Mixed LiCo0.6M0.4PO4 (M = Mn, Fe, Ni) phosphates: cycling mechanism and thermal stability

The electrochemical delithiation of LiCo0.6M0.4PO4 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.