Unraveling the Irreversible Storage of Dimethyl Carbonate-Solvated Hexafluorophosphate Anions in Graphite Electrode

LiPF6 dissolved in dimethyl carbonate (DMC) is one of the cheapest groups of electrolyte solutions in dual-ion batteries. Generally, the discharge capacity of anion storage delivered by the graphite cathode grows with increasing LiPF6 concentration. This fact is consistent with the irreversible storage of DMC-solvated PF6–, and then, the underlying mechanism is clarified by the electrochemical tests and ex situ X-ray diffraction (XRD) measurements of graphite cathodes as well as infrared (IR) and Raman spectroscopy characterizations of solutions. Moreover, quaternary ammonium salts have facile dissociation, which can effectively regulate the solvation state of the anion and the interaction between ion pairs in the electrolyte. A small amount of tetrabutylammonium hexafluorophosphate (TBAPF6) is introduced into the highly concentrated LiPF6-DMC solution to improve the performance of the graphite cathode. The discharge capacity of the Li/graphite cell has increased by approximately 50%. This effect is also correlated with the solvation state of the anion. This study provides an insightful clue for the choice of electrolyte solution in dual-ion batteries.