## 2 Electrodes and Electrolytes for Lithium-Based Batteries
### Solid Electrolytes
Kuwano and West in 1980 reported much higher ionic conductivity for Li\({}_{4}\)GeO\({}_{4}\)-Li\({}_{3}\)VO\({}_{4}\) systems with a total ionic conductivity of \(4\times 10^{-5}\) S/cm at 18 \({}^{\circ}\)C with the addition of an interstitial diffusion mechanism [176]. The introduction of V\({}^{5+}\) (e.g., Li\({}_{3+x}\)Ge\({}_{x}\)V\({}_{1-x}\)O\({}_{4}\)) contributed to stabilizing the structure in the presence of CO\({}_{2}\) in air. It is also reported that the Li\({}_{4}\)SiO\({}_{4}\)-Li\({}_{3}\)PO\({}_{4}\) solid solution presented the ionic conductivity 10\({}^{-6}\) S/cm at room temperature and had better stability against Li due to the absence of transition metal ions [75]. While maintaining the chemical stability in Li\({}_{4}\)Si/GeO\({}_{4}\)-Li\({}_{3}\)PO\({}_{4}\) solid solution, the ionic conductivity could be further improved to \(\sim\)10\({}^{-5}\) S/cm by substituting O with Cl, enlarging the four oxygen bottleneck size and lowering the diffusion barriers [177]. The most significant conductivity improvement of the LISICON structure was achieved with O replacement with larger and better polarizing ions, S, to form thio-LISICON. For example, the Li\({}_{2}\)S-GeS2-P\({}_{2}\)S\({}_{5}\) system (Li\({}_{3.25}\)Ge\({}_{0.25}\)P\({}_{0.75}\)S\({}_{4}\), called LGPS) reached a high ionic conductivity of \(2.2\times 10^{-3}\) S/cm at room temperature [178]. However, the high sensitivity to moisture in air and difficulties in the synthesis of sulfide electrolytes remain as challenges. In 1992, Bates et al. [179] reported the synthesis of lithium phosphorus oxynitride (LiPON, Li\({}_{3.3}\)PO\({}_{3.9}\)N\({}_{0.17}\)) by RF-magnetron sputtering of Li\({}_{3}\)PO\({}_{4}\), which showed conductivity of \(2\times 10^{-6}\) S/cm at 25 \({}^{\circ}\)C. Different from other electrolytes, LiPON has an amorphous structure, and its ionic conductivity is significantly affected by the amount of nitrogen [180; 181; 205]. Another route to improve the ionic conductivity of LiPON is to increase the Li concentration, as can be seen by the conductivity increase to \(6.4\times 10^{-6}\) S/cm at 25 \({}^{\circ}\)C when LiPON was deposited with Li\({}_{2}\)O in addition to Li\({}_{3}\)PO\({}_{4}\). Because of its easy deposition in thin films, LiPON can present a low resistance in the form of a thin film. Thus, LiPON is commonly used as the electrolyte for thin-film microbatteries (1-10 mAh) that can be used for smart cards, wearable devices, MEMS or implantable medical devices [205; 206; 207]. The deposition of LiPON and prototype battery performance will be discussed in detail later. Among perovskite-type solid electrolyte materials, Li\({}_{x}\)La\({}_{2/3-1/3x}\)TiO\({}_{3}\) (LLTO) exhibited very high bulk ionic conductivity. The LLTO is composed of the ideal structure cubic phase \(\alpha\)-LLTO with Pm3 m symmetry and tetragonal phase \(\beta\)-LLTO with a P4/mmm space group. In 1993, Inaguma et al. [182] showed the improved ionic conductivity of Li\({}_{0.34}\)La\({}_{0.51}\)TiO\({}_{2.94}\) compared to LISICON. The bulk and grain boundary activation energies of the cubic perovskite structure were reported to be E\({}_{\rm b}\) = 0.40 eV and E\({}_{\rm gb}\) = 0.42 eV, respectively. Importantly, the bulk ionic conductivity was reported to be \(1\times 10^{-3}\) S/cm at room temperature, but the total ionic conductivity was \(2\times 10^{-5}\) S/cm due to the high grain boundary resistance. Alonso et al. [208] identified the position of Li\({}^{+}\) in Li\({}_{0.5}\)La\({}_{0.5}\)TiO\({}_{3}\) using the neutron powder diffraction and suggested the Li\({}^{+}\) conduction pathway in LLTO. Jay et al. [209] proposed an additional diffusion pathway in the c-direction via a computational study that aligned more directly with experimental data. Lu et al. [183] synthesized Li\({}_{2-x}\)Sr\({}_{1-x-y}\)Li\({}_{y}\)TiO\({}_{3}\) to enhance the Li\({}^{+}\) diffusion by increasing A site.