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

Abstract 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 K 3 LnSi 3 O 9 (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−x GdP x Si 3−x O 9 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 2.8 GdP 0.2 Si 2.8 O 9 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 2.8 GdP 0.2 Si 2.8 O 9 /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.