Ultrafine layered LiCoO2 obtained from citrate precursors

A new method for the preparation of ultrafine LiCoO₂ with a layered crystal structure was developed, which consists in thermal pyrolysis of homogeneous lithium-cobalt-citrate precursors. Atomic scale mixing of Li and Co is achieved by citric acid acting as a chelating agent. Electron spectroscopy of concentrated Li-Co-citrate solutions with Li:Co:Cit=1:1:1 and Li:Co:Cit=1:1:2 reveals that the predominant species at pH=7 are Co(C₆H₅O₇)]⁻ and Co(C₆H₅O₇)₂]⁴⁻ complexes. Freeze-drying of the two types of solutions leads to the formation of LiCo(C₆H₅O₇).nH₂O and (NH₄)₃LiCo(C₆H₅O₇)₂.nH₂O precursors, where Co²⁺ ions are complexed by one and two triionized citrate ions, respectively, and Li⁺ ions serve as counter ions. Between 400–600 °C, the thermal decomposition of these metal-citrate precursors yields LiCoO₂ with layered and pseudo-spinel structure, the proportion between them being depending on: (i) the Co/citrate ratio; (ii) the concentration of the freeze-dried solution; (iii) the heating rate. At 400 °C, the most defectless layered LiCoO₂, consisting of hexagonal individual particles with dimensions of 120–170 nm, is a product of the bis-citrate decomposition with a slow heating rate. For this sample, heating up to 600 °C does not affect the crystal size dimensions. For ultrafine layered LiCoO₂ and LiCoO₂ obtained by solid state reaction at high-temperatures (850 °C), the deintercalation and intercalation reactions proceed in the 3.95 – 3.99 and 3.86 – 3.88 voltage intervals, respectively. For defect trigonal LiCoO₂, additional oxidation and reduction peaks at 3.7 – 3.8 and 3.4 – 3.5 V were observed. We did not succeed in preparing monophase LiCoO₂ with pseudo-spinel structure. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Sept. 14–21, 1996