Motif-Based Exploration of Halide Classes of Li₅M1_0.5M2_0.5X₈ Conductors Using the DFT Method: Toward High Li-Ion Conductivity and Improved Stability

The development of all-solid-state lithium-ion batteries (ASSLIBs) is highly dependent on solid-state electrolyte (SSEs) performance. However, current SSEs cannot satisfactorily meet the requirements for high interfacial stability and Li-ion conductivity, especially under high-voltage cycling conditions. To overcome the intractable problems, we theoretically develop the chemistry of structural units to build a series of MX6-unit mixed framework Li(5)M1(0.5)M2(0.5)X(8) (total 184 halides) for use as SSEs and recommend six halide candidates that combine the (electro)chemical stability with a low Li-ion migration barrier. Among them, three Li(5)M1(0.5)M2(0.5)F(8) compounds (M1 = Ca and Mg; M2 = Ti and Zr) exhibit expansive electrochemical windows with a high cathodic limit (6.3 V vs mu(Li)) and three-dimensional Li diffusion associated with moderate Li-migration barriers. To discuss their stability and compatibility (and in turn as a reference for experiments), the energy above the convex hull, the electrochemical stability window, the predicted (electro)reaction products, and the calculated reaction energies of Li(5)M1(0.5)M2(0.5)X(8) in combination with Li-metal and several cathodes are tabulated. We stress that the importance of the cation-mixed effect and specific moieties for the halide anion leads to a design principle for a halide class of Li-ion SSEs. We provide insight into selecting the optimal halide anion and cations and open a new avenue of broad compositional spaces for stable Li-ion SSEs.