponding SAED results and compositional illustrations for (D-E) discharging and (F-G) charging. Reproduced with permission: Copyright 2017, Nature Publishing Group. 46 OER, oxygen evolution reaction; ORR, oxygen reduction reaction; SAED, selected area electron diffraction; TEM, transmission electron microscopy.
Recently, Luo et al. 46 carried out a remarkable reaction mechanism study of the full-cycle reaction pathway for a solid-state Li-O 2 nanobattery, with RuO 2 decorated CNTs as a cathode, Li metal as an anode, and the Li 2 O covered on Li metal as a solid electrolyte (Figure 2A). Using aberration-corrected environmental TEM achieves in situ analysis of the phase formed in discharge and charge processes. They found out that LiO 2 was initially produced on CNTs during discharge and then disproportionated into Li 2 O 2 and O 2 ( Figure 2D), which induced the hollow structure with a Li 2 O outer shell and a Li 2 O 2 inner surface ( Figure 2B,E). While recharging, the Li 2 O 2 lost one Li + and one electron to form LiO 2 ( Figure 2F), which then released O 2 accompanied by shell collapse until complete decomposition ( Figure 2C,G). This research identifies the electrochemical pathway in the designed solid-state Li-O 2 system, providing insights into LABs chemistry. But limited by time resolution and the response to ordered structure, in situ TEM could not capture the transient intermediates and the amorphous products. The exquisite design for realizing real-time characterization of (electro)chemical evolution in solid-state LABs is urgently demanded, which could be based on advanced in situ X-ray absorption spectroscopy, surface-enhanced Raman spectroscopy, and so forth.

| Thermodynamics and kinetics
The thermodynamics and kinetics of LABs dominate their intrinsic chemical/electrochemical behaviors and provide guidelines for improving output performance. In liquidstate LABs, some in-depth studies focused on discharge products or solvent-driven principles lead ground-breaking research and design strategies for practical devices. For instance, the highest occupied molecular orbital (HOMO) level 56,57 and acid dissociation constant (pK a ) 58,59 have been proposed as the descriptor of thermodynamic driving forces for the oxidative stability and chemical stability of solvents, respectively. The Gibbs free energy of LiO 2 solubility in aprotic solvents has been tightly related to Li 2 O 2 formation routes (i.e., surface pathway in low-donor number [DN] solvents and solution pathway in high-DN solvents), 43 while a trade-off is required because the high-DN solvents are preferred to deliver higher capacities but have higher susceptibilities to nucleophilic attack. 60 These work offer valuable references for the thermodynamics and kinetics study targeted solid-state LABs which is still in the initial stage. Characterized by the solvent-free property of solid-state LABs, these research mainly direct on ion migration in solid electrolytes and then could share some findings in solid-state LIBs. Meanwhile, a previous study on ORR/OER that has been established in liquidstate LABs and is irrelevant to solvent should also be applicable. But considering the working condition is in an ambient atmosphere instead of pure oxygen, the development of thermodynamics and kinetics research for solidstate LABs are exceedingly challenging.
Free energy levels of adsorbed oxygen intermediate as LiO 2 * and Li 2 O 2 * on the cathode are usually computed to identify the thermodynamically favored Li 2 O 2 nucleation pathways and deduce the rate-determining step (RDS). [61][62][63][64][65][66] A balanced adsorption energy toward LiO 2 * is essential to drive both O 2 activation and Li 2 O 2 nucleation at low overpotentials. The weak LiO 2 * bind renders the electrochemical activation of O 2 into LiO 2 * as the limiting step, while the strong LiO 2 * bind induces the limitation from the reduction of LiO 2 * to Li 2 O 2 , similar to ORR and OER in aqueous systems that involve multistep electron transfer between oxygen intermediates. Theoretically, the overpotential for RDS can be used as a thermodynamic descriptor for Li 2 O 2 formation and decomposition. But an accurate computational model is the prerequisite, which should be carefully designed in a not-well-defined system (nonspecific surface structure, defects, under complicated gas and temperature conditions, etc.) that makes the computed overpotentials hard to be a tool for predicting the cathode performance in LABs. And it needs to be pointed out that the direct reverse reaction steps of ORR have often been used as models for OER while theoretical calculation, even though researchers reach a consensus that the charging process of LABs does not follow the reversed route of the discharge process. The simplification could bring unreliable results and mislead readers. On the premise of the kinetics of the discharge process is limited by LiO 2 * formation, Viswanathan et al. 67 proposed that the energy barrier could be predicted as demonstrated by Tafel kinetics (Equation 7): where [Li ] + and O * 2 are the reduction reactant; represents the kinetic barrier to the RDS at the equilibrium potential; α is the symmetry factor; e is the electron charge; U dis and U 0 stand for the discharge plateau voltage and the equilibrium potential, respectively.
Undoubtedly, the growth/dissolution thermodynamics of Li 2 O 2 on the Li 2 O 2 itself is pivotal for the outset of discharge/end of charge. Hummelshøj et al. 68   The surface energies of these nonstoichiometric surfaces are potentially dependent, thus enabling the prediction of dissolution barriers following different paths. In theory, low thermodynamic overpotentials (<0.2 V) existed for the charge at many Li 2 O 2 sites on the facets studied. This explained some low experimental overpotentials with forming conformally deposited Li 2 O 2 at early charge stages and provided a design strategy for building up Li 2 O 2 electrochemistry with preferable thermodynamics. In addition, the catalyst-participated dissolution of Li 2 O 2 is usual while not dependent on the unified standard. A general activity descriptor for Li 2 O 2 decomposition activated by 4d transition metal catalysts was proposed by Zhao et al. 69 They described Li 2 O 2 decomposition as stepwise Li + desorption and O 2 evolution, that, Li + → Li + → O 2 . The first-principles thermodynamic calculations revealed that the surface electron affinity (V SEA ) and surface ionic potential (V SIP ) of these catalysts should determine the activation of Li-O 2 bonds and the reduction of desorption barriers of Li + and O 2 , respectively. A balanced activity descriptor defined as surface electronegativity (V SE , V SE = (V SEA + V SIP )/2) was developed with a volcano correlation to reduced charge overpotential, identifying those catalysts with a V SE value of 1.7-2.2 V possess high OER activity ( Figure 3A). This descriptor provides a new guide for predicting OER catalysts with certain surface structures.
The kinetics of ion migration in solid electrolytes underpins the intrinsic mass transport of solid-state LABs.
Unlike in a liquid battery, there is only one type of ion (Li + ) with mobility in SSEs. 71 The diffusion can be depicted as ion hops between stable ground-state sites and/or metastable anion sites such as O 2− , S 2− , or polyanionic moieties. 70,72,73 The bonding environment of these sites determines the migration pathway of ions, as a result of the availability and connectivity of arranged anions. In an inorganic crystalline electrolyte, there are three typical migration mechanisms ( Figure 3B-D): (1) vacancy diffusion realized by the ion migration to adjacent vacant sites, (2) direct interstitial transfer between incompletely occupied sites, and (3) concerted or correlated interstitial mechanism that the migration of interstitial ions permute neighboring lattice ions into the adjacent sites. 70 In a polymer electrolyte, the successive coordination of mobile ions and polar groups on the chain segments enables ion migration. 74 The key descriptor for ion transport in SSEs is ionic conductivity (σ). For inorganic crystalline electrolytes, σ is defined by a modified Arrhenius relationship related to the product of charge (q), concentration (n), and mobility of charge carriers (u): where σ 0 presents the pre-exponential factor for intrinsic carrier density; m typically equals to −1; k B is the Boltzmann constant; T is the temperature; and E a indicates characteristic activation energy for ion conduction comprising the formation energy of mobile defects (E f ) and the energy barrier for their migration (E m ). For polymer electrolytes, the description of σ gives some discrepancy as shown in the following equation: where R stands for the ideal gas constant; and T is the reference temperature while a positive difference compared to the glass transition temperature of a polymer benefits the inside irregular movement of Li + . It is noteworthy that the ion migration in SSEs is a multiscale process and the obtained impedance in a routine electrochemical measurement is the result of all these factors. But the comprehensive understanding of mechanisms from the atomic scale to the device scale can guide the way for realizing fast ion transport. In a recent review, Famprikis et al. 70 provide detailed summarization and discussion of the mechanism study on multiscale ion transport accompanied by some progressed techniques, contributing to an in-depth and multidimensional inspiration for the development of practical SSEs.
