Current Status and Prospects of Solid-State Batteries as the Future of Energy Storage

Solid-state battery (SSB) is the new avenue for achieving safe and high energy density energy storage in both conventional but also niche applications. Such batteries employ a solid electrolyte unlike the modern-day liquid electrolyte-based lithium-ion batteries and thus facilitate the use of high-capacity lithium metal anodes thereby achieving high energy densities. Despite this promise, practical realization and commercial adoption of solid-state batteries remain a challenge due to the underlying material and cell level issues that needs to be overcome. This chapter thus covers the specific challenges, design principles and performance improvement strategies pertaining to the cathode, solid electrolyte and anode used in solid state batteries. Perspectives and outlook on specific applications that can benefit from the successful implementation of solid-state battery systems are also discussed. Overall, this chapter highlights the potential of solid-state batteries for successful commercial deployment in next generation energy storage systems.


Introduction
The dawn of the 21 st century coincided with the global civilization leapfrogging into the digital age. At the core of this digital revolution was the rapid adoption and wide deployment of lithium-ion batteries (LIBs) [1]. Ever since, both industries and the scientific community have been engaged in the quest for unlocking the secrets for developing batteries with more energy, -power, -life, and -safety. In the recent years, the battery R&D community is once again at the cusp of another technological revolution that could redefine the energy storage sector for decades to come. This revolution is largely fueled by the rapidly advancing efforts into a new battery technology called all Solid-State Batteries (ASSBs) [2]. While conventional lithiumion batteries have enjoyed unprecedented levels of research and industrial attention directed towards every aspect of their constituents, SSBs have largely been on the backburner. For decades, the battery research sector has been constantly attempting to integrate high-capacity lithium metal anodes (3860 mAh.g À1 ) to advance energy density targets of liquid electrolyte-based LIBs [3]. However, in such conventional batteries, the hydrocarbon derived organic liquid electrolytes pose significant safety and performance related challenges with lithium metal anodes and has also been a significant barrier preventing wide commercial deployment [4]. With renewables and electric vehicles (EV) set to dictate the timeline for the next industrial revolution, the battery R&D community has come to a profound consensus that conventional lithium-ion batteries are nearing their upper limits of performance [5]. The initial interest towards SSBs was based on the use of solid electrolytes (SE), as they are potentially thought to provide a straightforward approach towards realizing the safe integration of high-energy lithium metal anodes. In recent years, new classes of high-performance solid electrolytes have emerged with high room-temperature conductivities ($1 mS.cm À1 ) [6] comparable to that of conventional liquid electrolytes and high lithium-ion transference numbers ($1 for inorganic SE) [7]. This coupled with novel processing approaches targeting interfacial tuning and optimizations are propelling the new generation of SSB systems towards garnering a widespread support for commercial adoption. With the EV demand projected to skyrocket in the next few years, the need for the next-generation high energy batteries that would power these advanced automotive platforms is also growing [1,[8][9][10]. To this extent, several large-cap automotive companies including Toyota, Volkswagen, General Motors, Hyundai, and Ford have already made major investments in SSB technology companies with the aim to achieve full commercial deployment in the first half of the 21 st century.
Despite the optimism and promises around all SSBs, there are several major challenges pertaining to each cell component and processing approach that needs further optimizations to achieve the overarching performance goals. For SSBs, there can be several material-level issues that can cause major cell-level catastrophic failures. We had recently reported that an ideal solid-state battery (Figure 1a) that delivers a high energy density should consist of the following [11] -(i) a highcapacity thin lithium metal anode/seed layer (thickness $1-5 μm seed layer + 15-40 μm plated from the cathode), (ii) a stable solid electrolyte with high ionic conductivities (thickness $1-20 μm, ideally dry), (iii) a cathode composite with optimized loading and tailored architecture (thickness $45-200 μm) [12]. In order to achieve high energy densities in SSBs, it is generally understood that the cathode should be the most voluminous part. Current collectors employed in an SSB can generally be <10 μm thick applied in the form of thin coatings to mechanically robust electrodes [13]. Additionally, SSBs also offer the possibility of cell stacking in two different configurations depending upon performance requirementsconventional stacking and bipolar stacking [14]. Bipolar stacking in a single package using bipolar current collectors decreases the packing volume thus increasing the volumetric energy density. Though, the requirements to achieve high performance SSBs appear quite straight forward, significant fundamental issues and challenges pertaining to specific cell components remain unresolved in the SSB R&D space. Figure 1b depicts a schematic providing a brief overview of some underlying challenges that warrants systematic investigations and mitigation strategies to enable the wide commercial deployment of SSBs. From the aforementioned figure, it should be noted that interfacial properties play a dominant role in determining the final performance delivered by SSBs [15,16]. For example, if we consider the simple interface between the anode and the copper current collector, some of the challenges encountered may include (i) current collector corrosion/crackingdue to the repeated mechanical cycling stresses and Li plating related side reactions, (ii) Cu-Li adhesionimproper adhesion between these dissimilar metals can occur due to oxide layer formation during processing or due to cyclic mechanical and electrochemical loads during charge/discharge, (iii) volume changedrastic volumetric expansion/contraction can occur during charge/discharge processes at the anode interfaces owing to the soft and pliable nature of Li metal, etc. Similarly, other interfaces between the lithium metal anode and solid electrolyte, solid electrolyte and cathode composite, cathode composite and positive current collector also suffer from interface related issues which can hamper final cell performance. Additionally, the component specific bulk property variations could also be crucial factors that affect SSB performance and thus demands systematic investigations leading to strategic solutions prior to commercial adoption.
