Effect of Cation Disorder on Lithium Transport in Halide Superionic Conductors

Li2ZrCl6 (LZC) is a promising solid-state electrolyte due to its affordability, moisture stability, and high ionic conductivity. We computationally investigate the role of cation disorder in LZC and its effect on Li-ion transport by integrating thermodynamic and kinetic modeling. The results demonstrate that fast Li-ion conductivity requires Li-vacancy disorder, which is dependent on the degree of Zr disorder. The high temperature required to form equilibrium Zr disorder precludes any equilibrium synthesis processes for achieving fast Li-ion conductivity, rationalizing why only nonequilibrium synthesis methods, such as ball-milling, lead to good conductivity. Our simulations show that Zr disorder lowers the Li/vacancy order–disorder transition temperature, which is necessary for creating high Li diffusivity at room temperature. These insights raise a challenge for the large-scale production of these materials and the potential for long-term stability of their properties.