Mixed LiCo_0.6M_0.4PO₄ (M = Mn, Fe, Ni) phosphates: cycling mechanism and thermal stability

The electrochemical delithiation of LiCo_0.6M_0.4PO₄ phosphates (M = Mn, Fe, Ni) was studied by in situ synchrotron diffraction. In all three metallophosphates the oxidation–reduction of 3d-elements proceed via two-phase mechanisms leading to two-phase regions, corresponding to the Co²⁺/Co³⁺ and M²⁺/M³⁺ reactions. The Ni²⁺/Ni³⁺ reaction was not revealed, neither by the potentiostatic intermittent titration technique (PITT) nor by diffraction. In the two-phase reaction, the olivine-like structure of the cathode remains preserved, which is characteristic of this type of materials. Pronounced solid-solution domains were observed during both lithium extraction and insertion. The thermal stability of the charged cathodes is limited by the presence of Co³⁺ and its intrinsic instability in these compounds.