## 3 Interfacial Phenomena between Solid-State Electrolytes and Electrodes  
As described in previous sections, substantial effort has been devoted to developing high energy and power density electrodes, solid-state electrolytes with high ionic conductivity, good chemical stability and large electrochemical stability windows. However, the performance enhancement of batteries can be insignificant despite the dramatically enhanced performance of an individual component, i.e., electrodes or electrolytes. More importantly, the power density and cycle life of SSBs still have not met the requirements for practical applications. Such poor performances are mainly attributed to the large interfacial resistance between solid electrolytes and electrodes [227; 228; 229; 230] that originates from the mechanical force development or chemical composition changes. These configurational and chemical changes driven by electrochemical reactions are summarized in Figure 7[231; 232]. We will discuss mechanical and chemical factors associated with the large interfacial resistance in the following section. Mechanically driven interfacial resistance between solid electrolytes and electrodes originates from poor contact between two rigid materials and the volume changes of electrodes during the charge-discharge process [233]. This poor contact eventually leads to the formation and propagation of cracks [234] as well as the delamination of interfaces [235; 236]. Suffide-based electrolytes possess good mechanical ductility, thus can maintain good contact with electrodes without the degradation of the interfacial contact resistance [237]. In contrast, oxide-based electrolytes suffer from the poor adhesion of interfaces with electrodes as most ceramics are vulnerable to cracking due to the low ductility [238; 239]. The insufficient mechanical contact results in the delamination or \"dead\" area induced by isolated electrode contact points from solid electrolytes. Due to the lack of conduction paths, neither electrons nor Li\({}^{+}\) can be transferred across the dead areas, which in turn leads to the growth of interfacial resistance and capacity fading [240]. Furthermore, the large volume changes of electrode materials during repeated charge-discharge processes could also lead to the loss of effective contact between electrodes and solid electrolytes [241]. Zhang et al. [233] first demonstrated changes in the pressure and height of LCO/Li\({}_{10}\)GeP\({}_{2}\)S\({}_{12}\)/In and LCO/Li\({}_{10}\)GeP\({}_{2}\)S\({}_{12}\)/LTO under galvanostatic cycling where the volume expansions of LCO, LTO and In were found to be 2%, 0.2% and 105.6%, respectively. Due to the significant volume change of In, the LCO/Li\({}_{10}\)GeP\({}_{2}\)S\({}_{12}\)/In cell showed severe capacity fading. Similarly, Koerver et al. [235] detected the increased interfacial resistance and capacity fading caused by the contact loss at the NCM-811/B-Li\({}_{3}\)PS\({}_{4}\) interface. The occurrence of the interfacial resistance by the formation of interlayers is a well-known phenomenon in SSBs. One of the main reasons for the interfacial resistance is the formation of space charge regions (SCRs). SCRs originate from the depletion of lithium near the interface between the cathode and the electrolyte in SSBs due to the high potential gradient [242]. The potential difference at this interface causes Li\({}^{+}\) to move toward a higher potential region, causing lithium depletion and increasing the ionic resistance at the interface. Suffide-based electrolytes exhibit a weaker interaction with Li\({}^{+}\) and  
Figure 7: Schematic diagram of lithium metal battery and electrode/electrolyte interface issues. lower chemical potential of Li\({}^{+}\) compared with those of oxide cathodes, such as LCO. Thus, the Li\({}^{+}\) in sulfide-based electrolytes will migrate into the oxide cathode easily, resulting in the redistribution of Li\({}^{+}\) at the interface which forms lithium depletion layers, SCRs [243].