The structural changes of the Cr-doped and undoped LiMn1.5Ni0.5O4 crystallized in the initial disordered phase with space group of Fd 3 ¯ m have been investigated during the galvanostatic charge/discharge process at C/24 rate by using in-situ X-ray diffraction (XRD) measurements. The de-intercalation of lithium proceeds through a series of first-order phase transitions with two regions of two-phase coexistence. The phase diagram is analyzed and discussed, together with the differences among different results reported in the literature to distinguish between general intrinsic properties of spinel and sample-dependent properties due to deviation from stoichiometry, strain field effects, degree of cation ordering, and out-of-equilibrium effects. We argue that the Cr-doping stabilizes the lattice without impacting the capacity significantly, but decreases the energy density.