With the utilization of SSEs, the thermodynamics and kinetics study that reside in the solid-solid interface are needed reasonably. At the cathode side, the premise for an ideal SSE is that its HOMO level should be below the Fermi energy level of the discharge product to prevent SSE oxidation and unpredictable cathode electrolyte interphase (CEI), 75 which is similar to the research results in liquid-state LABs. 56,57 Analogously, its lowest unoccupied molecular orbital should be higher than that of the Fermi energy level of the Li anode, or the reduction of SSE will deteriorate its stability and generate uncontrollable SEI. The presence of CEI and SEI is likely to increase interfacial resistance to Li + migration and charge transfer, as a drag on electrochemical kinetics while the expected high-efficiency kinetics at the solid interfaces demands qualified conductivity for both electron and ion. Interfacial kinetics is a common problem for all solid-state batteries. However, the case for solid-state LABs is more complicated, as a combination of electron transfer, ion migration, O 2 circulation, and parasitic reactions. Thus, constructing a tight and integral "air-solid-solid" triple-phase contact at the cathode/electrolyte interface is a significant challenge. [76][77][78] Characterized by the electron-insulated property of Li 2 O 2 , the charge-discharge process is severely hindered at the triple-phase sites. The limited electrochemical reversibility of solid-state Li 2 O 2 inevitably results in escalating recession of reaction activity and collapse of triple-phase boundaries, making the physical contact shift from a connected configuration to a "point to point" model and then exceedingly decrease the interfacial kinetics.
Overall, the thermodynamics and kinetics of solidstate LABs are the foundation for understanding the origin of discharge/charge overpotentials and decayed stability. But the unrevealed mechanisms such as the electrochemical/chemical pathway at the cathode, intermediates behaviors in solid electrolytes, and interfacial reactions at solid-solid(-gas) contact regions, highly restrict the development of instructional theories for practical devices. We urgently need ingenious computational work and in situ characterization methods to identify the RDS indicators and qualify the optimal thermodynamical/kinetics factors for battery performance from the atomic scale to device scale. Then we can build up desirable electrochemical reactions and structural engineering for the specific bottleneck.

| Main challenges
As we have mentioned, the preponderance of intrinsic safety and stability enables solid-state LABs to become a practical option for next-generation energy storage techniques. However, coupled with complicated electrochemicalchemical reactions that require high integration of interfacial behaviors in multiscale, the solid-state LABs are confronted with several challenges (Figure 1).
(i) Mechanisms and kinetics of air cathode. The "air-solid-solid" triple-phase reaction accommodated at the cathode is the origin of high energy density for solid-state LABs. And its reversibility and consistency dominate the overall performance such as energy efficiency, rechargeability, stability, and life span. As mentioned above, even though some remarkable in situ electron microscopy studies have provided insights into the chemical evolution of solid products, 46,49,53 the types of intermediates and their evolution involved in discharge/charge processes are poorly understood. The insufficient knowledge about cathode mechanisms induces scarce research on thermodynamics and kinetics, lack of effective criteria, and presently, intensive studies on cathode targeting for building up highly active bifunctional catalysts and favorable porous structures that can perform well in electrochemical tests. But it is hard to reach the essence of cathode electrochemistry/chemistry underneath the demonstrated properties without theoretical guidance, like operating in a black box. Parasitic chemistry of contaminates in the air such as H 2 O and CO 2 , arouses additional interferences for the theoretical study of cathodes working in real air conditions. Thus, the well-designed in situ characterization methods and computational models with prudent variates control are highly recommended but extremely challenging. (ii) Ionic conductivity and stability of electrolytes. The realization of applicable SSEs is a critical precondition for intrinsically safe solid-state LABs. Key properties of SSEs include high ionic conductivity that has been associated with the critical current density initiating Li dendrite at the Li anode/ electrolyte interface, 79,80 negligible electronic conductivity which prevents short circuits arising from the growth of Li dendrite in SSEs, chemical compatibility with both cathode and Li anode, and wide electrochemical stability window. 78 Characterized by the sluggish ionic mobility, the Li + conductivity (σ Li ) of SSEs usually dwells in the range of 10 −6 -10 −4 S/cm, 2-4 order of magnitude below that for a liquid electrolyte (10 −2 S/cm). 81 Among these SSEs, inorganic sulfides with higher ion conductivity and better compatibility with Li anode have been a favored choice in solid-state LIBs, while their instability for air atmosphere and tendency to generate toxic H 2 S restrain the application in LABs. The most studied NASICON-type SSEs can supply decent σ Li while the cations prone to be reduced by Li anode at low potentials, like Ti 4+ in Li 1+x Al y Ti 2−y (PO 4 ) 3 (LATP) and Ge 4+ in Li 1+x Al y-Ge 2−y (PO 4 ) 3 (LAGP), rendering chemical stability concerns at the Li anode/electrolyte interface. Other stringent requirements for air tolerance, thermal stability, adequate mechanical properties, and capability of being processed into desirable thin electrolytes exert more challenges for SSEs. (iii) Lithium anode issues. Compared with liquid electrolytes, the SSEs possess a superior mechanical advantage over protecting Li anode by restraining the dendrite penetration, which leads to low Coulombic efficiency and safety concerns for all Li-metal batteries. Specifically, the solid-state inorganic ceramics were opined to be unaffected by Li dendrite propagation and benefited from their high Li transfer number, 78 high shear modulus with the magnitude of tens to hundreds of GPa, [82][83][84] and low fracture toughness. 85 However, the observations that Li dendrites preferably penetrate the SSEs through or along grain boundaries, surface defects, and coterminous pores, are challenging the conviction of impervious dendrite penetration in SSEs. [86][87][88] Except for the dendrite issue, the Li anode corrosion could be triggered by the shuttle behaviors of O 2 , H 2 O, and CO 2 into the solid matrix under air environment, along with volume change during Li stripping/ plating, inevitably retrograding Coulombic efficiency and cycle performance. Apparently, the Li anode assembled in a solid-state LAB faces more challenges than other Li-metal batteries operated as sealed systems, requiring more scrupulous study to impel secure utilization of Li anode. (iv) Interfaces in solid-state LABs. As shown in Figure 1, except for the triple-phase interface for ORR/OER, the solid-state LABs accommodate the other four types of interfaces as Li anode/electrolyte, electrolyte/electrolyte, addictive/electrolyte, and cathode/ electrolyte interfaces. The quantity of triple-phase interfaces determines the overall activity of ORR/ OER, which need consistent accessibility and integrity but is challenged by the coverage of solid products and structural collapse of the cathode.
Other interfaces can be categorized as follows: (1) voids, formed via unideal close packing of electrode and SSEs, as well as electrode pulverization during cell operation, could induce high contact resistance and dendrite growth; (2) grain boundaries, presenting a Li-deficient space-charge layer generated by Li + transfer between two adjacent particles with different electrochemical potentials, could greatly inhibit ion conductivity at the interface; (3) those produced by chemical/electrochemical reactions, coming from the mismatch chemical potential of electrode and SSEs, or the instability of SSEs under low/high voltages, could contribute to the formation of undesirable interfaces and exacerbate the charge transfer resistance. 89

| STRATEGIES FOR HIGH-PERFORMANCE SOLID-STATE LABs
Characterized by a hierarchical solid structure, half-open operating environment, and the utilization of Li metal anode, the solid-state LABs face miscellaneous challenges such as intractable interface physics and interface chemistry, parasitic reactions induced by air contaminants, anode dendrite, corrosion, and so forth. An indepth study of thermodynamics and kinetics would provide basic strategies for constructing highperformance solid-state LABs, but comprehensive guiding criteria are absent yet. Extensive efforts have been devoted to researching practical tactics for optimizing specific components and/or enhancing overall battery performance, usually accompanied by specific theoretical studies to find out how these tactics work. In general, there are mainly four effective routes to reaching highperformance solid-state LABs: (1) the improvement of cathode kinetics, with highly active bifunctional catalyst and desirable gas diffusion channels, has been acknowledged as an efficient method to obtain high energy efficiency and extended cycle life; (2) rational electrolyte design to deliver high σ Li and chemical/electrochemical stability, as well as competent mechanical properties which are pivotal for realizing an applicable solid-state LAB; (3) the optimization and protection of Li metal anode via microstructural design, surface functionalization, and introducing functional layer to build up a roust anode; (4) interfacial engineering targeted for low interfacial impedance and stable chemical/electrochemical environment, supports the fast kinetics and intrinsic stability of solid-state LABs.

| Improvement of cathode kinetics and durability
The reversible oxygen conversion reaction makes solidstate LABs distinct from traditional rocking-chair batteries based on ion intercalation, enabling ultrahigh energy density along with unremitting oxygen supply from the external air environment. Theoretically, the capacity of the cathode is no longer a limit for output energy while the available capacity from the Li anode is. However, in a practical solid-state LAB, the limited space for accommodating solid-state discharge products and their sluggish kinetics of formation/decomposition at the cathode severely restrict the overall battery performance. The catalytic behaviors and physical properties of the cathode determine its intrinsic electrochemical/chemical kinetics for ORR and OER, the efficiency of oxygen diffusion, the storage capability of discharge products, and durability for long-term operation. Thus, an ideal cathode for solid-state LABs should contain a highly active catalyst driving reversible oxygen conversion under low overpotentials, a rationally designed porous structure for preferred oxygen diffusion and accessibility to catalytic sites, superior conductivity for sufficient electron supply, and adequate void volume for reserving discharge products.
