In situ X-ray diffraction of Li(Mn 1.5 Ni 0.5 )O 4 was performed using a novel electrochemical cell based on coin cell hardware. The pristine material had a cubic spinel structure with a Ni 2þ oxidation state. As the cell was charged through its 4.75V plateau, a transition between spinels with Ni 2þ , Ni 3þ , and Ni 4þ 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 Ni 2þ 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.A global demand for portable, high power energy storage systems has made the development of improved lithium ion batteries (LIBs) an important research priority. 1 Consumer electronics, electrical vehicles, power grid regulation, and remote sensing applications all stand to benefit from LIBs with higher power, greater capacity, and longer life. To achieve this goal the structural behavior of the active electrode materials must be well understood at multiple levels from lattice, to particle, to composite. How the structure changes as the material is reversibly intercalated or alloyed with lithium as well as how the structure evolves over the course of many lithiation/delithiation cycles are important considerations in the design, development, and selection of active materials.Among current cathode materials for lithium ion batteries those based on the LiMn 2 O 4 spinel structure are advantageous in their nontoxicity, low cost, and ease of preparation. 2-7 The spinel structure of this material offers a 3D diffusion pathway for lithium intercalation/ deintercalation. Lithiation occurs reversibly as per the following reactionHowever, use of this material faces several challenges including somewhat limited rate capabilities, 8 dissolution of Mn 2þ ions into the electrolyte, 9,10 and a strong Jahn-Teller distortion at low potentials. 10-12 At low potentials the Mn 3þ /Mn 4þ redox couple produces Mn 3þ ions which may participate in the following disproportion reaction at the interface between the particle and the electrolyte 2Mn 3þ solid ! Mn 4þ solid þMn 2þ electrolyte At high degrees of lithiation, LiMn 2 O 4 will undergo a Jahn-Teller distortion that forces it from a cubic to tetragonal spinel structure as Li 2 Mn 2 O 4 is formed. 10-12 Any regions of material in this tetragonal phase, most likely ...