From nanoscale interface characterization to sustainable energy storage using all-solid-state batteries

1Department of NanoEngineering, University of California San Diego, La Jolla, CA, USA. 2Program of Chemical Engineering, University of California San Diego, La Jolla, CA, USA. 3Sustainable Power & Energy Center (SPEC), University of California San Diego, La Jolla, CA, USA. ✉e-mail: zhengchen@eng.ucsd.edu; shmeng@ucsd.edu The past several decades have seen conventional lithium ion batteries (LIBs) dominate the portable devices and consumer electronics market. Today, LIBs are gradually penetrating other technologies including electric vehicles and grid storage. The success of LIBs stems from the rapidly growing efforts in battery research and development, leading to vast improvements in materials performance and decrease in production costs. However, the justification for a more widespread adoption of LIBs entails overcoming fundamental obstacles such as safety hazards from battery fires and explosions, meeting the demand for higher energy density and achieving satisfactory performance in a wider temperature range for application in various climate conditions. In view of these concerns, all-solid-state batteries (ASSBs) are regarded as one of the future energy storage technologies that can compete with the state-of-the-art LIBs. Owing to the use of non-flammable solid-state electrolytes, ASSBs are well-placed to effectively eliminate battery safety concerns in electric vehicles, airline industry and grid storage applications in urban environments. Their wide operating temperature range also drastically reduces cooling power requirements with immediate benefits on costs and energy efficiency. While energy densities of ASSBs are virtually identical to those of LIBs at the materials level, they can be potentially increased by 50% or greater at the pack and system level from utilizing stackable formats without the need for individual cell packaging, reinforced metallic casings, or allocated space for coolants. Despite their many promising benefits, ASSBs still face a multitude of barriers before they can be transferred from the laboratory to commercial manufacturing lines (Fig. 1). In our view, the most pressing challenges are: (1) solid-state-electrolyte (SSE) chemistry: to date, no single SSE can meet the combination of properties necessary for commercialization, that is high ionic conductivity, ease of processability and wide electrochemical stability among others; (2) characterization challenges: ASSBs are inherently difficult to characterize given the buried and heterogenous nature of their interfaces, as well as their sensitivity toward electron beam damage; (3) scalable design: despite recent advances1,2, air sensitivity and the poor mechanical properties of SSEs still pose challenges in implementing roll-to-roll processability for scalable ASSB fabrication; and (4) sustainability: no ASSB recycling models exist yet—any sustainable ASSB model would require cell level recycling strategies beyond electrodes recovery. In this Review, we will take a close look at each of these obstacles and offer our perspective on how to approach them.

T he past several decades have seen conventional lithium ion batteries (LIBs) dominate the portable devices and consumer electronics market. Today, LIBs are gradually penetrating other technologies including electric vehicles and grid storage. The success of LIBs stems from the rapidly growing efforts in battery research and development, leading to vast improvements in materials performance and decrease in production costs. However, the justification for a more widespread adoption of LIBs entails overcoming fundamental obstacles such as safety hazards from battery fires and explosions, meeting the demand for higher energy density and achieving satisfactory performance in a wider temperature range for application in various climate conditions. In view of these concerns, all-solid-state batteries (ASSBs) are regarded as one of the future energy storage technologies that can compete with the state-of-the-art LIBs. Owing to the use of non-flammable solid-state electrolytes, ASSBs are well-placed to effectively eliminate battery safety concerns in electric vehicles, airline industry and grid storage applications in urban environments. Their wide operating temperature range also drastically reduces cooling power requirements with immediate benefits on costs and energy efficiency. While energy densities of ASSBs are virtually identical to those of LIBs at the materials level, they can be potentially increased by 50% or greater at the pack and system level from utilizing stackable formats without the need for individual cell packaging, reinforced metallic casings, or allocated space for coolants.
Despite their many promising benefits, ASSBs still face a multitude of barriers before they can be transferred from the laboratory to commercial manufacturing lines (Fig. 1). In our view, the most pressing challenges are: (1) solid-state-electrolyte (SSE) chemistry: to date, no single SSE can meet the combination of properties necessary for commercialization, that is high ionic conductivity, ease of processability and wide electrochemical stability among others; (2) characterization challenges: ASSBs are inherently difficult to characterize given the buried and heterogenous nature of their interfaces, as well as their sensitivity toward electron beam damage; (3) scalable design: despite recent advances 1,2 , air sensitivity and the poor mechanical properties of SSEs still pose challenges in implementing roll-to-roll processability for scalable ASSB fabrication; and (4) sustainability: no ASSB recycling models exist yet-any sustainable ASSB model would require cell level recycling strategies beyond electrodes recovery. In this Review, we will take a close look at each of these obstacles and offer our perspective on how to approach them.

