Reduced graphene oxide wrapped Na₃(VO)₂(PO₄)₂F microcubes for high-performance sodium-ion batteries

Area of Science:

• Materials Science
• Electrochemistry
• Energy Storage

Background:

• Sodium-ion batteries (SIBs) are a promising alternative to lithium-ion batteries.
• Na3(VO)2(PO4)2F (NVOPF) offers high working potential and energy density but suffers from poor conductivity and structural control.
• Addressing these limitations is crucial for practical SIB applications.

Purpose of the Study:

• To synthesize novel NVOPF structures with improved sodium storage performance.
• To enhance the electronic conductivity and structural stability of NVOPF cathodes.
• To investigate the electrochemical reaction mechanism and structural evolution of NVOPF during cycling.

Main Methods:

• Hydrothermal synthesis using phytic acid (PA) and citric acid (CA) to control NVOPF morphology.
• Incorporation of reduced graphene oxide (rGO) to improve electronic conductivity.
• Electrochemical testing including rate capability, cycling stability, and galvanostatic intermittent titration technique (GITT).
• Operando synchrotron X-ray diffraction (XRD) to analyze structural changes during sodiation/desodiation.

Main Results:

• Cuboid-like and truncated cube-like NVOPF structures were successfully synthesized, with smaller particle sizes (0.81 μm) showing enhanced kinetics.
• rGO-wrapped NVOPF (NVOPF/rGO) composites exhibited significantly improved electronic conductivity.
• The optimized NVOPF/rGO cathode delivered a reversible capacity of 130.3 mAh g⁻¹ at 0.2C and maintained 100.0 mAh g⁻¹ after 3500 cycles at 10C (0.0043% decay rate per cycle).
• Operando XRD confirmed a solid-solution reaction mechanism with highly reversible lattice parameters and a small volume change (2.28%) for NVOPF.
• A full cell utilizing NVOPF/rGO cathode and hard carbon anode demonstrated excellent cycling stability.

Conclusions:

• Phytic acid and citric acid enable controllable synthesis of NVOPF with tunable morphologies.
• Reduced graphene oxide effectively enhances the conductivity and electrochemical performance of NVOPF.
• NVOPF possesses a robust structure with excellent reversibility, making it a highly promising cathode material for SIBs.
• The developed NVOPF/rGO cathode demonstrates potential for practical, high-performance sodium-ion battery applications.