Molecular-level design for a phosphate-based electrolyte for stable potassium-ion batteries

The safety and cycling stability of potassium-ion batteries are of paramount importance. This study investigates a class of phosphate electrolytes with different alkyl chains to understand the correlation between electrochemical properties and alkyl chains. Based on the traditional trimethyl phosphate, the impact of structural modifications on performance improvement was explored in a single-salt, single-solvent environment matched with 1 M bis(fluorosulfonyl)imide potassium salt (KFSI). The research findings indicate that appropriately altering the alkyl chain structure of the phosphate [1 M KFSI-tripropyl phosphate (TPP)] can enhance its electrochemical performance. Through characterization and calculations, it was revealed that TPP's weak solvation and steric hindrance properties favor the formation of a robust interphase on the electrode. Coupling with its excellent ion conductivity, the 1 M KFSI-TPP electrolyte enables good cycling stability of K parallel to graphite, K parallel to Prussian blue (PB), and PB parallel to graphite full-cell configurations. This study provides essential guiding principles for electrolyte design and offers valuable insights into the molecular-level design of safe electrolytes.