Carbon materials (such as super P, Ketjen black, CNTs, graphene nanosheets, and reduced graphene oxide) [90][91][92][93] possessing desirable electronic conductivity, large surface area, light mass, and low cost, are widely employed in solid-state LABs as electronic conductive networks and catalysts. Moreover, the desirable interface with low interfacial impedance between carbon and SSE particles can be constructed by simple mechanical milling or heat treatment which is imperative in the fabrication of solidstate LABs. 94 We constructed a two-dimensional (2D) graphene-graphite composite layer on a LISICON electrolyte sheet via simple pencil drawing, which functioned as an air electrode for semi-solid-state LAB, which gave a discharge capacity of 950 mAh/g at 100 mA/g. 95 And we also proposed a kind of cathode composed of CNTs and LAGP (or its analog) ( Figure 4A,B), 96,97 which was sintered at high temperature to form Li ion conducting networks, affording relatively low charge transfer resistance. Compared to multiwall CNTs (MWCNTs), single-wall CNTs (SWCNTs) were perceived with higher electrical conductance and fewer defects for reducing side reactions. 96 The assembled solid-state LAB with a cathode of SWCNTs/LAGP showed a larger capacity of ∼2800 mAh/g for the first cycle than the battery with MWCNTs/LAGP cathode (1700 mAh/g) ( Figure 4C). 96 Despite the advantages of carbon materials, air electrodes with carbon inevitably encounter carbon decomposition arising from the side reactions between carbon and discharge intermediates under high voltages. The accumulation of Li 2 CO 3 from the reaction of carbon with Li 2 O 2 could readily elevate charging overpotentials and irreversibly consume Li ions, bringing an unexpected decline in the energy efficiency and cycling performance of solid-state LABs. Thus, carbon-free catalysts such as Pt, Au, Ru, RuO 2 , and transition metal oxides have been developed to eliminate this concern. 100-103 Liu et al. 104 synthesized a Co 3 O 4 cathode with a porous urchin-like microstructure enabling adequate active sites for reactions and storage space for discharge products. The integration with a hybrid solid electrolyte (HSE) membrane composed of inorganic Li 7 La 3 Zr 2 O 12 particles and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), empowered a solid-state Li-O 2 battery (LOB) to achieve a high initial specific capacity of 8000 mAh/g, close to that of the battery with liquid electrolyte. 104 Kim's 98 group designed a hybrid cathode by depositing an electronically conductive and catalytic layer (Ir and outermost IrO x ) on porous polyimide nanofibers (PI@IrO x NFs) ( Figure 4D). The PI@IrO x NFs supported the reversible conversion of discharge products as Li 2 O 2 and Li 2 CO 3 during cycling for more than 600 h in dry-air (O 2 /CO 2 /N 2 ) atmospheres ( Figure 4E), and decent shape deformability for the application in flexible LABs. 103 An all-in-one electrocatalytic design for meeting the needs of multifunctional cathodes is essential for solidstate LABs but is a challenge. Recently, Kim et al. 99 reported an ingenious strategy to simultaneously enhance the capacity and reversibility of solid-state LOBs ( Figure 4F). The carbon-free ceramic cathode built up on both electronically and ionically conductive Ru-based composite, and LiOH-based reaction chemistry triggered by the addition of water vapor ( Figure 4G,H), rendered remarkable promotion of specific capacity and battery stability of 200 mAh/g over 665 discharge-charge cycles, while other reported cathodes achieved only ∼50 mAh/g and ∼100 cycles. 104 Sun et al. 105 realized the dual modulation of electronic and ionic microenvironment on a novel Li-decorated RuO 2 (Li-RuO 2 ) cathode with an amorphous structure, creating plenty of open frameworks with unsaturated sites and defects for promoting electronic-ionic transport, which benefited fast kinetics at the gas-solid interface for solid-state oxygen electrolysis ( Figure 5G) and homogeneous distribution of discharge products on the cathode ( Figure 5A-F). The assembled LOB with A-Li-RuO 2 exhibits a high specific capacity of 15,219 mAh/g at 100 mA/g and low polarization overpotential between discharge and charge (1.2 V), far superior to these values for C-Li-RuO 2 (11,900 mAh/g, 1.4 V) and rutile RuO 2 (7896 mAh/g, 1.6 V) ( Figure 5H), as well as most reported solid-state LOBs ( Figure 5I). These studies provide instructive exemplification of integrated design strategy for solid-state electrolysis in solid-state LABs. However, the LiOH chemistry with fast kinetics or other desirable reactions is highly relevant to interfacial mass transport and electrochemical environment, which are contingent on the properties of solid electrolytes.

| Solid electrolyte design
An ideal solid electrolyte for LABs is characterized by high total ionic conductivity (10 −2 -10 −3 S/cm), negligible electronic conductivity, wide electrochemical window   zeolites, 116,117 LiPON, 118 as well as sulfides like Li 2 S-P 2 S 5 binary system 119,120 and thio-LISICON. 121,122 The advantages and disadvantages of these inorganic SSEs have been summarized in Table 1. The inorganic sulfides possess a notable preponderance of high ionic conductivity, favorable compatibility with Li anode, and wide electrochemical window, qualifying as a competitive choice in Li + conductive solid electrolytes. 77,130 However, the high sensitivity to humidity accompanied by risky leakage of toxic H 2 S, makes inorganic sulfides hard to be processed and utilized in the open air. Thus, inorganic oxides present as a more eligible choice for solid-state LABs with established supremacy of chemical stability in the air, along with decent ionic conductivity and electrochemical window, wherein NASICON, garnet, perovskite, antiperovskite, and zeolite are typical representatives.
NASICON-type solid electrolytes origin from the discovery of fast Na + transport in Na 1+x Zr 2 Si Goodenough et al. 131 in 1976, in which the Na + conductivity at 300°C reached up to 0.25 (Ω cm) −1 . As shown in Figure 6A, NASICON-type solid electrolyte displays a 3D skeleton with a rhombic unit cell composed of MO 6 octahedron and PO 4 tetrahedron by sharing vertex oxygen ions, giving an R c 3 space group with a general formula of AM(PO 4 ) 3 (A = Li, Na, or K, while M = Ge, Zr, or Ti). Depending on the distortion of the structure and the radius of the alkali metal ions, the alkali metal ions would occupy different positions and form ion migration channels between them, which constitute the ion conduction network of the NASICON structure. When the ionic conductor structure matches the size of the migrating ion, the ionic conductor can obtain the maximum diffusion coefficient and the lowest activation energy for delivering high ionic conductivity. Thus, in a Li + -conductive NASICON electrolyte, the mobility of Li + is mainly controlled by the narrowest point located in the conduction path known as the "bottleneck." LiTi 2 (PO 4 ) 3 and LiGe 2 (PO 4 ) 3 represent two excellent host structures with suitable structures for Li + migration, while the σ Li can be further improved via partial substitution of Ti 4+ /Ge 4+ ions by divalent or trivalent cations such as Al 3+ , Ga 3+ , In 3+ , Fe 3+ , Cr 3+ , Zn 2+ , and Ca 2+ to effectively widen the bottleneck. 84,[132][133][134] The LATP ceramic commonly used in solid-state LABs has a high ionic conductivity of over 10 −4 S/cm at room temperature. 132 And even a maximum conductivity of 10 −3 S/cm in polycrystalline Li 1.3 Al 0.3 -Ti 1.7 (PO 4 ) 3 prepared by field-assisted sintering was observed. 123 Similarly, the Li 1.5 Al 0.5 Ge 1.5 (PO 4 ) 3 demonstrated high ionic conductivity of 2.4 × 10 −4 S/cm at room temperature. Whereas, under the intimate contact of LATP or LAGP with Li anode, Al 3+, and Ge 4+ in the electrolyte are prone to be reduced by Li  Li-Al alloy or Li-Ge alloy while Ti 4+ tends to be reduced into Ti 3+ , inducing an unstable structure and even electrolyte cracking with the change of internal stress. 106,124 In addition, the electrolyte decomposition could produce oxygen, which further reacts with Li metal and then renders intense thermal runaway. 135 For this reason, NASICON electrolytes should not be placed at excessive temperatures when used in Li-metal batteries.
