## 3 Interfacial Phenomena between Solid-State Electrolytes and Electrodes
### Buffer Layers
#### 3.2.1 Buffer Layers at the Electrolyte/Cathode Interface
In order to suppress the SCRs and have good ionic conductivity at the interface, LiNbO\({}_{3}\) (10\({}^{-5}\)-10\({}^{-6}\) S/cm) [299] was also commonly used as a buffer layer for sulfide electrolytes. Using ab-initio calculation, Haruyama et al. [300] utilized a buffer layer of LiNbO\({}_{3}\) between LCO and \(\beta\)-Li\({}_{3}\)PS\({}_{4}\) (LPS) demonstrating the electronic properties of the interfacial regions. The authors predicted that the introduction of LiNbO\({}_{3}\) layers could relieve the structural disorder at the interface, balance the lithium distribution and suppress the growth of SCRs to boost Li\({}^{+}\) conduction through the interface. Experimental data by other groups confirmed that the LiNbO\({}_{3}\) buffer layer between the electrolyte and the cathode could weaken the space-charge effect. Takada et al. [301] showed that the LiNbO\({}_{3}\) buffer layer with a thickness of 20 nm on LMO could reduce the interfacial resistance from 10,000 \(\Omega\) to 200 \(\Omega\) by suppressing the SCRs formed at the high-voltage cathode/sulfide electrolyte interface. Recently, Vinado et al. [302] studied the interfacial behavior of Li\({}_{3}\)NbO\({}_{4}\)-coated LCO with Li\({}_{10}\)SnP\({}_{2}\)S\({}_{12}\) (LSPS) solid electrolyte. According to the experimental results in this study, the SSBs with Li\({}_{3}\)NbO\({}_{4}\)-coated LCO cathode was found to maintain its low interfacial resistance (210 \(\Omega\) cm) after 10 cycles in contrast to the uncoated LCO SSBs showing the increased interfacial resistance (680 \(\Omega\) cm). To reduce the interfacial reaction and prevent the formation of the SEI layer in SSBs, Takahashi et al. [303] investigated the effect of Li\({}_{3}\)PO\({}_{4}\), LiNbO\({}_{3}\) and Al\({}_{2}\)O\({}_{3}\) as buffer layer between LCO and LiBH\({}_{4}\) solid electrolyte on the interfacial properties. Li\({}_{3}\)PO\({}_{4}\) and LiNbO\({}_{3}\) were selected due to their relatively high chemical and thermal stabilities and high ionic conductivity but low electron conductivity. Al\({}_{2}\)O\({}_{3}\), however, is a poor ionic conductor relative to these two materials. The authors found that all the buffer layers can reduce the interfacial resistance with Li\({}_{3}\)PO\({}_{4}\) giving the lowest interfacial resistance (21 \(\Omega\)). Similarly, a nanoscale Al\({}_{2}\)O\({}_{3}\) buffer layer deposited on LCO by PLD also enhanced the cyclic performance of SSBs by reducing the SEI layer formation [304]. Using PLD, Chen et al. [305] introduced the Li\({}_{3}\)PO\({}_{4}\) buffer layer at the interface between LCO and 80 Li\({}_{2}\)S-20P\({}_{2}\)S\({}_{5}\) solid electrolyte reporting the improved cyclability and decreased interfacial resistance. The authors claimed that these enhancements could be attributed to suppressing the degradation of the sulfide electrolyte during cell cycling. According to the experimental results, SSBs with Li\({}_{3}\)PO\({}_{4}\) buffer layer showed higher discharge capacities (172 mAh/g) than those without the buffer layer (155 mAh/g). Recently, Kim et al. [306] reported a dramatically reduced interfacial resistance (\(\sim\)10 times lower than pristine cell without the buffer layer) with a 5-nm-thick LiNbO\({}_{3}\) buffer layer on LCO leading to a discharge capacity of 109 mAh/g. To effectively enhance the interfacial stability and electrochemical performances, Li et al. [307] constructed a novel NMC@LCO@LiNbO\({}_{3}\) cell by using a sol-gel method. According to their fabrication method, NMC-811 was first coated with LCO to form core-shell NMC@LCO with a Ni-deficient surface. The authors claimed that the LCO layer provides interfacial stability with Li\({}_{10}\)GeP\({}_{2}\)S\({}_{12}\) sulfide electrolyte while maintaining the high specific capacity from the NMC core. Then, in view of their previous work [308], the surface of NMC@LCOwas coated with a LiNbO\({}_{3}\) buffer layer to optimize the interface further.