Density Functional Theory and Electrochemistry Studies on LiFe_xMn_1-xPO_4 Solid Solutions

The thermodynamic stability and lithiated/delithiated potentials of LiFe_xMn_1-xPO_4 were studied with density functional theorical calculations. The results show that the formation free energy of the LiFe_xMn_1-xPO_4 solid solution is slightly higher than that of the phase-separated mixture of LiFePO_4 and LiMnPO_4, and the two forms may co-exist in the actual LiFe_xMn_1-xPO_4 materials. The calculation manifests that the lithiated/delithiated potentials of LiFe_xMn_1-xPO_4 solid solutions vary via the Mn/Fe ratio and the spatial arrangements of the transition metal ions, and the result is used to explain the shape of capacity-voltage curves. Experimentally, we have synthesized the LiFe_xMn_1-xPO_4 materials by solid-phase reaction method. The existence of the LiFe_xMn_1-xPO_4 solid solution is thought to be responsible for the appearance of additional capacity-voltage plateau observed in the experiment.