To resolve the contact problem, introducing an interlayer like liquid/polymer electrolyte and SEI has been certified as a practical strategy for stabilizing the electrolyte/Li anode interface. The details will be discussed in  125 Li + preferentially occupies the fourfold coordinated cation sites until the coordination number reaches three. But the tetrahedrally coordinated Li hardly participates in ion migration, resulting in low ionic conductivity of Li 3 -type garnets, such as Li 3 Nd 3 Te 2 O 12 delivering 1 × 10 −5 S/cm at 600°C merely and high activation energy of 1.22 eV. 125 In 2003, Thangadurai et al. 126 reported the Li 5 -type garnet as a novel family of fast Li + conductor, namely Li 5 La 2 M 2 O 12 (M = Nb, Ta), in which three 4-fold coordinated Li + and two 8-fold coordinated Li + form 3D ion migration channels, presenting decent σ Li of ≈1 × 10 −6 S/cm at room temperature. However, a practical SSE for Li batteries works on the premise of qualifying a σ Li of no less than 10 −4 S/cm. In 2007, Murugan et al. 136 explored a sheet-like Li 7 -type garnet, that is, Li 7 La 3 Zr 2 O 12 (LLZO), in which the Li atoms located at the tetrahedral site and distorted octahedral site, respectively, formed a loop structure accommodating the 3D network of Li + migration pathway and then achieved superior σ Li of 7.74 × 10 −4 S/cm at room temperature and low activation energy of 0.34 eV. High-valence dopants have been intensively studied to increase Li vacancy concentrations and reduce short-range ordering for enhancing σ Li . The highly improved σ Li close to 1-2 mS/cm for Ta/Te/Ga-doped LLZO like Li 6.4 La 3 Zr 1.4 Ta 0.6 O 12 (1.0 × 10 −3 S/cm), 137 139 has been observed. Despite the desirable σ Li , the susceptibility to humidity and CO 2 of garnet-type Li + conductors F I G U R E 6 Crystal structures of (A) NASICON-type, (B) garnet-type, (C) perovskite-type, and (D) zeolite-type SSEs. SSE, solid-state electrolyte.
inevitably produces a carbonaceous layer and then increases the interfacial resistance, and unexpected formation of Li dendrites originating from uneven interfacial contact. The introduction of a transition layer with strong chemical bonding energy to Li metal can remarkably alleviate the interfacial problem as demonstrated in Sections 3.2.3 and 3.4.2.
Perovskite-type ionic conductors, ideally presenting a Pm m 3 space group with the general formula of ABO 3 (A = Ca, Sr, La; B = Al, Ti), comprise cubic unit cells in which A ions and B ions take 12-fold and 6-fold coordinated sites, respectively ( Figure 6C). The elements doping brings Li + into perovskite and achieves a type of Li + conductor, namely Li 3x La 2/3−x□1/3−2x TiO 3 (LLTO). The migration of Li + proceeds along the A-site vacancies and goes through the bottleneck containing four oxygenadjacent ions. Thus, the type of introduced A-site cations, Li + and/or vacancy concentration as well as the interaction between them, have a prominent impact on the σ Li of LLTO. 27 The enhanced σ Li of LLTO can be obtained via controlling the concentrations of vacancies and Li + ions, with enlarged bottleneck when A sites are partially occupied by rare earth ions or alkaline earth ions possessing large radius. For example, Li et al. 140 applied a codoping strategy of Ta 5+ and Sr 2+ , realizing four-fold enhancement of σ Li from 3.7 × 10 −5 -1.4 × 10 −4 S/cm in LLTO. Although elemental doping can improve the crystal conductivity of LLTO, its grain boundary problem is basically unresolved and becomes the most intractable issue, which not only deteriorates total σ Li but also becomes the inducement for Li-dendritic propagation. 84,141 However, it is hard to distinguish the origin of the grainboundary core as the discrepancy in defect segregation energy between the bulk and grain boundaries, or the formation of an insulating phase at the grain boundaries since its extremely small width of 2-3 unit cells. 142,143 Effective tools for ascertaining the origin are urgently needed to design practical LLTO keeping from the poor grain-boundary transport. In addition, like LATP, the instability of LLTO toward the Li anode stemming from the reduction of Ti 4+ into Ti 3+ , further limits the application of LLTO in solid-state LABs. In 2012, halogen-based Li-rich antiperovskites with a general formula of Li 3 OA (A = Cl or Br) were proposed as highperformance superionic Li + conductors, showing a high σ Li of more than 1 × 10 −3 S/cm at room temperature and low activation energy of 0.2-0.3 eV. 128 The simulation results of ab initio molecular dynamics indicated that antiperovskites with perfect crystal structures were not desirable Li + conductors, while Li vacancies and structural disorders facilitated Li + migration by lowering the enthalpy barriers along favorable pathways, rendering high σ Li . 129 This implies that the σ Li of antiperovskites can be rationally tuned by Li vacancy concentration and structural disorder. Li et al. 144 transformed Li 2 OHCl antiperovskites from an orthorhombic phase to a cubic phase via partially substituting OH − by F − , which increased the tolerance factor for favoring disordered OH − orientation. In this way, the rotations of relevant hydrogen bonds get promoted and then decrease the activation energy for a Li + transfer to an adjacent vacancy site. 144 Whereas, the Li 3 OA antiperovskites also face stability problems due to their thermodynamical metastability, 129 as Li 3 OCl could decompose into Li 2 O 2 , LiCl, and LiClO 4 when the applied voltage surpasses 2.5 V. 114 Thus, effective strategies for optimizing the stability of antiperovskites is essential to bring their superiority of superionic conductivity into full play.
Zeolites emerge as a novel type of SSEs for solid-state LABs as first explored by Chi et al. 27,116 As shown in Figure 6D, the structure of zeolites is characterized by corner-sharing tetrahedra of Al, Si, and P joined by oxygen bridge bonds as the primary building units, giving an open framework with well-defined micropores. 145 Benefiting from the superiority of ordered micropores and incessant ion-migration pathway, as well as intrinsic stability toward Li metal and air atmosphere, zeolites demonstrate fascinating compatibility in solid-state LABs. 116,146 The Li + exchanged zeolite membrane (LiXZM) presents a high σ Li of 2.7 × 10 −4 S/cm and a low electronic conductivity of 1.5 × 10 −10 S/cm, accompanied by the high compatibility to Li anode and air, provided an all-in-one strategy for solving interfacial issues, Li dendrites, and stability concerns in solid-state LABs. 116 A solid-state LAB was integrated by in situ assembly of LiXZM with cast lithium and CNTs ( Figure 7A) and delivered an ultrahigh capacity of 12,020 mAh/g ( Figure 7B) and an extended cycle life of 149 cycles at 500 mAh/g with a fixed capacity of 1000 mAh/g in ambient air, much outperformed LAGPbased solid-state LABs of 13 cycles ( Figure 7C). Owing to favorable flexibility and electrochemical performance, the solid-state LAB with integrated cathode and LiXZM (C-LiXZM) showed a promising prospect for practical energy storage devices ( Figure 7D).
To sum up, inorganic oxides including NASICON, garnet, perovskite, antiperovskite, and zeolite are typical SSEs for solid-state LABs. Compared with perovskite and antiperovskite, NASICON and garnet SSEs normally possess higher σ Li , while the instability toward Li metal and/or air arouses more interfacial problems to be solved. Structural modification through optimizing bottlenecks underpinned by element doping, building up 3D ion migration channels, as well as modulating Li + and/or vacancy concentrations is an acknowledged strategy to obtain enhanced σ Li and structural stability in inorganic SSEs. However, the guiding criteria are still absent that require extensive studies on different crystal systems from both theoretical and experimental points. The successful utilization of zeolites paves a new way to construct practical SSEs with high σ Li and intrinsic stability. But the in-depth research on conductive behaviors related to different cage sites, and the balance between mechanical strength and conductivity of zeolite SSEs are indispensable. poly(methyl methacrylate) (PMMA), poly(vinyl chloride) (PVC), poly(vinylpyrrolidone) (PVP), poly(tetrafluoroethylene) (PTFE), and so forth.

| Polymer electrolytes
As the first discovered SPE with conductive behavior of alkali ions, 148 PEO has shown several advantages such as strong Li + solvating ability, preferred dimensional stability, and mechanical properties, thus has been widely explored in solid-state LABs ( Table 2). For instance, Balaish et al. 166 reported a PEO-based LOB displaying a higher discharge voltage (≈80 mV) and a lower charge voltage (≈400 mV) at 80°C than the liquidstate LOB. However, PEO is with a semi-crystalline structure, wherein the amorphous phase with activated chain segments above the glass transition temperature (T g ) supports Li + itineration through continuous Li-O bonds formation and breaking. Therefore, typical linear PEO exhibits insufficient σ Li based on the high crystallinity, which hardly provides free volume for ionic transition with a stiff structure, especially at low temperatures. 167 Apparently, decreasing the crystallinity of PEO is an effective strategy to construct a desirable SPE with enhanced σ Li . A series of approaches such as the introduction of plasticizers [168][169][170][171] or nanofillers 172,173 containing large anionic groups, polymer blending, [174][175][176] and designing block copolymers 177,178 have been implemented to improve the σ Li of PEO-based SSEs in LIBs while not gotten studied in LABs yet. Meanwhile, PEO-based SPEs are readily oxidized by the discharge products of LABs, inevitably generating stability concerns. The auto-oxidation of PEO in an oxygenated environment arising from the accelerated radical formation at the applied potential higher than OCV has been validated by Harding et al. 150 They also studied the chemical stability of some common polymers utilized in LABs in the presence of Li 2 O 2 . 179 Of the polymers researched, PEO may suffer from some cross-linking, while PAN containing electrophilic nitrile group allowed nucleophilic attack, and the halogenated polymers (PVC, PVDF, and PVDF-HFP) underwent dehydrohalogenation reactions with the existence of Li 2 O 2 , showing a reactivity tendency of PAN>>PVC≈PVDF>PVDF-HFP>>PVP. Meanwhile, PMMA with methyl and methoxy functionalities contributed to the reduction of potential reaction pathways, while PTFE and perfluorosulfonic acid resin (Nafion) with a lack of α-H or β-H adjacent to the electron-withdrawing fluoride groups, allowed the superior stability toward the nucleophilic environment of LABs. This study provides a guideline for the bottom-up design of stable polymers for LABs characterized by the inhibition of general reactivity toward functional groups with nucleophiles. But it is more complicated when the SPEs work in a practical condition instead of Li 2 O 2 alone, like stable PMMA is challenged by accelerated decomposition by any impurity such as water in the cell, with nucleophilic substitution by strong bases like O 2 •− at the carbonyl center. [179][180][181] Thus, the stability mechanisms of these polymers under working conditions are expected to be adequately understood, which requires valid in situ evidence but remains challenging.