In this chapter, we discuss the critical challenges, recent advances, and avenues for improvement for the various classes of cathodes, solid electrolytes and anodes that would facilitate the commercial adoption of next generation SSBs. We also discuss the key processing and fabrication criteria for assembling full SSBs cells with our recent modelling endeavors into achieving practical cell level energy densities. Finally, we discuss some perspectives on the challenges that remain unsolved as well as the future trends in SSB development. Through this chapter, we envision, the readers would get a comprehensive understanding of the recent trends, remaining challenges, and a clear perspective of the future prospects of all Solid-State Battery R&D.

Cathode materials for SSB
The cathode materials play a pivotal role in the energy density, power density, cycle life, and calendar life of a battery. Oxide-based lithium-ion intercalation materials are the cathode of today's choice and can mainly be segregated into five board classes based on their crystal structure. Apart from these, sulfur and some organic compounds are also used as cathode materials in batteries. Every cathode material and its crystal structure have inherent advantages and disadvantages from the standpoint of electrochemical energy storage. The basic concept of battery intercalation chemistry is very old [17]. The first intercalation reactions involving solid hosts (graphite) and guest molecules or ions (sulfate ions) were shown by Schauffautl in 1841. However, in the 1960s, the intercalation materials gained attention due to the altering of their electronic and optical properties through guest ion intercalation [18][19][20]. The transition-metal disulfides and oxides (such as MS 2 and WO 3 ) were first investigated for intercalation of H + , Li +, and Na + ions [17]. It was noted that the intercalation of these monovalent ions into the crystal structure of WO 3 altered the electronic structure and conductivity, resulting in the material changing from an insulator to metal depending upon the amount and types of monovalent cation intercalated. These intercalation reactions were also accompanied by structural changes with modifications to crystal chemistry [21].

Crystal structure cathode materials
The lattice atom and its coordination domain are the basic structural unit of a crystal. The lattice atoms are arranged periodically in specific combinations (e. g. space group) for the formation of crystals. It should be noted that, in general, the electronic structure and interaction for the bond formation energy in the structure unit ultimately determines the intrinsic chemical and physical properties of crystals [22]. For lithium-ion and sodium-ion batteries, the cathode materials can be formed by combinations of Li, Na, transition metal, and anion structure units. It should be further noted that the crystal structure and chemical composition of the cathode materials play an important role in the ionic and electronic transport properties and conduction mechanisms. The five different crystal structures of cathode materials are displayed in Figure 2 [23]. The layered structure cathode exhibits twodimensional ionic and electronic conductivity and diffusivity (Figure 2a). On the other hand, materials belonging to the spinel structure demonstrate threedimensional ionic and electronic conductivity and ionic diffusivity (Figure 2b). In contrast, the olivine structure cathodes showcase preferably one-dimensional ionic conductivity and diffusivity and two-dimensional electronic conductivity (Figure 2c). The β II -Li 2 FeSiO 4 and tavorite-type LiFeSO 4 F structures (Figure 2d and e) are not vigorously used in the lithium-ion battery cathode. The layer and spinel structure cathodes exhibit an interstitial type of mechanism of ionic conductivity and diffusivity. The olivine structure materials display vacancy migration type ionic conductivity and diffusivity. Unlike conventional lithium-ion battery (with liquid electrolyte), all solid-state battery (SSB) is impacted by dimensional conductivity, material hardness and mechanical properties which will be discussed in detail in the later sections.

