Thermodynamic stability of sulfide electrolyte/oxide electrode interface in solid-state lithium batteries

Thermodynamic analyses were made on the sulfide electrolyte-oxide electrode interfaces in all solid-state lithium batteries using chemical potential diagrams, constructed under the local thermodynamic equilibrium approximation. The chemical potential diagrams were first made for the Li-S-O system and the Li-Co-O system, to show that the stable region for the Li2S phase and the combined region of LiCoO2/CoO2 phases do not have a common overlap area in the same log a(Li) vs. log p(O2) plot. Three dimensional diagrams of the Li-Co-S-O system and the Li-Co-P-S-O system were constructed by fixing log a(Li) at selected values. Based on the constructed diagrams, the chemical nature of the sulfide electrolyte-oxide electrode interface could be considered from both the high temperature electrochemical and chemical points of view. This aided the interpretation of recently reported microscopic analyses on the interfaces. The causes of the “high resistive layer” and cobalt diffusion can be reasonably interpreted as chemical processes, caused by the high Li potential during charging. The results present in this paper are also compared to recent results from theoretical calculations on the energetics of interface structure. The possible applications of the here presented chemical diagrams are discussed in terms of the stoichiometric numbers and energies of the local interface atomic arrangement.