Comparative Study on Sulfide and Oxide Electrolyte Interfaces with Cathodes in All-Solid-State Battery via First-Principles Calculations

The interfacial resistance between the solid electrolyte (SE) and the cathode of all-solid-state battery can crucially affect its performance. However, the microscopic mechanisms due to which this resistance occurs depend on the SE type and are still debatable. In this study, we performed a comparative analysis of the characteristics of sulfide electrolyte (Li3PS4) and oxide electrolyte (Li3PO4 and Li7La3Zr2O12) interfaces with a typical oxide cathode (LiCoO2). We considered the Li vacancy formation associated with the Li chemical potential and the cation exchange related to the reaction layer formation, and used the density functional theory based on first-principles calculations. Compared to the case of sulfide SE interfaces, the oxide SE interfaces have fewer Li sites that have lower vacancy formation energy and are stable against the mutual cation exchange with the oxide cathode. These results indicate that the oxide electrolytes show less dynamical Li+ depletion upon initial charging and less formation of the reaction layer compared to those of sulfide electrolytes, which can be associated with the relatively low interfacial resistance observed experimentally. In addition to the material dependence, we also investigated the effect of the orientations of the SE and cathode at the interface. We demonstrated that the orientations strongly affect the ease of Li vacancy formation and mutual cation exchange. Interfaces of the buffer layer material of Li4Ti5O12 with a Li3PS4 SE and the LiCoO2 cathode were also evaluated. The results show that such oxide buffer layers suppress the Li vacancy formation, leading to less Li+ depletion. The present comparative analysis provides electronic and Li+ tendencies around the interfaces between the SE and the cathode, which will be useful for interface design in the future.