Aliovalent Cation Substitution in Cubic Spinel Li ₂ MgCl ₄ to Realize High-Conductivity, Low-Cost Chloride Solid Electrolytes

With values exceeding 1 mS cm^-1, Li₂Sc_2/3Cl₄ has the highest conductivity among spinel-type cubic chlorides, but it is not cost-effective due to the high price of Sc. In contrast, another cubic spinel, Li₂MgCl₄, is cost-effective but has very low conductivity of about 10^-4 mS cm^-1. In this first-principles study, we assessed the potential of Li₂Mg_{1-1.5<i>x</i>}M_<i>x</i>Cl₄ (0 < <i>x</i> < 2/3) containing both inexpensive Mg and expensive M (= Sc and Y) as high-conductivity and low-cost cubic spinel solid electrolytes. In Li₂Mg_{1-1.5<i>x</i>}M_<i>x</i>Cl₄, the site preference of the Li ions increases in the order of tetrahedral 48f < octahedral 16c < tetrahedral 8a < octahedral 16d, resulting in most of the Li ions occupying the 16d and 8a sites. For M = Sc (Y), the conductivity increases rapidly as <i>x</i> increases from 0 to 0.125, followed by a gradual increase for <i>x</i> > 0.125 until the highest conductivity of 1.36 mS cm^-1 (6.69 mS cm^-1) is reached for Li₂Sc_2/3Cl₄ (Li₂Y_2/3Cl₄) at <i>x</i> = 2/3. As <i>x</i> increases, the proportion of Li (Mg and Sc/Y) ions at the 16d site increases (decreases), which causes the diffusing Li⁺ ions to encounter Li⁺ ions rather than the Mg²⁺ or Sc³⁺/Y³⁺ ions, thereby reducing the strong cation-cation repulsion and activating the 16d → 48f → 16c jump that is important for improving conductivity. Compared with Sc, Y is considerably cheaper and has a greater conductivity-enhancing effect. This suggests that substituting an appropriate amount of Y for Mg in Li₂MgCl₄ can realize cubic spinel solid electrolytes that have both high conductivity and low cost.