## 4 Electrodes and Electrolytes for SSTFBs  
To meet the increasing demand for portable (micro-)electronic applications in today's information-rich and mobile society, developing rechargeable battery systems with high energy density and reduced dimensions is crucial. For such battery systems, SSTFBs are one of the most attractive battery systems owing to their shape, versatility, flexibility and lightness [315; 316; 317]. Since being first introduced in 1983 [318], SSTFBs have been continuously studied over the past four decades [317; 319; 320; 321; 322; 323; 324]. In recent years, considerable progress has been made in the development of SSTFBs along with advances in thin-film technologies [325; 326]. SSTFBs provide unique advantages such as outstanding cycle life and safety compared to conventional LIBs [327; 328]. Moreover, SSTFBs enable the miniaturization of LIBs [317; 323; 324] required for applications, including implantable medical devices, wireless microsensors, microelectromechanical system devices and flexible electronics [318; 329; 330].  
SSTFBs are composed of multiple micron-sized electrochemical cells consisting of a cathode and an anode electrode separated by an electrolyte. A thin-film electrochemical cell is generally fabricated on a solid substrate like glass, ceramic or even polymer. The first layer is usually a current collector, then followed by the electrode, electrolyte, electrode and another layer of a current collector [317]. Generally, the thickness of thin films in SSTFBs is in the range of nanometers to microns. Such thin layers can significantly enhance the charge transfer kinetics [59; 331] which prevents the local overcharging and discharging issue reported in conventional battery systems [317; 331]. In addition, SSTFBs use dense thin films without a polymeric binder and thus can be used as an ideal system for the fundamental  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Electrode**} & \multirow{2}{*}{**Electrolyte**} & \multirow{2}{*}{**Buffer Layer**} & \multirow{2}{*}{**Coating Method**} & \multicolumn{2}{c}{**Interface Resistance**} & \multirow{2}{*}{**Ref.**} \\  & & & & **w/ Buffer** & & **w/o Buffer** \\  & & & & **Layer** & **Layer** & \\  & & & & **(f-cm\({}^{2}\))** & **(f-cm\({}^{2}\))** & \\ \hline Li metal & Li\({}_{2.75}\)Ca\({}_{0.25}\)Zr\({}_{1.75}\)Nb\({}_{0.25}\)O\({}_{12}\) & Al\({}_{2}\)O\({}_{3}\) & ALD & 1 & 1710 & [255] \\ Li metal & Li\({}_{3}\)La\({}_{3}\)Ta\({}_{2}\)O\({}_{12}\) & ZnO & ALD & 20 & 2000 & [88] \\ Li metal & Li\({}_{1.3}\)Al\({}_{0.3}\)Ti\({}_{1.7}\)(PO\({}_{4}\))\({}_{3}\) & Al\({}_{2}\)O\({}_{3}\) & ALD & 117,810 & 314,160 & [311] \\ Li metal & Li\({}_{6.28}\)Al\({}_{0.24}\)La\({}_{3}\)Zr\({}_{2}\)O\({}_{12}\) & LiNbO\({}_{3}\) & sputtering & 91 & 1078 & [312] \\ Li metal & Li\({}_{6.4}\)La\({}_{3}\)Zr\({}_{1.4}\)Ta\({}_{0.6}\)O\({}_{12}\) & ITO & sputtering & 32 & 1192 & [314] \\ \hline \hline \end{tabular}
\end{table}
Table 5: Comparison of the interface resistance with (w/) and without (w/o) buffer layer on solid-state electrolyte/anode.  
understanding of energy storage mechanisms. Furthermore, SSTFBs have higher volumetric and gravimetric power density (Figure 11) compared to other battery systems [332]. The use of thin-film electrochemical cells is therefore a promising and practical strategy to fully utilize the advantage of lithium-based batteries for diverse applications.