Electronic and ionic transport properties
In a solid active cathode particle, the ion and electron moving together when charging or discharging the battery and this phenomenon is called ambipolar diffusion. Therefore, optimum ionic and electronic transport properties are prerequisites for high performances batteries, particularly SSBs. Nonetheless, it is very rare to get such good combinations in the presently available list of cathode materials. Some of the reported electronic and ionic conductivity and ionic diffusivity of spinel, layer, and olivine structure materials are compared and displayed in Figure 3 and Table 1 as a function of inverse temperature and lithium concentration, respectively.  Figure 3a that the electronic conductivity of spinel (LMNO) and layer structure (NMC and NCA) cathodes show the discrete pattern of conductivity as a function of lithium content. It is discernible from Figure 3b that the layer structure materials exhibit a gradual increase of conductivity with increasing degree of delithiation. On the other hand, the electronic conductivity of the spinel structure can be manipulated by varying the degree of disorder and degree of delithiation. It should be noted that the spinel phase exhibits two crystallographic polymorphs, ordered and disordered depending on the distribution of Ni and Mn in the crystal structure. The ordered LiMn 1.5 Ni 0.5 O 4 (LMNO (O)) exhibit approximately fifteen times lower electronic conductivity than the disordered LiMn 1.5 Ni 0.5 O 4-δ (LMNO (D)) phase (Figure 3a) in the lithiated states. Also, the electronic conductivities of the ordered spinel LNMO (O), measured at a given temperature, increases gradually with increasing the degree of delithiation (Figure 3a). However, when x = 0.3 and beyond, the conductivity is almost leveled where it hardly changes with the removal of lithium in the measured range. It appears that partial lithium off-stoichiometric phases are favorable for better highrate performances from comparing electronic conductivities for layered and ordered spinel cathodes. In disparity, the electronic conductivity of the disordered spinel (LNMO (D)) is reduced suddenly upon slight delithiation and falls to the level of the electronic conductivity demonstrated by lithiated ordered spinel phase. Following which, the electronic conductivity exhibits alike trends for both the ordered and disordered phases (Figure 3a). The electronic conductivity of the olivine phase as a function of lithium concentration is not available in the literature. However, the electronic conductivity of olivine phase LiFePO 4 exhibits the lowest conductivity as a function of temperature (Figure 3b) and layer structure NCA shows the highest conductivity. The ionic conductivity and diffusivity of spinel phases appear to be favorable from the SSB standpoint compared to layered and olivine structure cathodes (cf. Figure 3c and d and Table 1).

Electrochemical performances and particle morphology
The energy and power density are important metrics to determine the scope of a specific application for cathode materials [35][36][37][38][39]. The theoretical and experimentally obtained capacity, operational average cell voltage (against carbon electrode), and energy density of major cathode materials are compared in Table 2. It is discernible from Table 2 that all the phases of a particular crystal structure belonging to the same material family do not deliver the same energy density. The layer structure cathodes exhibit higher capacity compared to the spinel and olivine. However, the operational cell voltage is relatively lower than the spinel materials and some of the olivine phases. The energy density of Li 2 MnO 3 , NCA, and NMC based phases is higher than other cathode materials. Apparently, it is seen from Figure 3 that the spinel structure materials should exhibit higher power density than layer and olivine phases since it displays higher ionic diffusivity and conductivity as a function of lithium content as well as temperature. Nonetheless, particle morphologies play a crucial role in the power density and cycling stability of a battery. Three different types of particle morphologies are depicted in Figure 4 as an example. It should be noted that each type of particle morphology has some inherent advantages and disadvantages for a particular material. It is well known that the spherical dense particle morphology (Figure 4a) is beneficial for the longterm cycling of layer structure materials with liquid electrolyte (LE). On the other hand, the nanometer-thick plate-like particle (Figure 4b) is good for olivine materials as they exhibit very low ionic and electronic properties transport (see Figure 3 and Table 1). Rodlike particle morphologies (Figure 4c) of spinel materials are advantages for achieving high power densities. It is worth mentioning that the operational scenario of ASSB is very different than the conventional LE-based batteries. Unlike LE-based batteries, in SSBs, the solid electrolyte cannot penetrate inside the secondary particles as shown in the schematic (Figure 4d-f). Therefore, high power SSB cannot be achieved with a spherical particle in which ionic diffusion length would be longer and all particles might not be completely ionically wired. Submicron cathode particles (single crystals) are highly desirable for high performances SSB. Details are discussed in section 3 for requirements of cathode particles and the fabrication of composite electrodes for high-performance ASSB.

