High Conductivity in a Fluorine-Free K-Ion Polymer Electrolyte

Solid polymer electrolytes (SPEs) could play a major role in the transition to safer and high-energy-density potassium-based batteries. However, most polymeric K-ion electrolytes are based on fluorine-containing anions and flammable organic solvents, whose safety is nowadays in question. Herein, we report a facile solvent-free synthesis of a series of several poly(ethylene oxide) (PEO)-based SPE solid solutions with KBPh4 salt as potassium-ion source, including the formation of two crystalline (PEO)$_{n}$/KBPh$_{4}$ complexes. The ionic conductivity of these novel K-ion SPEs above and below the melting point of PEO is rationalized in light of their glass-transition temperature and chemical composition. We highlight that below the melting point of PEO, the crystalline complexes may not be intrinsically ionically conducting, but they act as ion sinks preventing the polymer cross-linking and the formation of contact ion pairs and lower the glass-transition temperature leading to a conductivity of 1.1 × 10$^{-4}$ S cm$^{-1}$ at 55 °C. A high ionic conductivity of 1.8 × 10$^{-3}$ S cm$^{-1}$ is achieved at 80 °C for the optimum (PEO)$_{30}$/KBPh$_{4}$4 composition (3.2 mol % KBPh$_{4}$). Besides, the SPE compositions were found to be stable up to 4 V vs K$^{0}$ metal electrode at 60 °C. The (PEO)$_{30}$/KBPh$_{4}$ composite electrolyte employed in an all-solid-state symmetric cell with Prussian Blue electrodes showed reversible K$^{+}$-ion (de)intercalation, where a reversible capacity of 20 mAh g$^{-1}$ and low voltage hysteresis were achieved for 20 cycles. Considering the material availability, ease of synthesis, and promising electrochemical properties, this work may encourage future research on fluorine-free polymer electrolytes for K-ion batteries