Effect of SEI Component on Graphite Electrode Performance for Li-Ion Battery Using Ionic Liquid Electrolyte

In this study, we investigated the role of solid electrolyte interface (SEI) ﬁlms in the cells fabricated with the ionic liquids ([N 2,2,1(2O1) ][TFSI] and [Li(G4)][TFSI]). The SEI was introduced beforehand to the graphite electrode, which facilitated the intercalation of Li-ions into the graphite electrode in [Li(G4)][TFSI]. In addition, the discharge capacity in the case of the cell fabricated with [Li(G4)][TFSI] was almost similar to that observed in the case of the cell fabricated with organic electrolyte. The SEI performed the pulling apart combination of glyme and Li-ion. Identiﬁcation analysis of the SEI component having action of extracting Li-ion from [Li(G4)] was carried out.

With the growing global interest in environmental and energy issues, the importance of storage technology is increasing. Lithium ion batteries (LIBs) are one of the most promising electric storage devices, since they have feature as high energy density, long cycle lives and low self-discharge rate etc. [1][2][3] Therefore, LIBs are used as a power source for portable device, electric vehicle and so on. However, the number of fire accident caused by LIBs is not negligible. These are largely attributed to organic electrolyte. A large number of organic electrolyte have a safety hazard: flammability, volatility. [3][4][5] In order to improve the safety of LIBs, the safer alternative electrolytes that replace the organic electrolytes are strongly required.
Recent years, ionic liquids are generating a lot of attention in terms of safety. Ionic liquids excel in thermal stability, non-volatility and wide liquid region. [6][7][8][9][10][11] Some of these characteristics are suitable for use over the wide temperature region. Furthermore, they also have electrochemical abilities: high ion-conducting, wide electrochemical window. Ion conduction is closely related to battery performance: internal resistance, discharged capacity and energy density. Using ionic liquid of wide electrochemical window, various electrode materials can contribute to LIBs. From the above of characteristics, ionic liquids have possibilities as a novel electrolyte instead of organic electrolyte for LIBs and some of these were reported that can intercalate Li-ions into graphite electrode. Some ionic liquids containing bis(fluorosulfonyl)imide (FSI) anion such as [EMIm][FSI] and [P13][FSI] exhibit good charge and discharge properties have been reported. [12][13][14][15][16][17] However, since FSI anion is inferior in thermal stability to bis(trifluoromethylsulfonyl)imide (TFSI) anion, ionic liquid containing TFSI anion is considered to be effective in improving the safety of LIBs. Many ionic liquids containing TFSI anion cannot occur intercalation reaction of Li-ions into the graphite layers. Because they cannot form solid electrolyte interface (SEI) on the graphite electrode. This causes decomposition of the ionic liquid to proceed on graphite electrode.
In typical LIB using organic electrolyte, solvated Li-ions are decomposed and deposited as SEI on graphite electrode surface in the first cycle. SEI promote desolvation of Li-ions from solvated Li-ions and enables intercalation of Li-ions into graphite layers. Moreover, the SEI also have a role of suppressing the decomposition of the electrolyte after the second cycle. Therefore, the SEI is indispensable for stable operation of LIBs. [18][19][20][21][22][23][24][25][26] In this study, we tried the appearance of charge/discharge capacity in ionic liquids using graphite negative electrode. Generally, it is difficult to occur reversible charge and discharge reaction in ionic liquids using graphite negative electrode. The possibility of reversible charge and discharge reaction was examined by introducing graphite negative electrode formed stable solid SEI beforehand derived from organic electrolyte.
Since  Fig. 1. In addition, the electrode was prepared as mixer of active material (0.95 g) and binder (0.05 g) at the mass ratio of 9.5: 0.5. The diameter of the electrode was 12 mm. The total mass of active material was 1-2 mg. Flat cells using the abovementioned materials were assembled in an Ar-filled glove box. The SEI were formed on the electrode by performing a typical charge and discharge cycle at 0.2 C in 1 M LiPF 6 /EC-DMC. After charge and discharge cycle, the electrode was taken out from the flat cell and was cleaned with DMC in an Ar-filled glove box.
The redox reaction occurring in the battery was analyzed using cyclic voltammetry (Hokuto Denko, HZ 7000, between 0.005 to 2 V vs. Li/Li + ). Furthermore, the charge and discharge measurement (Hokuto Denko, HZ 7000, CC 0.2 C, CV 0.02 C) was performed to confirm the charge and discharge behavior of each electrolyte. Impedance measurements (Hokuto Denko, HZ 5000, 10 mHz−100 kHz) was performed to evaluate the electrode resistance in each electrolyte. The intercalation of Li-ions into the graphite electrode was investigated using X-ray diffraction (XRD; Rigaku: RINT-TTR III). The surface condition of the electrode was confirmed by scanning electron microscope (SEM; Hitachi High-technologies: S-4800Type II). Analysis of SEI components was carried out by SEMenergy dispersive X-ray spectroscopy (EDX; Horiba: EMAXEvolution X-Max) and X-ray photoelectron spectroscopy (XPS; Nihondenshi: PS-9000MX).  , the peaks were observed at various potentials. The reduction peak observed only in the first cycle at around 1.6-1.0 V is thought to be due to decomposition reaction of free glyme which isn't coordinated with Li-ions. Since the amount of free glyme not coordinated to Liions is negligible, this reaction is considered to be observed only in the first cycle. Furthermore, the reduction peak could be confirmed also in the low potential region (1.0−0.005 V). This peak seems to be derived from the reaction between solvated Li-ions (Li(G4) + ) and graphite negative electrode. This reaction is generally confirmed only in the first cycle as SEI production reaction, but this system was also confirmed in the second and third cycles. Therefore, it was found that [Li(G4)][TFSI] can't produce favorable SEI on the graphite negative electrode. In general, reversible oxidation/reduction reactions found around 0.3−0.005 V couldn't be confirmed in this system. To prevent the decomposition of the electrolyte, the SEI formed in the CGB-10 electrode were introduced in the ionic liquids.   After SEI formation Furthermore, the intercalation of Li-ions into the graphite electrode was confirmed using XRD. Figure 5 shows the XRD patterns of (i) fresh CGB-10,  respectively. These results indicate that the SEI separated Li-ion and glyme, in addition to enabling the intercalation of Li-ions into the graphite electrode. Figure 6 shows SEM images of Fresh CGB-10 and SEI formed CGB-10 charged and discharged in 1 M LiPF 6 /EC-DMC and  Table I, we considered that these precipitates were decomposition products of [Li(G4)], not decomposition of [TFSI]. On the other hand, in the     This result revealed that ROCO 2 Li which was the decomposition products of the 1 M LiPF 6 /EC-DMC was particularly necessary to adapt the [Li(G4)][TFSI] as an electrolyte.

Conclusions
The graphite electrode in [Li(G4)][TFSI] achieved theoretical capacity by introducing stable SEI derived from 1 M LiPF 6 /EC-DMC. The capacity was identified as intercalation of Li-ions into the graphite electrode in ionic liquid electrolyte. From the results of XPS, it was found that ROCO 2 Li was required as SEI in order to adapt [Li(G4)][TFSI] as an electrolyte for LIBs. However, the SEI could not prevent the preferential intercalation of [N 2,2,1,(2O1) ] cations into the graphite electrode. This result suggest that SEI derived 1 M LiPF 6 /EC-DMC do not conform to full range ionic liquids. Consequently, in ionic liquids that do not undergo reversible charge and discharge reaction of Li-ions, the possibility of charge and discharge could be indicated by introducing the SEI beforehand. These results confirm the importance of SEI in ionic liquids.