The Er-doped LiMnPO4/C cathode materials were synthesized by a sol-assisted hydrothermal synthesis method followed by annealing at a low temperature using maleic acid as chelating agent and carbon source for the first time. The structure and electrochemical performances of the obtained samples were investigated systematically including XRD, SEM, EDS, galvanostatic charge and discharge tests, cycle voltammetry and electrochemical impedance spectroscopy. Results show that the doped materials especially LiMn0.97Er0.03PO4/C tend to grow in the orientation of rectangle with the uniform particle size. Among all the samples, LiMn0.97Er0.03PO4/C delivers the highest discharge capacity of 142.5mAhg−1 at 0.05C and a high capacity retention of 90.8% after 100cycles at 1C. Even with the high current density of 5C, its capacity remains at 105.6mAhg−1 in comparison with 46.3mAhg−1 of LiMnPO4/C. Compared with LiMnPO4/C, the Er-doped samples demonstrate significantly enhanced electrochemical performance for lithium-ion batteries. This is likely due to the fact that the large ionic radius of erbium ions could enlarge the volume of lattice and alleviate the effect of Jahn-Teller, which is beneficial to improve Li-ion diffusion coefficient and reduce the charge transfer impedance. This material shows promising potential for use in high-voltage cathodes for lithium-ion batteries.