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 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. This work, for the first time, proved that the sulfur cathode undergoes an intrinsic three-step solid-solid redox reaction following the thermodynamic principle, where S₈ first reduces to long-chain polysulfides (Na₂S₅ and Na₂S₄), then to Na₂S₂, and finally to Na₂S, resulting in a three-plateau voltage profile. However, under conventional battery test conditions, i.e., temperatures ≤ 60°C and C-rates ≥ C/20, the Na₂S₂ phase is bypassed due to kinetic limitations, leading to a direct conversion from Na₂S₄ to Na₂S, resulting in the commonly observed 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, providing valuable insights into the sulfur reaction mechanisms in the ASSSBs.