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 K0.62Mg0.54Sb0.46O2 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.