The electronic structures of pure and Mn-doped LiFePO4 are studied using density functional theory (DFT). The results demonstrate that the pure LiFePO4 has a band gap of 0.725 eV, while the 25% Mn doped LiFe0.75Mn0.25PO4 has the smallest band gap (0.469 eV), and the weakest FeO and LiO bond, which indicates that the electronic conductivity and the ionic conductivity of the doped LiFePO4 are improved due to doping. On the other hand, the experimental results also show that the LiFe0.75Mn0.25PO4 has the best electrochemical performance and it delivers a very high capacity of 158 mAh? g-1 and a high energy density of 551 Whkg-1.