Solid state electrolyte chemistry
Scientists have recently discovered a plethora of SSE materials with ionic conductivities greater than 10 -2 S cm -1 at 25°C, comparable to that of liquid electrolytes 3 . However, high ionic conductivity alone is not sufficient to make cells practical. Recent reports on highly conductive oxide-or sulfide-based SSEs show that poor interfacial stability makes them impractical 3 . Such instability arises from two types of reactions: (1) interfacial reactions between the electrode and SSE, which occur spontaneously upon physical contact and (2) electrochemical decomposition of the SSE itself during cell cycling at high voltage. Although some oxide-based electrolytes such as those from the garnet and NASICON class can be stable over a wide range of voltages 4 , this voltage window is still not satisfactory. Figure 2a illustrates the electrochemical stability windows of some SSEs compared to common electrode materials. Although polymer-composite-based SSEs represent an important class of materials for solid-state batteries, these materials have already been extensively covered in recent review articles [5][6][7] . Thus, we will focus on inorganic solid electrolytes, which are comparatively novel and not as well understood. In this section, we discuss the fundamentals The recent discovery of highly conductive solid-state electrolytes (SSEs) has led to tremendous progress in the development of all-solid-state batteries (ASSBs). Though promising, they still face barriers that limit their practical application, such as poor interfacial stability, scalability challenges and production safety. Additionally, efforts to develop sustainable manufacturing of lithium ion batteries are still lacking, with no prevailing strategy developed yet to handle recyclability of ASSBs. To date, most SSE research has been largely focused on the discovery of novel electrolytes. Recent review articles have extensively examined a broad spectrum of these SSEs using evaluation factors such as conductivity and chemical stability. Recognizing this, in this Review we seek to evaluate SSEs beyond conventional factors and offer a perspective on various bulk, interface and nanoscale phenomena that require urgent attention within the scientific community. We provide a realistic assessment of the current state-of-the-art characterization techniques and evaluate future full cell ASSB prototyping strategies. We hope to offer rational solutions to overcome some major fundamental obstacles faced by the ASSB community, as well as potential strategies toward a sustainable ASSB recycling model. of each type of interfacial reaction and evaluate the methods to prevent them.
Cathode interfacial reactions. The first type of interfacial chemical reaction stems from intrinsic chemical reactivity between the high-voltage cathodes and SSEs 8,9 . These spontaneous reactions result in the formation of transition metal oxides, sulfides, phosphates and other undesirable products on the cathode-SSE interface that increase cell polarization and limit rate capability. Oxide anions from the layered transition metal oxides form stronger electrostatic attractions with lithium ions than sulfide anions due to the hard-soft acid-base principle. Thus, transfer of Li + from sulfides to oxides occurs until equilibrium is reached, thickening the resistive layer and suppressing any ionic conduction across the interface. As the transfer of Li + from sulfide to oxide anions is charge balanced by the electronic conductive network of the cathode material, the most natural approach to prevent this effect is to adopt an electronically insulating but ionically conductive coating layer at the SSE-cathode interface. Protective coating materials such as Li 2 SiO 3 , Li 4 Ti 5 O 12 (LTO), LiTaO 3 , LiAlO 2 , Li 2 O-ZrO 2 , and LiNbO 3 (LNO) have been proposed 10 . For example, a 20-nm-thick LTO coating on LiCoO 2 (LCO) could reduce cathode interfacial resistance by an order of magnitude compared to the uncoated cathode 11 . In another example, an LNO coating of thickness as low as 5 nm was adequate to prevent interfacial reactions 12 .
Protective coatings on the cathode can also alleviate cation interdiffusion problems along the SSE-cathode interface. Cation interdiffusion occurs from exchange between the transition metal ions and SSE cations. Interdiffusion can occur over spatial scales of up to 100 nm, forming a highly resistive layer that blocks Li + from crossing the interface. Interdiffusion energies at the LCO (110) and β-Li 3 PS 4 (010) interface have been calculated and it has been found that Co and P ion diffusion exchange is highly thermodynamically favourable 13 . Adopting a protective layer such as LNO can prevent the Co and P interdiffusion, as Nb and P interdiffusion are slow due to the strong bonds between Nb and O anions. Such protective layers were later experimentally validated with transmission electron microscopy (TEM), where long range Co distribution was tracked at the interface protected by a Li 2 SiO 3 coating 14 .
Despite these results, there is still a lack of consensus on how to select protective coating materials. For example, incorporation of a charge carrier (such as in LiAlO 2 ) in Al 2 O 3 improves cell efficiency, 15,16 most likely because of the energetically favourable activation of Al 2 O 3 with Li + from the cathode during cell cycling. This was experimentally supported by the effectiveness of an Li 2 SiO 3 coating on LCO in improving the cell rate performance compared to SiO 2 14 . Regardless of the material, any coating must be sufficiently thin to avoid negative impacts on cell impedance, as oxides often exhibit low ionic conductivities (for example, 10 -8 S cm -1 for LTO). Therefore, more conductive (~10 -6 S cm -1 ) phosphatebased coating materials, such as lithium thiophosphate (LPS), have been proposed; these exhibit both high theoretical oxidative stability and low ion migration barriers 17 . As experimentally shown for the NCM811-LPS interface, oxide cathodes tend to react with PS 4 3groups in sulfide-based SSEs due to the exchange of O and S atoms between the SSE and the cathode to form PO 4 3units 18 . Because the bond dissociation energy of a P-O bond (597 kJ mol -1 ) is larger than that of P-S bonds (346 kJ mol -1 ), O-S exchange is highly favourable. Thus, the reactivity of phosphate-based coating materials in which P atoms are already bonded to O atoms is expected to be low.
