It has long been recognized that silicon, with a gravimetric capacity of nearly 11 times that of graphite, better safety and abundance, is the most obvious choice for the next-generation anode for lithium batteries. Silicon particles, in the anode, are covered by a thin oxide film which stops, or significantly slows, the reaction with oxygen in the air. During cell assembly, the silicon anode may react spontaneously with the electrolyte. The goal of this work was to study the spontaneous reaction between oxide-covered silicon with lithium-battery electrolytes, the kinetics of the reactions and the composition and impedance of the film formed. The passivating film has a very high impedance, it consists of polymers containing C-O, C=O, C-F-x and O-C-F moieties, SiFx, SiOxFy, some SiO2 and LiF. It was concluded that cells containing silicon anodes should be charged as soon as possible after electrolyte-filling to create a less resistive and more stable SEI that will slow further reaction of the silicon anode with the electrolyte. (C) 2019 The Electrochemical Society.