## 2 Electrodes and Electrolytes for Lithium-Based Batteries
### Solid Electrolytes
In the same year, Murugan et al. [55] reported the synthesis of Li\({}_{7}\)La\({}_{3}\)Zr\({}_{2}\)O\({}_{12}\) (LLZO) that has high ionic conductivity (\(10^{-4}\) S/cm at room temperature) as well as relatively good thermal and chemical stability compared to perovskite electrolytes. LLZO exhibits two crystallographic phases, a low conductive tetragonal phase and a high conductive cubic phase shown in Figure 6[215,216]. Thus, it is essential to stabilize the cubic phase to maintain a high conductivity. Figure 6: Crystal structure of (**a**) cubic Li\({}_{7}\)La\({}_{3}\)Zr\({}_{2}\)O\({}_{12}\) (LLZO), (**b**) coordination polyhedra around the Li1 and Li2 sites and (**c**) Li\({}^{+}\) migration through the LLZO crystal structure. Reprinted from ref. [216] with permission of The Chemical Society of Japan. Allen et al. [191] reported that Ta substitution for Zn (Li\({}_{6.75}\)La\({}_{3}\)Zr\({}_{1.75}\)Ta\({}_{0.25}\)O\({}_{12}\)) could stabilize cubic LLZO, resulting in a conductivity of \(8.7\times 10^{-4}\) S/cm at 25 \({}^{\circ}\)C. Li et al. [217] also reported 10\({}^{-3}\) S/cm of conductivity at room temperature with the specific composition of x = 0.6 in Li\({}_{7.X}\)La\({}_{3}\)Zr\({}_{2.X}\)Ta\({}_{X}\)O\({}_{12}\). Ga-substituted LLZO, Li\({}_{6.55}\)Ga\({}_{0.15}\)La\({}_{3}\)Zr\({}_{2}\)O\({}_{12}\), also presented a similar conductivity 1.3 \(\times\) 10\({}^{-3}\) S/cm at 24 \({}^{\circ}\)C by introducing lithium vacancies through the substitution of Li by Ga [192]. These levels of ionic conductivities are close to that of the carbonate-based liquid electrolyte, making LLZO attractive enough to be used for SSBs. Although it was believed that garnets are stable in air previously, several studies indicated their sensitivity to moisture and CO\({}_{2}\) in air [189; 218; 219; 220; 221]. Even after 24 h of exposure in air, Al-substituted LLZO (\(\sim\)150 \(\upmu\)m grain size) showed one order magnitude increase of area-specific resistance due to the Li\({}_{2}\)CO\({}_{3}\) formation on the surface [189]. When in contact with Li metal, Ta containing LLZO was stable with Li, whereas Nb containing LLZO was discolored to black [222]. With cathodes, the garnet structure was stable with LCO up to 900 \({}^{\circ}\)C but became unstable when in contact with Mn, Ni, (Fe, Mn) and (Co, Mn)-based cathodes [223; 224]. The anti-perovskite (Li\({}_{3}\)OX, X = Cl, Br, I) has a general perovskite structure, but the positions of cations are filled with anions. In 2012, lithium-rich anti-perovskite (LiRAP) structures were reported by Zhao et al. by a molten salt synthesis method, showing a conductivity of \(1.94\times 10^{-3}\) S/cm of conductivity with Li\({}_{3}\)OCl\({}_{0.5}\)Br\({}_{0.5}\) at room temperature [193]. The higher conductivity of the mixed-halogen phase (Li\({}_{3}\)OCl\({}_{0.5}\)Br\({}_{0.5}\)) than Li\({}_{3}\)OCl or Li\({}_{3}\)OBr can be due to its topological property providing enough free space for Li\({}^{+}\) to move compared to the anti-perovskites with solely Cl\({}^{-}\) or Br\({}^{-}\). Furthermore, Li et al. [194] showed an extended electrochemical stability window up to 9 V vs. Li/Li\({}^{+}\) with fluorine-doped anti-perovskites, Li\({}_{2}\)(OH)\({}_{0.9}\)F\({}_{0.1}\)Cl, that can be used in ultrahigh energy density storage systems. The high ionic conductivity, low cost, low melting temperature, and large electrochemical stability windows are the merits of anti-perovskites; however, their highly hygroscopic nature requires extreme care during sample fabrications and handling. Minimizing the interaction between the mobile charge carriers and the anion framework is important to enhance the ionic conductivity of solid-state electrolytes. Sulfides generally offer higher ionic conductivities than oxides because the interaction between S\({}^{2-}\) and Li\({}^{+}\) is weaker than that of O\({}^{2-}\) and Li\({}^{+}\). Another merit of sulfide electrolytes is that the grain boundary resistance is lower than that of the oxide-based electrolytes.