Mixtures of Ionic Liquid in Combination with Graphite Electrodes: The Role of Electrolyte Additives and Li-salt

Presently, commercially available lithium-ion batteries use graphite based anodes in combination with organic carbonate (e.g. Propylene Carbonate, PC, Ethylene Carbonate, EC) electrolytes. In this kind of Li-ion batteries the solid electrolyte intephase (SEI) formation process on the surface of graphite is crucial since it strongly influence the performances of the batteries systems [1]. It is known that the use of electrolyte additives (e.g. containing vinylene groups) improves the design of the resulting SEI on the graphite and leads to more efficient cycling of the material [2]. For that, several types of additives have been already studied and tested and intense research is now focused on the optimization of their design for a more effective film-forming efficiency. Ionic Liquids (ILs), room temperature molten salts typically showing a very low vapor pressure, high thermal stability, wide electrochemical windows and good conductivity at room and sub-room temperatures [3-4]. These properties make them very attractive candidates for the use as electrolytes in electrochemical devices such as batteries, particularly to increase the safety and the operative temperature range. So far, different types of ILs have been already used in combination with graphite electrodes with promising results [5-7]. However, only few reports studied the SEI formation process on graphite electrodes when ILs are used in the electrolytes as well as the contribution of additives to the SEI formation in such electrlytes [8]. Recently, we investigated the role of the additive Vinylene Carbonate (VC) in ILs-based electrolyte. The results of our studies indicated that when ultrapure ILs are used as electrolytes in combination with graphite electrodes, the need of additives in the electrolyte solution is strictly related to the film-forming ability of the ILs [9] themselves. For instance, in electrolyte solution based on the ultrapure ionic liquid N-butyl-N-methylpyrrolidinium bis(trifluoromethansulfonyl)imide (PYR14TFSI) the use of VC appears to be indispensable because such IL does not display film-forming ability. To the contrary, in electrolyte solution based on the ultrapure N-methyl-Npropylpyrrolidinium bis(fluorosulfonyl)imide (PYR13FSI) the presence of VC was not strictly required because this IL displays film-forming ability. In order to investigate the film-forming ability of PYR13FSI and the possibility of using this IL as additive or co-solvent in pure IL-based solutions, we prepare different mixture of PYR13FSI -PYR14TFSI with and without VC. These solutions have been used in combination with the graphite electrode and their influence on the specific capacity, the cycling efficiency and the cycling stability of the electrodes have been investigated. As example, Fig. 1 shows the cyclic voltammetry at 50 μV sec of graphite electrode in 0.3 M LiTFSI + PYR14TFSI + 5%wt. VC (A); 0.3 M LiTFSI + PYR14TFSI [50%] PYR13FSI [50%] + 5%wt. VC (B) and 0.3 M LiTFSI + PYR13FSI + 5%wt. VC (C). Also the influence of two different Lithium salt (Lithium bis(trifluoromethansulfonyl)imide, LiTFSI and Lithium exafluophosphate, LiPF6) on the performance of graphite electrode in the mixtures of PYR13FSI PYR14TFSI has been investigated. These studies clearly evidence that the Li-salt strongly influence the performance of graphite electrode in combination with ILs-based solutions.