The cycling performance of high-energy silicon (Si) based lithium-ion (Li-ion) battery is greatly hindered by the instability of the solid electrolyte interphase (SEI). Large volume change of Si during (de)-lithiation causes continuous cracking and re-formation of the SEI on the anode surface, eventually resulting in loss of Li inventory and extensive consumption of electrolyte. Our work aims to devise, ex situ, an artificial polymeric SEI that retains its integrity against the large volume expansion of Si (~300%) during lithiation, passivates the anode surface, and thus prolongs the cycling and calendar life of Si-based anodes. Electrophoretic deposition (EPD) was used to coat model silicon thin film surfaces with a layer of chitosan, an ionically conductive cationic polymer, with and without the addition of CH₃COOLi in the precursor solution. Morphological study of the coated surface at nano and macroscale via AFM-nanoIR and SEM-EDX show CH₃COOLi promotes conformal electrodeposition of chitosan. Electrochemical testing shows a boost in capacity retention and lower charge transfer resistance in the presence of the chitosan synthetic SEI. XPS and ATR-FTIR spectroscopy suggest that CH₃COOLi caps the -NH₂ group of the deposited chitosan via an amidation reaction which suppresses excess electroreduction of the cell electrolyte. Our work provides a pathway for controlling the chemistry and properties of the SEIs in batteries.