We investigated the electrochemical characteristics of a natural graphite electrode in room-temperature ionic liquids not containing additives. N, N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)amide (DEME-TFSA) containing lithium bis(trifluoromethylsulfonyl)amide (Li-TFSA) as the electrolyte and a natural graphite electrode as the negative electrode material were employed. The charge-discharge tests showed that the discharge capacity and the charge-discharge efficiency of the natural graphite electrode at the 1st cycle were 318 mAh g -1 and 75.6%, respectively. The cycle performance showed that the discharge capacity and the charge-discharge efficiency were stably maintained at ca. 320 mAh g -1 and 100%, respectively, until the initial 10th cycle. The ex-situ X-ray diffraction measurements showed that lithium-graphite intercalation compounds, such as LiC 12 and LiC 6 , were formed after the 1st charge. The structural change in the natural graphite electrode was reversible because graphite recovered to its original structure after the 1st discharge. These results clarified that the graphite electrode could operate as a negative electrode for lithium-ion secondary batteries in DEME-TFSA containing Li-TFSA without organic solvents. Highlights:Initial characteristics of graphite at 25 o C with a capacity close to theoretical.A stable cycle performance was obtained. DEME-TFSA containing Li-TFSA provides reversible lithium ion intercalation/extraction without any additives. DEME-TFSA containing Li-TFSA may provide suitable SEI film. DEME-TFSA containing Li-TFSA may prevent undesirable side reactions.
We investigated the charge-discharge characteristics of the natural graphite negative electrode in N, N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium bis (trifluoromethylsulfonyl) amide (DEME-TFSA) containing lithium ion as the electrolyte for nonflammable lithium-ion batteries. The charge-discharge tests showed that the discharge capacity was stably maintained at ca. 300 mAh g -1 until the initial 10th cycle and the charge-discharge efficiencies reached ca. 100% after the initial cycles. The FE-SEM / EDX mapping results showed that the surface deposit (O, F, and S) deriving from DEME-TFSA was formed during the 1st charge. The XPS analysis results indicated that the surface deposit would compose of LiF, O, and S. It was found that the LiF-based compound containing O and S as a surface deposit was formed on the natural graphite electrode due to the cathodic decomposition of the TFSA -anion as a side reaction during the 1st charge.
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.
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