With a low to moderate cross-linking density, SPE can be swollen by a plasticizer in abundance presenting as a quasi-solid-state gel polymer electrolyte (GPE). Balanced with the mechanical preponderance of a polymer network and the high ionic conductivity of liquid electrolytes, these GPEs usually provide high σ Li , low interfacial resistance, and preferred flexible properties while avoiding the evaporation and leakage of liquid solvents, as well as effectively protect Li anode from air contaminants in an open system. In previous research studies, many host polymers as PAN, PMMA, PVDF, PVDF-HFP have been explored in GPE-based LABs ( Table 2). [182][183][184][185] Among them, PVDF-HFP possesses lower crystallinity and T g , superior chemical and mechanical stability, and thus gets intensively studied. Of the developed plasticizers, ionic liquid (IL) electrolytes can induce a more stable and conductive SEI on the surface of the Li anode accompanied with high hydrophobicity against water crossover, thus promising more eligible GPEs to be used in open environment. 182 For example, Zhang et al. 186 reported a combination of IL (PMMITFSI), silica, and PVDF-HFP polymer matrix, enabling a GPE with high σ Li (1.83 × 10 −3 S/cm) and good Li anode protection. Amanchukwu et al. 183 presented a pioneering work on the control of oxygen reduction chemistry (from 2e to 1e mechanism) via the formation of IL-superoxide complexes in a IL (PYR 14 TFSI)-LiTFSI-PMMA GPE. According to Pearson's hard soft acid base theory, they proposed that Li 2 O 2 would form if Li + presented when O 2 was reduced to O 2 •− (hard acid-hard base), or the IL-superoxide complexes would govern the consequent process (soft acid-soft base) ( Figure 8A). This study indicates an energy-intense system for LABs and expands the future for Na-air batteries and K-air batteries.
Recently, Hoffknecht et al. 187 pointed out that the Li + transport in GPE with TFSI-based ILs still followed a sluggish chain pathway as solid PEO, while the ILs with more coordinating anions like TFSAM competing with PEO for Li + solvation, could achieve higher Li + transport efficiency and had been applied in LIBs. This research provides a new sight for design high-performance GPEs, and may inspire future advances in the field of LABs. In addition, the RMs are also applied to increase the energy efficiency and prolong the cycle life of GPE-based LABs. Fu et al. 188  in biological systems, then applied oxidized activated carbon (OAC) serving as a SOD mimetic. As shown in Figure 8C, the redox cycle of OAC enabled efficient reduction of soluble LiO 2 into LiOH instead of Li 2 O 2 ( Figure 8B), which could mitigate side reactions triggered by soluble LiO 2 , and thereby achieved prolonged life span. As a promising power resource for wearable devices, flexible LABs based on bendable GPEs provide alluring advantages of high energy density, favorable safety, and deformation ability. [190][191][192][193][194][195][196][197][198] Wang et al. 192 integrated a rippled air electrode composed of aligned CNT sheets stacked on a GPE, and a Li array electrode, endowing a highly flexible fiber-type LAB with high electrochemical performance under stretching, bending, or twisting condition ( Figure 8D-H).
Despite the attractive merits of polymer electrolytes on low cost, high processing ability, and favorable tolerance to volume change, their application is retarded by low σ Li , matrix decomposition, and so forth. The polymer-in-salt 199 concept could be a feasible strategy to increase the σ Li of polymer electrolytes with T g well below the ambient temperature but has not been explored in LABs yet. The most studied solution for a practical solid electrolyte is constructing the hybrid inorganic/polymer system, which will be overviewed below.

| Hybrid solid electrolytes
As previously discussed, the inorganic ceramic electrolytes are characterized by good thermal stability, desirable Young's modulus, and superior ionic conductivity, but their rigidity results in untight contact with electrodes rendering high interfacial resistance. Meanwhile, the GPEs offer excellent flexibility and good wettability toward electrodes, but inferior ionic conductivity and low mechanical strength to stop Li dendrite penetration. Obviously, both an inorganic ceramic electrolyte and a GPE alone cannot meet all expectancies for an ideal SSE while the two are complementary. Thus, integrating the HSE with an inorganic SSE and a GPE is a promising strategy for taking advantage of both sides while remedying their shortages mutually. [163][164][165][200][201][202] An eligible HSE should be qualified with (i) indispensable stability in O 2 or air, 203 (ii) a rigid inorganic component with 3D ion transport channels covering the surface of Li anode evenly to enable uniform Li + distribution and high mechanical strength, 204,205 (iii) adequate flexibility and smoothness to ensure tight contact with electrodes, 164 and (iv) homogeneous distribution of inorganic electrolyte and GPE to avert local disorder of Li + . [206][207][208] Here, some paradigm designs based on typical SSEs such as NASICON-type LAGP, garnet-type LLZO, perovskite-type LLTO, and amorphous SSE are illustrated for in-depth understanding.
Wang et al. 164 fabricated a novel HSE with a rigid nanosized LAGP core and ultrathin flexible PVDF-HFP shell. The force-separation curve certified that the surface of LAGP solid particles was homogeneously wrapped by a soft PVDF-HFP polymer with a thickness of around 5 nm (Figure 9A-C). Apparently, the core-shell interface strongly hinged on the ratio of LAGP to PVDF-HFP polymer as well as the size of LAGP. To understand the interface mechanism of LAGP@PVDF-HFP, three control groups characterized by the high-content and micro-sized LAGP (HSE-I), low-content and nano-sized LAGP (HSE-II), and GPE in the absence of LAGP were employed. The symmetric Li/Li battery with LAGP@PVDF-HFP exhibited the longest cycle life ( Figure 9F) without distinct Li dendrites formation ( Figure 9E). The mechanism study suggested that the nanocrystallization of LATP contributes to a full-time uniform distribution of Li + while HSE-I suffers from the later formation of sphere dendrites; and the homogeneous coverage of PVDF-HFP on LAGP ensures tight and stable Li anode/HSE interface, while the insufficient density of HSE would induce local Li + disorder rendering more severe dendrite problem than GPE ( Figure 9D,E). As a demonstration, the quasi-solid-state LOB with LAGP@PVDF-HFP afforded a long cycle life (146 cycles) at 300 mA/g with a fixed capacity of 1000 mAh/g ( Figure 9G), and markedly outperformed the control groups.
Zhao et al. 165 constructed an HSE comprising Benefiting from these advantages, the assembled LOB with PSSE/GPE delivered a long cycle life of 194 cycles, which was far superior to that with GPE ( Figure 10F). Le et al. 201 proposed an assembly of PVDF-HFP polymer with Al-doped LLTO (A-LLTO) particles covered with a modified SiO 2 (m-SiO 2 ) layer ( Figure 10D), which effectively protected A-LLTO from chemical decomposition while enhancing its adhesion to PVDF-HFP. The obtained HSE achieved a high σ Li of 1.22 × 10 −3 S/cm and remarkable suppression of the growth of Li dendrites compared to GPE ( Figure 10B,C). Compared to highly crystallized inorganic SSEs, the amorphous SSE with low grain boundary resistance is preferred to provide higher Li + diffusivity. Thus, our group 163 combined the amorphous LiNbO 3 with poly(methylmethacrylate-styrene) on a polyethylene (PE) substrate, realizing a quasisolid-state HSE with desirable thermal stability keeping from shrinkage or degradation even after heating at 120°C for 1 h ( Figure 10A). The stable interfacial resistance supported by this HSE effectuated a stable cycle performance of the assembled LOB over 100 cycles.
