A core-shell nanocomposite of Li2MnO3 is synthesized by coating FePO4 nanoparticles on the surface of our prepared Li2MnO3 nanoplate. This core-shell structured nanocomposite can eliminate the large irreversible capacity for the Li-rich materials. The FePO4 on the surface of Li2MnO3 can also serve as a host for Li ions that deintercalate from Li2MnO3 during the initial charging process and the initial Coulomb efficiency can be 100%. The composite as the cathode material for lithium ion batteries exhibits an attractive reversible capacity of 180 mAh g−1. Its cycling performance is also stable without any evident capacity fading, even when cycling in the high voltage range of 2.0–4.8 V vs. Li+/Li. This nanocomposite is free of Co- and Ni, different from other Li-rich Li2MnO3 based cathode materials, and shows great attraction for applications of low cost cathode materials for lithium ion batteries.