Fast Potassium‐Ion Conduction in K₃LnSi₃O₉ (Ln = Y and Gd) Enabled by P‐Doping Toward Ultrastable Quasi‐Solid‐State Batteries

Solid-state potassium-ion batteries are promising options for large-scale energy storage due to their high safety and abundance of potassium resources. However, solid-state potassium-ion batteries are still in their infancy and the reported electrolyte materials are very limited, making the exploration of solid electrolytes with high ionic conductivity and physical/electrochemical stability a major challenge. Here novel triclinic K₃LnSi₃O₉ (Ln = Y and Gd) potassium-ion solid electrolyte is reported with low activation energy and high stability. A rational vacancy design strategy is adopted to synthesize K_3-xGdP_xSi_3-xO₉ and the result of DFT calculation shows that the diffusion pathways of potassium ions on the ac plane exhibit a fish scale-like network structure. Specifically, the K_2.8GdP_0.2Si_2.8O₉ delivers a high ionic conductivity of 2.9 × 10^-5 S cm^-1 at 25 °C, accompanied by a stable potassium stripping/plating (a long-life cycle over 2000 h). As a result, the assembled quasi-solid-state KC/K_2.8GdP_0.2Si_2.8O₉/PB cell achieves a remarkable cycling performance at a high current density of 1 C (500 cycles, 95.9% capacity retention). These results would no doubt boost research for high-safety and high-energy-density solid-state potassium-ion batteries.