Lithium Transport Properties in LiMn_1−αFe_αPO₄ Olivine Cathodes

We report a comparative study of the electrochemical lithium diffusion properties within the olivine structure of LiMn_0.5Fe_0.5PO₄, LiFePO₄, and LiMnPO₄ materials prepared by the solvothermal pathway. The study includes careful analysis performed by potentiodynamic cycling with galvanostatic acceleration (PCGA), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and the galvanostatic intermittent titration technique (GITT), carried out in order to investigate the features of the Fe³⁺/Fe²⁺ and Mn³⁺/Mn²⁺ redox processes and the lithium ion transport within the olivine structure. The electrochemical investigation reveals a shift of the redox potential of Fe³⁺/Fe²⁺ and Mn³⁺/Mn²⁺ couples toward higher and lower values, respectively, in LiMn_0.5Fe_0.5PO₄ with respect to the bare materials. Interestingly, the study shows the dependence of the lithium diffusion coefficients on the state of charge of the materials as well as on the adopted technique. Accordingly, CV leads to lithium diffusion coefficients of the order of 10^–12 cm² s^–1 for LiMnPO₄, 10^–9 cm² s^–1 for LiFePO₄, and 10^–11 cm² s^–1 for LiMn_0.5Fe_0.5PO₄. EIS mainly indicates lower values of lithium diffusion coefficients, i.e., 10^–13 cm² s^–1 for LiMnPO₄, 10^–12 cm² s^–1 for LiFePO₄, and 10^–13 cm² s^–1 for LiMn_0.5Fe_0.5PO₄. GITT provides a wide range of Li⁺ diffusion coefficient, depending on the Li_1–<i>x</i>MePO₄ stoichiometry, that is, 10^–14–10^–10 cm² s^–1 for LiMnPO₄ and LiFePO₄ and 10^–13–10^–10 cm² s^–1 for LiMn_0.5Fe_0.5PO₄. The wide diffusion coefficient window obtained by changing the state of charge and the adopted technique sheds light on the complex trend of the lithium diffusion in olivines and indicates that the technique may actually influence the materials evaluation