Anode interfacial reactions. In general, there are two kinds of interfaces formed between SSEs and lithium metal: ionically conductive and mixed conductive ones. For example, Li 10 GeP 2 S 12 (LGPS) has an ionic conductivity greater than 10 -2 S cm -1 and decomposes upon contact with lithium metal, 4 forming Li 2 S (an insulator), Li 3 P (an ionic conductor) and an Li-Ge alloy (an electronic conductor); collectively making the interface a mixed conductor 19 . Because of the presence of a conductive component, as the interface grows, the anode impedance increases until cell failure. The same principles apply for Si-, Sb-, Sn-and As-containing SSEs, as they also form electronically conductive alloys with lithium. As such, despite their high ionic conductivity, these electrolytes are not suitable for lithium metal ASSBs. On the other hand, LPS-based glasses or glass ceramics (xLi 2 S.yP 2 S 5 ), along with argyrodite-based SSEs (Li 6 PS 5 X; X = Cl, Br, I) are more stable. When these SSEs meet lithium metal, their decomposed products include Li 2 S, Li 3 P, and LiX (X = Cl; Br, I), which are all electronically insulating, 4 thus passivating the SSE interface.
Room temperature long cycle performance of lithium metal ASSBs still remains challenging. One reason is the difficulty of maintaining good wettability at the lithium-SSE interface during cell cycling in order to achieve homogenous and dense lithium plating. In an effort to overcome this problem, conformal alumina coating on lithium has been shown to reduce interfacial resistance by almost two orders of magnitude in oxide-based SSEs 20 . However, such treatments are only feasible with garnet and other oxide-based SSEs due to their better chemical stabilities against lithium. For sulfide-based SSEs, any high temperature treatment would promote unwanted reactions and form a thick solid electrolyte interface (SEI) that renders cells unusable. While some studies demonstrated good lithium-SSE contact by simple cold pressing 21,22 , the effectiveness of this methodology in lowering interfacial resistance and allowing uniform lithium plating especially in long term cell cycling is still unclear. Another solution could be the use of a thin Li + conductive polymer layer between the lithium anode and the SSE. This strategy was previously shown to enable extended cell cycling in Li|polyethylene oxide (PEO)|Li 7 La 3 Zr 2 O 12 (LLZO) and Li|PEO|β-LPS configurations 23,24 . However, using conductive polymers often requires cells to be cycled at elevated temperatures. The good cyclability of these cells is often attributed to PEO's ability to either protect lithium or achieve good wettability. Even though there have also been reports of bare Li|SSE cell configurations showing good cell cyclability at elevated temperatures 25,26 , practical devices must be operated at room temperature. Prevailing theories suggest that lithium dendrites propagate as a result of deposition within pores near the lithium-SSE interface 27 , with experimental reports observing lithium deposits originating from pores at the interface that propagate along grain boundaries due to inhomogeneous current distribution 28 . However, studies using grain-free single crystal   Li 6 La 3 ZrTaO 12 also reported lithium penetration through the SSE 29 . Interestingly, only lithium phosphorous oxynitride (LiPON) appear to be able to effectively prevent lithium dendrite formation and enable cell cyclability beyond 10,000 cycles 30,31 . It is probably the amorphous, conformal and virtually defect free nature of the material that is responsible for such performances.
In spite of the challenges faced at the anode interface, metallic lithium is still indispensable to increase energy densities of ASSBs. Moreover, lithium metal ASSBs can potentially be used in an anodefree configuration. Such ASSBs have been demonstrated in thin film formats nearly two decades ago in a LiPON|LCO battery 30 . Here, surface passivation in solid electrolytes prevents side reactions from continuously forming, allowing retention of the lithium reservoir within the cell after the first charge cycle. This situation differs fundamentally from that in liquid electrolytes where continuous formation of new SEI and dead lithium at each cycle requires sacrificial lithium replenishment 32 . Adoption of anode-free cell configurations would immensely simplify large scale ASSB processing and lower its cost, because it would eliminate sophisticated lithium metal lamination and interface treatment steps, especially when lithiophobic SSEs are used 33 . However, the same challenges encountered in other lithium metal ASSBs, such as dendritic cell shorting, also apply in anode-free cell configurations.

SSE electrochemical decomposition.
