Carbon coating and nano-scale particle size are two impactful factors in improving the rate capability of LiFePO4 cathode materials for lithium-ion batteries. However, both factors decrease the tap density of the materials and are possibly causing unfavorable effect on the volumetric capacity of the cathode materials and thus the batteries, which is undesirable in commercial application. In the present study, LiFePO4 materials with moderate particle size of sub-micron and trace carbon content (0.5–0.9wt.%) are synthesized by a mechanical activation method. High-electronic conductivity iron phosphides (Fe2P/FeP) are in situ introduced into the LiFePO4 materials and the amount is modified by the calcination temperature. Electrochemical testing shows that Fe2P/FeP plays an important role in improving the high-rate capability of LiFePO4 with moderate particle size. The product calcined at 700°C, which has a high-tap density of 1.37gcm−3 correlating to a specific surface area approximately of 4m2 g−1, possesses discharge capacities of 110 and 100mAhg−1 at discharge rates of 5C and 10C, respectively. The introduction of Fe2P/FeP in an amount of ca. 5wt.% rather than carbon coating and the moderate particle size of LiFePO4 are promising approaches to obtain LiFePO4 cathode material of high-rate capability without unduly compromising its volumetric capacity.