Three-Step Thermodynamic vs. Two-Step Kinetic-Limited Sulfur Reactions in All-Solid-State Sodium Batteries
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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. S 8 first reduces to long-chain polysulfides, Na 2 S 5 and Na 2 S 4 , then to Na 2 S 2 , and finally to Na 2 S, 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 Na 2 S 2 phase is skipped, leading to a direct conversion from Na 2 S 4 to Na 2 S and resulting a two-plateau voltage profile. First-principles calculations reveal that the formation energy of Na 2 S 2 is only 4 meV/atom lower than the two-phase equilibrium of Na 2 S 4 and Na 2 S, 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.
