Spherical LiNi0.5Mn1.5O4 (LNMO) cathode material gradient-doped with Mg is successfully synthesized via a co-precipitation method. The average Mg doping concentration in the LNMO is 2 mol% with 10 mol% on the particle surface and a gradual reduction to 0% in the core. The Mg gradient-doped (GD) LNMO shows an improved electrochemical performance with discharge capacities of 121.5 and 91.4 mAh g−1 at discharging rates of 0.1 and 4.0C, respectively. The corresponding values for a pristine LNMO sample are 125.7 and 80.8 mAh g−1. After 80 charge/discharge cycles at room temperature, the Mg(GD)-LNMO retains 92% of the initial capacity, compared with a retention rate of 80% for the pristine LNMO. A high-temperature (55°C) cycling test shows that the Mg(GD)-LNMO has a lower loss of capacity (8%) compared with the pristine LNMO (24%) after 20 cycles. Electrochemical impedance spectroscopy (EIS) reveals that the Mg-rich surface of Mg(GD)-LNMO suppresses the reaction of the electrolyte with the LNMO and decreases the total resistance. In addition, gradient-doping LNMO with Mg can also improve Li ion diffusion based on results of the potentiostatic intermittent titration technique (PITT). Finally, differential scanning calorimetry (DSC) analysis shows that the exothermic peak of the Mg(GD)-LNMO is shifted at higher temperatures and that the amount of heat is decreased by comparison with the values for pristine LNMO.