Theoretical study on stability and ion transport property with halide doping of Na3SbS4 electrolyte for all-solid-state batteries

Abstract

All-solid-state Na ion battery (ASS-NIB) is a new class of battery which is a potential alternative to the conventional all-solid-state Li ion battery. Herein, we focused on Na₃SbS₄, a reported candidate solid electrolyte for ASS-NIB application, and performed a comprehensive theoretical study, primarily based on density functional theory calculations, to evaluate (electro)chemical stability, defect chemistry, and Na ion transport property. The calculated results reveal that when in contact with a layered cathode compound (e.g., NaCrO₂), sulfur in Na₃SbS₄ tends to migrate across the interface, leading to interface atomic rearrangement, interface disordering and/or decomposition. The material is also predicted to decompose under reductive voltage conditions (0 V vs. Na⁺/Na), in agreement with experiment. Kinetic modeling for stresses and electron density distribution showed that the interface between Na₃SbS₄ and Na metal anode under typical electrodeposition surface roughness would lead to dendrite initiation and growth. Aside from controlling Na vacancy concentration, halide doping at the S site was predicted by DFT molecular dynamics (MD) calculations to directly affect the Na⁺ ion activation energy. This can be ascribed to the size modulation of the Na site-to-site pathway bottleneck. Cl and Br halide dopants are both determined to be promising for conductivity optimization, with low DFT-MD Na⁺ ion activation energy (∼0.1 eV at 4% Na vacancy). Thermodynamic analysis shows the possible factors that can influence the final conductivity of Na₃SbS₄: secondary phases and intrinsic defects. Overall, our findings offer valuable insights for the rational design of solid electrolytes.