Development of a ReaxFF force field for Na-S chemistry and its application to chemo-mechanical evolution of sodiated...

Sodium-sulfur (Na-S) batteries have emerged as a promising alternative to lithium-sulfur systems owing to the natural abundance and low cost of sodium. However, their practical implementation is hindered by an incomplete understanding of the coupled structural, mechanical, and electrochemical evolution of sulfur cathodes during sodiation. In particular, the conversion of sulfur to sodium polysulfides (Na_xS) is accompanied by substantial volume expansion and structural reorganization, which generate internal stresses and contribute to electrode degradation. Despite their importance, the atomistic mechanisms governing chemo-mechanical evolution of Na-S cathodes remain poorly understood. Here, we develop a ReaxFF force field for Na-S chemistry, trained against extensive quantum mechanical (QM) data and validated through independent predictions of charge distributions, voltage profiles, and volumetric expansion. Using large-scale molecular dynamics (MD) simulations, we systematically investigate the structural, transport, electrochemical, and mechanical properties of Na_xS phases across a wide range of sodium concentrations. Our simulations reveal that sodiation induces a progressive transition from molecular sulfur to Na-rich ionic structures through extensive S-S bond cleavage and Na-S bond formation. This structural reorganization facilitates Na-ion transport and leads to the emergence of a percolating ionic network. Mechanical analysis further demonstrates that sodiation induces significant strengthening and enhanced ductility of Na_xS phases. These findings provide fundamental insight into the interplay between chemistry and mechanics in Na-S cathodes. Overall, this work offers a critical atomistic understanding of the sodiation-induced mechanical evolution necessary to develop mechanically resilient sulfur cathodes for next-generation Na-S batteries.