Low interfacial resistance between the solid sulfide electrolyte and the electrode is critical for developing all-solid-state Li batteries; however, the origin of interfacial resistance has not been quantitatively reported in the literature. This study reports the resistance values across the interface between an amorphous Li3PS4 solid electrolyte and a LiCoO2(001) epitaxial thin film electrode in a thin-film Li battery model. High interfacial resistance is observed, which is attributed to the spontaneous formation of an interfacial layer between the solid electrolyte and the positive electrode upon contact. That is, the interfacial resistance originates from an interphase mixed layer instead of a space charge layer. The introduction of a 10 nm thick Li3PO4 buffer layer between the solid electrolyte and positive electrode layers suppresses the formation of the interphase mixed layer, thereby leading to a 2800-fold decrease in the interfacial resistance. These results provide insight into reducing the interfacial resistance of all-solid-state Li batteries with sulfide electrolytes by utilizing buffer layers.
Keywords: Li3PS4; LiCoO2; all-solid-state battery; buffer layer; interfacial resistance; interphase layer; sulfide electrolyte; thin-film Li battery.