Layered Li[Li0.16Ni0.21Mn0.63]O2 and Li[Li0.2Ni0.2Mn0.6]O2 compounds were successfully synthesized by radiated polymer gel (RPG) method. The effect of deficient Li on the structure and electrochemical performance was investigated by means of X-ray diffraction, X-ray absorption near-edge spectroscopy and electrochemical cell cycling. The reduced Ni valence in Li[Li0.16Ni0.21Mn0.63]O2 leads to a higher capacity owing to faster Li+ chemical diffusivity relative to the baseline composition Li[Li0.2Ni0.2Mn0.6]O2. Cyclic voltammograms (CV) and a simultaneous direct current (DC) resistance measurement were also performed on Li/Li[Li0.16Ni0.21Mn0.63]O2 and Li/Li[Li0.2Ni0.2Mn0.6]O2 cells. Li[Li0.16Ni0.21Mn0.63]O2 shows better electrochemical performance with a reversible capacity of 158 mA hg−1 at 1C rate at 20 °C.