High Voltage Na3V2O2x(PO4)2F3-2x (0≤x≤1) Positive Electrode Materials for Na-Ion Batteries

Sodium-ion batteries have arisen as good candidates to become alternative systems that could complement commercial Li-ion systems [1]. The use of Na instead of Li in rocking chair batteries could mitigate the possible shortage of lithium in an economical way. For this purpose, the search for commercially viable Na-ion batteries demands finding and optimizing new electrode materials and electrolytes.

Among the possible cathodic materials, the family of compounds Na₃V₂O_2x(PO₄)₂F_3-x (Fig. 1) presents high operating voltages and specific capacity values, which are key parameters to get a high energy density battery [2]. In this study, two sodium-vanadium fluorophosphate materials were prepared:

-           Carbon-free and carbon-coated Na₃V₂O₂(PO₄)₂F phase (x = 1), where vanadium oxidation state is V⁴⁺.

-           A mixed valence V³⁺/V⁴⁺ carbon-coated material belonging to the Na₃V₂O_2x(PO₄)₂F_3-2x  family where 0 ≤ x ≤ 1.

Structural characterization of the composites was performed by powder X-ray diffraction (XRD). A double titration method as well as ²³Na and ¹⁹F NMR measurements were necessary for the determination of the oxidation state of the vanadium. The morphology of the materials was analyzed by Scanning Electron Microscopy (SEM). The electrochemical measurements were conducted using Swagelok-type cells versus a metallic sodium anode. Ex-situ XRD and XANES data of electrodes charged and discharged at selected voltages allowed to deeper study the structural evolution of the material during cycling.

The two investigated compounds show similar electrochemical features such as high operating voltages (3.6 and 4.1 V vs. Na/Na⁺) and a high theoretical specific capacity (about 130 mAh/g). However, some differences can be observed in the potential vs. capacity curves (Fig. 2). A comparison between the up-to-date literature data [3,4] and the electrochemical data obtained from both the mixed valence and V⁴⁺materials will be established. The relationships composition-structure-electrochemical behaviour will be presented in detail and discussed.

[1] V. Palomares, P. Serras, I. Villaluenga, K.B. Hueso, J. Carretero-González and T. Rojo Energy and Environ. Sci. 5 (2012) 5884.

[2] P. Serras, V. Palomares, A. Goñi, I. Gil de Muro, P. Kubiak, L. Lezama and T. Rojo J. Mater. Chem. 22 (2012) 22301.

[3] K. Chihara, A. Kitajou, I. D. Gocheva, S. Okada, J-I Yamaki J Power Sources, (2012) doi: 10.1016/j.jpowsour.2012.10.034.

[4] R. A. Shakoor, D-H Seo, H. Kim, Y-U Park, J. Kim, S-W Kim, H. Gwon, S. Leec and K. Kang .J. Mater. Chem. 22 (2012) 20535.