Lithium iron phosphate-carbon (LiFePO4-C) Composites with various amounts of Fe2P are synthesized by ball-milling coupled with microwave heating to serve as cathodes for lithium-ion batteries. LiFePO4-C in which Fe2P is restrained below a critical concentration gives as very high discharge capacity of 165 mAh g(-1), excellent rate capability (85.4% C/50-rate discharge capacity at 2C) and stable cyclic retention for 250 cycles. Above the critical concentration however, electrochemical performance deterioated. Analysis and debate, based on a comparison of the physical and electrochemical properties among LiFePO4-C composites with the variation of Fe2P, proceeded to the conclusion that below the critical concentration, Fe2P enhanced the conductivity of LiFePO4-C, whereas above the critical concentration, it blocked the one-dimensional Li+ pathways in LiFePO4 and might hinder Li+ movement in LiFePO4. Therefore, in order to obtain a LiFePO4-C composite that enhances electrochemical performance, it is concluded that the amount of Fe2P should be carefully controlled below its critical concentration. (C) 2008 Elsevier B.V. All rights reserved.