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Title: β-NaVOPO4 obtained by a low-temperature synthesis process: A new 3.3 V cathode for sodium-ion batteries

Vanadyl phosphates (VOPO4) represent a class of attractive cathodes in lithium-ion batteries. However, the exploration of this type of materials in sodium-ion batteries is rare. Here, we report for the first time the synthesis of orthorhombic β-NaVOPO4 by first chemically extracting lithium from beta-LiVOPO4 and then inserting sodium into the obtained β-VOPO4 by a microwave-assisted solvothermal process with NaI, which serves both as a reducing agent and sodium source. Intermediate NaxVOPO4 compositions with x = 0.3, 0.5, and 0.8 have also been obtained by controlling the amount of NaI in the reaction mixture. Joint Rietveld refinement of synchrotron X-ray diffraction (XRD) and neutron diffraction confirms that the fully sodiated β-NaVOPO4 is isostructural with the lithium counterpart β-LiVOPO4. Bond valence sum maps suggest that sodium ions possibly diffuse along the [010] direction in the lattice, similar to the ionic conduction pathway in β-LiVOPO4. Although the initial discharge capacity is low due to the protons in the structure, it steadily increases with cycling with a long plateau at 3.3 V. As a result, ex situ XRD data of cycled β-VOPO4 and β-NaVOPO4 electrodes confirm the reversible reaction in sodium cells involving the V4+/V5+ redox couple.
Authors:
 [1] ;  [2] ;  [1] ;  [1]
  1. Univ. of Texas, Austin, TX (United States)
  2. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
Publication Date:
OSTI Identifier:
1261529
Grant/Contract Number:
AC05-00OR22725
Type:
Accepted Manuscript
Journal Name:
Chemistry of Materials
Additional Journal Information:
Journal Volume: 28; Journal Issue: 5; Journal ID: ISSN 0897-4756
Publisher:
American Chemical Society (ACS)
Research Org:
Oak Ridge National Laboratory (ORNL), Oak Ridge, TN (United States). Spallation Neutron Source
Sponsoring Org:
USDOE
Country of Publication:
United States
Language:
English
Subject:
25 ENERGY STORAGE