Anion\nIntercalation into a Graphite Electrode from\nTrimethyl Phosphate

Abstract

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