Lithium Transport Properties in LiMn1−αFeαPO4 Olivine Cathodes

We report a comparative study of the electrochemical lithium diffusion properties within the olivine structure of LiMn0.5Fe0.5PO4, 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 LiMn0.5Fe0.5PO4 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 LiMn0.5Fe0.5PO4. 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 LiMn0.5Fe0.5PO4. GITT provides a wide range of Li+ diffusion coefficient, depending on the Li1–xMePO4 stoichiometry, that is, 10–14–10–10 cm2 s–1 for LiMnPO4 and LiFePO4 and 10–13–10–10 cm2 s–1 for LiMn0.5Fe0.5PO4. 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.