Effect of Cation Structure Modification of Ionic Liquids for Lithium-Ion Batteries

Ionic liquid (ILs) have received much attention next to traditional carbonates based electrolytes, due to their high oxidation potential (above 5V vs Li/Li⁺ for typical pyrrolidinium based ILs) ^[1-2], excellent thermal stability^[3], negligible vapor pressure and non-flammable nature^[3]. Nevertheless, the high price of ILs represents a major drawback for the commercialization of these electrolyte materials. Furthermore, for the use of graphite anodes cells, severe reductive decomposition of ILs can be triggered by an intercalation of cation of the ILs into graphite layer^[4], which results in graphite exfoliation. As a result, less reversible capacity can be achieved during the charge/discharge process.

In recent years, many approaches have been pursued to optimize (1-butyl-1-methylprrolidinium bis(trifluoromethane-sulfonyl)imide (Pyr₁₄TFSI)) based electrolyte to obtain a suitable electrolyte formulation for application in lithium-ion batteries^[4-5]. More attention has been paid to accommodate the incompatibility with carbon anodes, such as mixture use of ILs and organic solvents^[1] , addition of certain amount of SEI additives (e.g. VC^[6-7],  ClEC^[7-8], FEC and ES^[6-7]) and substitution of conductive salts^[4-5]. The basic strategy is to form an ideal surface layer on graphite^[4], which can offer sufficient ability to prevent the intercalation of the cation of ILs and avoid a continuous electrolyte decomposition of electrolyte. In order to enrich the knowledge of the ionic liquid performance in lithium ion batteries, the concept of this work is set on IL with organic functional group modifications. The structure medication is considered to be an easy way for synthesis approaching and favorable for fast change of physical and chemical properties.

In this work, we synthesized a series of ester group modified ILs from Pyr₁₄TFSI, based on our computational screening results. The synthesized ILs methyl-methylcarboxymethyl prrolidinium bis(trifluoromethane-sulfonyl)imide (MMMPyrTFSI)) and   methyl-propylcarboxymethyl prrolidinium bis(trifluoromethane-sulfonyl)imide (MPMPyrTFSI)) show high oxidation stability (ca 5.4 V vs Li/Li⁺). TGA measurements show that modified ILs are thermally stable above 300^oC. Moreover, electrolytes based either on MMMPyrTFSI or MPMPyrTFSI show excellent electrochemical performance in Li/LiFePO₄cells, display a stable reversible capacity of 150 mAh/g at 1C and 110 mAh/g at higher rate of 5C. For the graphite anode, MPMPyrTFSI based electrolyte shows stable capacity of 330 mAh/g at 0.1C with an efficiency of 99% during the 100 cycles. These interesting results illustrate cation modification of ILs as an effective way for improved performance of ILs in lithium-ion batteries.

References

 

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