Ethylmethyl Carbonate’s Role in Hexafluorophosphate Storage in Graphite Electrodes

In dual-ion batteries, the electrolyte solutions of LiPF6 dissolved into ethylmethyl carbonate (EMC) have demonstrated excellent compatibility with a graphite positive electrode. To further optimize the solutions, typical concentrations of LiPF6 are applied in Li/graphite cells. Traditional electrochemical tests, including galvanostatic charge–discharge and cyclic voltammetry, preliminarily verify that 2 M LiPF6-EMC is an appropriate choice for electrolyte solution. Furthermore, in situ XRD combined with EQCM clarified the crystal structural evolutions of the graphite electrode and the mass change during the PF6– anion storage in the graphite electrode. EMC transport is discovered in the opposite direction of the anion transport between the graphite electrode and the electrolyte solutions. NMR and IR analyses were explored to characterize the solvation states of anions in the solutions. In addition, the long-term cycling and rate capability of the graphite electrode are tested to address EMC’s role. A suitable concentration of EMC is vital for the satisfactory performance of the graphite positive electrode.