Three-Step Thermodynamic vs. Two-Step Kinetics-Limited Sulfur Reactions in All-Solid-State Sodium Batteries

The investigation of all-solid-state sodium-sulfur batteries (ASSSBs) is still in its early stage, where the intermediates and mechanism of the complex 16-electron conversion reaction of the sulfur cathode remain 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. The sulfur cathode undergoes a three-step solid-solid redox reaction following the thermodynamic principle. S8 first reduces to long-chain polysulfides, Na2S5 and Na2S4, then to Na2S2, and finally to Na2S, resulting in a three-plateau voltage profile when temperatures ≥ 90°C or C-rates ≤ C/100. However, under kinetics-limited conditions, temperatures ≤ 60°C and C-rates ≥ C/20, the Na2S2 phase is skipped, leading to a direct conversion from Na2S4 to Na2S and resulting 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 kinetics-limited conditions. This work clarified the thermodynamic and kinetics-limited pathways of the 16-electron conversion reaction of the sulfur cathode in ASSSBs, thereby facilitating the development of high-performance ASSSBs.