Effect of Concentrated Diglyme-Based Electrolytes on the Electrochemical Performance of Potassium-Ion Batteries

The effect of salt concentration in diglyme-based electrolytes on cycling performance of promising KVOPO4 and K1.69Mn[Fe(CN)6]0.85·0.4H2O positive electrodes (cathodes) and a hard carbon negative electrode (anode) for next-generation potassium-ion (K-ion) batteries is investigated. A decrease in free solvent molecule number with increasing electrolyte concentration is found, which results in a better aluminum current collector stability, formation of thinner solid electrolyte interface (SEI) passivation layers, and further inhibition of solvent degradation redox processes occurring at the electrode surface upon cycling. The KVOPO4 and K1.69Mn[Fe(CN)6]0.85·0.4H2O cathodes exhibit an enhanced specific discharge capacity (54 and 105 mA·h·g–1, respectively) in K-ion cells at the highest electrolyte concentrations (2 and 2.5 M KPF6 in diglyme, respectively) at a 0.1 C rate. However, the behavior of the hard carbon anode is noticeably affected by the salt concentration over the first few cycles, a phenomenon tentatively attributed to the SEI layer formation and the presence of irreversible intercalation sites for K+ ions in the hard carbon framework. Finally, electrochemical tests on K-ion full cells consisting of the K1.69Mn[Fe(CN)6]0.85·0.4H2O cathode, a hard carbon anode, and an ether-based electrolyte show capacity retention of 86% over 300 cycles at a 0.6 C rate.