**Abstract**

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.

**Keywords:** solid electrolyte, composite cathode, lithium-ion, batteries, lithium anode

#### **1. Introduction**

The dawn of the 21st 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�<sup>1</sup> ) 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<sup>1</sup> ) [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, 8–10]. 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 21st 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 requirements – conventional stacking and bipolar stacking [14]. Bipolar stacking in a single package using bipolar current collectors decreases the packing volume thus increasing the volumetric energy

#### **Figure 1.**

*(a) An ideal solid-state battery with optimized configuration and (b) remaining material level challenges in solid state batteries.*

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

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/cracking – due to the repeated mechanical cycling stresses and Li plating related side reactions, (ii) Cu-Li adhesion – improper 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 change – drastic 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.

#### **2. 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 [18–20]. The transition-metal disulfides and oxides (such as MS2 and WO3) 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 WO3 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].

### **2.1 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**).

#### **Figure 2.**

*The crystal structures of major cathode materials: (a) layered α-LiCoO2; (b) cubic LiMn2O4 spinel; (c) olivine-structured LiFePO4 ; (d) β II -Li2FeSiO4; and (e) tavorite-type LiFeSO4F. Li ions are shown as light green spheres, CoO6 octahedra in blue, MnO6 octahedra in mauve, Fe*�*O polyhedra in brown, PO4 tetrahedra in purple, SiO4 tetrahedra in yellow, SO4 tetrahedra in gray, and in (e) fluoride ions in dark blue. Black lines demarcate one unit cell in each structure. (Reprinted with permission from Ref [23]. Copyright 2014, published by The Royal Society of Chemistry. This image is taken from the article titled "Lithium and sodium battery cathode materials: computational insights into voltage, diffusion and nanostructural properties", and it is under the Creative Commons Attribution 3.0 International License. To view a copy of this license, visit https:// creativecommons.org/licenses/by/3.0/.).*

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

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 -Li2FeSiO4 and tavorite-type LiFeSO4F 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.

#### **2.2 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 3.**

*(a) Electronic conductivity of layer and spinel structure cathodes as a function lithium content, (b) Electronic conductivity of layer, spinel, and olivine structure cathode materials as a function of inverse temperature, (c) ionic diffusivity of layer and spinel structure cathode as a function of lithium content and (d) ionic conductivity of layer, spinel, and olivine structure cathode materials as a function of inverse temperature. LiFePO4 = LFP, LiNixMnyCozO2 = NMC, LiNi0.75Co0.25Al0.05O2 = NCA, LiMn1.5Ni0.5O4 = LMNO (O), and LiMn1.5Ni0.5O4-<sup>δ</sup> = LMNO (D). Each of the figures, a, b c and d were made by authors from the following references. [24] = LFP, [25] = (NMC333, NMC523), [26] = (NCA-NEI, NCA-TODA), [27] = (LNMO (O), LNMO (D)), [28] = (LNMO (O), LNMO (D)).*


#### **Table 1.**

*Comparison of ionic diffusivity of the selected major classes of cathode materials at specific temperature.*

It is seen from **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 LiMn1.5Ni0.5O4 (LMNO (O)) exhibit approximately fifteen times lower electronic conductivity than the disordered LiMn1.5Ni0.5O4-<sup>δ</sup> (LMNO (D)) phase (**Figure 3a**) in the lithiated states. Also, the electronic conductivities of the ordered spinel LNMO (O), measured at a given

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

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 LiFePO4 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**).

## **2.3 Electrochemical performances and particle morphology**

The energy and power density are important metrics to determine the scope of a specific application for cathode materials [35–39]. 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.


#### **Table 2.**

*Comparison of theoretical and experimental capacity of the major cathode materials and their average operational cell voltage.*

However, the operational cell voltage is relatively lower than the spinel materials and some of the olivine phases. The energy density of Li2MnO3, 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.
