An Nb5+-doped cathodic material, Li1-xNbx+yFe1-yPO4, was synthesized via the high-temperature solid-state method. The crystal structure, micro-defects and doping ion occupancy of doped LiFePO4 have been studied via the Rietveld crystal structure refinement, HRTEM, EPR and positron annihilation lifetime spectroscopy (PALS) methods. The effect of ion doping on the electrochemical properties was studied by measuring the changes in the electronic localization and structural defects. The results of the Rietveld crystal structure refinement showed that Nb5+ occupied positions on both Li and Fe in [Li0.99-xNbx][Fe1-yNby]PO4, with an occupancy rate on Fe approximately six times greater than that in the Li position. The HRTEM results indicated that, when Nb5+ was doped into the lattice well, there were no other phases on the grain surface. Furthermore, the EPR results indicated that Nb5+ occupying the Fe position broke the Fe2+-O-Fe2+ electron spin correlation, which distorted the crystal ligand field around Fe2+. The PALS investigation showed that Nb5+ could be doped into the internal LiFePO4 lattice to produce cation vacancy defects. The internal defects and outer electron energy of the cations in the lattice were changed because of the Nb5+ doping, which could improve the electrochemical performance of the material. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.052204jes] All rights reserved.