Lithium bis(oxalato) borate (LiBOB) was investigated as an additive to improve the performance of LiNi0.5Mn1.5O4 cathodes cycled to high voltage (4.9 V vs Li). Incorporation of low concentrations of LiBOB (0.25-1.0%, wt) results in a significant improvement in the capacity retention and cycling efficiency of LiNi0.5Mn1.5O4/Li cells. Electrochemical impedance spectroscopy indicates that the addition of LiBOB decreases cell impedance. Ex-situ surface analysis of cycled cells suggests that the cathode surface films are thinner in cells cycled with added LiBOB. The addition of LiBOB inhibits detrimental reactions of the electrolyte with the surface of LiNi0.5Mn1.5O4 cathodes at high voltage. (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.015202esl] All rights reserved.