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.