Stable cycling of Si nanowire electrodes in fluorine-free cyano-based ionic liquid electrolytes enabled by vinylene carbonate as SEI-forming additive

Abstract

Herein, the mixture of the fluorine-free cyano-based ionic liquid N-butyl-N-methylpyrrolidinium tricyanomethanide (Pyr14TCM), lithium dicyanamide (LiDCA) (1:9 salt:IL mole ratio) and 5 wt% vinylene carbonate (VC) is proposed as an electrolyte for the stable electrochemical alloying of silicon nanowire (Si NW) anodes. Detailed electrochemical characterization of the electrolyte (long-term galvanostatic cycling and impedance tests of Si NW-Li half-cells) demonstrates a remarkable cycling performance of the Si anode delivering 1500 mAhg−1 after 500 cycles with 99.5% Coulombic efficiency. The electrode/electrolyte interface is thoroughly investigated via scanning electron microscopy (SEM), energy dispersive X-ray (EDX) mapping, and X-ray photoelectron spectroscopy (XPS). The postmortem analysis reveals the key role of VC in controlling the IL decomposition, resulting in a bilayer solid electrolyte interphase (SEI) formation. The inner layer is mostly composed of graphitic carbon serving as a conductive coating for Si, and inorganic compounds such as Li3N providing high Li-ion conductivity. The outer-layer, is rich in polymeric species ensuring the good mechanical stability and flexibility to withstand the extreme volume change of Si during de-/alloying process, thus explaining the observed prolonged cycling performance.