Mg-substituted LiNi0.5−x Mg x Mn1.5O4 (x = 0, 0.02, 0.05, and 0.10) are prepared via a spray pyrolysis method followed by a two-step heating at 600 °C for 6 h and 800 °C for 8 h in air. The compositions, morphology, and crystalline structure are studied with ICP-OES, SEM, XRD, and NPD, respectively. The XRD and NPD results are refined with Rietveld method to reveal the crystallography and phase composition of the prepared samples. The electrochemical properties of the obtained samples are investigated with results of capacity retention and cyclic voltammetric studies. Samples synthesized in this study manifest promising rate capability that may be due to the presence of major F d 3 ¯ m phase with a minor amount of P4332. Among the prepared samples, LiNi0.4Mg0.1Mn1.5O4 demonstrates the lowest capacity fading rate among the prepared samples at 0.1 ∼ 5C rates when cycled between 3.5 and 5.1 V at temperatures of 30 and 60 °C.