 Layered LiNi0.5−xCaxMn0.5O2 (0 ≤ x ≤ 0.2) cathode materials were prepared through a combination of co-precipitation and a solid-state method. The prepared cathode materials were investigated in detail by X-ray diffraction (XRD), Rietveld refinement, inductively coupled plasma, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), cyclic voltammetry and charge–discharge measurements. The results of XRD, Rietveld refinement, XPS and SEM measurements revealed that Ca-doping can increase the stability of the structure and lower the amount of Li/Ni cation mixing. Furthermore, Ca-doping was not observed to affect the morphology or oxidation states of the LiNi0.5Mn0.5O2. The electrochemical measurements showed that the pristine LiNi0.5Mn0.5O2 material has the lowest discharge capacity of 88.6 mAh g−1 between 3 and 4.5 V at a constant density of 0.2 C, which was improved 38% by doping with 3 mol% of Ca. Additionally, the capacity retention of the 3 mol% Ca-doping is 20% higher than that of the pristine LiNi0.5Mn0.5O2 material in the voltage range of 3.0–4.5 V. Furthermore, we investigated the source of the enhancement of the electrochemical properties from Ca-doping. The improvement may be attributed to increased structural stability, lowered Li/Ni cation mixing, decreased polarization, reduced migration resistance and faster lithium-ion diffusion.