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, Li 2 Sc 2/3 Cl 4 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, Li 2 MgCl 4, 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 Li 2 Mg 1–1.5 x M x Cl 4 (0 < x < 2/3) containing both inexpensive Mg and expensive M (= Sc and Y) as high-conductivity and low-cost cubic spinel solid electrolytes. In Li 2 Mg 1–1.5 x M x Cl 4, 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 x increases from 0 to 0.125, followed by a gradual increase for x > 0.125 until the highest conductivity of 1.36 mS cm –1 (6.69 mS cm –1 ) is reached for Li 2 Sc 2/3 Cl 4 (Li 2 Y 2/3 Cl 4 ) at x = 2/3. As x 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 Mg 2+ or Sc 3+ /Y 3+ 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 Li 2 MgCl 4 can realize cubic spinel solid electrolytes that have both high conductivity and low cost.