LiFePO₄/carbon (LFP/C) nanocomposites were fabricated using bis(hexamethylene triamine penta (methylene phosphonic acid)) (BHMTPMPA) as a new and environment-friendly phosphorus source. The activation energy of the fabricated LFP/C was first investigated in depth based on the theoretical Arrhenius equation and experimental results of the LFP/C composite particle size distribution to explore the grain growth dynamics of the LFP/C particles during the sintering process. The results indicate that the activation energy is lower than 3.82 kJ mol⁻¹ when the sintering temperature is within the range of 600–800 °C, which suggests that the crystal growth kinetics of the LFP/C particles is diffusion-controlled. The diffusion-controlled mechanism results from the mutual effects of chelation with Fe²⁺ cations, in situ formation of carbon layers and high concentration of hard aggregates due to the use of an organic phosphorous source (BHMTPMPA). The diffusion-controlled mechanism of the LFP/C effectively reduces the LFP particle size and hinders the growth of anomalous crystals, which may further result in nanosized LFP particles and good electrochemical performances. SEM and TEM analyses show that the prepared LFP/C has a uniform particle size of about 300 nm, which further confirms the effects of the diffusion-controlled mechanism of the LFP/C particle crystal growth kinetics. Electrochemical tests also verify the significant influence of the diffusion-controlled mechanism. The electrical conductivity and Li-ion diffusion coefficient (D_Li⁺) of the fabricated LFP/C nanocomposite are 1.56 × 10⁻¹ S cm⁻¹ and 6.24 × 10⁻¹¹ cm² s⁻¹, respectively, due to the chelating structure of the phosphorus source. The fabricated LFP/C nanocomposite exhibits a high reversible capacity of 166.9 mA h g⁻¹ at 0.2C rate, and presents an excellent rate capacity of 134.8 mA h g⁻¹ at 10C.