Producing high-performing silicon anodes by tailoring ionic liquids as electrolytes

Nanostructured silicon is a promising anode for the next generation of high-energy lithium ion batteries. The challenge for implementation of Si anode is the control of the continuous chemical reactivity at the electrode/electrolyte interface during lithiation and delithiation. Given their relevant physicochemical properties such as high stability, good transport properties and nonvolatility, ionic liquids can potentially alleviate the instability of the solid electrolyte interface layer due to the large volume changes of Si upon cycling. Since the properties of ionic liquids are modulated by the anion and cation, or both, a suitable selection must be made for each application. Here, we report the electrochemical performance of triethyl-n-pentylphosphonium bis(fluorosulfonyl)imide [P₂₂₂₅][FSI] and bis(fluorosulfonyl)imide N-methyl-N-butylpyrrolidinium [BMPYR][FSI] ionic liquids as electrolyte solvents for silicon/poly(acrylonitrile), Si/PAN, composite electrode. After 1000 charge/discharge cycles, these composite anode-ionic liquid systems exhibit a specific delithiation capacity of approximately 1000 mAh•g^{− 1} at 1.0 A·g⁻¹ with a Coulombic efficiency approaching 100%. This demonstrates the superior performance of ionic liquids compared to classical organic alkyl carbonate solvent-based electrolytes and that are also the best among the reported state-of-the art ILs for silicon electrodes.