Lithium rich material xLi₃NbO₄·(1-x) LiMnO₂ (0 < x < 1) was successfully synthesized by solid state method. Stoichiometric amounts of Li₂CO₃, Mn₂O₃ and Nb₂O₅ were mixed by ball milling, and the mixture was calcinated at 900℃ for 5 h under Ar atmosphere. X-ray diffraction (XRD) results indicate that the samples with 0.25 < x < 0.67 can be indexed as a cubic structure with Fm-3m space group. Electrochemical results show that the samples of x=0.25 and 0.43 have better electrochemical performance, both delivering 216 mAh·g^-1 in the initial cycle between 2 V and 4.8 V. Although voltage decay is an intrinsic drawback of lithium excess materials, the sample of x=0.43 decays slower. We speculate that Li₃NbO₄ helps stabilizing the crystal structure. Ex-situ XPS and XAS studies show that the charging process can be divided into two stages. In the first stage, below 4.3 V, Mn³⁺ is oxidized to Mn⁴⁺, in the second stage, O^2- is oxidized. The reversible oxidation of O^2- is the origin of the achievement of large reversible capacity. Co³⁺ doped material 0.43Li₃NbO₄·0.57LiMn_1-yCo_yO₂ (y=0.25; 0.5) was also synthesized by the same procedure. The structure of the doped material maintains the cubic structure with smaller lattice constant and the variation of lattice constant is in proportion to the amount of Co³⁺. Galvanostatic charge and discharge tests show that 0.43Li₃NbO₄·0.57LiMn_0.75Co_0.25O₂ also delivers a large capacity of 215 mAh·g^-1 in the first cycle between 2 V and 4.8 V, but the voltage plateau in the charging process decreased from 4.3 V to 4.1 V, it can be attributed to the weak dissociation energy of Mn-O bond and the overlap of Co^3+/4+ 3d and O^2- 2p energy band. The electrochemical impedance spectroscopy results show that a moderate amount of Co³⁺ doped into the material decreases the charge transfer resistance. After doped with Co³⁺, the rate capability is improved.

Key words: lithium-ion batteries, Li₃NbO₄, rock-salt structure, lithium excess materials, electrochemical performance