The rapid microwave-assisted hydrothermal synthesis of NASICON-structured Na3V2O2 x (PO4)2F3-2 x (0 < x ≤ 1) cathode materials for Na-ion batteries

Daria Burova; Iaroslava Shakhova; Polina Morozova; Anna Iarchuk; Oleg A Drozhzhin; Marina G Rozova; S Praneetha; Vadivel Murugan; Jean-Marie Tarascon; Artem M Abakumov

doi:10.1039/c9ra02257k

The rapid microwave-assisted hydrothermal synthesis of NASICON-structured Na₃V₂O_{2 x} (PO₄)₂F_{3-2 x} (0 < x ≤ 1) cathode materials for Na-ion batteries

RSC Adv. 2019 Jun 20;9(34):19429-19440. doi: 10.1039/c9ra02257k. eCollection 2019 Jun 19.

Authors

Affiliations

¹ Center for Energy Science and Technology, Skolkovo Institute of Science and Technology Nobel Str. 3 143026 Moscow Russia y.shakhova@skoltech.ru +7 495 280 14 81 ext. 3244.
² Chemical Department, Moscow State University Leninskie Gory, 1 119234 Moscow Russia.
³ Advanced Functional Nanostructured Materials Laboratory, Centre for Nanoscience and Technology, Madanjeet School of Green Energy Technologies, Pondicherry University (A Central University) Kalapet Puducherry 605014 India.
⁴ Collège de France, Chimie du Solide et de l'Energie, UMR 8260 11 Place Marcelin Berthelot 75231 Paris Cedex 05 France.

Abstract

NASICON-structured Na₃V₂O_2x (PO₄)₂F_3-2x (0 < x ≤ 1) solid solutions have been prepared using a microwave-assisted hydrothermal (MW-HT) technique. Well-crystallized phases were obtained for x = 1 and 0.4 by reacting V₂O₅, NH₄H₂PO₄, and NaF precursors at temperatures as low as 180-200 °C for less than 15 min. Various available and inexpensive reducing agents were used to control the vanadium oxidation state and final product morphology. The vanadium oxidation state and O/F ratios were assessed using electron energy loss spectroscopy and infrared spectroscopy. According to electron diffraction and powder X-ray diffraction, the Na₃V₂O_2x (PO₄)₂F_3-2x solid solutions crystallized in a metastable disordered I4/mmm structure (a = 6.38643(4) Å, c = 10.62375(8) Å for Na₃V₂O₂(PO₄)₂F and a = 6.39455(5) Å, c = 10.6988(2) Å for Na₃V₂O_0.8(PO₄)₂F_2.2). With respect to electrochemical Na⁺ (de)insertion as positive electrodes (cathodes) for Na-ion batteries, the as-synthesized materials displayed two sloping plateaus upon charge and discharge, centered near 3.5-3.6 V and 4.0-4.1 V vs. Na⁺/Na, respectively, with a reversible capacity of ∼110 mA h g^-1. The application of a conducting carbon coating through the surface polymerization of dopamine with subsequent annealing at 500 °C improved both the rate capability (∼55 mA h g^-1 at a discharge rate of 10C) and capacity retention (∼93% after 50 cycles at a discharge rate of C/2).