Suitability for solid-state battery
One of the major advantages of SSBs are the safe use of high voltage cathode materials (e. g. LMNO, LRM, and LCoP) which are not feasible in conventional liquid electrolytes due to their limited electrochemical stability window. Thus, SSBs are expected to provide a high energy density at the system level. However, the loose interfacial contact of oxide-based cathode and electrolyte (e. g. NMC/garnet-type electrolyte) creates a severe problem for cell voltage polarization and at room and low temperatures. In addition, a recent report has shown that solid electrolytes (SE) have instability issues while in direct contact with high voltage cathode materials [55]. In such a scenario, one sustainable solution might be the formation of a protective surface coating on cathode materials which would require stabilizing the cathode materials and SE interface by suppressing the possible oxidative reactions in the SSB. Furthermore, the primary cathode particles in the form of single crystals would be more beneficial for SSB than the secondary particles either in spherical or rod-like shapes. Unlike primary nanoparticles, the sphere of secondary particle has many separations or semi-separation regions between neighboring primary particles which is detrimental for SSB operation. Thus, single crystal cathodes are advantageous for use in SSBs because of their good crystallinity, high reaction homogeneity, mechanical strength, and better structural and thermal stability. All these salient features can remarkably improve the electrochemical performance and safety of the SSB.

Anode materials and designs for SSB
Anode materials can be broadly classified into three major types based on the mechanism of ion storage and electrochemical reactions occurring within the material [58]. The most common and prevalent type of anode material is the intercalation anode (Figure 5a) [59][60][61]. These materials typically possess layered structure into which Li-ion can reversibly insert (intercalate) during cycling of the battery [62]. Graphite, like several other materials (viz. LTO [63,64], TNO [65]) is an intercalation type anode material. Conventional Li-ion batteries employ graphite as the anode material for hosting Li-ions for reversible intercalation and storage of electrochemical energy. Graphite has a theoretical capacity of 372 mAh g À1 which is higher than most cathode materials making it suitable as an anode material [66]. Graphite has demonstrated high coulombic efficiency and cycling performance making it ubiquitous in secondary lithium-ion batteries These materials typically possess lower theoretical capacities; however, they are generally more stable and efficient electrodes. Alternate anode materials can be of deposition or conversion type depending on whether the mobile ion is depositing directly as a metal or as an alloy of a component respectively [3,58,67]. Alkali metals (Li, Na, etc.) are examples of deposition type anodes and they possess high theoretical capacity and relatively lower redox potentials [68]. Conversion type materials typically for alloys with the mobile ion (viz. In, Se, Si, etc.) and these also possess high theoretical capacity [69]. The major drawback for the deposition and conversion type anode materials are the electro-chemo-mechanical stability which makes them harder to integrate into functional devices compared to intercalation-type anode materials. Overall, typical reactions for each anode type can be given as: Deposition Reaction : Li þ þ e À ➔ Li Capacity $ 3860 mAh g À1 À Á Conversion Reaction : x Li þ þ Si þ x e À ➔ Li x Si Capacity $ 3579 mAh g À1 À Á Solid-state batteries rely on transitioning to high-capacity anode materials of the deposition or conversion type in order to achieve the expected improvements in the energy density. A comparison of nominal capacity of several key deposition, intercalation and conversion type anode materials is provided in Figure 5b. It should be noted that the nominal capacities are plotted on a logarithmic scale. The comparison highlights that intercalation anode materials have an order of magnitude lower capacity compared to some conversion/deposition anode materials (375 mAh g À1 for graphite; 3860 mAh g À1 for Li, 3590 mAh g À1 for Si). Mechanical properties of materials are also of key interest for solid-state batteries in order to design solid electrolytes that can mitigate filament growth. Conversion type anode materials typically show higher Young's modulus and shear modulus compared to intercalation and deposition type anodes (Figure 5b). Solid electrolyte materials should ideally have shear modulus higher than the anode material in order to mitigate the growth of filaments as proposed by Monroe and Newman [70,71]. It should be noted that the focus of anode studies with respect to solid-state batteries in the literature is primarily with lithium metal [3]. Relatively fewer reports on intercalation and conversion anode materials are reported and further work is anticipated in these material systems moving ahead.