Very recently, Kondori et al. 209 presented an intriguing study on an HSE that Li 10 GeP 2 S 12 (LGPS) nanoparticles embedded in a PEO matrix, realizing a four-electron reaction with Li 2 O formation under room temperature. In this HSE, LGPS chemically bonded with a silane-coupling agent, mPEO-TMS, which protected LGPS from decomposition at the electrode/SSE interfaces. The Li 2 O discharge product was detected with increasing peak intensity over 1 h of discharging in the in situ Raman spectroscopy ( Figure 11A,B). This four-electron reaction was confirmed by acid titration coupled with ultraviolet-visible spectroscopy and ex situ differential electrochemical mass spectroscopy (DEMS) (Figure 11C), and its reversibility was further elucidated by in situ DEMS result ( Figure 11D). The X-ray diffraction patterns also indicated the highly reversible Li 2 O electrochemistry while discharge and charge, and the small amount or amorphous nature of LiO 2 and Li 2 O 2 products ( Figure 11E). The authors proposed that the four-electron reactions were driven by the abundance of O 2 at the interface, excess Li + originating from high σ Li of HSE, and F I G U R E 11 (A) In situ Raman spectroscopy experiments and (B) relative Raman peak intensities at different time intervals during discharge with a limited capacity of 125 mAh/g at 1 A/g. Calculated electron transfer number using (C) acid titration coupled with UV-vis spectroscopy (inset: ex situ DEMS result) during discharge with a limited capacity of 1 Ah/g at 1 A/g, and (D) in situ DEMS during charge with a limited capacity of 5 Ah/g at 1 A/g. (E) XRD patterns of the discharged/charged cathodes at different cycle numbers during (F) the galvanostatic cycling over 1000 cycles with a limited capacity of 1 Ah/g at 1 A/g. (G) The Coulombic efficiency, energy efficiency, and polarization gap during cycling. Reproduced with permission: Copyright 2023, AAAS. 209 DEMS, differential electrochemical mass spectroscopy; UV-vis, ultraviolet-visible; XRD, X-ray diffraction. mixed electron-conductor behavior at the cathode, but the lack of detailed mechanism study. The battery was rechargeable for 1000 cycles ( Figure 11F) with a low polarization gap that increased from 50 mV initially to ≈430 mV at the end, while the energy efficiency gradually dropped from 92.7% to 87.7% ( Figure 11G).
These pioneering research studies validate the advanced nature of HSEs over individual inorganic SSEs and GPEs. However, a practical HSE is not a simple mixture with uncertain structure, but a well-designed integration with a homogeneous distribution of the hybrid structure for stable and efficient interfacial behaviors, adequate compactness for blocking O 2 crossover and avoiding local disorder of Li + , as well as sufficient rigidity for suppressing Li dendrites. As a result, the construction of HSEs is challenged by structural control, processing difficulty, and a balance between the merits of inorganic SSEs (high σ Li , thermal stability, and Young's modulus) and GPEs (preferred flexibility and wettability). When faced with future commercialization, the overall cost and air compatibility should also be vital concerning factors.

| Li anode optimization and protection
As one kind of Li metal battery, undoubtedly, solid-state LAB dictates the need for a robust and chemically stable Li anode that is qualified for curbing Li dendrites formation and bearing the volume change while sequent Li plating/stripping. Meanwhile, the half-open structure of solid-state LAB introduces concerns about Li anode corrosion and mechanical strength decay, induced by the permeation of air contaminants such as H 2 O and CO 2 into the free volume of SSE matrices. Specifically, a small amount of H 2 O can trigger a fast N 2 -attacking reaction (Equations 10 and 11) to form Li 3 N and LiOH. 210 2Li N + 6H O 6LiOH + NH .
Several strategies have shown alluring prospects for Li anode optimization and protection, such as Li alloying for elevating anode stability, fabrication of super-hydrophobic SSEs or introducing a functional layer to avoid anode corrosion, and construction of preferred interface environment for realizing low interfacial resistance and inhibited dendrite growth (see details in Section 3.4.2).
Introducing alloying elements was found to effectively diminish the discrepancy in surface energy of Li anode and SSEs, contributing to higher wettability and enhanced (electro)chemical stability at the anode side ( Figure 12A). 211 The Li alloys such as Li-In, [212][213][214][215][216] Li-Si, [217][218][219][220][221] Li-Al, [222][223][224]214,[225][226][227] have been widely studied as advanced substitutes for Li metal anodes in solid-state batteries, while Li-Si 228 and Li-Al 210 also have shown preponderance over Li metal in liquid-state LABs with suppressed O 2 /air-attacking-induced polarization. However, the alloying with inactive elements would inevitably sacrifice the specific capacity of the Li anode, exacerbating the limitation of anode capacity in LABs. Herein, Ma et al. 229 proposed a Li-Na alloy serving for a novel aprotic bimetal Li-Na-O 2 battery with enhanced cycling stability ( Figure 12B). The electrostatic shield effect that the adsorbed Li + ions on the tips were prone to exclude incoming Na + and force Na + to deposit away from the tips, successfully endowing the suppression of anode dendrites and cracks. This research provides a paradigm shift in designing Li alloy anodes but needs to be further explored in solid-state LABs.
The introduction of super-hydrophobic SSEs or functional layer is with a direct effect on repelling the permeation of H 2 O. For example, our group 230 constructed a super-hydrophobic SSE effectuated by the combination of super-hydrophobic SiO 2 matrix and Li + conducting ILs ( Figure 12E). The quasi-solid electrolyte (QSE) showed a decent σ Li of 0.91 × 10 −3 S/cm, excellent thermal stability below 230°C, and super hydrophobicity with a contact angle over 150°, underpinning a safe and long-life solidstate LOB in a humid atmosphere. Similarly, Shu et al. 194 developed a QSE (PS-QSE) with SiO 2 filler and PVDF-HFP matrix, enabling desirable corrosion resistance of Li metal ( Figure 12F,G) and improved anodic reversibility for over 200 cycles. Wang et al. 190 designed a low-density polyethylene (LDPE) film ( Figure 12C) with low H 2 O permeability (0.825 g/(m 2 d)) while high O 2 permeability (40.3 Barrer). The favorable selectivity could effectively restrain Li corrosion induced by H 2 O, and impede the Li 2 CO 3 formation in ambient air ( Figure 12D).
Although the protective strategies mentioned above markedly retard the Li corrosion caused by humidity and undesirable gas crossover, they arouse some side effects as well, such as the sacrifice of energy density and high interfacial resistance. Constructing a stable Li metal/SSE interface with homogeneous Li + distribution for inhibiting dendrite growth, and sufficient compactness for blocking contaminants is a more promising way, which will be discussed in Section 3.4.2.

| Interfacial engineering
Except for the essential components for LABs, the chemical, electronic, and mechanical properties of the involved interfaces in a solid-state LAB are of critical importance for determining its long-term electrochemical performance and viability. As previously mentioned, the solid-state LABs accommodate four kinds of interfaces as (i) triple-phase interfaces for ORR and OER, (ii) voids forming via the untight contact of electrodes and SSEs, (iii) grain boundaries in SSEs, and (iv) the electrode/SSE interfaces. Due to the poor wettability, excessive stiffness, and/or inferior Li + conductive behavior of SSEs, the solid-state LABs face with more intractable interface problems than liquidstate LABs, albeit decreased influence from the crossover of discharge products and air contaminants. Especially, the high interfacial resistance between cathode/SSE and Li anode/SSE, as well as the instability of SSE toward oxidative products or Li anode toward O 2 , H 2 O, CO 2 , and so forth requires rational strategies for interface optimization. Here, these strategies will be categorized and discussed for cathode/SSE interfaces and Li anode/SSE interfaces.

| Interface optimization between cathode and electrolyte
The quantity, availability, and stability of triple-phase interfaces dominate the overall performance of air cathodes, while challenged by inadequate contact with SSE, sluggish Li + transfer, and structural collapse of cathode/SSE interface. Except for constructing cathodes with high electrocatalytic performance and desirable physical structure as discussed in Section 3.1, improving the interfacial conductivity and contact of cathode/SSE interface through constructing abundant triple-phase boundaries, adding the electrolyte with high σ Li , cathode coating, and so forth is an effective tactic for accelerating cathodic kinetics.
For example, Wang et al. 231 developed a cathode with in situ introduced porous plastic crystal electrolyte (composed of succinonitrile, LiTFSI, PVDF-HFP, etc. denoted as SLPB) ( Figure 13A). The high softness and firm adhesion of SLPB ( Figure 13B) enabled tight interfacial contact between SSE and electrode, then formed continuous and abundant triple-phase boundaries at the cathode side. The electrolyte resistance and interface resistance of SLPB-based LAB were effectively decreased compared to the traditional solid-state LAB with LATP or PEO ( Figure 13C), presenting a high specific capacity (5963 mAh/g) and long cycle life of up to 130 cycles. Zhao et al. 232 applied an ionically superconductive Li 3 InCl 6 electrolyte as the interface modifier for N-doped CNTs (NCNTs) cathode and LAGP SSE ( Figure 13D). The high ionic conductivity (1.3 × 10 −3 S/cm) and solution-based preparation method of Li 3 InCl 6 enabled its uniform distribution within the cathode and continuous contact with LAGP. The interfacial resistance of cathode/SSE was significantly decreased from 2056 to 569 Ω ( Figure 13E), realizing fast cathode kinetics and prompting reversibility of discharge products ( Figure 13F). Meanwhile, the point-to-point triple-phase junction of SSEs, cathode, and Li 2 O 2 particles usually leads to high charge polarization and quick failure of solid-state LABs. To conduct optimization, they developed a transformation of the cathode/SSE interface from three-phase contact to two-phase contact by building a hybrid cathode as LiTaO 3 -coated NCNT (NCNT@LiTaO 3 ), wherein the NCNT core acted as an electronic conductor while the LiTaO 3 coating layer performed as an ionic conductor ( Figure 14A). 233 The in situ environmental TEM results visually verified the continuous growth of Li 2 O 2 film in NCNT@LiTaO 3 enabling highly reversible decomposition from a twophase process ( Figure 14B), while pure NCNT suffered from particle-shaped Li 2 O 2 and its residual followed by a three-phase process ( Figure 14C). This optimized cathode/SSE interface achieved steady charge transfer and intact catalytic regions, endowing a remarkable increase in Coulombic efficiency from 38.6% to 80.8%.
