Formamide-engineered VOPO4 cathodes with high volumetric capacity and mass loading for aqueous zinc-ion batteries

Aqueous zinc-ion batteries (AZIBs) are gaining traction as next-generation energy storage systems due to their inherent safety, cost-effectiveness, and high theoretical capacity. However, their practical application is limited by poor cycling stability and sluggish ion diffusion kinetics, especially under high mass loading conditions. These issues are mainly attributed to restricted ion transport pathways within the electrode structure and structural degradation caused by repeated zinc-ion insertion and extraction.

This study introduces a novel strategy to overcome these challenges by intercalating formamide (FA) into VOPO₄. The FA-VOPO₄ nanosheet cathodes were designed with expanded interlayer spacing (9.3 Å), where FA molecules partially replace interlayer water. This unique structural modification enhances both structural stability and ion transport pathways. The synergistic hydrogen bonding between FA and residual water significantly improves Zn²⁺ diffusion kinetics and charge transfer properties.

The FA-VOPO₄ electrodes delivered remarkable electrochemical performance. At a mass loading of 7 mg/cm², they achieved a specific mass capacity of 463 mAh/g and a volumetric capacity of 733 mAh/cm³, approximately 8 times higher than that of the VOPO₄·2H₂O electrode. Even at a high mass loading of 20 mg/cm², the FA-VOPO₄ cathode maintained a volumetric capacity of 535 mAh/cm³. After 1000 cycles at 1 A/g with a mass loading of 10 mg/cm², the FA-VOPO₄ electrode retained 82.1% of its capacity, demonstrating superior cycling stability.

This work provides valuable insights into electrode design strategies for high-performance AZIBs. The successful synthesis of FA-VOPO₄ nanosheets with enlarged interlayer spacing offers a promising pathway toward high-energy-density, safer, and more durable aqueous energy storage systems. These findings contribute to the development of more efficient energy storage technologies with potential applications in grid storage and portable electronics.