Layered K₂Mg₂TeO₆ Solid Electrolyte Enables Long-Life Solid-State Potassium Batteries

Solid-state potassium batteries are potentially supplementary to lithium batteries in large-scale energy storage due to their low cost, high safety, and energy density. However, potassium-ion solid electrolytes are quite limited and deliver lower ionic conductivities than lithium or sodium counterparts. Herein, K 2 Mg 2 TeO 6, an underexplored potassium-ion solid electrolyte with comparable high ionic conductivity, the capability to suppress metallic K dendrites, and mechanical-chemical and electrochemical stabilities against the metallic K anode, is reported. High potassium-ion conductivities of 1.65 × 10 –6 S cm –1 at 20 °C and 5.15 × 10 –5 S cm –1 at 120 °C are achieved. An ultrastable K stripping/plating behavior is implemented for 500 h under 0.02 mA cm –2, and the critical current density reaches 0.14 mA cm –2 at 60 °C. Cells coupling the KC composite anode and Prussian blue cathode deliver good rate performance (1 C, ∼64 mAh g –1 ) and long-term cycling stability of 400 cycles with a capacity retention of 94%.