A new phosphate-formate precursor method for the preparation of carbon coated nano-crystalline LiFePO4

Mixed phosphate-formate precursors, LiFePO4Hx(HCOO)x.yH2O (x ∼ 1, 1 < y < 1.3 and x < y), were prepared by freeze-drying of solutions containing Li+, Fe2+, PO43- and HCOO- in a ratio of 1:1:1:2. Fe(HCOO)2.2H2O was used as an iron source. The concentration of the freeze-dried solutions allows affecting the morphology of phosphate-formate precursors. Powder XRD, IR spectroscopy, DTA, BET measurements, SEM and XPS analyses were used for the characterization of the precursors and LiFePO4. IR spectroscopic study of the precursors shows that the deprotonated phosphate and formate ions are coordinated around the metal ions. The thermal heating of the lithium-iron phosphate-formate precursors at 350 oC for a short period yields stoichiometric defectless LiFePO4 having a high-specific surface area of about 35 m2/g, a mesoporous structure and containing up to 1.5 mass% carbon. By variation of the solutions concentration subjected to freeze-drying, the morphology of LiFePO4 and the carbon content are changed. The morphology of target LiFePO4 comprises micrometric aggregates, which are composed of nanometric particles with close particle size distribution in the range of 60-140 nm. The nanometric particles are interconnected in a way to form mesoporous network. This method is suitable for the preparation of LiFePO4 as cathode material for lithium-ion batteries since it allows affecting both the particle dimensions and carbon content.