The key challenges with deposition type anodes, and specifically Li metal will be discussed next. Controlling electrodeposition and electrodissolution morphology for Li metal is imperative to achieving stable solid-state batteries. Specifically, stable morphologies are required at the areal loading of $5 mAh cm À2 of reversible cycling capacity at $5 mA cm À2 plating current density with high coulombic efficiency is far from realization [11]. One major concern with lithium metal is the propensity for filament growth leading to cell failure [72][73][74]. Filament formation can have significant negative impacts to rate performance, power density and coulombic efficiencies of SSBs. Filament growth typically stems from non-uniform deposition rate at the Li | SE interface. Interfacial kinetics heterogeneity at the Li metal solid electrolyte interface initiates several degradation pathways including filament formation limiting the stability and performance of solid-state batteries. In addition to growth of filaments, high rate electrodissolution from the Li metal can lead to formation of pores that can cause onset of failure [75]. A direct evidence of this was obtained from X-ray tomography measurements of Li | LLZO | Li symmetric cells (Figure 5c). Tracking pore evolution during cycling which showed clear cycling behavior (increase in porosity with stripping and decrease in porosity with plating). Mass transport within the Li metal is thus a key challenge and understanding creep and flow behavior of Li is necessary to tune the performance of the system. Interphase formation can also occur during integration of Li metal with solid-electrolytes [15]. Depending on the thermodynamic stability of the solid electrolyte material with lithium metal, three possible interphases can result. These are (i) thermodynamically and kinetically stable (no reaction @ Li | SE interface), (ii) Unstable (unmitigated reaction), and (iii) kinetically metastable (controlled reaction @ Li | SE interface) [3,68,73]. With the exception of few materials (viz. LLZO, LiPON), most solid electrolytes undergo reaction with Li metal due to inherent chemical and thermodynamic instability. For some materials, like NASICON-type LAGP and LATP materials as well as LPS thiophosphates, chemical and electrochemical reaction with Li metal leads to an unmitigated growth of an ionically insulating interphase coupled with volume expansion of the material [76][77][78][79]. This leads to higher impedances, local stress generation and inhomogeneous current distributions that can cause failure through filament formation, shorting or mechanical fractures. On the other hand, addition of stabilizing agents to the solid electrolyte or introduction of interlayers to these solid electrolytes can lead to formation of a meta-stable interphase that is a mixed ionic and electronic conductor leading to stable solidstate batteries.
Lithium metal stabilization is enabled by several strategies that can be broadly classified into: (i) electrolyte modification [80,81] (ii) interface modification [82,83] and (iii) operating parameter modification [84][85][86]. Electrolyte modification is afforded by additives that can promote the formation of kinetically metastable interphases [73]. For instance, LiI addition to LPS material in conjunction with microstructure control led to improvement of critical current density from < 0.5 mA cm À2 to > 4 mA cm À2 . Similarly, halide addition to a range of solid electrolytes have shown improved performance in terms of ionic conductivity and critical current density. Interface modification is typically carried out by introducing the use of an interlayer barrier film at the anode | solid electrolyte interface. Atomic layer deposition of materials like Al 2 O 3 , Si, Li x Al (2-x/3) O, LiXO 3 (X = Ta, Nb) has shown to improve the performance of lithium metal anodes [87][88][89][90][91][92]. However, typically the introduction of interlayers is carried out by cost-, time-and equipment-intensive processes that limit the large-scale deployment of such strategies.
Another key strategy is modification of operating conditions primarily, temperature and pressure. Indeed, numerous studies have shown the importance of a critical stack pressure in order to mitigate the mass transport limitations within lithium metal by enhancing creep flow at higher pressures (Figure 6a and b) [86,93,[96][97][98]. Overpotential at constant lithium stripping current (0.1 mA cm À2 ) shows reversibly changing overpotentials with modification of the stack pressure. Similarly, overpotential as a function of applied current density shows a reduction of overpotential with increasing stack pressure. Silicon and indium-based anodes also show promising performance (Figure 6d) [94,95]. In summary, anode materials for solid-state batteries need to provide high capacity with high-rate capabilities. Further work on stabilization of anodes under these conditions and demonstration of scalable integration approaches is required for deployment.

