A series of Al-substituted spinel powders LiNi 0.5−x Al 2x Mn 1.5−x O 4 (0 ≤ 2x ≤ 1.0) have been prepared and the effects of Al concentration on the structural, electrochemical and thermal properties are investigated. The XRD patterns show that impurity arises when 2x ≥ 0.6. The FTIR and Raman spectra indicate that the introduction of Al in the LiNi 0.5 Mn 1.5 O 4 increases the disordering degree of Ni/Mn ions, changing the spinel structure from P4 3 32 to Fd3¯m. Cyclic voltammetry tests show that the voltage step between Ni 2+ /Ni 3+ and Ni 3+ /Ni 4+ have a sudden leap at 2x = 0.075, responding to the structural difference of the spinels. The Al concentration is optimized in the range of 0.05 ≤ 2x ≤ 0.1, in which the cyclic stability and rate capability of the LiNi 0.5−x Al 2x Mn 1.5−x O 4 spinels are significantly improved. At room temperature the LiNi 0.45 Al 0.10 Mn 1.45 O 4 presents the best cycle performance with the capacity retention of 95.4% after 500 cycles at 1C rate, and the best rate capability with the discharge capacity of 119 mAh g −1 at 10C rate, which is about 93.7% of its capacity at 0.5C. The thermal properties of the spinels have been tested by C80 calorimeter and the results show that introduction of Al in LiNi 0.5 Mn 1.5 O 4 can effectively suppress the exothermic reaction below 225 °C, thus improve the safety of the high voltage cathode material.