A Versatile New Synthesis of Carbon Rich LiFePO4 Enhancing Its Electrochemical Properties.

In this contribution we report a new reproducible synthetic route to prepare LiFePO4/C composites. As starting materials an intimate stoichiometric mixture of lithium carbonate and an hybrid organic-inorganic Fe(II) organophosphonate, i.e. Fe[(RPO3)(H2O)] (R = methyl or phenyl group) were used. The idea behind it was to provide a single source for phosphorus, iron and carbon. The hybrid organic-inorganic compounds of formula Fe[(CH3PO3)(H2O)] and Fe[(C6H5PO3)(H2O)] are easy to handle because they are stable to the air and moisture [1,2]. The mixture of Li2CO3 and Fe[(RPO3)(H2O)] (R=CH3-, C6H5) was heated in a tubular furnace under N2 gas at temperatures above 600°C for at least 16 h [3]. Nano-crystalline LiFePO4 samples were obtained. One of the most interesting aspects of this new synthetic method is the formation of elemental carbon during the decomposition of Fe(II) methyl- and phenylphosphonate. In the final LiFePO4 samples amounts of 2.5 wt. % C in the case of Fe[(CH3PO3)(H2O)] precursor and 12 wt. % C in the case of Fe[(C6H5PO3)(H2O)] precursor have been found. This means that part of the theoretical carbon content in the reaction mixtures (8.8 wt. % and 29.6 wt. % respectively) is lost during the heating process. TG measurements on the initial precursor mixtures showed in both cases the elimination of the water molecules coordinated to the Fe(II) ions at temperatures up to 200°C, while at higher temperatures (200-800°C) weight losses of 30 and 45 % have been observed. An exothermic effect at 400°C (Fe[(CH3PO3)(H2O)]) and 450°C (Fe[(C6H5PO3)(H2O)]) has been evidenced in the DTA curves and might be related to the initial formation of Li2O from the corresponding carbonate followed by the decomposition of the organophosphonate precursor and final formation of LiFePO4. The exact mechanism of the reaction is still under investigation. The crystalline LiFePO4 powders were further characterized by X-ray powder diffraction analysis (XRPD) and Scanning Electron Microscopy (SEM). Our LiFePO4 samples crystallize in the orthorhombic space group Pnma and the LiFePO4 phase consists of spherical aggregates of about 0.2 μm diameter. Composite cathode tapes were made by roll milling a mixture of 75 wt. % active material and 10 wt. % binder (Teflon, DuPont). Carbon (KJB Carbon) was added until 15 wt. % final carbon content was reached. Electrodes disks of typically diameter of 10 mm were punched and the electrode weight ranged from 7.4 to 10.7 mg. T-shaped battery cells with lithium metal as counter and reference electrode were used for electrochemical characterization. The cells, filled with a 1M solution of LiPF6 in ethylene carbonate / diethyl carbonate (1:1), were automatically cycled by means of a battery cycler (Maccor 4000). Composite cathode preparation, cell assembly, tests and storage were performed in the dry room (R.H. < 0.1 % at 20°C). To test the effect of different discharge rates, a cell was subjected to various discharge rates, i.e. C/10, 1C, 3C, 10C, 20C and 30C. The cell was always charged using the same procedure to insure identical initial conditions: a constant current step at 1C rate until the voltage reached 4.0 V, followed by a constant voltage step until the current fell below C/10 rate. Figure 1 shows the Ragone plot for the cell discharged at different rates. The specific energy and specific power are based on the weight of the active material. The specific energy calculated at C/10 rate was about 550 Wh kg-1. The specific power calculated at 30C rate was in excess at 14,000 W kg-1 while the specific energy was about 28 % of the energy delivered at C/10. The excellent electrochemical performance of the LiFePO4/C composites can be ascribed to the tailored synthesis addressed to enhance the electrochemical properties of the material. The main advantage of this synthetic route is the formation of elemental carbon from the organic constituent of the precursor. The carbon particles interact with the LiFePO4 grains just during their formation providing a good electronic contact between the grains and the carbon added for the composite electrode fabrication. The low particle size and the enhanced surface conductivity both result in the outstanding performance of the LiFePO4/C cathode. In conclusion, electrodes made of the cathode material reported here showed very high specific energy, specific power and capacity retention upon cycling. The new synthetic method is very simple and the Fe(II) organophosphonate precursors are air-stable. The good electrochemical performance of LiFePO4/C indicates that the reported method is very promising for developing high power lithium-ion batteries. Figure 1. Ragone plot for the cell discharged at different rates. LiFePO4 was prepared starting from the Fe(II) phenylphosphonate. The cathode loading of LiFePO4 was 7.1 mg cm-2. Electrode area was 0.16 cm2. ACKNOWLEDGEMENTS We thank the Ministero Italiano per la Ricerca Scientifica e Tecnologica (MIUR) for financial support. REFERENCES 1. C. Bellitto, F. Federici, M. Colapietro, G. Portalone, and D. Caschera, Inorg. Chem., 41, 709 (2002). 2. A. Altomare, C. Bellitto, S.A. Ibrahim, and M.R. Mahmoud, Inorg.Chem., 39, 1803 (2000). 3. Italian Patent no. RM2003A000048 (2003).