Besides, an all-in-one design with the integration of catalysts and SSE to serve as both cathode and electrolyte emerges as a promising strategy, with intrinsically uniform distribution of cathode/SSE interfaces and decreased overall thickness to deliver dramatically suppressed interfacial resistance. Zhu et al. 234 first proposed this battery configuration, in which the LATP scaffold supported efficient Li + transport, while a carbon coating onto it ensured electron transfer and catalytic activity for ORR and OER at the same time ( Figure 15A,B). This design achieved a thin electrolyte layer with a 90% decrease from that in conventional solid-state batteries and a highly porous cathode with a porosity of 78%. The LOB based on this integrated structure delivered a cycle life of 100 cycles at 0.15 mA/cm 2 with a fixed capacity of 1000 mAh/g carbon ( Figure 15C). Similarly, they proposed an ultrathin integrated structure (≈19 μm) with porous LATP and carbon, accompanied by the Si-oil film coating to repel H 2 O and CO 2 from reaching reaction sites ( Figure 15D,E), contributing to a LAB with high practicability in ambient air when cycled at 0.3 mA/cm 2 with a fixed capacity of 5000 mAh/g carbon for 50 cycles (125 days). 235 Although these strategies markedly enhance the interfacial contact and stability between cathode and SSEs. But considering the ORR/OER is characterized by a surface reaction and requires space for accessing O 2 and reserving solid products, the balance between interfacial compactness and catalyst availability is of great challenge. The trade-off strategies have not been studied yet.

| Interface optimization between anode and electrolyte
It is acknowledged that Li metal is highly electropositive and reactive, rendering the spontaneous reaction with most SSEs at room temperature and the formation of SEI. Most binary ionic conductors show intrinsic chemical stability with Li metal as the involved anions present a fully reduced state. For widely used ternary and quaternary SSEs, the stability of Li metal is contingent on the formation energy of decomposition products. As per the previous statement, the LATP and LAGP react with the Li anode at room temperature, resulting in the formation of Li-Al or Li-Ge alloy and the reduction of Ti 4+ into Ti 3+ . 106,124 The binary ionic conductors usually enable a chemically stable interface with Li anode while no SEI layer, where Li + could efficiently transfer from Li metal to SSEs. The SSEs (LiPON, Li 3 PS 4 , etc.) with some electronically insulating decomposition products but at least one ionically conductive component, produce a stable SEI once formed. For SSEs (like LLZO, LATP, and LAGP) that are most used in solid-state LABs, with a thermodynamically favorable reductive decomposition, result in a mixed SEI accommodating both electronically and ionically conductive components which are unstable and tend to grow continuously during cycling then generate increasing interfacial resistance. 89 Meanwhile, the development of most exceptional SSEs with high σ Li is hindered by the large interfacial resistance between Li anode and SSE, due to the rigid nature of SSEs. Although some simple mechanical methods have been proposed to optimize the wettability of Li anode/SSE interface, such as simple nanopolishing 236 and ultrasonic-assisted fusion welding 237 we reported previously, the long-lasting interfacial stability required by LABs is challenging.
There are mainly two ways to avoid such an SEI and the unexpected contact problem to bring these SSEs with the advantage of high σ Li for practical battery applications. One way is to utilize the Li alloy to realize stable Li stripping and plating under kinetically stabilized SEI, but remains a controversial failure under large current density. 88,89,238 The other way is to construct a thin buffer layer between Li anode and SSEs but without a distinct sacrifice of ionic conductivity, such as ultrathin metal/metal oxide coating, Li + -conductive solid interlayer, gel polymer interlayer, and so forth.
For example, our group 239 sputtered an amorphous Ge thin film on the surface of LAGP, then suppressed the reduction of Ge 4+ by Li and realized greatly dropped interfacial resistance from 2506 to 147 Ω cm 2 in symmetric Li/Li batteries ( Figure 16B). A quasi-solid-state LAB further demonstrated this advantage and realized a stable cycle life of 30 cycles in the air. Similar enhancement was achieved in the garnet system by Luo et al., 240 through depositing a thin Ge layer (20 nm) on an LLZO garnet, endowing intimate contact of Li metal and SSE ( Figure 16A). With the formation of Li-Ge alloy, the decreased interfacial resistance from ≈900 to ≈115 Ω cm 2 and stable Li stripping/ plating was achieved. Similarly, they engineered a Ca, Nb-substituted LLZO (LLCZNO) garnet with Al coating, which induced enhanced wettability of the LLCZNO surface ( Figure 16C-H) via forming Li-Al alloy layer ( Figure 16I). 241 The computation result indicated the high stability between Li-Al alloys and garnet SSE with mutual reaction energies of −60 to −40 meV/atom ( Figure 16J), while these minor interfacial reactions supported kinetic stabilization and improved wettability at the interface. Then a markedly reduced interfacial area-specific resistance from 950 Ω cm 2 for the Li|garnet SSE to 75 Ω cm 2 for the Li|Al-coated garnet SSE was produced.
The oxide coating (Al 2 O 3 , ZnO, etc.) known for its high stability and wettability to Li metal, can act as a desirable buffer layer between Li anode and SSEs. Han et al. 242 introduced an ultrathin Al 2 O 3 coating on LLCZNO garnet by atomic layer deposition (ALD) and enabled tight Li anode/SSE contact leading to a remarkable decline of interfacial resistance from 1710 to 1 Ω cm 2 ( Figure 17C,D). They also demonstrated an all-in-one strategy for simultaneously improving the interfacial property with intimate Li anode/SSE contact and a high tolerance for anode volume change. 243 For this concept, an ultrathin and conformal ZnO surface coating was built up in 3D porous garnet SSE via ALD, which notably promoted the wettability between garnet SSE and the molten Li from bulk surface to internal structure by forming Li-Al alloy ( Figure 17A,B), endowing a low interfacial resistance of about 20 Ω cm 2 . In addition to oxides, Le et al. 244  Except for the (electro)chemical properties, the mechanical nature is also pivotal for determining the performance of coating layers, because Li metal experiences incessant volume change during cycling that will accumulate internal stress in the coatings. The stress can induce crack formation in a stiff coating layer with high Young's modulus as aforementioned, while the cracks serve as hot spots for the nucleation and growth of Li dendrite, resulting in severe performance decay and safety concerns for solid-state LABs. Mechanically, polymer coating layers with lower stiffness can accommodate the stress evolution preferably. For instance, Zhou et al. 245 coated a single-Li + conducting polymer film composed of PEO and lithium poly(acrylamide-2methyl-1-propane-sulfonate) (PAS) on LLZTO, realizing good adhesion of PEO-PAS to LLZTO and avoiding direct contact between Li anode and the grain boundaries residing in LLZTO pellet ( Figure 18A). The PEO-PAS polymer coating with markedly improved Li + transfer number is obtained to decrease interfacial resistance and suppress Li dendrite formation ( Figure 18B). Liu et al. 246 stored an apropos amount of liquid electrolyte inside a PVDF-HFP matrix as a polymer coating for LLCZNO, affording a significant decline in interfacial resistance from 1.4 × 10 3 to 214 Ω cm 2 for the Li anode ( Figure 18C,D). To sum up, introducing an artificial SEI layer into solidstate LABs is an effective strategy for optimizing the Li anode/SSE interface. The criteria for an ideal coating layer include high thermodynamical stability with both Li metal and SSEs, electrical insulation, sufficient Li + conductivity, and good mechanical ductility for tight interface contact. High chemical bonding to Li metal, metal/oxide coating, and GPE buffer layer have shown prominent improvement in the wettability and stability of the Li anode/SSE interface. The integrated HSE combining the merits of both ceramic SSE and GPE should also be regarded as an effective all-inone demonstration for realizing overall high σ Li and anode protection simultaneously, as discussed in Section 3.2.3. Besides, constructing 3D Li anode/SSE interfaces is an effective strategy for alleviating the Li + concentration polarization at the interface and endowing homogeneous Li + flux on the Li anode. The interfacial fluctuation can be well reduced by applying a 3D Li anode, 247-249 a 3D SSE, 250,251 or hosting Li in a 3D SSE framework. 243,252 Although these design strategies give promising prospect in developing practical Li metal anode for solid-state LABs, there remain great challenges in structural engineering and processing technologies.