Despite the fact that protective coating layers can effectively reduce interfacial reactions, they cannot prevent interfacial electrochemical decomposition. Electrochemical decomposition during cell charging is an unavoidable intrinsic thermodynamic phenomenon of the SSE. The SSE at each electrode experiences a strong oxidative or reductive environment that leads to the formation of unwanted interfacial products, resulting in poor first cycle Coulombic efficiencies and high impedance growth. To reduce the effects of electrochemical decomposition, SSE thermodynamics can be altered through compositional changes. For instance, introducing oxygen dopants stabilizes the electrolyte, because oxides are more thermodynamically stable than sulfides due to their lower anion polarization. In one study, P 2 O 5 was introduced in place of P 2 S 5 during SSE synthesis, forming oxygenated units within the bulk Li 3 PS 4 glass ceramic oxysulfide system 34 . The electrochemical stability window of oxysulfides with the Electrodes operating ranges Oxides    composition Li 10 GeP 2 S 12−x O x (LGPS) is wider than that of the oxygen-free counterpart, with only a small trade-off in bulk ionic conductivity 35 . Alternatively, SSE thermodynamics can be controlled by using different SSEs with suitable redox stability at each electrode. Figure 2b compares the improvements in first cycle Coulombic efficiency by using SSEs with higher oxidative stability. For example, halide-based Li 3 YCl 6 resulted in reduced SSE decomposition and an improved first cycle Coulombic efficiency of 94%, from 84% when sulfide-based Li 3 PS 4 was used 36 .
Although studies using conventional cyclic voltammetry (CV) measurements have claimed SSE electrochemical stabilities up to 5 V or higher, recent reports using both modified CV measurements and first principles calculations have shown a narrower electrochemical voltage window. For example, LGPS, previously claimed to be stable up to 5 V with lithium metal 37 , was found to decompose at 2.2 V versus Li/Li + (ref. 38,39 ). These inconsistencies in stability were also found in oxide-based SSEs such as Li 7 La 3 Zr 2 O 12 (LLZO) and Li 1+x Al x Ti 2-x (PO 4 ) 3 (LATP), where oxidative decomposition was reported to be higher (>5 V) than their predicted values (<3.7 V) 40,41 . These contradictory reports can be explained by the fact that experimental methods are sometimes unable to account for the sluggish redox kinetics of SSEs. In one example, instead of the common CV with metal current collectors, a modified process where SSE was mixed with carbon and applied on the cathode was consistent with the electrochemical stability windows derived from first principles calculations 39 . The use of carbon provides electronic pathways that facilitate redox of the SSE, allowing oxidation to be detected at its intrinsic thermodynamic potentials.
However, poor redox kinetics can in turn be applied to tune SSE oxidation. As decomposition products tend to deposit around the carbon additives or the surface of cathode particles, an ideal approach would involve minimizing SSE exposure to conductive surfaces, while providing long range electronic pathways between electrode materials and the current collector. As seen in Fig. 2c, the kinetics of SSE decomposition can be reduced by selecting a specific surface area of the carbon additives 42 . Carbon materials with low surface area (<100 m 2 g -1 ) and long range conductive capability, such as carbon nanotubes or vapour grown carbon fibre are ideal additives for ASSBs, because they reduce SSE decomposition and maintain high capacity utilization .

Characterization challenges
Challenges to characterize ASSBs often relate to the difficulty of probing buried and/or beam sensitive interfaces. Even though a wide spectrum of available tools exists, they are limited in characterizing specific chemistries within solid-solid interfaces. These include metallic lithium dendrites formation and growth of amorphous interfacial products within SSEs at both bulk and localized sites. Only in a few cases is it possible to characterize the true dynamic states within ASSBs with in situ and operando techniques. As such, recent reviews covering broad characterization topics have called for novel techniques to address this gap in knowledge [43][44][45] . In this section, we cover the state-of-the-art developments in experimental design for the characterization of ASSBs and offer possible solutions to address some of the most urgent problems.
Characterizing bulk ASSBs. To date, electrochemical methods such as galvanostatic cycling or impedance spectroscopy remain the primary characterization methods of buried interfaces within ASSBs 46 . However, such techniques are limited, because they cannot provide elemental or morphological information that are vital to properly evaluate cell failure mechanisms. Recently, X-ray computed tomography (CT) has been used to provide non-destructive three-dimensional (3D) in situ spatial visualization of dynamic morphological changes at solid-solid anodic interfaces 47 . Latest advancements in nano-CT capabilities (Fig. 3a) can achieve spatial resolutions below 50 nm over large sample volumes, potentially allowing the analysis of the entire cell packs. Unfortunately, poor X-ray absorption of lithium still makes it difficult to differentiate metallic lithium from sub-micron sized voids within the SSE. Thus, CT methods can be more effective when applied to cathodic interfaces where there is a larger X-ray contrast between cathode particles and SSEs. Several studies have quantified voids or cracks within the cathode composite using CT 48,49 .
