Defect chemistry and ion transport in low-dimensional-networked Li-rich anti-perovskites as solid electrolytes for solid-state batteries

Abstract

Solid-state batteries are attracting significant attention due to their plethora of potential advantages, including energy density gains and safety enhancements. The heart of this technology can be found in its solid electrolytes and thus its progress is intrinsically attached to the discovery and understanding of novel solid electrolyte materials. In recent years, Li-rich anti-perovskites have become promising solid electrolyte candidates as they combine high ionic conductivity, structural versatility and stability against Li metal. Here, the energetics of defect formation, stability, Li-ion transport and fluorine doping in a range of Li_xOX_x−2 (X = Cl or Br; x = 3–6) anti-perovskites with zero- to three-dimensional-networked structures are explored via atomistic simulations. Our calculations show that these materials generally present Li–halide Schottky defect pairs as the dominant native defects. We find that defect formation is generally energetically more favourable in the Li_xOCl_x−2 series compared to the equivalent structures in the Li_xOBr_x−2 set. Additionally, our molecular dynamics simulations reveal the strong relationship between Li-ion dynamics and dimensionality in these materials, namely, increasing Li-ion diffusion and decreasing activation energy with reduced dimensionality. Enhanced Li-ion diffusion is observed for the low-dimensional-networked Br-based materials (4.81 × 10⁻⁹ and 6.08 × 10⁻⁹ cm² s⁻¹ for Li₅OBr₃ and Li₆OBr₄ at 300 K, respectively) compared to their Cl-based counterparts (9.85 × 10⁻¹⁰ and 2.06 × 10⁻⁹ cm² s⁻¹ for Li₅OCl₃ and Li₆OCl₄ at 300 K, respectively), illustrating the importance of lattice polarisability in these soft and unstable materials. This is further exemplified by the significant reduction in Li-ion diffusion and increase in activation energies observed in F-doped Li_xOX_x−2.