In situ X-ray diffraction of Li(Mn1.5Ni0.5)O-4 was performed using a novel electrochemical cell based on coin cell hardware. The pristine material had a cubic spinel structure with a Ni2+ oxidation state. As the cell was charged through its 4.75V plateau, a transition between spinels with Ni2+, Ni3+, and Ni4+ oxidation was observed. As the oxidation of the nickel increased, the lattice parameter of the corresponding spinel diminished. During discharge, the spinel reversed its phase changes until only the Ni2+ spinel was observed. As the discharge potential reached a plateau at 2.75V a tetragonal spinel phase was formed, which upon subsequent cell charging was completely converted back to a cubic spinel phase. Lattice parameter changes of each phase were calculated and showed a characteristic strain release during phase changes. After 15 full cycles the transition between cubic spinels was no longer complete and the formation of the tetragonal spinel phase was no longer detected. This suggests a gradual change from an ordered to disordered MNO structure and lithium trapping in the active material. These cycle-induced changes to phase transition behavior can be related to capacity fade and overall cell performance. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3596376] All rights reserved.