To probe buried interfaces, neutron-based techniques are especially useful. Neutron depth profiling (NDP) is effective to study lithium dendritic growth, as it is non-destructive and lithium-sensitive. In one example, cell shorting was diagnosed using NDP, where time-resolved lithium concentrations at the SSE-lithium interface were obtained as lithium was plated and stripped. It was found that increased accumulation of metallic lithium within the buried SSEs under high current densities was the likely cause for poor stripping reversibility and eventual cell shorting 50 . Another study used NDP to compare the dynamic evolution of lithium concentration in lithium-SSEs interfaces of LiPON, LLZO and Li 3 PS 4 during Li plating. It was observed that lithium plating in LiPON remained uniform and unchanged throughout the process, in agreement with previous literature on lithium-LiPON interfaces. Microstructural lithium dendrite formation within buried SSEs can also be characterized using 7 Li nuclear magnetic resonance (NMR). Making use of nondestructive probes along with its high sensitivity toward Li + in bulk SSEs, 7 Li NMR chemical shift imaging was used to track lithium growth in Li 6.5 La 3 Zr 1.5 Ta 0.5 O 12 (Fig. 3b). The increase in microstructural growths were subsequently correlated with symmetric cell plating and stripping voltage profiles, and it was found that voltage spikes often seen in ASSBs before cell failure can be attributed to dendrites that fuse (break) due to lithium melting during localized cell shorting 51 . However, the limited spatial resolution in such imaging techniques restricts detection to the micron-range (~100 μm). More sensitive techniques are needed to observe the initial onset of lithium nucleation, as these formations are likely to be nanometresized deposits within the SSEs.
Characterizing meso to nanoscale interfaces. To explore ASSB mesoscale properties, we need to revert to cryogenic focused ion beam (FIB) techniques (Fig. 3c). These methods can help quantify porosity and volume changes within the ASSBs at spatial resolutions under 1 nm without damaging the SSE or SEI. By keeping the sample temperatures low enough, local heating during exposure to Ga + ion beams can be prevented, allowing imaging of lithium metal at 100 K 52 . Considering the heterogeneous nature of the SEI layers, especially for thick electrodes, Xe + plasma FIB can be applied for large volume serial section tomography as the ion milling rates are at least 60 times greater than conventional Ga + FIB. Large volume tomography combined with windowless energy dispersive spectroscopy (EDS) can be used to monitor the SEI formation or cracking, electrode porosity and tortuosity evolution, and SEI layer elemental distribution changes in the 3D meso-structure. An example is illustrated in Fig. 3c where 3D reconstruction was used to quantify porosity as well as pores interconnectivity within an ASSB 53 . However, any reconstruction method using the FIB would be limited by the spatial resolutions of its detector-a scanning electron microscope (SEM) in this case. Additionally, the secondary electron signal intensities at cryogenic temperatures are low and charges from the probe beam can accumulate on electronically insulating SSEs, limiting the effectiveness of the method.
One strategy to overcome these challenges is the use of a solidstate nanobattery configuration for interface characterization (Fig. 3d). To do so, FIB is first used to slice a cross-sectional lamella from a thin-film ASSB. This lamella preserves the full function of the ASSB and is thin enough (~100 nm) to be used in a TEM. This approach was used to show electrochemical cyclability  of a cross section LCO/LiPON/amorphous-Si thin film battery 54 . A wedge-shaped nano ASSB prepared by FIB was then placed onto a potentiostat coupled platform, allowing in situ cycling of the nano ASSB within the TEM. Using scanning TEM electron energy-loss spectroscopy (EELS), the decomposition products consisting of lithium oxides and peroxides and oxidized cobalt species could be observed at the cathode-SSE interface (Fig. 3d). Although such in situ TEM characterization has been demonstrated for thin-film LiPON based batteries, its principles can be applied for sulfide and oxide based ASSBs too, where sputtering or other vapour deposition methods can be used to fabricate nanobattery interfaces for the analysis.
Spectroscopy techniques. Various spectroscopy-based in situ methods have been adopted to probe electron beam sensitive and amorphous interfacial products. Of these, in situ Raman spectroscopy is a simple and robust method to provide good contrast in SSEs due to the strong scattering signals of covalently bonded interfacial products. With this technique, formation of various lithium thiophosphates, such as P 2 S 6 4or PS 4 3-, can be easily distinguished during lithium plating at a gold-LGPS interface (Fig. 3e). Thiophosphate signals would otherwise be very difficult to distinguish with EELS, because this technique is sensitive to the P-S bond, which is present in both species. Limitations of in situ Raman include the inability to detect certain Raman inactive materials, especially ionic species such as lithium salts. Raman techniques also tend to be noisy especially from heterogeneous SSE interfaces, owing to the presence of absorbing or blocking materials such as carbon. One approach to overcome this issue is combining higher energy surface sensitive XPS with in situ deposition of lithium metal onto sulfide based LGPS (Fig. 3f). This allows detecting both chemical bonds and intrinsic band-structure, as well as observing the chemical decomposition products such as Li 3 P, Li 2 S, and Li−Ge alloy at the lithium -SSE interface 19 . However, its capability to capture subsurface (>10 nm) interfaces formation is limited by the escape depth of ejected photoelectrons. To address this issue, X-ray absorption spectroscopy (XAS) has been used in fluorescence mode to study oxidative decomposition of Li 3 PS 4 54 , where oxidized products of S and P 2 S 5 were found in buried interface, in agreement with previous computational work 55 .
