Boosting K+‐Ionic Conductivity of Layered Oxides via Regulating P2/P3 Heterogeneity and Reciprocity for Room‐Temperature Quasi‐Solid‐State Potassium Metal Batteries

Solid-state potassium metal batteries are promising candidates for grid-scale energy storage due to their low cost, high energy density and inherent safety. However, solid state K-ion conductors struggle with poor ionic conductivity due to the large ionic radius of K⁺-ions. Herein, we report precise regulation of phase heterogeneity and reciprocity of the P2/P3-symbiosis K_0.62Mg_0.54Sb_0.46O₂ solid electrolyte (SE) for boosting a high ionic conductivity of 1.6×10^-4 S cm^-1 at 25 °C. The bulk ionic conducting mechanism is explored by elucidating the effect of atomic stacking mode within the layered framework on K⁺-ion migration barriers. For ion diffusion at grain boundaries, the P2/P3 biphasic symbiosis property assists in tunning the SE microstructure, which crystallizes in rod-like particles with lengths of tens of micrometers facilitating long-distance ion transport and significantly decreasing grain boundary resistance. Potassium metal symmetric cells using the modified SE deliver excellent cycling life over 300 h at 0.1 mA cm^-2 and a high critical current density of 0.68 mA cm^-2. The quasi-solid-state potassium metal batteries (QSSKBs) coupled with two kinds of layered oxide cathodes demonstrate remarkable stability over 300 cycles, outperforming the liquid electrolyte counterparts. The QSSKB system provides a promising strategy for high-efficiency, safe, and durable large-scale energy storage.