We have investigated the electrochemical characteristics of a series of high capacity xLi2MnO3-(1-x)Li(Mn0.375Ni0.375Co0.25)O2 (0.0 ≤ x ≤ 1.0) integrated cathodes. Among several interrelated factors (viz. nominal molar content of Li2MnO3 and Li(Mn0.375Ni0.375Co0.25)O2 constituents, activation of Li2MnO3 component, crystallinity of the particles etc) an optimum particle size is argued to be most critical to yield better electrochemical performance of the synthesized cathodes. Through X-ray diffraction in conjunction with micro-Raman spectroscopy and high resolution transmission microscopy analyses we have demonstrated that with the increase of nominal Li2MnO3 contents, the size of the ordered nano-domains (inside the active matrix) and the average size of the composite cathode particles increase systematically. The size of the ordered nano-domains, cathode particles, and electrochemically triggered layer to spinel phase transformation influence the electrochemical characteristics of these cathodes. The average particle size of 0.5Li2MnO3- 0.5Li(Mn0.375Ni0.375Co0.25)O2 particles have been systematically varied by tuning the calcination time temperature combination. The optimized cathode yields a discharge capacity ∼ 300 mAhg−1 with capacity retention about 96% after 50 charge-discharge cycles at 10 mAg−1 rate. The cathode with optimal particle size also exhibits a decent rate capability with room temperature discharge capacity ∼ 200 mAhg−1 at 300 mAg−1 rate.