Effects of trimethoxy methyl silane additive on solid electrolyte interphase (SEI) layer formation on nanocrystalline Si thin-film electrode in 1 M LiPF6/ethylene carbonate:diethyl carbonate(1:1) electrolyte were studied using attenuated total reflection Fourier transform infrared (IR) spectroscopy. IR analyses indicated that silane effectively protected the Si surface, which produced a stable SEI layer composed of organics with alkyl carbonate and carboxylic acid metal salt functionalities, and PF-containing species, leading to a stable cycling at 0.1-1.5 V vs Li/Li+ delivering >2400 mAh/g over 200 cycles. The film surface obtained after cycling without silane, however, showed primarily PF-containing species, which is responsible for inefficient passivation of the Si surface, resulting in a rapid capacity fade.