Polymer Electrolytes for Potassium Batteries: Incorporating Ionic Liquids to Enhance the Room Temperature Ionic Conductivity

Exploring the next generation of batteries based on sustainable materials is crucial for transitioning to a post-lithium-technology era. Potassium-based technology is one of the candidates that could fulfil the sustainability criteria. The electrolyte plays an essential role in battery performance, being responsible for ionic transport, cycling performance, working temperature, and safety. Polyethylene oxide (PEO)-based solid polymer electrolytes (SPEs) have been intensively studied. However, one of the drawbacks of SPEs is their poor ionic conductivity at room temperature (RT). Quasi-solid and solvent-free polymer electrolytes are a promising solution to this issue, combining the benefits of a liquid phase and SPEs. This work focuses on the development of cross-linked PEO, fluorinated K salt, and ionic liquid (IL)-based solvent-free SPEs for potassium batteries. The designed cross-linked ternary solvent-free SPEs are thoroughly characterized both physicochemically and electrochemically, achieving ionic conductivities of up to 1.6 × 10^-3 S cm^-1 at 20 °C. The solvent-free SPEs were tested in K cells at 20 °C, using a Prussian White (PW) cathode as a proof-of-concept. The effects that different fluorinated anions, such as bis(fluoromethanesulfonyl) imide (FSI^-) and bis(trifluoromethanesulfonyl) imide (TFSI^-), have on the electrochemical performance were analysed by investigating the solid electrolyte interphase (SEI) formed on the K metal surface through electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS) and magic-angle spinning solid-state nuclear magnetic resonance spectroscopy (MAS-ssNMR).