A series of metal-doped LiMn1.5Ni0.5O4 (metal = Co, Al, Cu and Mg) positive electrode materials for lithium ion batteries were synthesized and their structural changes during the galvanostatic charge/discharge process at C/24 rate were investigated by using in situ X-ray diffraction (XRD) measurements. The phase diagram shows that similar series of first-order phase transitions with two regions of two-phase coexistence are observed during intercalation/de-intercalation of lithium among all the doped cathode materials. However, minor differences of the phase evolution and the electrochemical properties point to the different roles of the dopant ions. 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 the degree of cation ordering, out-of-equilibrium effects, electro-negativity and radii of the dopant ions. Among the metal-substituted samples, we argue that the Co-doping is the most promising approach with improved electrochemical property.