Interface Reinforcement of a Prussian Blue Cathode Using a Non-Flammable Co-Solvent Cresyl Diphenyl Phosphate for a High-Safety Na-Ion Battery

A Prussian blue (PB)-based sodium-ion battery (SIB) is a promising candidate in large-scale electric energy storage. However, its development is hindered by the poor cyclability of the PB cathode and safety problems with a flammable electrolyte. Herein, a non-flammable co-solvent cresyl diphenyl phosphate (CDP) in the electrolyte is proposed to increase the safety associated with PB-based SIBs, due to its effective flame retardancy for the electrolyte and good compatibility with the Na anode. Moreover, the deteriorated electrochemical performance of PB is obviously suppressed with cathode interface reinforcement after using CDP, resulting in excellent cyclability with a capacity retention of 83.1% after 700 cycles. The capacity fading of PB can be effectively suppressed with a protective film of phosphates produced by CDP decomposition, forming FePO₄, which can activate low-spin Fe-ion sites in the PB lattice. This work effectively improves the battery safety and interface stability between the PB cathode and the electrolyte using the non-flammable CDP co-solvent, as well as the clarification of a capacity fading mechanism for general PB with considerable Fe­(CN)₆ vacancies and crystal water.