Unraveling the Reaction Mechanism of Sulfur Cathode in All-Solid-State Sodium-Sulfur Batteries via Spectroscopic and Theoretical Investigation

The investigation of all-solid-state sodium-sulfur batteries (ASSSBs) is in the early stage, where the intermediates and mechanism of the complex 16-electron conversion reaction of the sulfur cathode are still unclear. Herein, this study for the first time presents a comprehensive investigation of the sulfur reaction mechanism in ASSSBs by combining electrochemical measurements, ex-situ synchrotron X-ray absorption spectroscopy (XAS), in-situ Raman spectroscopy, and first-principles calculations. Based on the thermodynamic mechanism, the sulfur cathode undergoes a three-step solid-solid redox reaction, with S8 first reducing to Na2S5 and Na2S4, then to Na2S2, and finally to Na2S, resulting in a three-plateau voltage profile. However, under kinetic limitations, the Na2S2 phase is skipped, leading to a direct conversion from Na2S4 to Na2S and a two-plateau voltage profile. First-principles calculations reveal that the formation energy of Na2S2 is only 4 meV/atom lower than the two-phase equilibrium of Na2S4 and Na2S, explaining its absence under conventional cycling conditions. This work clarified the thermodynamic and kinetic limited pathways of the 16-electron conversion reaction of the sulfur cathode in ASSSBs, thereby facilitating the development of high-performance ASSSBs.