High‐Entropy Solid‐State Na‐Ion Conductor for Stable Sodium‐Metal Batteries

Solid-state sodium-metal batteries (SSBs) hold great promise for their merits in low-cost, high energy density, and safety. However, developing solid electrolyte (SE) materials for SSBs with high performance is still a great challenge. In this study, high-entropy Na4.9 Sm0.3 Y0.2 Gd0.2 La0.1 Al0.1 Zr0.1 Si4 O12 was synthesized at comparatively low sintering temperature of 950 °C with high room-temperature ionic conductivity of 6.7×10-4 S cm-1 and a low activation energy of 0.22 eV. More importantly, the Na symmetric cells using high-entropy SE show a high critical current density of 0.6 mA cm-2 , outstanding rate performance with fairly flat potential profiles at 0.5 mA cm-2 and steady cycling over 700 h under 0.1 mA cm-2 . Solid-state Na3 V2 (PO4 )3 ||high-entropy SE||Na batteries are further assembled manifesting a desirable cycling stability with almost no capacity decay after 600 cycles and high Columbic efficiency over 99.9 %. The findings present opportunities for the design of high-entropy Na-ion conductors toward the development of SSBs.