**2.4 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

#### **Figure 4.**

*Particle morphologies of cathode materials (a-c) adapted from the references [55–57] and schematics showing the scenario of liquid and solid electrolytes for ionically wiring of cathode spherical particle (d-f).*

*Current Status and Prospects of Solid-State Batteries as the Future of Energy Storage DOI: http://dx.doi.org/10.5772/intechopen.98701*

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.

## **3. 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**) [59–61]. 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�<sup>1</sup> 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:

Intercalation Reaction : Li <sup>þ</sup> 6 C ➔ LiC6 Capacity � 372 mAh g�<sup>1</sup> (1)

Deposition Reaction : Li<sup>þ</sup> <sup>þ</sup> <sup>e</sup>� ➔ Li Capacity � 3860 mAh g�<sup>1</sup> (2)

$$\begin{aligned} \text{Conversion Reaction} &: \text{x } \text{Li}^+ + \text{Si} + \text{x } \text{e}^- \rightarrow \text{Li}\_{\text{x}} \text{Si} \left( \text{Capacity} \sim \text{3579 } \text{mA} \text{g}^{-1} \right) \end{aligned} \tag{3}$$

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

#### **Figure 5.**

*(a) Schematic diagram showing anode material type and operation mechanism. (b) Nominal capacities and mechanical properties of some common anode materials. (c) Schematic diagram highlighting the challenges with metallic anodes in terms of flux imbalance at the interface of solid electrolyte and the formation of reactive interphase at the electrode | electrolyte boundary.*

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�<sup>1</sup> for graphite; 3860 mAh g�<sup>1</sup> for Li, 3590 mAh g�<sup>1</sup> 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�<sup>2</sup> of reversible cycling capacity at �5 mA cm�<sup>2</sup> plating current density with high coulombic efficiency is far from realization [11]. One major concern with lithium metal is the propensity

#### *Current Status and Prospects of Solid-State Batteries as the Future of Energy Storage DOI: http://dx.doi.org/10.5772/intechopen.98701*

for filament growth leading to cell failure [72–74]. 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 [76–79]. 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 [84–86]. 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�<sup>2</sup> to > 4 mA cm�<sup>2</sup> . 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 Al2O3, Si, LixAl(2-x/3)O, LiXO3 (X = Ta, Nb) has shown to improve the performance of lithium metal anodes [87–92]. 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, 96–98]. Overpotential at constant lithium stripping current (0.1 mA cm�<sup>2</sup> ) 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

#### **Figure 6.**

*Summary of key results from anode integration studies in solid state batteries. (a) Potential response of Li | LLZO | Li cell under constant current of 0.1 mA cm2 under varying stack pressures. Reprinted with permission from [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 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.

#### **4. 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

*Current Status and Prospects of Solid-State Batteries as the Future of Energy Storage DOI: http://dx.doi.org/10.5772/intechopen.98701*

**Figure 7.**

*Schematic diagram highlighting the differences in properties of three major classes of solid electrolytes: (a) Garnet, (b) Sulphides, and (c) NASICONs.*

for not only Na-ion but also Li-ion all-solid-state batteries [103, 104]. Li1.3Al0.3Ti1.7(PO4)3 (LATP) and Li1.5Al0.5Ge1.5(PO4)3 (LAGP) are the two most popular NASICON electrolytes for Li-metal solid-state batteries. These electrolytes have high ionic conductivities (<sup>10</sup><sup>3</sup> S cm<sup>1</sup> ) but suffer from stability issues as Ti4+ and Ge4+ undergo reduction in contact with Li metal anode [105–107]. 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 Na1+xZr2SixP3xO12 (0 ≤ x ≤ 3) is a promising electrolyte material providing high ionic conductivity (10<sup>4</sup> S cm<sup>1</sup> ) at room temperature owing to the facile 3D ion conducting pathways. The general formula for these NASICON type materials is AM1M2(PO4)3 where A can be a monovalent cation Li+ , Na+ , K+ , Rb+ , Cs+ , Ag+ , Cu+ , H+ , H3O+ , NH4 + , or a divalent cation such as Mg2+, Ca2+, Sr2+, Ba2+, Pb2+, Cd2+, Zn2+, Mn2+, Fe2+, Co2+, Ni2+ or Cu2+ 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 R3C 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 H2S 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


**Table 3.** *Solid state electrolyte material for All-Solid-State Lithium Ion batteries.*


*Current Status and Prospects of Solid-State Batteries as the Future of Energy Storage DOI: http://dx.doi.org/10.5772/intechopen.98701*

#### **Table 4.**

*Oxidation and reduction potential of well-known solid electrolytes against Li/Li+.*

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**.

#### **5. 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 galvanostatic charge-discharge curves of NMC-811 | Li-In cells with LPS solid electrolyte (separator, catholyte: 30 %, active material: 70 %). SSB cell shows a large first cycle irreversibility (30 %) compared to an analogous conventional cell (15 %). Subsequent rate testing shows strong capacity loss at high C-rates with 0 mAh g<sup>1</sup> 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-coated NCA shows a first cycle irreversibility similar to NMC materials with subsequent cycles showing higher coulombic efficiency (98%). 80% retention over 200 cycle was observed for this cell at the 5 MPa stack operating pressure and 3.5 mg cm<sup>2</sup> active material loading. The results suggest optimization of the operating conditions (pressure, temperatures) in order to mitigate the formation of filaments and extend SSB lifetimes. Similar studies have been carried out for different cathode and solid electrolyte material combinations that highlight the need of tailoring cathode microstructure, interfaces, reactivity as well as mechanics of the composite cathode. Dixit et al. investigated LFP based cathode composites in conjunction with hybrid solid electrolytes (PEO-LLZO) with varying mechanical properties (**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. MoOS2 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

#### **Figure 8.**

*(a) Schematic diagram highlighting the challenges in integration of all solid-state batteries. (b) Schematic diagram of a typical implementation of a solid-state battery cell. (c) Summary of experimentally reported energy and power density of solid-state batteries. Note that the target performance region for solid state batteries is shaded. (d) Polarization curves and rate performance of NMC-811 | Li-In cells with LPS solid electrolyte (separator, catholyte: 30%, active material: 70%). Reprinted with permission from [114]. (e) Cycling performance of LFP | PEO-LLZO | Li cells for hybrid solid electrolytes with three different molecular weights 300K, 1M and 500 K. Reprinted with permission from [115].*

*Current Status and Prospects of Solid-State Batteries as the Future of Energy Storage DOI: http://dx.doi.org/10.5772/intechopen.98701*

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

#### **Figure 9.**

*(a) Schematic diagram showing the differences in SSBs with and without anode incorporated in the system. 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]. (d–e) Cycling performance of an anode free NCA|LLZO cell after initial charging cycle at 0.05 mA cm2. Stack pressure of 4 MPa was used for the tests. Reprinted with permission from [119]. (f) Schematic diagram showing bipolar stacking of solid-state batteries. Typical polarization curve for bipolar stacked SSBs with (g) NMC and (h) LFP based cathode materials. Reproduced with permission from [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– 123]. Subsequent investigations into materials, architectures and cell design for bipolar stacking needs to be carried out for high energy and power density SSBs.