Spectroscopy methods can also be utilized to qualitatively deconvolute chemical reactions from electrochemical reactions. This can be done through careful selection of their respective states of charge. For example, to probe chemical reactions between charged cathodes and SSEs without contributions from electrochemical decomposition, characterization can be conducted for mixtures of pristine SSEs and charged cathodes harvested from liquid electrolyte cells. Likewise, purely electrochemical reactions can be analyzed by characterizing SSE-carbon composites in the absence of transition metal oxide cathodes 55 .

Scalable design-organic/inorganic composites
To achieve practical energy densities, SSE layers need to be thinner than 50 μm. However, the poor mechanical properties of inorganic SSEs makes them brittle, posing challenges for processing ASSBs in large formats. Polymer composites can improve mechanical flexibilities of SSE composites, allowing roll-to-roll manufacturing, with good elasticity and adhesion to current collectors. Figure 4a illustrates a typical manufacturing process of ASSBs using wet slurry processing methods similar to those in conventional LIBs 56 . However, unlike conventional LIBs manufacturing where common solvents such as water or N-methyl-2-pyrrolidone (NMP) and binders such as carboxymethyl cellulose (CMC) or polyvinylidene fluoride (PVDF) are used, careful consideration must be taken to ensure these solvents are chemically compatible with SSEs. While polyethylene oxide (PEO), polyvinylpyrrolidone (PVP), CMC and PVDF have been adopted in oxide-based ASSBs, 57,58 they cannot be used with sulfidebased SSEs. Electron-rich atoms within the polymer backbone, namely O, N and F, tend to form electrostatic interactions with the strong nucleophilic cations containing aliovalent atoms such as P or transition metals. This can result in localized SSE-binder agglomerations that impede both ionic conductivity and adhesive properties of the polymer. As such, favourable ASSB binders involve polymers with low or no electronegative functional groups, such as poly(acrylonitrile-co-butadiene) (NBR), polystyrene-blockpolybutadiene (SBS) or styrene-ethylene-butylene-styrene (SEBS) reported in several cases; usually only 2 wt% of binder is needed to achieve sufficient adhesion [59][60][61] . While using non-polar binders may lead to poor adhesion between electrodes and current collectors, stack pressures typically applied in ASSBs can circumvent these concerns. Homogeneous electrode distribution is also a crucial factor for ASSBs, because poor particles wettability impedes capacity utilization. As such, binders that can be processed dry are promising candidates for ASSBs because they enable uniform distribution of the SSEs and active particles within the cathode composites 62 .

Role of polymers in scalable fabrication.
Whenever solution processing is used, considerations for solvent and binders compatibility with sulfide-based SSEs must be made 63 (Fig. 4b). Only solvents with polarity index below 3.1 are fully compatible with sulfide-based SSEs 64 . In addition, the stability of the SSEs in ambient conditions needs to be considered. Sulfides spontaneously undergo hydrolysis when exposed to moisture, producing H 2 S gas, a safety hazard when not properly vented. ASSB manufacturing should be done under dry-room conditions, similar to that of conventional LIBs. Trace amounts of moisture even in ppm levels can already be detrimental for the extremely hygroscopic sulfide SSEs. Studies have demonstrated improved moisture stability of sulfide SSEs using partial substitution of stable oxides into the bulk SSE 65 . Alternatively, to avoid modifying the intrinsic electrochemical properties of the SSE itself, hydrophobic binders can be adopted during processing 60 .
Resistive effects of polymers. In general, it is expected that mixing a SSE with a binder decreases its conductivity by about an order of magnitude. This is due to the impedance contribution across the polymer-SSE interface as a result of the presence of insulating binder along the grain boundaries 61 . However, solvated ionic liquids (SILs) can reduce binder-induced impedance as well as fill any pore generated in the polymer composite. This strategy has been utilized to increase ionic conductivities by one to two orders of magnitude of oxide-based polymer-SSE composites 41,66 . However, this can be difficult to achieve in sulfide-based SSEs, as polar solvents that solvate lithium salts will chemically degrade the sulfides. This contradiction can be resolved by controlling the ratio between the salt and solvent. As the proportion of salt to solvent increases until saturation, a critical salt-solvent complex point is reached, forming a SIL with no available free solvent left to react with or dissolve sulfide SSEs. In an example, when four moles of triglyme G3 ethers were added to one mole of LiTFSI, a saturated solvent-salt complex, Li(G3) 4 was formed. This complex was found to not affect the sulfide SSE 67 . However, incorporation of any solvated ionic liquids in ASSBs such as LiG3 would also inevitably lower the thermal stability of the entire system. Thus, an alternative is the use of ionically conductive binders by solvating the insulating binders with lithium salts (Fig. 4c) 68 . In one case, a composite of Li 6 PS 5 Cl and NBR-Li(G3)TFSI solvated binder was applied 69 , improving the cell rate capability and capacity utilization while maintaining good thermal stability. To eliminate the binder impedance contribution, binderfree ASSB cell fabrication has also been proposed. While binders play an important role during the manufacturing process, it is no longer needed once the cell is assembled. As a binder only acts as inactive material, it would be ideal for it to be removed after assembly. Figure 4d depicts a heat treatment process that allows for the removal of volatile poly (propylene carbonate)-based binder within the cell after fabrication 70 . However, as the volume occupied by the binder is not filled by SSEs after removal, increased porosity within the ASSBs could potentially occur. As such, this temperature treatment technique can be done under stack pressure, allowing the thermally softened SSEs to deform and fill any pores generated within the solid electrolyte bulk.

