Diluent Conformational Flexibility Enhances Interfacial Kinetics in Safe High-Voltage Potassium Batteries

Introducing diluents into high-concentration electrolytes to promote anion decomposition is a conventional strategy for stabilizing electrode interfaces, but this approach overlooks how the conformational dynamics of the diluent itself regulate cation solvation. Here, three electrolytes based on a potassium salt, tributyl phosphate, and fluorinated ether diluents are designed and investigated. Across the three electrolytes, the conventional radial distribution functions and coordination numbers show only limited differences, but the probability distribution analysis of the instantaneous solvation state defined by the composition shows that introducing the diluent with maximum conformational flexibility can achieve a rich and uniform set of states in the electrolyte. This is beneficial for achieving the highest mean Tafel-derived exchange current density, fast interfacial charge-transfer kinetics and good electrochemical performance. Specifically, this type of electrolyte enables graphite cells to exhibit excellent rate performance and long-term cycling stability of 1400 cycles; a 4.5 V Prussian blue-based cell to operate stably; and a potassium-ion full cell to cycle for 900 cycles with a capacity retention rate of 93.6%. This work establishes diluent conformational flexibility as a molecular design dimension for regulating instantaneous solvation-state distributions and interfacial kinetics in safe phosphate-based electrolytes.