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 using four different and facile one-step soft chemical routes. The as-synthesized compounds, Na_2.95(VO)₂(PO₄)₂F (I), Na_2.93(VO)₂(PO₄)₂F (II), and Na_2.58(VO)₂(PO₄)₂F (IV), crystallize in the tetragonal crystal system in the P4₂/mnm space group, while Na₃(VO)₂(PO₄)₂F (III) crystallizes in the orthorhombic crystal system in the Amam space group, as confirmed by Rietveld refinement of the high-resolution synchrotron powder X-ray diffraction and single-crystal X-ray diffraction data. Detailed crystal structure analysis, spectroscopic techniques, such as XANES, DRS, and 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 mA h g⁻¹ with an average voltage of 3.8 and 4 V, respectively, when cycled between 4.5 and 2.5 V. When the batteries are cycled between 4.5 and ∼1 V, a capacity exceeding 150 mA h 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 at faster C-rates and upon long cycling for Na-ion and Li-ion hybrid cells without much optimization of the particle size or chemical-carbon coating, demonstrating that the frameworks favor facile ion conduction of both Li- and Na-ions in their channels without degradation upon repeated cycling. Composition- and synthesis-dependent differences are observed in the electrochemical performances in Na- and Li-ion cells. These results demonstrate the importance of the structural framework of polyanion compounds that are highly stable in aqueous media and can be synthesized hydrothermally in highly pure forms with excellent electrochemical activities.