Solid electrolytes
Solid ion conductors have been synthesized in a wide range of chemistries (Figure 7) [99]. Currently, most promising electrolyte that have been investigated thoroughly are NASICON (LATP, LAGP) [100], Garnets (LLZO) [101] and Sulfides (LPS) [102]. However, each of these electrolytes have distinct limitations which are hindering their deployment in ASSBs. NASICONs have been widely investigated  [84]. (b) Influence of stack pressure on voltage increase for varying current densities for Na | β Alumina | Na cell with the inset showing the critical current density as a function of applied stack pressure. Reprinted with permission from [93]. (c) Porosity for two lithium metal electrodes as a function of cycling steps obtained from X-ray tomography measurements and machine learning segmentation. Reprinted with permission from [75]. (d) Silicon | LPS | Li cell cycling behavior. Silicon particles are spray coated on steel current collectors. Areal loading and current density for the test were 55 μg cm2 and 0.06 mA cm2 respectively. Reprinted with permission from [94]. (e) Potential profile during pulsed lithiation on In metal at 0.2 mA cm2. LPS was used as the solid electrolyte. Reprinted with permission from [95].
for not only Na-ion but also Li-ion all-solid-state batteries [103,104]. Li 1.3 Al 0.3 Ti 1.7 (PO 4 ) 3 (LATP) and Li 1.5 Al 0.5 Ge 1.5 (PO 4 ) 3 (LAGP) are the two most popular NASICON electrolytes for Li-metal solid-state batteries. These electrolytes have high ionic conductivities ($10 À3 S cm À1 ) but suffer from stability issues as Ti 4+ and Ge 4+ undergo reduction in contact with Li metal anode [105][106][107]. First principles studies on these LATP and LAGP materials report operating voltage windows between 2.17 -4.21V and 2.7 -4.21V respectively. Garnet type electrolytes have garnered great attention due to their high electrochemical voltage window which enables high power density ASSB and enables long cycling life. However, garnet electrolytes have ionic conductivity one order of magnitude lower than LATP and LAGP. Garnet electrolytes also suffer from environmental instability which makes the processing of garnet type solid electrolyte cost intensive [108]. While a plethora of solid electrolyte classes have been explored over the years, the sodium superionic conductors or the NASICON class are slowly being re-examined for their high ionic conductivities, mechanical robustness and good chemical and electrochemical stabilities [109]. In particular, materials belonging to the NASICON family with phosphate anions are being extensively explored as potential electrolytes and cathode materials for Li, Na, and Mg-ion batteries owing to their high ionic conductivity, thermal and environmental stability [110]. The NASICON type Na 1+x Zr 2 Si x P 3Àx O 12 (0 ≤ x ≤ 3) is a promising electrolyte material providing high ionic conductivity (10 À4 S cm À1 ) at room temperature owing to the facile 3D ion conducting pathways. The general formula for these NASICON type materials is AM1M2(PO 4 ) 3 where A can be a monovalent cation Li + , Na + , K + , Rb + , Cs + , Ag + , Cu + , H + , H 3 O + , NH 4 + , or a divalent cation such as Mg 2+ , Ca 2+ , Sr 2+ , Ba 2+ , Pb 2+ , Cd 2+ , Zn 2+ , Mn 2+ , Fe 2+ , Co 2+ , Ni 2+ or Cu 2+ or it can also be vacant. M1 and M2 can be filled with di-, tri-, tetra-or pentavalent transition metal ions within the boundaries of charge balance. NASICONs can crystallize in three different crystal structures, based on the synthesis method, annealing temperature and choice of A, M1 and M2 resulting in α, β and γ-NASICON. Of these, γ-NASICON has the highest symmetry with R3̅ C space group which is highly suitable for achieving high ionic conductivities. It is important to explore the possibility of Na-based NASICON materials to be able to conduct Li-ions as well.
