A novel garnet-type high-entropy oxide as air-stable solid electrolyte for Li-ion batteries

Abstract

Li-ion batteries are considered prospective candidates for storage systems because of their high energy density and long cycling life. However, the use of organic electrolytes increases the risk of explosion and fire. Hence, all-solid-state Li-ion batteries have attracted considerable attention because the use of solid electrolytes avoids the combustion of electrolytes and explosions, and enhances the performance of batteries. Garnet-type oxides are commonly used solid electrolytes. The common Ta-doped Li7La3Zr2O12 can react easily with CO2 and H2O in air, and its ionic conductivity decays after contact with air. In this study, a novel garnet-type, high-entropy oxide, Li6.4La3Zr0.4Ta0.4Nb0.4Y0.6W0.2O12 (LLZTNYWO), is successfully synthesized as a solid electrolyte for Li-ion batteries,using a conventional solid-state method. Ta, Nb, Y, and W are used as substitutes for Zr, which significantly increase conductivity, have high stability in air, and a lower sintering temperature. LLZTNYWO achieves higher Li-ion conductivity at 1.16 × 10-4 S cm-1 compared to mono-doped Li6.6La3Zr1.6Ta0.4O12 (6.57 × 10-5 S cm-1), Li6.6La3Zr1.6Nb0.4O12 (2.19 × 10-5 S cm-1), and Li6.2La3Zr1.6W0.4O12 (1.16 × 10-4 S cm-1). Additionally, it exhibits higher ionic conductivity compared to equimolar Li5.8La3Zr0.4Ta0.4Nb0.4Y0.4W0.4O12 (1.95 × 10-5 S cm-1). The Li-ion conductivity of LLZTNYWO remains constant for 30 days in the atmosphere without decay, thereby revealing its excellent air stability. Furthermore, LLZTNYWO exhibits a remarkable electrochemical window of up to 6 V vs Li/Li+ and excellent electrochemical stability against Li metal after cycling at 0.1 mA·cm-2 for 2 h, which indicates that it is a promising solid electrolyte for Li-ion batteries.