## 4 Electrodes and Electrolytes for SSTFBs
### Thin-film Electrolytes
Later, using a sol-gel method they also synthesized polycrystalline LAGP thin films. In particular, the sample sintered at 900 \({}^{\circ}\)C for 8 h showed the highest bulk (material itself) and total (material + pores) conductivity of \(7.76\times 10^{-4}\) and \(4.18\times 10^{-4}\) S/cm, respectively, at room temperature. Further increases in the sintering temperature and time led to the deterioration of the conductivity of films due to the formation of an impurity phase and the increase of the interspace within grains [400]. Polycrystalline LAGP thin films were also synthesized via aerosol method without the post-annealing treatment and showed much lower total conductivity (\(5\times 10^{-6}\)-\(2\times 10^{-7}\) S/cm) relative to the samples with the post-annealing treatment [401]. Using RF-sputtering, Ling et al. [398] and Chen et al. [399] fabricated amorphous LATP thin films. Ling et al. reported an ionic conductivity of \(6.47\times 10^{-6}\) S/cm for LATP films deposited at 500 \({}^{\circ}\)C, while Chen et al. reported a higher ionic conductivity (\(2.46\times 10^{-5}\) S/cm) for LATP films deposited at 300 \({}^{\circ}\)C.  
Li\({}_{2}\)O-V\({}_{2}\)O\({}_{5}\)-SiO\({}_{2}\) (LVSO), one of the LSICON type material systems, has also been deposited as thin films with PLD [402; 403; 404; 405; 406] and RF sputtering [407; 319; 408]. Kuwata et al. [404] deposited LVSO thin films on top of Pt-coated quartz glass substrates using PLD. According to this study, the LSVO thin films were found to be amorphous with a conductivity of \(3\times 10^{-7}\) S/cm at room temperature. Kawamura et al. [406] also reported the deposition of LVSO thin films via PLD. The authors utilized three different post-annealing temperatures (200, 400 and 600 \({}^{\circ}\)C), which resulted in the increased crystallinity of LVSO films. The conductivity of the as-deposited amorphous LVSO thin films was a \(10^{-7}\) S/cm, which is \(\sim\)2 order of magnitudes lower than the bulk value. Ohtsuka et al. [319; 407; 408] reported the growth of crystalline LVSO thin films via RF sputtering. The films were grown on multiple substrates including sapphire, SiO\({}_{2}\) glass, Pt-Pd-coated SiO\({}_{2}\) glass, SnO\({}_{2}\)-coated SiO\({}_{2}\) glass and stainless steel. Thin films on SiO\({}_{2}\) glass, sapphire, and stainless steel showed a dominant (002) orientation. The LSVO/sapphire samples were then annealed at different temperatures of 410, 510 and 610 \({}^{\circ}\)C. As annealing temperature goes up, (011), (020) and (220) orientations started to show up. The major XRD peak of (002) has an increase in peak intensity and a decrease in peak width, which indicates better film crystallinity and larger grain size. The influence of deposition temperature was also investigated. The peak intensity of the (002) LVSO plane first increased as the deposition temperature increases. However, the (002) LVSO peak was not observed in samples deposited at temperature higher than 300 \({}^{\circ}\)C, showing very minor (011) and (210) peak with the deposition temperatures of 300 and 390 \({}^{\circ}\)C, respectively. As for conductivity, the as-deposited films at 60 and 200 \({}^{\circ}\)C showed \(\sim\)\(10^{-6}\) S/cm of conductivity at room temperature. The films annealed at 600 \({}^{\circ}\)C (samples deposited at 60 and 200 \({}^{\circ}\)C) showed a higher conductivity of \(10^{-5}\) S/cm, which is one order magnitude higher than the bulk conductivity value of LVSO, as shown in Figure 12.  
LiPON is the most widely used electrolyte for SSTFBs due to its good stability, easy growth of thin films and good ionic conductivity at room temperature. There have been extensive studies on LiPON thin films prepared by RF sputtering [179; 409; 410], PLD [411; 412] and ALD [413]. The nitrogen to phosphorus (N/P) ratio can be controlled by the N\({}_{2}\) gas pressure during the deposition for RF sputtering and PLD [179; 409; 410; 411; 412]. The N/P ratio can also be varied by controlling the dose of N\({}_{2}\) during the ALD process [413].