Multifunctional Diluent Enables High‐Voltage and Flame‐Retardant Potassium‐Ion Batteries

Abstract

ABSTRACT Potassium‐ion batteries (PIBs) emerge as a highly promising candidate system in large‐scale energy storage, owing to their abundant resources, low cost, and potentially high operating voltage. However, the current development of PIBs still faces core bottlenecks such as unstable electrode‐electrolyte interphase (EEI) at high voltages and flammability of electrolytes. Herein, a multifunctional solvent of ethoxy(pentafluoro)cyclotriphosphazene (PFPN) is used as the flame retardant, diluent, and interface regulator to design a non‐flammable localized high‐concentration electrolyte. Crucially, the introduction of PFPN (intrinsic flame retardant) not only significantly enhances the proportion of solvation structure of contact ion pair and cation‐anion aggregate, but also promotes the formation of inorganic‐rich EEI, thereby strengthening its inherent safety, ionic transport kinetics, interfacial desolvation, and high‐voltage tolerance. Consequently, the optimized non‐flammable electrolyte enables highly stable cycling of K||graphite (450 cycles), K||Mn‐based Prussian blue analogs (Mn‐PBA) cells (2.0–4.5 V, 3000 cycles), and Mn‐PBA||graphite full‐cells (2.0–4.3 V, 2200 cycles). This study provides guidelines for designing multifunctional electrolytes with both safety and high performance for PIBs and beyond.