Structural and Electronic-State Changes of a Sulfide Solid Electrolyte during the Li Deinsertion-Insertion Processes

All-solid-state batteries with sulfide solid electrolytes (SEs) are promising next-generation energy storage devices that are safe and have a cycle life and energy densities. To improve their electrochemical performances, we investigated the electrochemical reactions of the SEs and their structural changes. The detailed changes in the structure and electronic states have not yet been measured experimentally because of the difficulty in observing the microscopic area formed at the interface between the electrode materials and the SEs. Thus, we prepared composite electrodes composed of Li3PS4 SE and carbon to increase the electrochemical reaction area. The structural and electronic-state changes in Li3PS4 during the Li deinsertion-insertion processes were revealed using X-ray diffraction and Raman, X-ray photoelectron, and X-ray absorption spectroscopies. We found that the sulfide ions in Li3PS4 contribute to charge compensation during the charge-discharge processes. The S-S bonds between PS4 units associate and dissociate via the retention of the covalent bonds between the P atoms and S-S bonds. Although the anion redox behavior is generally discussed with regard to transition metal chalcogenides, here we first report the reversible association and dissociation of S-S bonds in typical element compounds. This knowledge contributes to an improved understanding of the anion redox reactions and the interfacial resistances between high-voltage positive electrodes and SEs.