Mixed LiCo(0.6)M(0.4)PO(4) (M = Mn, Fe, Ni) phosphates: cycling mechanism and thermal stability

The electrochemical delithiation of LiCo(0.6)M(0.4)PO(4) 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 Co(2+)/Co(3+) and M(2+)/M(3+) reactions. The Ni(2+)/Ni(3+) 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 Co(3+) and its intrinsic instability in these compounds.