Electrochemical and electrical properties of. Nb- and/or C-containing LiFePO 4 composites
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Electrochemical and electrical properties of. Nb- and/or C-containing LiFePO4 composites C. Delacourt, C. Wurm, L. Laffont, F. Sauvage, J.-B. Leriche, R. Bouchet, M. Morcrette, J-M. Tarascon & C. Masquelier Laboratoire de Réactivité et de Chimie des Solides, UMR CNRS #6007, Université de Picardie Jules Verne, 33 Rue St. Leu, 80039 Amiens Cedex 9, France
Abstract LiFePO4-based powders prepared through various synthesis conditions are presented. Depending on whether the precursors contain carbon or not, LiFePO4-based composites obtained contain significant amounts of carbon as well. We did not succeed in doping LiFePO4 with Nb and produced, instead, crystalline β-NbOPO4 and/or an amorphous (Nb,Fe,C,O,P) matrix around LiFePO4 particles. The total electrical conductivity is of ~10-9 S.cm-1 at 25°C with an activation energy of ca. 0.65 eV for pure LiFePO4 and LiFePO4/βNbOPO4 composite. C-containing LiFePO4 samples, including those that had been tentatively doped with Nb, are much more conductive (up to 1.6.10-1 S.cm-1) with an activation energy ∆Ε ~ 0.08 eV.
Introduction Since the original work of Padhi [1], phospho-olivines LiMPO4 (M = Fe, Mn, Co, Ni) appear to be potential candidates to be used as positive electrode materials for lithium batteries. LiFePO4 was the most investigated [2-6] and its commercialization is now close to reality, due to very high performances in terms of reversible capacities for extensive electrochemical cycling (~ 140 mAh.g-1 at C/2 rate over more than 1000 cycles) [7-9]. The main drawback of these olivine-type materials nests in their low intrinsic conductivity. We present part of our systematic and extensive investigation of “Li1-yNbyFePO4”/ z %C composites (0 ≤ y ≤ 0.05, 0 ≤ z ≤ 4.74 wt. % C) through structural and nanostructural techniques (XRD and HRTEM as well as impedance spectroscopy (AC) and DC polarization. Our goal was to identify, for each composite, the factors which prevented / promoted good electrochemical performances when the composite was used as a positive electrode in Li batteries.
Experimental The Li, Fe and P precursors were reagent grade LiH2PO4, Li2CO3, LiNO3, LiOH.H2O, Fe(NO3)3.9H2O, FeC2O4.2H2O, H3PO4 and NH4H2PO4. Reagent grade Nb(OC6H5)5 and NbCl5, used for Li substitutions for Nb, were stored and weighted in an Ar-filled glove box. Two methods were used to prepare pure LiFePO4, LiMnPO4 and Nb - and / or C - LiFePO4 composites, as gathered in Table 1. Samples B, C, D1, D2, E1, E2, and J were synthesized by Method #1, as follows : the first step consisted in the slow evaporation under continuous stirring of an intimate mixing of very soluble Li, P, and Fe precursors (LiH2PO4, NH4H2PO4 and Fe(NO3)3.9H2O) in demineralized
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water. The as-obtained amorphous homogeneous mixture was then calcined at 350°C for 10 h, in order to dehydrate and decompose ammonium, nitrate into gaseous NH3 and NOx. Final samples were obtained after a subsequent annealing at either 500, 700°C or 800°C for 10 h. For Nb-containing samples, e.g. D1, D2, E1 and E2, NbCl5 was
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