Analysis of Electrochemical Behavior of Natural Graphite Electrode in N, N-Diethyl-N-Methyl-N-(2-methoxyethyl) Ammonium Bis (trifluoromethylsulfonyl) Amide Containing Lithium Ion

1. Introduction  Recently, room-temperature ionic liquids (RTILs) have attracted much attentions as an electrolyte for lithium-ion secondary batteries due to their non-flammability and negligible volatility. However, the electrochemical behavior of the natural graphite electrode in many types of RTILs is not clear in spite of many researchers' vigorous efforts. This would be due to the electrochemical organic cation intercalation into the graphite layers before the formation of an effective solid electrolyte interface (SEI) layer on the graphite particles during the 1st charging.

     In this study, we have analyzed the electrochemical behavior of the natural graphite electrode in the N, N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium bis (trifluoromethylsulfonyl) amide (DEME-TFSA) containing lithium ion.

2. Experimental  The electrolyte was prepared by dissolving lithium bis (trifluoromethylsulfonyl) amide (LiTFSA) in DEME-TFSA. The R2032 coin type cell which consisted of the natural graphite electrode (NG-3, average size 3 mm) using the poly (vinylidene fluoride) binder as s working electrode, a pressed lithium metal foil as a counter electrode, and 1 mol dm^-3LiTFSA/DEME-TFSA as the electrolyte was used for the electrochemical measurements such as cyclic voltammetry (CV) and the charge-discharge cycle tests. The surface analysis of the NG-3 electrodes was carried out by energy dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS).

3. Results and Discussion  An irreversible reduction peak at the 1st cycle was observed in the CV. It might be due to the reduction of the RTIL. The charge-discharge cycle tests showed that the discharge capacity and the charge-discharge efficiency of the NG-3 electrode at the 1st cycle were 318.1 mAh g^-1 and 75.6%, respectively. The surface of the NG-3 electrode was analyzed by the EDX mappings after charging down to 5 mV and after discharging up to 2.0 V, respectively. The EDX mapping results showed that C, O, F, and S were detected on the surface of the NG-3 electrode. The reduction products were uniformity formed on the graphite particle. Figure 1 shows the high resolution XPS spectra of F 1s region for the NG-3 electrode in the 1 mol dm^-3 LiTFSA/DEME-TFSA electrolyte before soaking, after charging down to 5 mV, and after discharging up to 2.0 V, respectively. The peak at around 688 eV was assigned to a C-F bond derived from PVdF. The peak intensity at around 688 eV was decreased after charging down to 5 mV and discharging up to 2.0 V, indicating that the surface of the NG-3 particles was covered. In contrast, the peak intensity at around 685 eV was increased after charging down to 5 mV and discharging up to 2.0 V. The peaks were assigned to the fluorine of a Li-F bond derived from LiF. The results suggest that a layer of LiF was present on the surface. The small peaks were identified for -CF₃ derived from a TSFA^-anion at 689eV, indicating that they would be due to the residue of the RTIL.

     Based on these results, it was found that the LiF-based compound as a reductive product was formed on the natural graphite electrode due to the cathodic decomposition of a TFSA^-anion as a side reaction after the 1st charging.

Acknowledgement  This work was partially supported by Grant-in-Aid for Scientific Research (C) from the Japanese Ministry of Education, Science, Sports and Culture.