We report the synthesis of silicon nanowires using the supercritical-fluid-liquid-solid growth method from two silicon precursors, monophenylsilane and trisilane. The nanowires were synthesized at least on a gram scale at a pilot scale facility, and various surface modification methods were developed to optimize the electrochemical performance. The observed electrochemical performance of the silicon nanowires was clearly dependent on the origination of the surface functional group, either from the residual precursor or from surface modifications. On the basis of detailed electron microscopy, X-ray photoelectron spectroscopy, and confocal Raman spectroscopy studies, we analyzed the surface chemical reactivity of the silicon nanowires with respect to their electrochemical performance in terms of their capacity retention over continuous charge-discharge cycles.