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

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 LixOXx-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 LixOClx-2 series compared to the equivalent structures in the LixOBrx-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 x 10(-9) and 6.08 x 10(-9) cm(2) s(-1) for Li5OBr3 and Li6OBr4 at 300 K, respectively) compared to their Cl-based counterparts (9.85 x 10(-10) and 2.06 x 10(-9) cm(2) 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 LixOXx-2. This work reveals the relationships between defects, Li-ion dynamics and dimensionality in anti-perovskite solid electrolytes. Significant increases in defect concentrations and ion transport are observed with decreasing dimensionality from 3D to 0D.