| SUMMARY AND PERSPECTIVE
As the most energy-rich "beyond Li-ion" batteries, LABs have motivated intensive research for approaching the high theoretical energy density rivaling combustion engines by clean electricity sources. Compared to liquidstate counterparts, solid-state LABs provide a more practical prospect with superior air compatibility, intrinsic safety, wide electrochemical window, favorable mechanical properties, environmental benignity, and cost efficiency. However, characterized by the complicated electrochemical-chemical reactions at the cathode, and assorted interfacial behaviors residing in the Li anode/ electrolyte, electrolyte/electrolyte, addictive/electrolyte, and cathode/electrolyte interfaces, the solid-state LABs are confronted with several intractable challenges. Many prominent research groups have contributed paradigms of mechanism studies and effective strategies for propelling the development of solid-state LABs. In this review, we proposed the main challenges and related strategies of this field in detail.
1. The insufficient mechanisms and kinetics study.
The energy-rich property of solid-state LABs originates from the reversible oxygen conversion reaction at the cathode, while its reaction routes and involved intermediates evolution are poorly understood. The insufficient knowledge about discharge-recharge mechanisms, leads to the absence of criteria for enhancing cathodic thermodynamics and kinetics at the atomic level. And a simplification in the theoretical calculation that modeling OER with direct reverse reaction steps of ORR in many papers could bring misleading results against a consensus in this field that OER does not follow the reversed route of ORR. Except for the cathode, the in-depth studies on mechanisms and kinetics for SSE and Li anode are hardly developed. But considering the shared properties with solid-state LIBs, tremendous work in this field would provide a valuable reference. Overall, to identify the specific bottleneck and construct targeting tactics for battery performance from the atomic scale to the device scale, we urgently need to develop ingenious computational methods and in situ characterization techniques. 2. To-be-improved ionic conductivity and stability of electrolytes. An ideal SSE for LABs should possess high σ Li (10 −2 -10 −3 S/cm), negligible electronic conductivity, wide electrochemical window (>5 V vs. Li/Li + ), eligible compatibility with Li anode and cathode, durability to oxidative intermediates and air contaminants, adequate stiffness, and so forth. The inorganic ceramic electrolytes such as NASICON (LAGP, LATP, etc.), garnet, perovskite, and antiperovskite are the most developed SSEs for LABs, showing the merits of good thermal stability, desirable Young's modulus and σ Li , while their rigidity leads to a to-be-improved interfacial resistance. Meanwhile, the chemical instability of the Li metal of LAGP/LATP, the susceptibility to humidity and CO 2 of garnet, the grain boundary problem of perovskite, and the thermodynamical metastability of antiperovskite inevitably impede the application of these SSEs. Structural modifications such as element doping, modulating Li + and/or vacancy concentrations, and building up artificial interlayers could effectively enhance their σ Li and structural stability. However, the guiding criteria are not established yet and demand systematic studies with both theoretical and experimental efforts. Zeolite emerges as a promising candidate for constructing practical SSEs with high σ Li and intrinsic stability, which not only contributes to LABs but also brings a chance for LIBs. To prompt its development, in-depth research on mechanisms like conductivity related to different cage sites, and the balance between mechanical strength and conductivity should be comprehensively carried out. Compared to inorganic SSEs, the SPEs often show attractive merits of excellent ductility and favorable wettability toward electrodes, while suffering from low σ Li , matrix decomposition, and Li dendrite penetration. Strategies such as decreasing their crystallinity, and being swollen by a plasticizer to form GPEs could produce applicable polymer electrolytes for LABs. With complementary properties, the integrated HSE of an inorganic SSE and a GPE is a promising strategy that meets most expectancy for an ideal SSE. It should be noted that a practical HSE is not a simple mechanical mixture but with a well-designed hybrid structure for stable and efficient interfacial behaviors, which need to be meticulously researched. 3. Lithium anode issues. Although the Li dendrite propagation should be alleviated to a large extent in solid-state LABs, with reaping the mechanical benefits of SSEs, the observations of Li dendrites penetration residing in grain boundaries, surface defects, and coterminous pores, repudiate the impervious dendrite penetration in SSEs. Meanwhile, when operated under ambient conditions, the Li anode corrosion accompanied by mechanical strength decay could be induced by the permeation of O 2 , H 2 O, and CO 2 into the free volume of SSE matrices. The continuous Li stripping and plating also result in undesirable volume change. These negative effects inevitably retrograde the overall Coulombic efficiency and cycle performance of solid-state LABs. Strategies for Li anode optimization and protection include substituting Li metal with Li alloys to enhance anode stability, utilizing super-hydrophobic SSEs or introducing a buffer layer to avoid anode corrosion, and constructing artificial SEI to decrease interfacial resistance and restrain dendrite growth, and so forth. Apparently, the Li anode worked in solid-state LABs with a half-open feature faces more complicated circumstances than other sealed Li-metal batteries, requiring more scrupulous study to qualify a practical Li anode. 4. Complicated interfaces in the battery. The chemical, electronic, and mechanical properties of the involved interfaces, especially that between electrolyte and cathode/anode, are crucial for determining the output energy density and long-term viability of a solid-state LAB. The poor wettability, incompatibility to Li anode/cathode products, excessive stiffness, and/or inferior Li + conductive behavior of SSEs, often produce high interfacial resistance and an unstable interfacial environment. Adding auxiliary electrolytes with high σ Li , cathode coating, integration of catalyst and SSE, and so forth are presented as effective methods for accelerating cathodic kinetics. But the tradeoff strategies for balancing interfacial compactness and catalyst availability are absent yet, which needs further in-depth investigations. Introducing an artificial SEI layer with high thermodynamical stability, electrical insulation, sufficient Li + conductivity, and eligible mechanical ductility for intimate contact, has been proposed as an applicable tactic for optimizing the Li anode/SSE interface. The metal/oxide coating, GPE buffer layer, and all-in-one design of HSE have shown prominent improvement in the wettability and stability of the Li anode/SSE interface. Despite these strategies for interface engineering giving promising prospects in developing practical solid-state LABs, there still are many challenges in structural engineering and processing technologies that need to be overcome.
When it comes to practical application, the energy density of a full LAB has been predicted to be 610 Wh/kg and 680 Wh/L, with distinct preponderance over Li-S battery (450 Wh/kg and 450 Wh/L) or the LIB with Li anode and Li-rich NMC cathode (300 Wh/kg and 500 Wh/L). 23 But the maximum performance is guaranteed by operating in the air as an open battery. Characterized by the nonvolatile and (electro)chemically stable nature of SSEs, solid-state LABs show an alluring prospect in the LAB devices working with air. The activities of air contaminants such as CO 2 and H 2 O in solid-state LABs are highly restricted compared to their liquid-state counterparts due to their low reservation ability in SSEs, but not totally obviated. The residual H 2 O molecules at the air cathode could react with Li 2 O 2 and form LiOH, which has been proposed to enable an energy-rich reaction with four electron transfers and fast discharge/charge kinetics. 99,189 But LiOH is prone to evolve into Li 2 CO 3 with the existence of CO 2 , then would greatly elevate polarization potential and retrograde cycling ability. For that reason, the reversible LiOH chemistry is running in pure O 2 merely. At the Li anode side, H 2 O also brings unexpected anode erosion via a fast N 2 -attacking reaction to form Li 3 N and LiOH. Thus, to obtain stable and durable battery performance in an air atmosphere, the penetration of CO 2 and H 2 O should be strictly restrained (other varied contaminants like NO x and organic gas have not been considered in present studies). Utilizing SSEs with high air compatibility is a prerequisite, wherein not only the (electro)chemical stability to CO 2 and H 2 O is required but also the hydrophobic property is indispensable when a protective layer is absent. The dense physical structure and/or the hydrophobic groups of SSEs can be effective against H 2 O, however, their blocking behavior is the lack of in-depth understanding. To get into that, the H 2 O diffusion pattern in the free volumes or specific sites of SSEs should be well explored to provide guidance for SSE design. Besides, assembling a water-proof but oxygen-permeable layer at the air-facing side of the cathode is an ideal solution for LABs. A thin thickness of the layer is highly preferred, for reducing the influences on mass transfer and total energy density. Whereas, the knottiest problem is the undesirable size discrepancy between O 2 (0.346 nm) and H 2 O (0.265 nm) molecules, 253 nullifying the pore sieving strategy and bringing a great challenge for building up O 2 -selective but H 2 O-screening gas channels. Undoubtedly, the surface engineering of these water-proof layers with nonpolar molecular structure is required, to exhibit high selectivity for nonpolar O 2 molecules while repelling polar H 2 O molecules. The exquisite physicochemical design of building up an exclusive highway for O 2 at the cathode is a radical solution for realizing practical LABs, while lacking intensive study in both fundamental and experimental research yet.
Although great challenges remain, we believe the booming development of in situ characterization techniques and computational methods will provide further insights into the mechanisms for solid-state LABs. Underpinned by the theoretical guidance and the progress of material science, in the future, well-designed strategies targeting the specific bottleneck in different systems will endow practical LAB devices. When that happens, this energy-rich technology would revolutionize power supply from wearable devices to varied vehicles, and even largescale energy storage systems.