Solid electrolytes have limited ionic conductivity at atmospheric temperatures which inhibits the rate capability of ASSB for practical applications (see Table 3). Ion transport in polymer electrolyte happens by the complexation of oxides while in inorganic electrolytes, it happens across the crystal lattice sites [111,112]. Ion transport depends on the available lattice sites and activation barrier for hoping from one lattice to other. Investigation on increasing available lattice sites for ion transport should be carried out for improving ion-transport. Garnet structures can be synthesized for different cubic structures to improve ion-conductivity. Sulfides have shown highest ionic conductivity among solid electrolytes but have major issues with sensitivity ambient environment since it produces H 2 S when exposed to humidity. Low bending stiffness of ceramic electrolytes are hindering the processing with roll-to-roll manufacturing. Polymers can be easily processed but they don't have very high ionic conductivity (see Table 3). Thus, composite electrolytes can be one of the solutions to this conundrum.
Solid electrolyte must be chemically and electrochemically stable with the anode and cathode material at the operating potentials. Thermodynamic calculations based on the phase equilibria at the oxidation and reduction potential of  well-known electrolytes showed that no electrolyte is simultaneously stable at both reductive potential of $0 V (vs Li) at the negative electrode and at typical positive potential of $4 V [113]. That's why chemically and electrochemically stable and lithium ion conducting interphase must be formed as depicted in the Figure 7. From the first principle calculation, reduction and oxidation potential of well know solid electrolytes are given in the Table 4.

Full cell integration
Material families that can meet ion transport criteria comparable to the state-of-the-art liquid electrolytes have been identified for solid ion conductors. Integration of these materials into a high-performance battery stack is still far from realization. The primary limitation in this regard is the lack of fundamental understanding of the interplay between charge transfer kinetics and mass transport within the system, specifically at the electrode | electrolyte interfaces in addition to other challenges (Figure 8a). Typical implementation of lab-scale solid-state batteries is not in traditional coin-cell or pouch-cell formats. Solid-state batteries are typically operated in "pressure cells" that encase the cell system in a container on which a mechanical load is applied (Figure 8b) in addition to temperature. Generally, SSBs are reported to function at operating pressures of >100 MPa and elevated temperatures (>50°C). A quick survey of the reported SSB performance shows that the achieved specific energy and power density of SSBs fall short of required metrics of operations for SSBs of >400 Wh/kg gravimetric energy density and >200 W/kg power density (Figure 8c) [11]. Janek et al. have carried out extensive work to understand and decouple the influence of interphase formation and its impact on cycling of SSBs [116,117]. Typical SSB cycling performance for sulphide based SSBs is depicted in Figure 8d [114]. The  analogous conventional cell ($15 %). Subsequent rate testing shows strong capacity loss at high C-rates with 0 mAh g À1 at 1C (Figure 8d). Subsequent long-term cycling at 0.1 C shows a strong capacity fade (1-2% each cycle) that is not observed for the conventional cell. The origin of this behavior is identified as a resistive layer formed on the cathode at the high charging voltages which is validated by in situ impedance spectroscopy, SEM and XPS measurements. NCA cathode material with LPSCl solid electrolyte and Li metal anode was investigated in full cells at 5 MPa stack pressure [85]. LNO (Figure 8e) [115]. The results indicated that solid electrolyte with higher adhesion properties at the interface shows improved performance due to improved wetting and contact with the cathode. SSB micro-batteries are also investigated as a potential architecture to maximize areal capacity and electrochemically active surface areas for niche applications. MoOS 2 cathode material in conjunction with PVDF-based solid electrolyte and mesoporous carbon anode was used to fabricate 3D micro-batteries using thin-film coating processes [118]. The results from this study showed improved areal capacity of over an order of  magnitude for 3D micro-battery compared to a traditional 2D architecture processed identically. Recently, Dixit et al. carried out a numerical study on investigation on impact of cathode architecture on the energy density of solid-state batteries [14]. They identified a necessity for a large variation in particle size of cathode components in order to achieve higher density composite cathodes as well as to achieve high contact area between the solid-electrolyte and cathode active material. Additionally, the influence of excess anode material to the resultant cell-level energy density was investigated (Figure 9a-c). Transitioning to low/no-excess anodes systems can provide significant improvements in terms of cell-level energy density. Dense solid electrolytes ($LLZO) result in high volumetric energy density while low-density solid electrolyte ($PEO) in conjunction with high voltage/capacity cathode materials. Limited demonstrations of completely anode-free cells are observed in literature. Cycling of an in-situ formed Li anode in a NCA | LLZO anode free cell is highlighted here (Figure 9d and e). The investigated anode free system shows typically low cathode utilization due to unoptimized cathode architecture with highly reversible cycling (coulombic efficiencies $100%) over 50 cycles [119]. It should be noted that due to changes in "accessible" lithium, certain discharge cycles show higher capacities than the corresponding charge cycle. Another important consideration in solid-state battery architecture is the concept of bipolar stacking. The use of solid electrolyte mitigates the shorting and electrolyte leaking in unit cells allowing for series stacking and reduction of inactive materials in the cell (packaging, sealing, conductor elements). This can lead to improvement in both gravimetric energy density as well power density due to reduction in inactive materials as well as overall resistance of the modules. Initial results with excess-area stainless steel Effect of transitioning to a no-excess anode system from a 100% excess anode system on (b) gravimetric energy density and (c) volumetric energy density for a range of material combinations. Reprinted with permission from [14]. ( [120,121]. current collector as a bipolar plate shows promising polarization profiles for NMC622 as well as LFP -based SSBs with polymer based solid electrolytes [120][121][122][123]. Subsequent investigations into materials, architectures and cell design for bipolar stacking needs to be carried out for high energy and power density SSBs.

