Structure and Stability of Olivine Phase FePO 4
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Structure and Stability of Olivine Phase FePO4 Gene M. Nolis, Natalya A. Chernova, Shailesh Upreti, M. Stanley Whittingham Department of Chemistry and Institute for Materials Research, State University of New York at Binghamton, Binghamton, NY 13902-6000, USA. ABSTRACT LiFePO4 has shown considerable promise as a cathode material in Li-ion batteries due to its stability, low toxicity and high cyclability. However, the data on thermodynamic stability of olivine phase FePO4 (o-FePO4), the delithiated form of o-LiFePO4, remains scarce and contradictory. In this work, o-FePO4 was synthesized by chemical delithiation of o-LiFePO4 and characterized structurally and thermally. X-ray diffraction and absorption data indicate pure olivine phase, but with residual amount of Fe2+, most likely due to incomplete delithiation. Differential scanning calorimetry and thermal gravimetric analysis reveal that o-LixFePO4 decomposes exothermally above 550 °C with about 9% weight loss, the products being trigonal phase FePO4, Fe7(PO4)6, and LiPO3. INTRODUCTION LiFePO4, of the olivine phase, has received much research attention as a cathode material in Li-ion batteries thanks to its structural stability and common elemental components; furthermore, theoretical charge capacity for this material approaches 170 mAh g-1-, higher than that obtained by LiCoO2 [1]. However, poor electronic and ionic conductivity is an intrinsic issue for this cathode material. Typically, carbon-coating and nano-sizing techniques are used to optimize the electrochemical performance of LiFePO4 in Li-ion batteries [1]. The LiFePO4 structure is built by FeO6 octahedra connected by PO4 tetrahedra, with Li located in one-dimensional channels along the b-axis. Delithiation of the olivine phase LiFePO4 (o-LiFePO4) leads to the formation of isostructural FePO4 (o-FePO4). Furthermore, monoclinic and orthorhombic FePO4 may be produced by dehydration of monoclinic and orthorhombic FePO4⋅2H2O, respectively [2]. FePO4 also exists as trigonal (t-FePO4) phase, similar to α-quartz, with both Fe and P atoms in a tetrahedral environment of oxygen atoms. The trigonal phase undergoes a reversible α-β transition, similar to that of quartz. Monoclinic, orthorhombic, and olivine FePO4 phases all transform to t-FePO4 upon heating [2]. However, the thermodynamic stability of o-FePO4 remains a subject of controversy. For instance, theoretical calculations predict t-FePO4 more thermodynamically stable than its olivine counterpart [3]. On the other hand, in 2006, high-temperature oxide melt calorimetry results indicated the olivine phase of FePO4 as more thermodynamically stable than trigonal [4]. Oxygen evolution from the cathode material, in its charged state, is a major safety issue for Li-ion batteries. Even though LixFePO4 is much more stable than oxide cathodes and LixMnPO4, which decomposes with oxygen release above 210 °C [5], the thermal decomposition of LixFePO4 is not well understood. Computations by Ceder’s group suggest that a metastable decomposition route of LixFePO4 would lead
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