High Performance of Lithium Iron Phosphates for HEV with Quality Control Made by Magnetometry
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High Performance of Lithium Iron Phosphates for HEV with Quality Control Made by Magnetometry Christian M. Julien1, Alain Mauger2, Karim Zaghib3, and François Gendron1 1 INSP, University Paris 6, 140 rue de Lourmel, Paris, 75015, France 2 MPPU, CNRS, 140 rue de Lourmel, Paris, 75015, France 3 IREQ, 1800 Bd Lionel-Boulet, Varennes, J3X 1S1, Canada ABSTRACT Optimized LiFePO4 positive electrode for Li-ion batteries was obtained after severe control of the fundamental properties of material. The nanoscopic structure and magnetic properties of a series of carbon-coated LiFePO4 particles prepared under various conditions were analyzed with XRD, FTIR, Raman and SQUID magnetometry. We evaluate intrinsic and extrinsic properties. The existence of low content of nanosized ferromagnetic particles was evidenced by magnetic measurements in samples grown from iron(II) oxalate; such ferromagnetic clusters do not exist in the optimised samples grown from FePO4(H2O)2. Other impurity phases such as Fe2P, Li3Fe2(PO4)3, FeP2O7 were also detected for particular conditions of preparation. The nature and properties of the carbon deposited at the surface of particle is investigated by Raman spectroscopy. The impact of the carbon coating on the electrochemical properties is reported. Li-ion cells show excellent cyclability after 200 cycles at 60 °C without iron dissolution. INTRODUCTION Presently, the olivine-like LiFePO4 compound is being investigated extensively as a positive electrode material for Li-ion batteries because of its low cost, environmentally benignity, and relatively high theoretical specific capacity of 170 mAh/g [1-2]. The current debate for the utilization of LiFePO4 in large-size batteries (for hybrid electric vehicles (HEV), for instance) is mainly focused on the perceived poor rate capability because of a low electronic conductivity. Another aspect concerns the material purity and the non-migration of iron ions through the electrolyte. The high-temperature performance is also a critical issue because batteries may be operated at elevated temperatures (around 60 °C). The early drawback of highly resistive LiFePO4 has been currently resolved by coating the particle surface with carbon film [3-6]. A major difficulty with the synthesis of orthophosphate LiFePO4 comes from the existence of two oxidation degrees of iron in nature, namely Fe(II) and Fe (III) [1]. A careful control of the synthesis procedure is needed to avoid any impurity, which can poison the electrochemistry of the positive electrode in Li-ion batteries [7]. Various types of Fe3+containing compounds may be incorporated such as Fe2P, Fe3P, Li3Fe2(PO4)3, γ-Fe2O3, LiFeP2O7, Fe2P2O7, etc. Some of these compounds could be beneficial from the view-point of enhanced electronic conductivity (as Fe2P, for instance) but highly detrimental for capacity retention and iron dissolution into the non-aqueous electrolyte. This work is devoted to the optimization of the structure and morphology of phosphoolivine LiFePO4 cathode materials. We investigate the nan
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