The reaction of an electrolyte (1 M LiPF6 in ethylene carbonate/dimethyl carbonate/diethyl carbonate, 1:1:1) with the surface of LiNi0.5Mn1.5O4 at various voltages (4.0-5.3 V vs Li) was investigated. Coin cells (Li/LiNi0.5Mn1.5O4) stored at low voltage (4.0-4.5 V vs Li) show low residual current, and ex situ analysis of the LiNi0.5Mn1.5O4 surface by X-ray photoelectron spectroscopy (XPS) and IR spectroscopy suggests low concentrations of electrolyte decomposition products. Coin cells (Li/LiNi0.5Mn1.5O4) stored at high voltage (4.7-5.3 V vs Li) show increased residual current, and ex situ analysis of the cathode surface by XPS and IR supports the presence of poly(ethylenecarbonate). (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3428515] All rights reserved.