LiXMg0-1,Ni0.4Mn1.5O4 spinel (P4(3)32) was chemically and electrochemically lithiated in the range 1 < x less than or equal to 2.25 and subjected to detailed X-ray and neutron diffraction analysis to understand the electrochemical behavior in the 3 V region. Extensive migration of Ni and Mn during lithium insertion was found, resulting in the disappearance of the initial Ni-Mn ordering and the formation of Ni-rich and Ni-poor domains, leading to two Jahn-Teller-distorted tetragonal phases with different Ni:Mn ratios. Such extensive Ni and Mn migration was not known for these spinels, and strongly influences the initial cycling behavior. The newly formed tetragonal phase with a Ni:Mn ratio of similar to0.07 has a higher cyclability and a higher capacity, and is therefore suggested to have a favorable composition for this intercalation range, as is confirmed in the literature. In addition, lithium is found to occupy multiple positions inside the distorted oxygen octahedron of this phase, a finding previously known only for lithium in anatase TiO2.