Understanding the relationship between structure, ionic conductivity, and synthesis is the key to the development of superionic conductors. Here, a series of Li_3-3xM_1+xCl₆ (−0.14 < x ≤ 0.5, M = Tb, Dy, Ho, Y, Er, Tm) solid electrolytes with orthorhombic and trigonal structures are reported. The orthorhombic phase of Li–M–Cl shows an approximately one order of magnitude increase in ionic conductivities when compared to their trigonal phase. Using the Li–Ho–Cl components as an example, their structures, phase transition, ionic conductivity, and electrochemical stability are studied. Molecular dynamics simulations reveal the facile diffusion in the z-direction in the orthorhombic structure, rationalizing the improved ionic conductivities. All-solid-state batteries of NMC811/Li_2.73Ho_1.09Cl₆/In demonstrate excellent electrochemical performance at both 25 and −10 °C. As relevant to the vast number of isostructural halide electrolytes, the present structure control strategy guides the design of halide superionic conductors.

Lithium halide electrolytes with high ion conductivity and good cathode compatibility have shown great potential for solid-state batteries. Li₃YBr₆, with a conductivity of 0.39 mS/cm at room temperature, synthesized by mechanical milling (BM-Li₃YBr₆), which can be further increased by heat treatment. The annealing parameters are tailored to obtain pure Li₃YBr₆ (AN-Li₃YBr₆) with a higher conductivity of 3.31 mS/cm by annealing the BM-Li₃YBr₆ at 500 °C for 5 h. The higher conductivity of AN-Li₃YBr₆ compared to the previously-reported results is due to the lower activation energy. NMR and simulation results show that the lithium ion migration between Li-1 and Li-2 sites along the [001] direction is the major obstacle for lithium diffusion in AN-Li₃YBr₆. The K- and L₃-edge X-ray absorption near-edge structure (XANES) of Y for BM-Li₃YBr₆ and AN-Li₃YBr₆ showed that Y exists with similar local structures. The increased vibrations of AN-Li₃YBr₆ due to increased temperatures increase the rate of lithium jumping from one site to another, yielding higher lithium ion mobility. Lithium nuclear density maps prove that the mobile lithium on the 4g(Li) site is more sensitive to the varying temperatures. Both BM- and AN-Li₃YBr₆ are incompatible with Li, however, an annealing process can improve the electrochemical stability. Both the experimental and simulation results confirm the anode incompatibility between In and AN-Li₃YBr₆. To mitigate the cathode and anode incompatibility with AN-Li₃YBr₆, a LiNbO₃ coating layer and a Li_5.7PS_4.7Cl_1.3 buffer layer are introduced at the cathode side and anode side, respectively, to assemble all-solid-state batteries with improved capacity and cyclability.