Composites that consisted of Li2MnO3 and LiMn0.5Ni0.5O2 were prepared by a co-precipitation method followed by high-temperature calcination. Several preparation parameters including chemical compositions, calcination temperature, and periods of calcination were systematically investigated to uncover the relationships between structure and electrochemical properties. Increasing the lithium concentration led to an increased content of the layered-type component Li(Mn,Ni)O2, while the cation mixing in the layers decreased. Comparatively, high-temperature calcination increased the content of Li2MnO3 component. Alternatively, periods of calcination had little impacts on the structure of composites, but significantly altered the morphology and particle size. As a consequence of the structural modifications by these preparation parameters, the composites were tailored to have an optimum electrochemical performance as the electrodes for lithium-ion batteries.