Available on-demand - EN03.09.30
Late News: Crystal Structures and Local Environments of NASICON-Type Na3FeV(PO4)3 and Na4FeV(PO4)3 Positive Electrode Materials for Na-Ion Batteries
Sunkyu Park1,2,3,Jean-Noël Chotard1,4,5,Dany Carlier2,4,5,Iona Moog3,Mathieu Duttine2,Antonella Iadecola4,François Fauth6,Christian Masquelier1,4,5,Laurence Croguennec2,4,5
Laboratoire de Reactivite et de Chimie des Solides1,Institut de Chimie de la Matière Condensée de Bordeaux2,TIAMAT Energy3,RS2E, Reeseau Francais sur le Stockage Electrochimique de l’Energie4,ALISTORE-ERI European Research Institute5,CELLS-ALBA Synchrotron6
NASICON-type Na3V2(PO4)3 (NVP) is promising as positive electrode material for Na-ion batteries due to its robust crystal structure, providing long cycle life and great rate capability.[1,2] However, it is able to exchange only two Na+ ions per two Vanadium transition metals leading to a moderate specific capacity of 118 mAh/g. Therefore, it has been an important challenge in the research field to improve the specific capacity while keeping the solid framework of NASICON structure. Recently, several new NASICON-type materials such as Na3+xMgxV2-x(PO4)3, Na4MnV(PO4)3,[4-6] and Na4MnCr(PO4)3, have been reported attempting to gain higher specific capacity and have better sustainability. Surprisingly, Na4FeV(PO4)3 has not been reported yet in literature. We thus decided to study the Fe/V-mixed system demonstrating possible extraction of three Na+ ions per two transition metals. Sol-gel and solid-state methods with various synthesis parameters were investigated to synthesize the target material, Na4FeV(PO4)3. We obtained the NASICON-type phase with small amount of secondary phases (NaFePO4 and Na3PO4). To obtain the pure phase, Na3FeV(PO4)3 has been synthesized and electrochemically sodiated to form Na4FeV(PO4)3. Herein, we present for the first time crystal structures of the NASICON materials, Na3FeV(PO4)3 and Na4FeV(PO4)3, using high resolution Synchrotron X-ray diffraction (SXRD) as well as the electrochemical performances. 2.64 Na+ were extracted from the Na4FeV(PO4)3 upon charge process, corresponding to a specific capacity of 146.3 mAh/g. We could clearly observe a superstructure in Na3FeV(PO4)3 due to Na+ ordering, which is different from the structure reported in the literature. Thermal analyses of Na3FeV(PO4)3 were performed combining in situ temperature XRD and differential scanning calorimetry (DSC). An ordered/disordered phase transition is observed at 123°C with an increase symmetry from monoclinic (Space Group: C2/c) to rhombohedral (Space group: R-3c). The superstructure reflections from the ordered phase completely disappear at high temperature, which reversibly appear again as temperature goes down. The fully electrochemically sodiated Na4FeV(PO4)3 crystallizes in a rhombohedral unit cell (R-3c) with all the sodium sites almost fully occupied. Since Vanadium and Iron share the same crystallographic site synchrotron X-ray absorption spectroscopy (XAS) at V and Fe K-edges and Mossbauer spectroscopy were performed. These technics allow to differentiate the local V, and Fe environments, respectively, and their oxidation states in both Na3FeV(PO4)3 and Na4FeV(PO4)3. Extended X-Ray Absorption Fine Structure (EXAFS) analysis shows that average bond distance of Fe – O increases from 1.99 Å in Na3FeV(PO4)3 to 2.06 Å in Na4FeV(PO4)3 while that of V – O remains unchanged as 2.02 Å suggesting the reduction of Fe3+ to Fe2+ without noticeable distortion of the MO6 octahedra. The details of the local environments and oxidation states evolution and the electrochemical performances will be discussed.
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