Battery recycling and sustainability
With rapidly increased LIBs adoption, the need for sustainable battery recycling is a matter of utmost importance. Spent LIBs contain lithium, cobalt, nickel and other transition metals that are not only economically valuable but are also limited in terms of their natural availabilities. Unfortunately, today's LIBs are not designed to be recycled easily 71 . To re-design batteries for recycling instead of disposal, battery manufacturers will need to modify their established production protocols, with evident economic backslash. While this might be too late for conventional LIBs, it is prudent to design potential recycling strategies such that future ASSB manufacturers can consider it as part of a sustainable production-to-recycling manufacturing process. Figure 5a depicts the battery manufacturing chain from materials mining to disposal. While recycling efficiencies for lead acid batteries exceed 99% in major parts of the world 72 , such as in Europe and the USA where recycling is led by strong government mandates, much more can be done to improve recycling rates for end-of-life LIBs and future ASSBs. Moreover, commercial LIBs should be regulated to include labelling of their chemical classifications, to allow ease of recycling and sorting based on their core materials. This would require the co-operation of battery manufacturers and policy makers, in efforts to streamline recycling of respective LIB chemistries. The responsibility can be further extended to original equipment manufacturers as well as EV manufacturers to incentivize battery buy-back programmes as part of their in-house recycling or repackaging for secondary use.

Battery recycling challenges.
The most common recycling technologies adopted today to break down recovered cathode materials and regenerate them for reuse are pyrometallurgical and hydrometallurgical methods. Recent studies on such methods have reported valuable metal recovery rates exceeding 95% using acid leaching followed by chemical precipitation methods [73][74][75] . The recovered metals in their precursor forms are subsequently used in cathode re-synthesis with co-precipitation, solgel or solid-state heat treatment steps to reform the active materials.  However, these methods tend to be energy intensive, costly and use toxic chemicals processing, which can be difficult to handle 76 . Additionally, materials recovery efficiency as a fraction of the entire cell still remains low, due to low recycling rates of other components in the cell such as the liquid electrolytes, lithium salts, separator and additives. To address these shortcomings, the US Department of Energy's ReCell Center has set out core principles of battery recycling that involves design for recyclability, direct regeneration and recovery of other components (https://recellcenter.org/research). Achieving these goals requires a paradigm shift in the way researchers think about battery recycling, to go beyond metals recovery or materials processability, and to consider LIBs and ASSBs recycling from a wider perspective 75 . Given the lack of reports on ASSBs recycling in the literature, this framework presents an opportunity to explore possible pathways for recycling ASSBs using the goals of ReCell as a starting reference point.
Fully recyclable ASSB model. For any ASSB recycling model to be sustainable and practical, several technical challenges need to be addressed: (1) designing ASSB cell chemistries that allow for efficient materials disassembly with reduced processing steps; (2) avoid using toxic, expensive and difficult-to-process organic solvents; (3) recovering all components in a battery, rather than just the cathode, in a cost-effective manner; and (4) designing a scalable, closed loop ASSB recycling model applicable to a variety of ASSB formats (Fig. 5b).
