The chemical structure of SiO ͑silicon monoxide͒ anodes has been analyzed using X-ray photoelectron spectroscopy ͑XPS͒. Vapor deposition was used to form SiO anodes on Cu film. XPS analysis was performed on anodes at each of three stages: after deposition, after initial charge, and after discharge. The results of this analysis were then evaluated in terms of the anode's respective electrochemical characteristics. It has been found that some Si remains oxidized in the full charge state and that lithium silicates are formed. The lithium silicates serve as a buffer with respect to changes in anode volume.The high energy density of lithium-ion rechargeable batteries would seem to make them especially promising candidates for meeting future requirements for large capacity batteries. However, the capacity of the material conventionally used for their anodes, graphite, is increasingly approaching its theoretical limit of 372 mAh/g, which would be insufficient for meeting the needs of future portable electronic equipment. 1,2 That is why much research has been devoted to the development of new anode materials, and substances which can absorb and retain lithium appear to be particularly promising in this respect. Silicon, which has a capacity ten times that of graphite, is one such substance. 3,4 Unfortunately, however, with silicon the expansion and contraction of lithium alloy during chargedischarge processes would lead to a pulverization of the active materials, and the resulting poor electrical contact between the current collector and the active materials would mean the degradation of capacity over a relatively small number of cycles. 5 As an alternative, SiO appears particularly promising because of its long cycle life. 6 It also has the advantage of absorbing and retaining a large quantity of lithium, which gives it a higher energy density than lithium-graphite. In order to determine the reasons for the good cycle characteristics of SiO, however, it is important to learn more about the chemical structure of SiO anodes. In this study, we have used X-ray photoelectron spectroscopy ͑XPS͒ to determine that chemical structure, and we have also investigated the electrochemical reaction of SiO with lithium.
ExperimentalA SiO thin film about 2 m in thickness was prepared on a flat Cu film by vapor deposition. The background pressure was 1 ϫ 10 −6 Pa, and the deposition pressure was 1 ϫ 10 −4 Pa. X-ray diffraction ͑XRD͒ analysis of as-deposited SiO thin film was performed with Rigaku, RINT-2050. SiO anodes cut from this twolayer film were incorporated into coin-type electrochemical cells having lithium metal counter electrodes. The electrolyte was 1 M LiPF 6 in ethylene carbonate ͑EC͒:diethyl carbonate ͑DEC͒ ͑30:70 by volume͒. The electrochemical performance of these coin cells was measured for a 1/40 C current rate during initial charge and discharge operations, which were performed for a 0-2.5 V range. After initial charge and initial discharge, the anodes were removed in an Ar atmosphere, washed a number of times with DE...
