The present rechargeable lithium-ion battery systems commonly use small amount of electrolytes based on suitable lithium salts (generally LiPF₆) and organic solvents (generally alkylcarbonates, such as EC, DMC), which represent a major concern for device safety due to the flammable and volatile nature of these organic liquids. The use of these organic carbonates electrolytes allows the realization of high-performance batteries, but due to its flammability, their use also poses serious safety risks and strongly reduces the battery operative temperature range. Therefore, alternative electrolytes have been proposed and tested in the last decade [1].

Ionic liquids doped with a lithium salt are alternative electrolytes for lithium-ion batteries that offer some advantages compared to organic carbonates based liquid electrolytes. Ionic liquids display a low vapor pressure which leads to nonflammability. The main drawback of ionic liquid-based electrolytes is the low lithium-ion conductivity due to relatively high viscosity of ionic liquids. A lot of research has been devoted to developing ionic liquids with low viscosity. However, so far, these ionic liquids could be obtained only at the expense of lower thermal and/or electrochemical stability. In recent years, mixtures of organic carbonates-based electrolytes and ionic liquids have been proposed as a solution to overcome the lithium-ion transport limitations of ionic liquid-based electrolytes while improving safety due to a lower flammability than that of organic carbonates electrolytes [1, 2].

Herein, we report the results of physical-chemical and electrochemical investigations performed on mixed electrolytes based on an ionic liquid, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMP-TFSI), and organic electrolytes (LiTFSI in EC/DMC) with nanostructured silicon-anode for lithium-ion batteries. The ionic conductivity, lithium-ion transference number, viscosity and electrochemical stability windows of all different ionic liquid content mixtures were investigated and compared with carbonates-based electrolyte. The specific capacity and cycling stability of the nanostructured silicon-anode were investigated at different C-rates at room temperature. A reversible capacity of 3480 mAh g^-1 (of Si) at C/10 and 1600 mAh g^-1 at 5C is obtained with cells having electrolyte mixture with a composition of 1:1. This study indicates that safety and electrochemical performance of the Si-anode for Li-ion battery can be improved by using mixed ionic liquid and carbonates-based electrolytes.

[1] A. Guerfi, M. Dontigny, P. Charest, M. Petitclerc, M. Lagacé, A. Vijh, K. Zaghib, J. Power Sources 195 (2010) 845-852.

[2] R.-S. Kühnel and A. Balducci, J. Phys. Chem. C 118 (2014) 5742-5748.