Fortunately, compared to conventional liquid-electrolyte-based LIBs, recycling of ASSBs has several advantages. Due to its large stackable formats, ASSBs can be discharged at the pack level before disassembly; and its intrinsic non-flammability also mitigates safety hazards during packaging breakdown. Conversely, fire and gas evolution hazards in LIBs require individual cells to be submerged typically in salt solution to achieve zero state of charge. However, the major challenge for a recyclable ASSB model is separation of SSEs from electrodes and its subsequent recovery. Unlike liquid electrolytes that can be removed with organic solvents or supercritical drying during dismantling 74,77 , SSEs contains a host of various metallic and non-metallic elements (oxides, sulfides). Elemental separation and purification of these SSEs can be difficult with conventional recycling approaches. While solution type dissolution and precipitation methods of SSEs might be effective, the fact that transitionmetal oxide chemistries are similar in both oxide-based SSEs and cathode materials make it challenging to selectively separate each component via dissolution. Fortunately, this should not be the case in sulfide-based ASSBs, where sulfide dissolution can be done using cheap and safe solvents such as ethanol or acetonitrile to recover sulfide-based SSEs from spent ASSBs 78 . Unlike previous studies that reported chemical incompatibilities between polar solvents and sulfides 64 , it has been found that such polar solvents will only cause dissolution and not chemical degradation of certain SSEs, allowing them to be precipitated in its original chemical formula. Solutionbased processing of sulfide-based SSEs using such solvents has been already demonstrated in previous solution synthesis work [79][80][81] . This is often seen in sulfide-based SSEs comprising of PS 4 3conductive thiophosphate units that can be easily solvated with polar solvents and precipitated as either Li 3 PS 4 or Li 6 PS 5 X (X = Cl, Br or I) forms. By contrast, conventional LIB cells need to be separated into their subcomponents before washing away the electrolyte solvents and  salts. Moreover, ASSBs contain no ethers, salts or separators and its SSEs can potentially be fully recovered without energy intensive organics processing. This provides a promising opportunity for simple recovery and recycling of SSEs from spent ASSBs.

Direct regeneration of SSEs and cathodes.
A crucial requirement for any robust recycling model is the need to avoid the breakdown of spent electrodes and SSEs to their precursor forms. As such, direct regeneration methods would be superior to existing energy intensive pyrometallurgical or hydrometallurgical methods, as they substantially reduce the energy and processing costs of recycling. Conceptually, spent bulk SSEs do not undergo any structural change aside from fractional decomposition in the SEI. Thus, the majority of the SSE can be recovered and directly regenerated without sophisticated re-synthesis processes using dissolution-precipitation methods combined with mild post processing. Although sulfide SSE dissolution processes can result in conductivity losses of 1 to 2 orders of magnitude 79,80 , this is mainly due to the small grain sizes and poor crystallinity of recovered SSEs, not to chemical degradation of the SSE itself. It has been shown that mild annealing can be carried out for precipitated SSEs to regain its pure phase and ionic conductivities greater than 1 mS cm -1 (refs. 79,80 ). Such techniques can allow simple and low-cost recovery of lithium in its reusable SSE form.
Aside from the SSE, direct regeneration of spent electrodes is also possible. Previous nanoscale characterization work done on aged LIBs has shown that the degradation of transition metal oxides occurs mainly on the surface or subsurface, forming localized spinel or rock salt phases 82 . Therefore, total destruction and recovery of the bulk cathode are not necessary. Recent reports on direct re-lithiation of spent layered oxides (LCO, NMC) via solution, solid state and molten salt infusion methods provide promising approaches for cathode direct regeneration 83,84 . As an end-of-life lithium metal ASSB would be fully depleted of lithium, or contain only traces of unconsumed lithium (Fig. 5b), recycling strategies can be centred around processing the SSE and surface degraded cathodes. Although cathode coating materials used in ASSBs such as LiNbO 3 would remain on the surface of cathodes, direct regeneration using re-lithiation methods can be done together with these ionically conductive coating layers that facilitate Li + diffusion. This eliminates the need to break down either the protective coating layers or bulk cathode into precursors forms. Once the correct stoichiometric ratio of lithium and transition metals are achieved, annealing can be done to reach the target crystal structure and removal of any remaining impurities. While existing studies on SSE solution processing and cathode direct regeneration have demonstrated the technical feasibility of recycling an end-of-life ASSB, this has yet to be tested in a full-cell pack. However, the proposed recycling model here may provide potential ASSB manufacturers new strategies for future sustainable battery designs, lowering financial burdens of manufacturing environmentally friendly commercialized ASSBs without compromising energy density and overall cell performance.

Conclusions
The continued pursuit of sustainable energy storage technologies with increasing energy density and safety demands will compel an inevitable shift from conventional LIBs to ASSBs. Developing a single type of SSE capable of meeting all required properties remains challenging, but the combination of materials and nano-engineering shows great promise toward overcoming obstacles such as interfacial stability by controlling the thermodynamics and kinetics of SSE decomposition. We have highlighted the state-of-the-art characterization techniques to shed light onto the nano-scale phenomena within buried SSE interfaces and have proposed methods for in situ observation of unstable solid-solid interfaces. We have also discussed polymer-SSE composites, solvent-polymer combination selection criteria, and methods to reduce resistive effects of binders. The aim is to accelerate commercialization of ASSBs using scalable solution-based processes. Finally, we have introduced strategies for sustainable ASSBs recycling, and proposed a fully recyclable ASSB model that can potentially lower costs of battery recycling with safer and simpler methods compared to current technologies. Nanotechnology itself may not be an all-encompassing silver bullet for every challenge faced by ASSBs, however it is certainly becoming an enabler for deeper understanding of nanoscale phenomena, helping better design strategies that can translate into improvements in materials and cell level performance.