Inner Helmholtz Plane constructing LiF-rich solid electrolyte interphase of silicon anodes

Abstract

The electrolyte components are inevitably reduced and decomposed on the anode surface to form solid electrolyte interphase (SEI) during the first electrochemical reaction, and the composition of the SEI largely determine the cycle stability of the silicon anode. The competitive specific adsorption in the Inner Helmholtz Plane (IHP) adjacent to the electrode determines the derivatized reduction of the electrolyte, which in turn affects the growth characteristics of the SEI. Herein, the synergistic effect of spray drying and gas-phase fluorination method was cleverly used to construct uniform fluorocarbon (CF_x) barriers in porous silicon/carbon composites to tune the composition of the IHP. The introduction of CF_x effectively promotes the aggregation of Li⁺ in the IHP, where C-F bond is electrochemically transformed into LiF and occupy the dense layer of SEI. More importantly, the preferentially generated LiF benefits the specific adsorption behavior of hexafluorophosphate (PF₆^-) and fluoroethylene carbonate (FEC), further forming IHP-derived LiF at the inorganic interface. The robust LiF-rich skins are beneficial of accelerating Li⁺ transfer and alleviating the elastic-plastic deformation of silicon anodes originating from large volume change upon cycling. As a result, the silicon-based composite exhibits a remarkable cycling performance with a reversible capacity of 919 mAh g⁻¹ (a high capacity retention of 90 %) after 400 cycles at 3 A g⁻¹ and high rate capability of 721 mAh g⁻¹ at 5 A g⁻¹. This work may offer a feasible approach for fluorine-directing SEI formation of the electrode sides to deeply explore the interfacial chemistry and volume effects of Si/C anode.