The electrochemical properties of three isotopic Li₂FePO₄F compounds, as cathode materials under different space groups Pbcn, P and Pnma were investigated using first principle calculations. Their structures and average open circuit voltages for step delithiation reactions were explored, and the results are in good agreement with the reported experimental data. We estimate the substitution effect of Fe by Co in Pnma-Li₂FePO₄F. The substitution of Fe by Co in Li₂Fe_1−xCo_xPO₄F may enhance the discharge potential of the materials, and the rate of its volume change during the redox process is between 0.6% and 2.1%. Furthermore, from the projected density of states for Li₂Fe_0.5Co_0.5PO₄F, we found strong hybridization for Fe-3d and Co-3d bands near the Fermi level, which implies that the Co-doped Li₂Fe_1−xCo_xPO₄F may possess better electronic conductivity than the pure phase.