Potential Solid-State Electrolytes with Good Balance between Ionic Conductivity and Electrochemical Stability: Li5–xM1–xMx′O4 (M = Al and Ga and M′ = Si and Ge)

Exploring new solid-state electrolyte (SSE) materials with good electrochemical stability and high Li-ion conductivity for all-solid-state Li-ion batteries is vital for the development of technologies. Herein, we employ two lithium aluminates, α- and β-Li5AlO4 (α- and β-LAO), as the model framework, which have an orthorhombic crystal structure and isolated AlO4 tetrahedron units connected in lithium atoms, exhibiting large band gaps, low migration barriers (0.30–0.40 eV), fast Li-ion conductivity (LIC, in a magnitude of 10–4 S/cm), and a good electrochemical stability window (ESW, [0.01–3.20 V] vs Li+/Li). We tabulate the expected decomposition products at the interface, while considering cathodes in combination with the LAO electrolyte to discuss their compatibility. We also examine the electrochemical stability, H2O/CO2 stability, and Li-ion mobility of Li4.6Al0.6Si0.4O4 (LASO), Li5GaO4 (LGaO), and Li4.6Ga0.6Ge0.4O4 (LGaGeO) compounds. In general, there is usually a trade-off between the LIC and the ESW; however, LAO features a good balance between an outstanding LIC and a wide ESW, making the compound a promising candidate for next-generation SSE materials.