Hydrothermal Microwave-Assisted Synthesis of Na3+xV2-yMny(PO4)2F3 Solid Solutions as Potential Positive Electrodes for Na-Ion Batteries

Hydrothermal microwave-assisted synthesis of Mn-substituted Na3+xV2-yMny(PO4)(2)F-3 positive electrode materials for Na-ion batteries resulted in phase-pure solid solutions with y <= 0.6. According to joint Rietveld refinement from the X-ray and neutron powder diffraction data, the solid solution adopts a highly disordered tetragonal I4/mmm structure [a = 6.4695(1) angstrom, c = 10.6402(3) angstrom for Na3.6V1.4Mn0.6(PO4)(2)F-3]. Electron energy loss spectroscopy (EELS) revealed the Mn2+ and V3+ oxidation states, whereas the charge compensation for the heterovalent V3+ -> Mn2+ substitution occurs by a corresponding increase in the Na content. Electrochemical tests on carbon-coated Na3.6V1.4Mn0.6(PO4)(2)F-3 revealed the ability to reversibly (de)intercalate Na+ in the potential range of 2.5-4.6 V versus Na/Na+ with similar to 108 mA h/g discharge capacity, similar to 3.7% volume change, and >50% capacity retention at 10 C rate. Post-mortem EELS analysis revealed that the redox process up to 3.8 V is dominated by the Mn2+/Mn3+ pair, whereas in the 3.8-4.6 V region, the V3+/V4+ pair is active where V4+ is a result of disproportionation into V3+ and V5+. A promising application of the Mn-doped materials as positive electrodes in sodium-ion batteries is demonstrated by a steady-functioning Na3.6V1.4Mn0.6(PO4)(2)F-3/hard carbon full cell with the specific energy of 312 Wh per kg of cathode.

Automated summary

The hydrothermal microwave-assisted synthesis of Mn-substituted Na3+xV2-yMny(PO4)(2)F-3 positive electrode materials for Na-ion batteries resulted in phase-pure solid solutions with y <= 0.6. Characterization with EELS revealed the oxidation states of Mn2+ and V3+, with charge compensation achieved through an increase in Na content. The electrochemical tests demonstrated the reversible intercalation of Na+ with a discharge capacity of approximately 108 mA h/g and >50% capacity retention at 10 C rate.