Electrochemical and material analyses for sulfide-based solid electrolyte–cathode interface under high voltage

The active material coating of a cathode in all-solid-state lithium-ion batteries using sulfide-based solid electrolytes is known to be an essential technology. Electrochemical impedance spectroscopic measurements showed that two charge-transfer processes occurred at the interface between an argyrodite-type sulfide solid electrolyte and LiNi0.5Co0.2Mn0.3O2 both with and without a LiNbO3 coating. The process in the higher-frequency region was attributed to the solid electrolyte–coating and/or the coating itself, whereas that in the lower-frequency region was attributed to the lithiation/delithiation of the cathode active material. The two charge transfer resistances observed in the cell with a LiNbO3-coated cathode—after durability tests at various charging potentials—exhibited different trends. The charge transfer resistance was maintained at 4.55 V vs. Li/Li+, and only one at the higher frequency increased. The oxidation of sulfide at the solid electrolyte–coating interface was observed using time-of-flight secondary-ion mass spectrometry. However, the degradation at the active material surface was not observed using transmission electron microscope/electron energy loss spectroscopy. The results of the material analyses support the ascription of impedance spectra and their associated resistance trends in the durability test.