High voltage spinel LiNi(0.5)Mn(1.5)O(4) is a very promising cathode material for lithium ion batteries that can be used to power hybrid electrical vehicles (HEVs). Through careful control of the cooling rate after high temperature calcination, LiNi(0.5)Mn(1.5)O(4) spinels with different disordered phase and/or Mn(3+) contents have been synthesized. It is revealed that during the slow cooling process (<3 °C min(-1)), oxygen deficiency is reduced by the oxygen intake, thus the residual Mn(3+) amount is also decreased in the spinel due to charge neutrality. In situ X-ray diffraction (XRD) demonstrates that the existence of a disordered phase fundamentally changes the spinel phase transition pathways during the electrochemical charge-discharge process. The presence of an appropriate amount of oxygen deficiency and/or Mn(3+) is critical to accelerate the Li(+) ion transport within the crystalline structure, which is beneficial to enhance the electrochemical performance of LiNi(0.5)Mn(1.5)O(4). LiNi(0.5)Mn(1.5)O(4) with an appropriate amount of disordered phase offers high rate capability (96 mAh g(-1) at 10 °C) and excellent cycling performance with 94.8% capacity retention after 300 cycles. The fundamental findings in this work can be widely applied to guide the synthesis of other mixed oxides or spinels as high performance electrode materials for lithium ion batteries.