Performance of Graphite Positive Electrodes in LiPF6–Methyl Acetate/trimethyl Phosphate Solutions

Abstract

In dual–ion batteries (DIBs), it is a key issue to search for electrolyte solutions compatible with graphite positive electrode. The solutions of LiPF6 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 PF6– storage capacity about 100 mA h g–1. However, in a much shorter chain ester, methyl acetate (MA), the PF6– 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 LiPF6–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 LiPF6–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.