The cycling performance of Li/LiNi0.5Mn1.5O4 cells with 1.0 M LiPF6 in ethylene carbonate (EC)/ethyl methyl carbonate (EMC) (3:7) with and without added dimethyl methylphosphonate (DMMP) (0.5-1.0%) was investigated. Addition of DMMP resulted in improved capacity retention during cycling to high voltage. (4.9 V vs Li). Ex-situ surface analysis of LiNi0.5Mn1.5O4 electrodes after cycling via Scanning Electron Microscopy (SEM), X-ray Photoelectron Spectroscopy (XPS) and Infrared (IR) spectroscopy suggest that addition of DMMP inhibits electrolyte decomposition on the surface of the cathode. Addition of DMMP also inhibits the dissolution of Mn from LiNi0.5Mn1.5O4 particles stored in electrolyte at 85 degrees C. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.077212jes] All rights reserved.