Understanding Decomposition of Electrolytes in All-Solid-State Lithium-Sulfur Batteries

The decomposition behavior of electrolytes affects the cycle stability and electrochemical redox activity in all-solid-state lithium-sulfur batteries (ASSLSBs). However, there is a sparse understanding of the electrochemistry of ASSLSBs involving the oxidative decomposition of sulfide solid electrolytes (SEs) due to the lack of fundamental studies. Herein, we unveil the redox chemistry related to Li2S/S conversion reaction, electrolyte decomposition, and redox reaction of the decomposition product, based on differential capacity curves. The oxidation reaction of Li2S proceeds simultaneously with a continuous oxidative decomposition of sulfide SEs. Raman spectroscopy of the cell after cycling shows that SEs in the cathode convert the thiophosphates with a S-S thiol bond via the decomposition behavior of the electrolytes. The implication of this reaction chemistry is expanded toward understanding the Li2S activation process in ASSLSBs. Additionally, we demonstrate that Li2S/SE interface modification by different rotation speeds of ball milling in SE mixing allows us to control the decomposition kinetics of electrolytes. The severe decomposition of electrolytes causes cycle fading instead of increasing the electrochemical redox activity in the early period. The findings of this work highlight the need for interface engineering to avoid severe degradation of electrolytes in the cathode and enhance the ability of SEs as a redox mediator.