A Novel Codoping Approach for Enhancing the Performance of LiFePO4 Cathodes

By combining experimental and theoretical studies, we have demonstrated that donor-acceptor charge-compensating codoping is a promising approach to significantly enhance the rate performance of LiFePO4 cathodes. Our density-functional theory calculation predicts that codoping with Si on the P site and F on the O site modifies the nature of the conduction band edge of LiFePO4 from localized Fe 3d derived states to more delocalized F s and cation s derived states. This effect, thus changes the carrier transport from a poloron-like to a band-like mechanism, and consequently leads to significant improvement in the electrical conductivity of LiFePO4. Most importantly, our comparative doping experiments show that the electrical conductivity of Si P -FO codoped LiFePO4 exhibits at least 2 to 3 orders of magnitude increase in electrical conductivity as compared to that of un-doped LiFePO4. Because of the dramatic improvement of electrical conductivity, the optimal Si-F codoped LiFePO4 shows both a much higher rate-capability than un-doped LiFePO4 or LiFePO4 solely doped with either Si or F. Furthermore, we also believe that the charge-compensating codoping approach may be employed to improve the performance of other cathode materials suffering from inferior electrical conductivities due to localized conduction band states.