In Situ Raman Study of Graphite Negative-Electrodes in Electrolyte Solution Containing Fluorinated Phosphoric Esters

Abstract

The electrochemical characteristics of graphite composite electrodes and structural changes in the surface of edge plane highly oriented pyrolytic graphite electrodes were studied in ethylene carbonate-based electrolytes containing flame-retardant solvents of fluorinated phosphoric esters. The effects of the number of 2,2,2-trifluoroethyl groups in the phosphoric ester (PO(OCH3)3-n(OCH2CF3)n, n = 0–3) on the intercalation/de-intercalation reactions of lithium ion at graphite were investigated systematically. In the initial cyclic voltammograms of graphite composite electrodes, anodic currents due to the de-intercalation reaction of lithium ion were not observed in 2,2,2-trifluoroethyl dimethyl phosphate (PF-3DM, n = 1)- or bis(2,2,2-trifluoroethyl) methyl phosphate (PF-3M, n = 2)-containing electrolyte solution. In the Raman spectra, a band with a peak at around 1595 cm−1, which was assigned to an E band, was observed after potential cycling in electrolyte solution containing PF-3M or PF-3DM. On the other hand, the use of tris(2,2,2-trifluoroethyl) phosphate (TFEP, n = 3)-containing electrolyte solution resulted in the reversible intercalation/de-intercalation reactions of lithium ion, and no E band was observed in the Raman spectra after potential cycling. These results suggest that PF-3M and PF-3DM should be co-intercalated with lithium ion into graphite layers, which should interfere with reversible intercalation/de-intercalation reactions of lithium ion at graphite electrodes. The variation of the Raman shifts of phosphoric esters in electrolyte solution was analyzed to evaluate their solvating abilities, and the results revealed the relationship between the solvating ability and the charge/discharge performance of graphite electrodes.