Soft-Chemical Scalable One-pot Aqueous-medium Synthesis of Na₃(VO)₂(PO₄)₂F-Based Cathodes: Compositional Tuning and Electrochemical Performance in Na- and Li-Ion Batteries

Abstract

One of the most important cathode materials for Na-ion batteries, Na3(VO)2(PO4)2F, and its compositional variants have been synthesized employing four different and facile one-step soft chemical routes. The as-synthesized compounds, Na2.95(VO)2(PO4)2F(I), Na2.93(VO)2(PO4)2F(II), and Na2.58(VO)2(PO4)2F(IV), crystallize in tetragonal crystal system in P42/mnm space group, while Na3(VO)2(PO4)2F (III) crystallize in orthorhombic crystal system in Amam space group as confirmed by Rietveld refinement from the high-resolution synchrotron powder X-ray diffraction and single-crystal X-ray diffraction data. Detail crystal structure, spectroscopic techniques such as XANES, DRS, IR and chemical analysis reveal subtle differences in the compositions. The electrochemical performances of the four compounds are investigated in sodium and lithium-ion batteries, which exhibit high reversible capacities of more than 115 mAh.g–1 with an average voltage of 3.8 and 4 V, respectively, when cycled between 4.5 to 2.5 V. When the batteries are cycled between 4.5 to ~1 V, a capacity exceeding 150 mAh.g–1 can be achieved for Na- and Li-ion batteries showing the capability of displaying more than one electron process. In addition, the compounds show excellent capacity retention in faster C-rates and on long cycling for Na-ion and in Li-ion hybrid cells without much optimization of particle size or chemical-carbon coating demonstrating that the framework favor facile ion-conduction for both Li- and Na-ions in their channels without degradation on repeated cycling. The composition/synthesis-dependent differences are observed in the electrochemical performances in Na- and Li-ion cells. These results demonstrate the importance of structural framework of polyanion-compounds that are highly stable under aqueous medium and can be synthesized hydrothermally in highly pure form with excellent electrochemical activities.