## 4 Electrodes and Electrolytes for SSTFBs
### Thin-film Electrolytes
Similar to LLZO, LiPON thin films are also amorphous after the deposition. The ionic conductivity was found to increase with increasing the N/P ratio ranging from \(10^{-4}\) to \(10^{-7}\) S/cm [409; 413]. As shown in Figure 13, most the thin films showed a similar or higher conductivity value than the bulk polycrystalline reference, ranging from \(10^{-2}\) to \(10^{-7}\) S/cm. LLTO is one of the most commonly used perovskite-based solid electrolyte materials in SSBs due to its high bulk ionic conductivity [182]. Many attempts have been made to control the performance of SSTFBs with LLTO by modulating dopant concentration and film morphology [414, 415, 416, 417, 418, 419, 420, 421, 422, 423, 424, 425]. While various deposition methods can be applied to synthesize epitaxial LLTO thin films, PLD is the most preferred method. Ohnishi et al. [416] grew both a-axis and c-axis oriented epitaxial Li\({}_{0.33}\)La\({}_{0.56}\)TiO\({}_{3}\) thin films on (001) STO and (110) NdGaO\({}_{3}\) (NGO) single crystalline substrates, respectively. The conductivity of (001)-oriented LLTO thin film was found to be \(\sim\)\(3.5\times 10^{-5}\) S/cm, which is \(\sim\)2 orders of magnitudes lower than the bulk conductivity of LLTO. The author proposed that the lower conductivity of LLTO thin films is attributed to the lithium-deficient LLTO film composition and the compressive strain induced by the lattice mismatch between the film and the substrate. Wei et al. [414] also grew epitaxial Li\({}_{0.33}\)La\({}_{0.56}\)TiO\({}_{3}\) thin films with different strain states induced by the lattice mismatch between the  
Figure 12: Conductivity summary of bulk [405] and thin-film Li\({}_{2}\)O–V\({}_{2}\)O\({}_{5}\)–SiO\({}_{2}\) (LVSO) deposited via PLD [404, 405] and RF sputtering [319]. Figure 13: Conductivity summary of bulk [412] and thin-film LiPON deposited via PLD [411, 412], RF sputtering [179, 409, 410] and ALD [413]. substrate and the film, demonstrating the strain-dependent ionic conduction behavior of LLTO thin films. The ionic conductivity of LLTO films in LLTO\(|\)NGO along the perpendicular to the b-c plane (\(6.7\times 10^{-4}\) S/cm) was higher than that along the perpendicular to the a-c plane (\(4.3\times 10^{-4}\) S/cm). The authors proposed that the strain-dependent ionic conductivity is due to the larger compressive strain along the perpendicular to the b-c plane. (111)-oriented epitaxial Li\({}_{0.17}\)La\({}_{0.61}\)TiO\({}_{3}\) thin films were successfully synthesized on the (111) LMO(111) SrRuO\({}_{3}\) (SRO)\(|\)(111) Nb-STO electrode by Kim et al. [415]. Ohta et al. [417] demonstrated the conductivity of (100)-oriented Li\({}_{3x}\)La\({}_{2/3\sim x}\)TiO\({}_{3}\) (x = 0.1, 0.167) thin films on (001) STO substrate with an atomically flat surface. According to the authors, the conductivity of LLTO thin film (x = 0.1) was found to be \(2.5\times 10^{-2}\) S/cm at 190 \({}^{\circ}\)C, which is in good agreement with the bulk conductivity of single-crystalline LLTO \(3\times 10^{-2}\) S/cm. Although the PLD process tends to yield epitaxial thin films with good crystallinity and atomically flat surface, it is worth mentioning that the PLD process requires a target material with an excess amount of Li to balance the Li lost during the target sintering and deposition processes [424]. Besides PLD, deposition processes such as sol-gel method and RF sputtering were also used for the growth of LLTO thin films [421; 422]. In particular, Teranishi et al. [421] reported the growth of a polycrystalline LLTO thin film with a sol-gel process and Xiong et al. [422] used RF sputtering to fabricate an amorphous LLTO thin film. However, the ionic conductivity of these thin films was orders of magnitudes lower than that of the bulk and PLD-deposited thin films, as shown in Figure 14.  
LLZO is one of the commonly used garnet-based electrolyte materials for SSBs. LLZO exhibits good ionic conduction in addition to good structural stability, which makes LLZO a promising electrolyte material for SSTFBs [427; 428].