The LiMg x Mn 1−x PO 4 /C ͑x = 0, 0.02, 0.04, 0.12͒ composite cathode was successfully prepared by a combination of spray pyrolysis and wet ballmilling with heat-treatment. The composite cathode had narrow particle size distribution with an average particle size of 99 nm. The Mg doping on the Mn site led to the electrochemical performance enhancement of the composite cathode, which was confirmed by cyclic voltammetry, ac impedance spectroscopy, and charge-discharge tests. The Mg-doped composite cathode exhibited a high discharge capacity in lithium cell, which remarkably increased with an increase in the charge cutoff voltage under galvanostatic charge-discharge. The LiMg 0.04 Mn 0.96 PO 4 /C cathode exhibited a discharge capacity of 154 mAh g −1 ͑above 93% of the theoretical value͒ at 0.1C when charge-discharged galvanostatically to 4.9 V. Along with enhanced discharge capacity, the cell exhibited a good rate capability under the galvanostatic charge-discharge. Under the trickle mode conditions, the cell exhibited discharge capacities of 154, 136, 106, and 74 mAh g −1 at 0.05, 0.1, 1, and 5C, respectively. © 2010 The Electrochemical Society. ͓DOI: 10.1149/1.3294698͔ All rights reserved. Lithium-ion batteries ͑LIBs͒ are leading the rapidly growing market of portable power sources for various applications starting from the miniature devices, such as peacemakers and chipsets, to electricity powered automobiles and energy storage facilities. Rechargeable LIBs with liquid electrolytes were developed and introduced into the market by Sony Corporation 1 in 1991. Despite its outstanding properties compared with other types of batteries such as nickel-cadmium and metal hydride batteries, LIBs suffer from the application of high cost and toxic materials, mainly as positive electrodes. Therefore, the main demand in the field of LIBs is to develop low cost and environment-friendly materials to substitute commercial cathodes such as expensive and toxic cobalt-containing cathodes. Among other candidates to replace the high cost and toxic cobalt-based cathodes for LIBs, the olivine structured phosphates LiMPO 4 ͑M = Fe, Mn, Ni͒ are the most promising ones due to the reliable combination of theoretical capacity and cost, nontoxicity, and high electrochemical and thermal stabilities.2-5 However, high ionic and electronic resistance of these materials restricts achieving high electrochemical activity. This disadvantage has been successfully overcome for the LiFePO 4 cathodes via the synthesis of nanosized powders, the conductive layer coating of the compound particles surface and doping with cations. [3][4][5][6][7][8][9] It was shown that in the presence of carbon in the cation-doped olivine LiFePO 4 , the carbothermal reduction of iron phosphate at high temperatures leads to the formation of phosphides at the grain boundaries, and this nanonetwork could be responsible for the grain-boundary transport enhancement.9,10 The same process could be readily extendable to other olivines such as LiMnPO 4 .9 Chung et al. 11 showed tha...