2D Solid‐Electrolyte Interphase Built by High‐Concentration Polymer Electrolyte for Highly Reversible Silicon Anodes

Abstract Silicon anodes with a high capacity of 4200 mAh g −1 and a low potential of 0.3 V (vs Li + /Li) enable lithium‐ion batteries with improved energy density. However, the thickened 3D solid‐electrolyte interphase (SEI) formation on Si particles in the liquid electrolytes consumes the electrolyte/active Si and blocks the Li + /e − transport, resulting in fast capacity fading. Herein, a high‐concentration polymer electrolyte (HCPE) is designed to build 2D SEI on the Si anode surface instead of the particles, which accommodates the volume change and maintains the continuous Li + /e − transport pathways as well. The retarding effect of NO 3 − lowers the polymerization rate of 1,3‐dioxolane (DOL), enabling 6 m LiFSI dissolution. The high concentration of LiFSI takes part in constructing the solvation structure and pulls the DOL away, reducing the decomposition of DOL and poly‐DOL (PDOL) and inducing the generation of a LiF‐ and Li 3 N‐rich SEI with high mechanical strength and fast Li + transport capability. As a result, the cell using HCPE delivers a high capacity of 1765 mAh g −1 at 2C and maintains a high capacity of 2000 mAh g −1 after 100 cycles at 0.2C, which is superior to that of a liquid electrolyte (617 mAh g −1 ) and a low‐concentration polymer electrolyte (45 mAh g −1 ).