Lithium−Nickel Citrate Precursors for the Preparation of LiNiO₂ Insertion Electrodes

Noncrystalline solids with LiNi(C 6 H 4 O 7 ) 3/4 · x H 2 O, LiNi(C 6 H 5 O 7 )· x H 2 O, and (NH 4 ) 3 LiNi(C 6 H 5 O 7 ) 2 · x H 2 O ( x ≈ 5) chemical compositions were obtained by freeze drying of concentrated lithium−nickel−citrate solutions. The IR spectra of solid citrates reveal that triionized citrate ions chelate the Ni 2+ ions with their carboxylate groups acting as monodentate agents. Electronic spectra reveal monomeric 1:1 Ni(II)−citrate(−3) complexes in the mixed citrate powders. Finely disperse layered Li 1 - x Ni 1+ x O 2 solids were obtained by the thermal decomposition of the citrate precursors at 700−800 °C in an oxygen atmosphere. Almost stoichiometric Li 1 - x Ni 1+ x O 2 ( x ≤ 0.05) oxides with layered R 3̄ m structure are obtained from monocitrate precursors. The LiNiO 2 powder were used as active cathode material in lithium cells. Cyclic voltammograms showed several current peaks during the first cycle resulting from the formation of the intermediate phases usually found in quasi-stoichiometric LiNiO 2 . After the almost complete extraction of lithium, further cycles reveal a significant loss of capacity and changes in the number of current peaks as a result of the changes induced in the structure of the solid. EPR data on cycled LiNiO 2 show partial migration of Ni from 3a (nickel layers) to 3b sites (lithium layers).