Ordered Olivine‐Type Lithium–Cobalt and Lithium–Nickel Phosphates Prepared by a New Precursor Method

Single phases of olivine‐type LiCoPO4 and LiNiPO4 were synthesized by thermal treatment of homogeneous lithium–metal–phosphate–formate precursors obtained by freeze drying of aqueous solutions of the corresponding metal formates and LiH2PO4. The structure, thermal behavior, and morphology of the precursors were studied by IR spectroscopy, DTA, and SEM. Cobalt and nickel phosphate–formate precursors have a composition LiMHx(PO4)(HCOO)x·yH2O, where the formate and phosphate groups are mainly deprotonated. For the Co precursor the formate and phosphates ions are randomly coordinated to both Co and Li cations, whereas for the Ni precursor there is a preferential coordination of the formate and phosphate ions around the Ni2+ and Li+ ion, respectively. Thermal treatment of the precursors yields single phases of olivine‐type LiCoPO4 at 450 °C and LiNiPO4 at 700 °C. Structural analysis evidences that both LiCoPO4 and LiNiPO4 have an ordered olivine‐type structure without any Li to M disorder between the metal positions and lithium deficiency. The effect of the freeze‐dried solution concentration and annealing temperature on the structure, crystallite size, and morphology of LiCoPO4 and LiNiPO4 has been discussed. The morphology of the cobalt and nickel phospho‐olivines comprises isometric particles with mean sizes of 190 and 380 nm, respectively.