Nanocomposite Electrodes for Advanced Lithium Batteries: The LiFePO 4 Cathode
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Nanocomposite Electrodes for Advanced Lithium Batteries: The LiFePO4 Cathode Shoufeng Yang, Yanning Song, Peter Y. Zavalij and M. Stanley Whittingham* Institute for Materials Research, Binghamton University, Binghamton, NY 13902-1600, U.S.A. ABSTRACT LiFePO4 was successfully synthesized by high temperature and hydrothermal synthesis. A nanocomposite was formed by carbon coating this material; initial electrochemical results showed that up to 70% capacity could be obtained at 1.0 mA/cm2 current density. In contrast, the hydrothermally prepared LiFePO4 showed a lower capacity even at lower discharge rates due to a partial occupation of lithium sites by iron. This occupation, identified by Rietveld X-ray refinement, decreased both the rate and degree of intercalation and de-intercalation of lithium; chemical reaction with butyl lithium and bromine confirmed the electrochemical behavior. This investigation showed that the cathode could be prepared by high temperature synthesis, followed by a carbon black coating to achieve high capacity at high current density. INTRODUCTION LiFePO4 has been of much interest recently [1-3] due to its low cost, high performance and stability. Its theoretical capacity is 170 mAh/g, 40% more than that of the LixCoO2 presently used in commercial batteries, where ∆x ≈ 0.6. The discharge voltage is about 3.4 V, high enough for large-scale application, but not high enough to decompose the electrolyte during recharge. However, the low electronic conductivity results in low kinetics and hence small current densities. This can be partially overcome by increasing the temperature, but this might increase electrolyte decomposition. The low kinetics and their improvement at elevated temperature have been discussed previously [4,5]. Ravet et al [6] showed that with a conductive carbon coating, full capacity could be attained at 80oC at C/1 current density. We demonstrated the possibility of synthesizing LiFePO4 via hydrothermal synthesis in just a few hours [7,8]. When this hydrothermal material was coated with sucrose following the Ravet method [9], good cycling behavior was observed at 60% capacity and a current density of 0.14 mA/cm2. However, a recent study showed that the in the hydrothermal material there is some disorder of the lithium and iron atoms resulting in poor behavior of the as-synthesized material [10]. Nazar’s group [11] used a carbon gel technology to optimize LiFePO4 and showed that it could be cycled with 80% capacity at a 2C current density at ambient temperature. We have reproduced their results [10]. Dominko [12] recently showed that with a novel carbon coating by aqueous gelatin solution, LiMn2O4 and LiCoO2 had better performance, and the polarization was reduced. Here we used the same technology for LiFePO4 and demonstrated that 70% capacity was available at 1.0 mA/cm2 density.
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Contact author, [email protected] V7.9.1
EXPERIMENTAL LiFePO4 was prepared hydrothermally as previously reported by us [7] (material A), the sample was then coated with carbon [7] (material
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