The effect of synthesis temperature on the structural and electrochemical characteristics of the layered-spinel composite cathode system xLi[Li0.2Mn0.6Ni0.2]O2–(1 − x)Li[Mn1.5Ni0.5]O4 (0 ≤ x ≤ 1) has been investigated. With a joint neutron diffraction (ND) and X-ray diffraction (XRD) Rietveld refinement method, the composition and weight percent variations of the layered and spinel phases in this composite cathode system have been obtained as a function of x and synthesis temperature. While no significant composition and weight percent variations are found with the synthesis temperature, the electrochemical characteristics of both the layered and spinel phases in the composites are significantly affected by the synthesis temperature. In contrast to the layered sample (x = 1), the capacity of the layered phase in the composites increases with decreasing synthesis temperature due to an increase in surface area. Conversely, the effect of synthesis temperature on the spinel phase is similar in both the spinel sample (x = 0) and the composite samples. However, the lower synthesis temperature increases the cation ordering in the 16d octahedral sites of the spinel phase, which changes the voltage profiles below 3 V due to the decrease in the lattice distortion during lithium ion insertion into the empty 16c octahedral sites.