Anion-Regulated Helmholtz Layer Enables Rigid-Yet-Flexible SEI for Ultrastable Silicon Anodes in Lithium-Ion Batteries

Silicon anode volume expansion causes continuous solid electrolyte interphase (SEI) fracture, hindering their practical use. Competitive anion adsorption in the adjacent Helmholtz plane critically governs electrolyte reduction, thereby influencing the growth and properties of the SEI. We propose a polyethylenimine and phytic acid (PEIPA) composite strategy, forming a self-assembled nanolayer on silicon that combines molecular-scale anion regulation with mechanical reinforcement. The PEIPA layer selectively adsorbs PF₆^- anions, creating an anion-enriched Helmholtz layer at the Si/electrolyte interface, promoting LiF-rich SEI formation. Simultaneously, its flexible polymer matrix enables a "rigid-yet-flexible" composite SEI with a high Young's modulus (2.45 GPa) and a 21-fold increase in fracture energy. This interfacial PEIPA film significantly improves electrochemical performance, especially the rate capability and cycle life. At 10 C (42 A g^-1), the modified Si anode delivers 2000 mAh g^-1, far exceeding literature values. After 800 cycles at 0.5C, it retained 1318.5 mAh g^-1 (81.2% retention). Full cells with NCM811 cathodes further validate these advantages. This anion-regulation strategy employing functional nanolayers presents a promising avenue for optimizing SEI on Si-based anodes.