Three-step thermodynamic vs. two-step kinetics-limited sulfur reactions in all-solid-state sodium batteries

Abstract

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 under the extreme low rate (C-rates ≤ C/100) or at high temperature (≥ 90 °C), where S₈ is first reduced 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 per 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 solid-state sodium–sulfur reaction mechanisms.