Analysis of Surface Deposit Formed on the Binder-Free Si Nanoparticle Electrode in Room-Temperature Ionic Liquid Containing Lithium Bis(trifluoromethanesulfonyl)Amide

Abstract

1. Introduction Si is expected to be a promising alternative negative electrode material for lithium secondary batteries. Despite that, it suffers from poor cyclability due to the cracks caused by the large volume change during a charge-discharge reaction. On the other hand, 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 Si electrode in many types of RTILs is not enough in spite of many researchers' vigorous efforts. As one reason, the SEI film on the Si electrode in RTILs may be less effective than that on the graphite electrode in the conventional electrolyte. To solve the problem, the analysis of the surface deposit formed on the Si electrode during a charge-discharge reaction is desired. To analyze it in more detail, we focus on an electrophoretic deposition (EPD) method because the binder-free (BF) electrode not containing conductive additives and a binder can be prepared. In this study, we prepared the BF nano-Si film electrode by using the EPD method and then investigated the electrochemical behavior in the RTILs. By employing the BF nano-Si film electrode not containing carbon, the surface deposit on the Si electrode was analyzed. 2. Experimental An EPD bath was prepared as follows: a concentration of nano-Si (average particle size: 50 nm) in acetone bath of 1.0 g L-1. Nano-Si was deposited on a Cu foil as a current collector substrate. The electrolyte was prepared by dissolving lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) in N, N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium bis(trifluoromethanesulfonyl)amide (DEME-TFSA). The R2032 coin type cell, which consisted of the BF nano-Si film electrode (W.E.), a pressed lithium metal foil (C.E.), a 1 mol dm−3 solution of LiTFSA/DEME-TFSA (electrolyte), and celgard#2500 (separator), was used for the electrochemical measurements such as cyclic voltammetry (CV) and the charge-discharge cycle tests. For comparison, a 1 mol dm−3 solution of LiPF6/EC + DEC (50:50 vol. %) was used as a conventional electrolyte. The surface analysis of the BF nano-Si film electrode was carried out by energy dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS). 3. Results and Discussion The CVs of the BF nano-Si film electrodes in the above RTILs (DEME type) and above organic solvent electrolyte (organic solvent type) were measured. The reduction wave at the 1st cycle was observed in the cell voltage from 0.25 V to 5 mV regardless of the electrolyte type. Similarly, the oxidation wave at the 1st cycle was observed in the potential range from 0.20 V to 0.85 V in both electrolyte types. The reduction and oxidation currents would be due to the insertion and extraction reactions of Li+ ion into and from Si. Figure 1 shows the initial charge-discharge curves of the BF nano-Si film electrodes (ca. 0.23 mg cm-2) (a) in the DEME type and (b) in the organic solvent type at 25 oC. The 1st discharge capacity and the 1st charge-discharge efficiency of the BF nano-Si film electrode in the DEME type are 1507 mAh g-1 and 67.5%, respectively. In contrast, the 1st discharge capacity and the 1st charge-discharge efficiency in the organic solvent type are 1968 mAh g-1 and 73.2%, respectively. After the 2 cycle, the charge-discharge efficiency of the BF nano-Si film electrode increased and achieved to 98% regardless of the electrolyte type. These results suggest that the SEI film would be mostly formed on the BF nano-Si film electrode during the 1st charge. About the DEME type, the surface of the BF nano-Si film 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 BF nano-Si film electrode. The surface deposit was uniformly formed on the nano-Si particle. The high resolution XPS spectra of F 1s region were measured. The peaks were assigned to the fluorine of a Li-F bond derived from LiF, suggesting that the layer of LiF was present on the surface. It was clarified that the LiF-based compound as the surface deposit was formed on the BF nano-Si film electrode. Based on these results, it was found that the compound containing LiF, C, O, and S as a surface deposit was formed on the BF nano-Si film electrode due to the cathodic decomposition of a DEME+ cation as well as a TFSA- anion as a side reaction after the 1st charging. Figure 1