Lithium-rich manganese(IV) oxide Li2MnO3 has hardly any activity as the cathode active substance of lithium ion batteries (LIBs) but its reversible capacity can be greatly improved by introducing oxygen deficiencies. After the solid-state heat treatment of nanocrystalline Li2MnO3 by sodium borohydride (NaBH4), the resulting Li2MnO3-delta crystallites comparatively acquire distinguishable appearances in color and shape and slight differences in surface composition and lattice structure. As a LIB cathode within the potential range of 2.5-4.7 V, at 20 mA g(-1) pristine Li2MnO3 gives the specific discharge capacities of 3.3, 5.0 and 7.4 mAh.g(-1) in the 1st, 10th and 100th cycles, while the derivative Li2MnO3-delta delivers the relatively high values of 64.8, 103.8 and 140.2 mAh.g(-1) in the 1st, 10th and 120th cycles, respectively. Aside from the similar phenomenon of gradual electrochemical activation, substituting Li2MnO3-delta for Li2MnO3 means the great enhancements of charge-transfer ability and electrochemical performances. Especially, the cationic-anionic redox mechanisms of Li2MnO3 and Li2MnO3-delta are similar to each other, suggesting a possible solution to prepare high-performance xLi(2)MnO(3-delta)(1-x) LiMO2 solid solutions for application purposes.