Hydroxyl groups on carbon nanotube cathodes enable ultra-long-life aqueous Zn-I2 batteries

Science China Press

Aqueous zinc-ion batteries (ZIBs) have emerged as promising next-generation energy storage systems, owing to the natural abundance and high theoretical specific capacity of the Zn metal anode, and inherent safety of aqueous electrolytes. Among them, aqueous Zn-I₂ batteries stand out by virtue of long cycling stability, great rate performance, and a high theoretical capacity. However, I₂-based cathodes face several problems: poor electrical conductivity of I₂, the cathode volume changes during cycling, and the shuttle effect of I₃⁻.

To tackle these challenges, researchers at Jilin University and Changchun Institute of Applied Chemistry developed self-supporting carbon nanotube cathodes (SSCC) with controlled surface functional groups. Guided by density functional theory calculations, they compared pristine carbon nanotubes and nanotubes modified with hydroxyl (‒OH), carboxyl (‒COOH) and amine (‒NH₂) groups. The calculations showed that ‒OH-modified carbon nanotubes had the strongest adsorption affinity for I₂ species. The adsorption energy of I₃⁻ on ‒OH-modified nanotubes was −7.42 eV, and the interaction energy was −5.69 eV, stronger than those on pristine, ‒COOH-, and ‒NH₂-modified nanotubes.

Experimentally, the SSCC‒OH cathode had a high specific surface area of about 1005 m²/g, which helped confine I₂ species inside the nanotube channels. Spectroscopy and microscopy confirmed the functional groups and uniform I₂ loading. The optimized cathode, named SSCC-OH/I₂, delivered an initial reversible capacity of 175.5 mAh/g at 1 A/g and retained 89.34% of its capacity after more than 10,000 cycles, corresponding to over 3,500 hours. At a high current density of 10 A/g, it still achieved an initial capacity of 120 mAh/g with a cycling life of 93,400 cycles.

Mechanistic studies using in situ Raman spectra and ex situ X-ray photoelectron spectroscopy showed that iodine species were reversibly converted during charge and discharge. In the SSCC-OH/I₂ system, I₃⁻ completely disappeared at the end of charging and discharging, indicating that the shuttle effect was effectively suppressed. A pouch cell with a high I₂ loading of 150 mg delivered an operating voltage of 1.1 V at 3.612 mA/cm², an actual capacity of 23.8 mAh and 91.7% capacity retention after 350 cycles.

The researchers conclude that functional group chemistry, especially ‒OH modification, provides a simple and effective way to improve iodine conversion kinetics and stabilize aqueous zinc-iodine batteries. The strategy may guide the design of other long-life aqueous energy storage systems. The findings were published in Science China Chemistry.