New Series of Ternary Metal Chloride Superionic Conductors for High Performance All-Solid-State Lithium Batteries

Understanding the relationship between structure, ionic conductivity, and synthesis is the key to the development of solid electrolytes for all-solid-state Lithium batteries. Here, we investigate chloride solid electrolytes with compositions Li_{3 − 3x}M_1+xCl₆ (-0.14 < x ≤ 0.5, M = Tb, Dy, Ho, Y, Er, Tm). When x > 0.04, a trigonal to orthorhombic phase transition occurs in the isostructural Li-Dy-Cl, Li-Ho-Cl, Li-Y-Cl, Li-Er-Cl and Li-Tm-Cl solid electrolytes. The new orthorhombic phase shows a four-fold increase in ionic conductivity up to 1.3×10^{− 3} S cm^{− 1} at room temperature for Li_2.73Ho_1.09Cl₆ (x = 0.09) when compared to the trigonal Li₃HoCl₆. For isostructural Li-Dy-Cl, Li-Y-Cl, Li-Er-Cl and Li-Tm-Cl solid electrolytes, about one order of magnitude increase in ionic conductivities are observed for the orthorhombic structure compared to the trigonal structure. Using the Li-Ho-Cl components as an example, detailed studies of its structure, phase transition, ionic conductivity, air stability and electrochemical stability have been made. Molecular dynamics simulations based on density functional theory reveal that the different cations arrangement in the orthorhombic structure leads to a higher lithium diffusivity as compared to the trigonal structure, rationalizing the improved ionic conductivities of the new Li-M-Cl electrolytes. All-solid-state batteries of In/Li_2.73Ho_1.09Cl₆/NMC811 demonstrate excellent electrochemical performance at both room temperature and − 10°C. As relevant to the vast number of isostructural halide electrolytes, the present structure control strategy provides guidance for the design of novel halide superionic conductors.