## 3 Interfacial Phenomena between Solid-State Electrolytes and Electrodes
Unlike sulfide-based electrolytes, the influence of SCRs is smaller in oxide-based electrolytes because the chemical potential of Li\({}^{+}\) in oxide electrolytes is comparable with that in the cathode [244]. At the interface between LCO and Li\({}_{1+x+y}\)Al\({}_{7}\)Ti\({}_{2-y}\)Si\({}_{x}\)P\({}_{3-x}\)O\({}_{12}\), the thickness of SCRs in the sulfide-based electrolyte was thicker than 1 \(\upmu\)m determined by measuring the electric potential profile with transmission electron microscopy (TEM) [245]. A similar SCR thickness was also reported in LiCoPO\({}_{4}\)/Li\({}_{1+x}\)Al\({}_{x}\)Ti\({}_{2-x}\)(PO\({}_{4}\))\({}_{3}\) using Kelvin probe force microscopy (KPFM) [246]. On the contrary, the thickness estimated from the resistance (-10 \(\Omega\) cm\({}^{2}\)) at the LiPON/LCO interface was found to be in the range of nanometers [247]. De Klerk et al. [248] also estimated the nanometer-thick SCRs based on the interfacial resistance (17 \(\Omega\) cm\({}^{2}\)) between the solid electrolytes, i.e., garnet and NASICON (LLZO and Li\({}_{1.2}\)Al\({}_{0.2}\)Ti\({}_{1.8}\)(PO\({}_{4}\))\({}_{3}\)) and the cathode (LCO) or anode (graphite). Interfacial chemical reactions derived from the interdiffusion between electrodes and solid electrolytes can also contribute to high interfacial resistance [249]. These interfacial reactions can form an interphase layer known as SEI at the electrode/electrolyte interface by consuming Li\({}^{+}\) and electrons from electrodes. The electrical properties of the SEI layer play a role in determining how the reaction between electrolytes and electrodes continues [250; 251; 252]. This SEI layer continues to grow until it blocks the Li\({}^{+}\) transport over the electrolyte/electrode. Park et al. [253] showed that an approximately 50-nm-thick layer forms in the vicinity of the LCO/Garnet interface due to the mutual diffusion of Co, La and Zr which leads to capacity fading. In addition, Wenzel et al. [254] also revealed that Li\({}_{3}\)P, Li\({}_{2}\)S and Li-Ge alloys form a SEI layer upon the reaction of Li\({}_{10}\)GeP\({}_{2}\)S\({}_{12}\) solid electrolyte with Li metal by in situ X-ray photoemission spectroscopy. As a general strategy to resolve the aforementioned issue at the electrode/electrolyte interface, nanometer-thick interfacial buffer layers have been grown to enhance the performance of the SSBs [255; 256]. Consequently, it is critical to employ thin-film growth techniques that can provide high purity and desired crystallinity of target materials in SSBs' assembly processes. Furthermore, the growth of thin films is a key success factor in building SSTFBs that have dramatically reduced charge-transfer resistance throughout the device. Therefore, understanding the precise control of thin-film growth and determining the impact of thin films on battery performances are requisite. In the following sections, we first review the most widely used thin-film deposition techniques. Then, we focus on the representative thin-film materials applied to buffer layers, evaluating the influence of deposition methods on the properties of thin films and the subsequent SSB performances.