Designing Sulfur-Substituted NASICON Electrolyte for Sodium-Ion Batteries: A Computational Approach

NASICON (Na super ionic conductor)-type sodium superionic conductors are promising candidates for all-solid-state sodium-ion batteries owing to their high ionic conductivity and structural tunability. While NASICONs have been extensively investigated, thio-NASICONs, where O2– is replaced by larger, more polarizable S2–, remain comparatively underexplored despite their potential for enhanced Na+ transport. Here, we combine density functional theory, ab initio molecular dynamics, and climbing image nudged elastic band (CI-NEB) simulations to probe the structural stability and ion transport characteristics of Na3Zr2PSi2X12 (X = O, S). Our results reveal that S2– substitution expands the lattice, widens critical migration bottlenecks, and increases lattice polarizability, ultimately softening the framework. At ambient conditions, these effects partially open the otherwise largely constrained Na+ migration pathway, facilitating enhanced ion diffusion. Quantitative analysis through CI-NEB-based simulations reveals an ∼20% reduction in the migration barrier for the narrowest bottleneck in Na3Zr2PSi2S12 relative to its oxide analogue. These findings illustrate that sulfur substitution simultaneously tunes the static channel geometry and dynamic structural fluctuations, thereby enhancing the Na+ ion migration. Overall, the study provides mechanistic insights into structure–ion transport relationships and offers reliable in silico guidelines for designing next-generation, high-conductivity sodium solid electrolytes.