Performance of Graphite Positive Electrodes in LiPF 6 –Methyl Acetate/trimethyl Phosphate Solutions

In dual–ion batteries (DIBs), it is a key issue to search for electrolyte solutions compatible with graphite positive electrode. The solutions of LiPF 6 dissolved in short chain esters like ethyl methyl carbonate or methyl propionate have emerged as suitable choices, in which graphite positive electrode can deliver a PF 6 – storage capacity about 100 mA h g –1 . However, in a much shorter chain ester, methyl acetate (MA), the PF 6 – storage capacity delivered by graphite electrode is limited to 60 mA h g –1 . In this paper, the flame–retardant solvent of trimethyl phosphate (TMP) is introduced into LiPF 6 –MA solutions to improve the electrochemical performance of graphite electrode. The reversible capacity can be enhanced to near 90 mA h g –1 in 3 M LiPF 6 –MA/TMP (6:4 by vol.). Traditional electrochemical tests in combination with ex situ/in situ X–ray diffraction (XRD) characterizations have been applied to probe the charge storage mechanism across the interfaces between graphite electrode and electrolyte solutions. Raman/fourier transform infrared (FTIR) spectra and nuclear magnetic resonance (NMR) spectra have been carried out to explore the solvation states of ions in the solutions.