Hydrothermal Microwave-Assisted Synthesis of Na_3+xV_2–yMn_y(PO₄)₂F₃ Solid Solutions as Potential Positive Electrodes for Na-Ion Batteries

Hydrothermal microwave-assisted synthesis of Mn-substituted Na3+xV2–yMny(PO4)2F3 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) Å, c = 10.6402(3) Å for Na3.6V1.4Mn0.6(PO4)2F3]. 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)2F3 revealed the ability to reversibly (de)intercalate Na+ in the potential range of 2.5–4.6 V versus Na/Na+ with ∼108 mA h/g discharge capacity, ∼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)2F3/hard carbon full cell with the specific energy of 312 Wh per kg of cathode.