Novel composite cathode materials, xLi2MnO3–(1−x)LiNi2/3Co1/6Mn1/6O2 (where x = 0.3, 0.5, and 0.7), were synthesized by sol–gel route and characterized by advanced techniques for rechargeable Li-ion battery applications. Phase purity of the composites was examined by XRD as well as Raman spectroscopy and the studies revealed good crystallinity and the formation of pure composite phases with monoclinic (C2/m) and hexagonal (R3m) crystal structures for Li2MnO3 and LiNi2/3Co1/6Mn1/6O2, respectively. Polyhedral agglomerates seen in the scanning and transmission electron microscopic images elucidated the better electrochemical properties of the composites. Valence states of transition metals in the composites were examined by X-ray photoelectron spectroscopy and the analysis suggested predominant oxidation states of Ni, Co, and Mn as 2+, 3+, and 4+, respectively. Galvanostatic charge–discharge tests, performed at different C-rates between 2.0 and 4.8 V, indicated high discharge capacity (∼250 mAh g−1), good rate capability, and excellent cycleability of the composite with x = 0.5 compared to the composites with x = 0.3 and 0.7. In-situ Raman spectroscopic studies revealed the activation of Li2MnO3 component in all composite cathode materials during the first cycle charging process with structural stability thereby enhancing performance of the composite with x = 0.5. These results demonstrated the feasibility of using 0.5Li2MnO3-0.5LiNi2/3Co1/6Mn1/6O2 composite as advanced cathode for high power Li-ion batteries.