Perspective and outlook
Battery research at present is at a stage where SSB technology can either redefine the next generation of batteries or fissile out without making an impact. The key difference between the two scenarios is whether the underlying challenges can be systematically mitigated or not. With more commercial enterprises investing heavily into SSB research, there is momentum slowly building to propel and position SSBs as the batteries of the future. Success of SSBs would depend upon the following factors: (i) improving the interfacial issues, (ii) developing unique processing capabilities while minimizing cost, (iii) addressing cell and pack level design challenges when integrating solid state components, and (iv) demonstrating performances exceeding that of advanced lithium-ion batteries. Some potential challenges and opportunities for SSBs include: With the electric vehicle revolution driving up the demand for batteries that can deliver high energy density, can fast charge, have long cycle and calendar life while maintaining low manufacturing costs, there is definite potential for SSBs to play a major role in achieving these goals. Moreover, the US Department of Energy's target goal for EV batteries include (i) of reducing battery cost to <$100/kWh and ultimately to <$80/kWh, (ii) increasing the range of electric vehicles to 300 miles and (iii) decreasing charge time to 15 minutes or less. A full solid-state battery that can meet these targets that are set for electric vehicles in the next decade is an ambitious endeavor especially when the best anode, cathode, and electrolyte chemistries for such an SSB are not obvious at present.
Urban air mobility is a rapidly pursued avenue that could redefine transportation as we know it. Conventional liquid electrolyte-based batteries are subjected to increasingly stringent safety requirements for use in operation of auxiliary electronics on present day aircraft platforms. Applications with demanding duty cycles such as electric vehicle take off and lift (EVTOL) platforms require batteries that can operate at extreme temperature gradients while being subjected to a multitude of mechanical stresses during operation. SSBs with their safety and energy density advantages are uniquely positioned to be the go-to batteries for such applications. Exploring SSBs for such applications would require coordinated efforts between industrial players and research institutes to initially define the performance requirements for such platforms followed by systematic material and engineering efforts to address the challenges.
i. Extreme environment batteries are a niche yet rapidly growing application due to the increasing public interest into commercial low-earth orbit and interplanetary travel. Certain terrestrial, upper altitude and space applications necessitate secondary energy storage technologies that can function under a very wide modality of extreme environmental conditions including but not limited to temperature, pressure, radiation loads, and mechanical loads. With some present day SSBs operating efficiently at elevated temperatures when compared to the room temperature performance, this presents a unique opportunity for deploying SSBs in these applications. Development of stable, safe, and durable energy storage technologies can have a transformational impact on the application sectors which can include orbital satellites, outer planetary/deep-space probes, land rovers, polar vehicles/end stations, among others.
ii. As an intermediate step, a less-demanding end use application such as SSBs for portable/wearable electronics can be pursued, however, this would risk diverting the focus of the battery R&D community from the biggest projected market for next generation battery systemselectric vehicles and grid storage.

Conclusion
In summary, despite the challenges, solid-state batteries have great potential for implementation in applications that demand high-energy and safe batteries. Successful deployment of practical SSBs is contingent on addressing the underlying challenges related to materials, processing, and cell engineering. Through this chapter, we have discussed the key issues pertaining to the specific SSB cell componentscathode, solid electrolyte and anode and their interfaces. We envision that our perspectives and outlook discussed in this chapter encourages the readers and inspires solutions that would lead to the eventual practical realization and wide commercial deployment of energy dense solid state battery systems.