A study in Nature Communications introduces amorphous organic-hybrid vanadium oxide (AOH-VO) as a candidate material for ultrafast-charging aqueous zinc-ion batteries (AZIBs).
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The AOH-VO material consists of one-dimensional (1D) chains arranged in a disordered structure with atomic- and molecular-scale pores. This architecture facilitates hierarchical ion diffusion pathways while reducing Zn2+ interactions with the solid framework.
Background
Ultrafast-charging batteries with durable cycle stability are critical for applications such as grid-scale energy storage, electric vehicles, and portable electronics. AZIBs are preferred among various types of batteries because of their high energy density, nonflammability, affordability, and aqueous electrolyte’s excellent ion conductivity.
Achieving ultrafast charging in AZIBs is challenging due to the high activation energy (Ea) associated with metal ion diffusion in active materials and charge transfer kinetics at the solid-electrolyte interfaces. These intrinsic barriers limit the performance of AZIBs, despite the efficient metal ion transport in aqueous electrolytes.
Hybrid materials that combine organic components with inorganic oxides have been explored to improve AZIB stability. However, steric hindrance in crystalline hybrids continues to impede ion migration kinetics. This study proposes the use of amorphous organic-hybrid metal oxides as an alternative, aiming to retain the benefits of organic-inorganic hybrids while mitigating steric hindrance effects.
Methods
AOH-VO was synthesized via a solvothermal method using ammonium metavanadate, ethylene glycol (EG), and H2O2. To investigate the effect of H2O2 as a reaction regulator, seven samples were prepared by varying H2O2 quantities, maintaining a 12-hour reaction time due to the low yield of vanadyl ethylene glycolate (VEG). A calcined sample, AOH-VO-300, was also derived by heating AOH-VO at 300 °C in air.
The synthesized materials were characterized using X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and X-ray absorption spectroscopy (XAS). In-situ XRD and FTIR analyses were conducted during charge/discharge cycling on a pouch cell with AOH-VO as the active material.
Electrochemical characterization was performed using coin cells assembled with the AOH-VO samples. Tests included galvanostatic charge-discharge cycling, long-term performance evaluation, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS) for AOH-VO, VEG, and AOH-VO-300 cathodes.
Temperature-dependent EIS measurements were conducted at 20, 30, 40, 50, and 60 °C, while temperature-dependent Zn2+ diffusion coefficients were determined for AOH-VO and VEG using the galvanostatic intermittent titration technique (GITT) at 25, 40, and 55 °C.
Density functional theory (DFT) calculations were performed using the Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation within the Vienna Ab Initio Simulation Package (VASP) to further elucidate the material properties.
Results and Discussion
Thermogravimetric analysis revealed that the chelating agent (EG-CL)-related weight loss of the AOH-VO was considerably reduced to 12.45 % relative to that of crystalline VEG (28.4 %). Additionally, the EG-CL mass ratio in VEG was about twice that in AOH-VO.
Meanwhile, the V content increased from 39.77 % in VEG to 41.63 % in AOH-VO, exhibiting decreased organic content in AOH-VO. Differential thermal analysis results further supported the transformation from crystalline to AOH-VO with 1D defective EG-chelated VO5 chains (1D-D-VO5).
DFT calculations exhibited a broader and longer V-O bond length in the 1D-D-VO5 units relative to the perfect 1D-VO5, depicting the sensitivity of V-O bond interactions to the local atomic environment. These computational results aligned with the notable redshift and peak broadening for the V-O and V-O-V bonds in the FTIR spectra of AOH-VO. Thus, AOH-VO contained randomly arranged 1D-D-VO5 units with defective EG-CL and diminished interchain interactions.
The near barrier-free diffusion from the electrolyte to the solid framework enabled ultrafast Zn2+, resulting in high rate performance for AOH-VO. The material retained 85.3 % of its initial capacity at 1 A/g and achieved a specific capacity of 121 mAh/g at an ultrahigh current of 100 A/g, with a full discharge completed in 4.3 seconds. This performance corresponded to a capacity retention of 23.2 % over a 1000-fold increase in current density from 0.1 to 100 A/g.
SEM images and FTIR spectra after 5000 to 27,325 cycles indicated well-preserved surface morphologies, minimal structural degradation, and stable VO5-related vibrations and EG-associated peaks, confirming the structural stability of AOH-VO during long-term high-rate cycling. Additionally, SEM and XRD analyses of the Zn anode revealed no dendrite formation after 5000 to 25,000 cycles, demonstrating excellent long-term cycling stability.
Conclusion
The researchers developed AOH-VO with abundant atomic- and molecular-level pores formed by randomly assembled 1D-D-VO5 chains with weak interactions. This architecture maximized the exposure of active sites to the electrolyte, significantly reducing Ea for Zn2+ diffusion into the solid framework. The near barrier-free Zn2+ dynamics enabled an ultrafast charge-discharge process.
The weakened interaction between Zn2+ ions and the electrode skeleton contributed to exceptional stability, with the cell maintaining performance over 17,000 cycles at 100 A/g. The assembled pouch cell demonstrated a capacity of 2.04 Ah at 2 A/g, achieving full charge in 9.5 minutes and stable operation for more than 5,000 cycles. These findings suggest that AOH-VO is a promising candidate for stable, safe, and ultrafast-charging AZIBs.
Journal Reference
Liu, M., et al. (2024). Amorphous organic-hybrid vanadium oxide for near-barrier-free ultrafast-charging aqueous zinc-ion battery. Nature Communications. DOI: 10.1038/s41467-024-55000-8, https://www.nature.com/articles/s41467-024-55000-8
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