The spinel LiMn1.5Ni0.5O4 has been prepared at low temperature using a new sol-gel process. X-ray diffraction analysis reveals that the obtained material presents a cubic spinel structure with a unit cell parameter lower than that of stoichiometric LiMn2O4. X-ray photoelectron spectroscopy confirms the divalent state of doped nickel in the material. Furthermore, LiMn1.5Ni0.5O4 compound can intercalate a second lithium both chemically and electrochemically, leading to the formation of a new Li2Mn1.5Ni0.5O4 material with the same cubic spinel structure. This material presents only one plateau at around 3 V vs. Li/Li+ with a large discharge capacity of 160 mAh/g and fairly good cyclability. This result is in contrast to the undoped Li2Mn2O4, where two plateaus at 4 and 3 V are observed because of the occurrence of a structural transition caused by the Jahn-Teller effect. The structure and the electrochemical behavior of Li1+xMn1.5Ni0.5O4 are investigated and compared to those of LiMn1.5Fe0.5O4 material prepared by the same sol-gel process.