Journal of Power Sources 163 (2006) 560–566 Optimized electrochemical performance of LiFePO 4 at 60 ◦ C with purity controlled by SQUID magnetometry K. Zaghib a,∗ , N. Ravet b , M. Gauthier c , F. Gendron d , A. Mauger e , J.B. Goodenough f , C.M. Julien d a Institut de Recherche d’Hydro-Qu´ ebec (IREQ), 1800 Bd Lionel-Boulet, Varennes, Que., Canada J3X 1S1 b Universit´ e de Montr´ eal, CP 6128, Succursale centre-ville, Montr´ eal, Que., Canada H3C 3J7 c Phostech Lithium, 135-D chemin du Tremblay, Boucherville, Que., Canada J4B7K4 d Institut des Nano-Sciences de Paris (INSP), UMR 7588, Universit´ e Pierre et Marie Curie, 140 rue de Lourmel, 75015 Paris, France e D´ epartement MIPPU, CNRS, Campus Boucicaut, 140 rue de Lourmel, 75015 Paris, France f The University of Texas at Austin, Austin, TX 78712, USA Received 31 July 2006; received in revised form 30 August 2006; accepted 15 September 2006 Available online 27 October 2006 Abstract The local structure and magnetic properties of a series of carbon-coated LiFePO 4 particles prepared under different conditions are analyzed with X-ray diffractometry (XRD), FTIR and Superconducting Quantum Interference Device (SQUID) magnetometry for comparison. While nano-sized ferromagnetic particles (-Fe 2 O 3 clusters) are detected by magnetic measurements in samples grown from iron(II) oxalate, such ferromagnetic clusters do not exist in the optimized samples grown from FePO 4 (H 2 O) 2 . FTIR analyses show that carbon does not penetrate significantly inside the LiFePO 4 particles despite the fact that it has been very efficient in reduction of Fe 3+ to prevent -Fe 2 O 3 clustering, thus pointing to a gas-phase reduction process. The impact of the carbon coating on the electrochemical properties is also reported. No iron dissolution was observed after 200 charge–discharge cycles at 60 ◦ C for cells containing lithium foil, lithium titanate or graphite negative electrodes. © 2006 Elsevier B.V. All rights reserved. Keywords: Lithium-ion batteries; Positive electrodes; Olivine structure; Magnetic properties; High-temperature performance 1. Introduction Among the well-known Li-insertion compounds, the olivine LiFePO 4 compound is currently being investigated extensively as a positive electrode material for Li-ion batteries because of its low cost, low toxicity, and relatively high theoretical spe- cific capacity of 170 mAh g -1 [1,2]. The current debate for the utilisation of LiFePO 4 in large-size batteries (for hybrid electric vehicles, 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 perfor- mance is also a critical issue because batteries may be operated at elevated temperatures (around 60 ◦ C). The early drawback of highly resistive LiFePO 4 has been resolved by coating the particle surface with carbon [3–5]. A seven-order-of-magnitude ∗ Corresponding author. Tel.: +1 450 652 8019; fax: +1 450 652 8424. E-mail address: zaghib.karim@ireq.ca (K. Zaghib). increase in the electronic conductivity is achieved by the addi- tion of a carbon coat by using, for example, sucrose to produce carbon-coated LiFePO 4 (C-LiFePO 4 ) raw materials by a spray pyrolysis technique [6]. Recently, significant effort has been underway to improve LiFePO 4 by developing a new synthesis route via carbon coating [7]. The 1D Li channels make the olivine performance sensi- tive not only to particle size, but also to impurities and stacking faults that block the channels. Various types of iron-based impu- rities have been identified in the olivine framework: for example -Fe 2 O 3 , Fe 3 O 4 , Fe 2 P 2 O 7 , Fe 2 P, Fe 3 P and Fe 75 P 15 C 10 . The polyphosphate framework Li 3 Fe 2 (PO 4 ) 3 has also been identi- fied [8]. Critical quality control of the product is necessary to obtain a complete understanding of synthesis conditions using combination of experiments such as Raman spectroscopy for an evaluation of the carbon film [9] and magnetic measure- ments, which are highly sensitive to a low impurity concentration [10,11]. In this paper, we report the results obtained on several sam- ples of LiFePO 4 with special attention to the new generation 0378-7753/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jpowsour.2006.09.030