Anion Intercalation into a Graphite Electrode from Trimethyl Phosphate

Abstract

Trimethyl phosphate (TMP) is a flame-retardant solvent frequently used in nonaqueous electric energy storage devices. Anions can hardly intercalate into a graphite positive electrode from neat TMP at ordinary conditions. In TMP solutions, dissolving lithium hexafluorophosphate (LiPF₆), lithium tetrafluoroborate (LiBF₄), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium bis(trifluoromethanesulfonimide) (LiTFSI), by means of increasing lithium salt concentration or increasing the charge cutoff voltage of Li/graphite cells, the TMP-solvated anions can successfully intercalate into graphite positive electrodes. Moreover, the effect of TFSI^- activation on a graphite electrode is addressed. Ex situ X-ray diffraction measurements in combination with traditional electrochemical tests are employed to investigate the crystal structure change and electrochemical performance of graphite electrodes, respectively. Nuclear magnetic resonance, Fourier-transform infrared, and Raman spectroscopy are employed to characterize the TMP solutions.