The electrochemical behaviors of a silicon thin-film electrode in organic lithium salt solution were explored with focus on irreversible reactions of the first lithium charge and discharge cycling by using electrochemical quartz crystal microbalance (EQCM) combined with various electrochemical techniques. A considerable increase in mass of the silicon electrode was observed during lithium charging even before lithium absorption into the electrode, which is ascribed to the buildup of electrolyte reduction products on the silicon surface. Galvanostatic charge-discharge experiments combined with ac impedance spectroscopy demonstrate a significant overpotential growth and an aggravating capacity for the lithium charge and discharge cycling, and suggest they are due to the sedimentation of electrolyte reduction product. Additives containing alkoxy silane functional groups were evaluated as a passivation agent for lithium rechargeable batteries utilizing a silicon anode. The presence of additives in electrolyte suppressed the mass accumulation to the silicon electrode caused by irreversible electrolyte reductions and improved the electrode for cycle life. Electrochemical analyses associated with EQCM as a function of the number of alkoxy functional groups of the additives illustrate that the silicon electrode is passivated by chemical reaction of the alkoxy silane functional group of the additives with hydroxyl groups at the electrode/electrolyte interface, and this passivation improves the cycle life. (C) 2008 The Electrochemical Society.