Electrochemical activity in high-voltage spinel LiNi 0.5 Mn 1.5 O 4 (LNMO) is strongly affected by the disordering of Ni/Mn and the presence of Mn 3+ ions. However, understanding the effect of the Ni/Mn disordering or the presence of Mn 3+ ions on electrochemical properties is not trivial because disordering is typically coupled with the presence of Mn 3+ ions. Here, we demonstrate for the first time that the doping of Li instead of Ni increases Ni/Mn disordering, which is decoupled from the presence of Mn 3+ ions. The resultant material has a particle size of~1-2 μm and can achieve 120 mAh g − 1 at 10 C for 50 cycles and further deliver about 60 mAh g − 1 even at a rate of~60 C (1 min discharge). Superior electrochemical performance is achieved by increased solid-solution phase transition behavior, which is caused by increased Ni/Mn disordering during delithiation. By decoupling, we find that the electrochemical properties in LNMO strongly depend on the phase transformation behavior and that the Ni/Mn disordering, rather than Mn 3+ ions, affects the phase transformation by increasing the solid-solution reaction. The fundamental understanding gained from this work could be applied to the development of other phase-separating compounds to improve their electrochemical performance. INTRODUCTIONFor a lithium-ion battery, a high energy density is an essential requirement for novel applications such as plug-in hybrid electric vehicles and electric vehicles. Increasing the redox potential of cathode materials is an effective way to achieve high energy density in the cell. For this purpose, high-voltage spinel LiNi 0.5 Mn 1.5 O 4 is a promising cathode material for LIBs 1-3 because it has a high redox potential of about 4.7 V, which makes its energy density (650 W h kg − 1 ) 20% higher than that of the conventional LiCoO 2 . However, the electrochemical properties of LNMO spinel depend on several factors, such as its structure, 3,4 the quantity of Mn 3+ ions, 5,6 the particle size 2,7 and the morphology of particles. 8 Among these factors, the structure of the spinel has a critical influence on its electrochemical performance. 9-11 The LNMO structure is dictated by the ordering of Ni and Mn at two octahedral sites, which results in two spinel forms: disordered and ordered. The Ni and Mn in the ordered spinel are well ordered in two distinct octahedral sites, 4b sites for Ni and 12d sites for Mn, whereas the Ni and Mn in the disordered spinel are randomly distributed in 16d octahedral sites. 1,12 In the ordered spinel, the ordering of Ni/Mn exists without Mn 3+ ions because a second annealing process at o700°C simultaneously leads to the ordering of Ni/Mn on two distinct octahedral sites and the oxidation of Mn 3+ ions into Mn 4+ ions. However, the disordered spinel shows both the disordering of Ni/Mn and the presence of Mn 3+ ions. The correlation