## 2 Electrodes and Electrolytes for Lithium-Based Batteries
### Solid Electrolytes
With the composition of Li\({}_{15/56}\)Sr\({}_{1/16}\)La\({}_{15/28}\)TiO\({}_{3}\) a total conductivity of \(4.84\times 10^{-4}\) S/cm was achieved with an activation energy of just 0.29 eV. Although its high bulk ionic conductivity is attractive, LLTO suffers from high grain boundary resistance and reactivity with Li metal. LLTO variants showed distinct discoloration when in contact with Li metal, and the Li intercalation at 1.7 V vs. Li/Li\({}^{+}\) into LLTO was observed, limiting their use with low potential anode materials [210; 211; 212]. To resolve the instability issue of LLTO with Li, Ti\({}^{4+}\) was substituted with Ge\({}^{4+}\)[184] or Zr\({}^{4+}\) and Ta\({}^{5+}\)[185] extending the cathodic stability limit of perovskites to 0-1 V vs. Li/Li\({}^{+}\). Studies that substitute Ti\({}^{4+}\) were able to stabilize the electrolyte but showed lower ionic conductivities [75]. Inada et al. [185] managed to achieve viable ionic conductivities by Zr-substitutionwith the composition of Li\({}_{38}\)Sr\({}_{7/16}\) Ta\({}_{3/4}\)Zr\({}_{1/4}\)O\({}_{3}\) (LSTZ). The newly formulated LSTZ produced a total conductivity value of \(2.7\times 10^{-4}\) S/cm at 27 \({}^{\circ}\)C. The enhancement in conductivity relative to similar Ti-substitution studies can be attributed to the use of powder bed sintering. Powder bed sintering produced samples with fewer impurities, less Li\({}^{+}\) loss during high-temperature reactions and denser structure with larger grains. One unfortunate attribute of this LSTZ material is that it requires a sintering temperature of 1300 \({}^{\circ}\)C, which is too high to be compatible with electrode materials in cases of co-sintering. In an effort to reduce the processing temperature, Yu et al. [213] formulated Li\({}_{38}\)Sr\({}_{7/16}\)Nb\({}_{3/4}\)Zr\({}_{1/4}\)O\({}_{3}\) (LSNZ), which can be sintered at 1200 \({}^{\circ}\)C, but showed reduced total conductivity (\(2\times 10^{-5}\) S/cm at 30 \({}^{\circ}\)C). La doping to Sr sites could lead to a slight increase in the conductivity. For Li\({}_{38}\)Sr\({}_{7/16-3}\)La\({}_{x}\)Zr\({}_{1/4}\)Nb\({}_{3/4}\)O\({}_{3}\) (x = 0.05) the ionic conductivity reached to \(3.33\times 10^{-5}\) S/cm at 30 \({}^{\circ}\)C. However, this value was still an order of magnitude lower than that of LLTO [183] and LSTZ [186]. Huang et al. [187] prepared Li\({}_{38}\)Sr\({}_{7/16}\) Ta\({}_{3/4}\)Hf\({}_{1/4}\)O\({}_{3}\) using a solid-state reaction, which exhibited a total conductivity of \(3.8\times 10^{-4}\) S/cm at room temperature. Recently, the same authors prepared a sample with the same composition but used spark plasma sintering at 1250 \({}^{\circ}\)C for 10 min [188]. As opposed to conventional sintering methods, this sintering method improved the mechanical strength, enhanced the conductivity to \(5.2\times 10^{-4}\) S/cm, and lowered the activation energy to 0.33 eV at room temperature. Recently, oxides with garnet related structures have gained considerable attention as the potential solid-state electrolyte. The general chemical formula of garnet is A\({}_{3}\)B\({}_{2}\)(XO\({}_{4}\))\({}_{3}\) (A = Ca, Mg, Y, La or rare-earth; B = Al, Fe, Ga, Ge, Mn, Ni or V; X = Si, Ge, Al) where A, B and X are 8-, 6- and 4-fold coordinated cation sites, which forms a face-centered cubic structure with the space group Ia-3d. Li\({}^{+}\) can diffuse through interconnected 3D pathways in the garnet structure [214]. Thangadurai et al. [190] reported a garnet-type electrolyte made with similar elements to LLTO but replacing Ti\({}^{4+}\) to form Li\({}_{5}\)La\({}_{3}\)M\({}_{2}\)O\({}_{12}\) (M = Nb, Ta). Its bulk conductivity was reported as \(10^{-6}\) S/cm at 25 \({}^{\circ}\)C with activation energies of E\({}_{\rm Nb}\) = 0.56 and E\({}_{\rm Ta}\) = 0.43 eV. In 2007, garnet conductors with bismuth Li\({}_{5}\)La\({}_{3}\)Bi\({}_{2}\)O\({}_{12}\) and Li\({}_{6}\)SrLa\({}_{2}\)Bi\({}_{2}\)O\({}_{12}\) was reported to have comparable activation energies (0.4-0.5 eV) and total ion conductivity of \(2.0\times 10^{-5}\) S/cm at room temperature [55].