The Li2Fe1−x Ni x P2O7/C (x = 0.00, 0.02, 0.04, 0.06, 0.08, 0.10) composites were successfully synthesized for the first time by a conventional solid state route. The solid state reaction mechanism, crystal structure, morphology, and electrochemical performance of the samples are characterized in detail. The XRD results show that Ni2+ is incorporated in the lattice of Li2FeP2O7 and the monoclinic structure does not change after doping. Among all the samples, the Li2Fe0.98Ni0.02P2O7/C composite delivers the highest discharge capacity of 110.2 mAh g−1 at 0.025C, and possesses a capacity retention ratio of 90.1% after 100 cycles at 0.05C, demonstrating the best rate performance and cycle stability in the potential range of 2.0–4.5 V. Electrochemical impedance spectroscopy (EIS) results reveal that the Li2Fe0.98Ni0.02P2O7/C sample has a lower charge transfer resistance and a higher Li-ion diffusion coefficient compared to other samples. The results indicate Ni2+-doping in Li2FeP2O7/C can effectively enhance the electrochemical performance of this monoclinic compound, especially at a high charge/discharge rate.