Composition Optimization of the Lithium-Rich Li3OCl1-XBrx Anti-Perovskite Superionic Conductors

The newly discovered lithium rich anti-perovskite (LRAP) superionic conductors are an extremely interesting class of materials with potential applications as solid electrolytes in Li-ion batteries. Here we present a rational composition optimization strategy for maximizing the Li⁺ conductivity in the LRAP guided by a combination of first principles calculations and percolation theory. Using nudged elastic band (NEB) calculations, we show that a Cl-rich channel with Br-rich endpoints configuration leads to low vacancy migration barriers in the LRAP structure. By incorporating the halide-environment-dependent NEB barriers in a bond percolation model, we predict that there are potentially higher conductivity Li3OCl_1-xBr_x structures near 0.235 ≤ x ≤ 0.395. This prediction is confirmed by AIMD simulation that finds Li₃OCl_0.75Br_0.25 to have a higher Li⁺ conductivity than Li₃OCl_0.5Br_0.5, the highest conductivity LRAP identified experimentally thus far. These results highlight that there is scope for further enhancing the conductivity in the LRAP chemistry. The general approach developed can potentially be extended to other ion-conducting systems, such as the structurally similar perovskite oxygen-ion conductors of interest in solid-oxide fuel cells as well as other superionic conductors.