Lithium diffusion in materials based on LiFePO4 doped with cobalt and magnesium

We have studied lithium intercalation/deintercalation kinetics in magnesium- and cobalt-doped lithium iron double phosphates in a cathode material for lithium ion batteries. The results demonstrate that the incorporation of divalent cations reduces the charge and discharge capacities of the samples, the effect being stronger in the magnesium-doped materials. In addition, magnesium doping markedly increases the resistivity of the material in both the lithiated and delithiated states, whereas the resistivity of the cobalt-doped materials is considerably lower in comparison with the undoped material, which leads to an increase in the charging/discharging rate of batteries despite the marked increase in particle size. These findings can be understood in terms of different doping mechanisms: it seems likely that cobalt substitutes for iron, whereas magnesium is accommodated predominantly in the lithium site.