KAIST Researchers Develop Sequential Ion Electrode for Next-Generation Aqueous Batteries

Researchers at the Korea Advanced Institute of Science and Technology (KAIST) have engineered an electrode material capable of storing zinc ions and protons in a controlled sequence within aqueous energy storage systems. The innovation addresses a longstanding efficiency bottleneck in water-based battery chemistries by converting previously unwanted proton reactions into an additional source of energy capacity.

Aqueous zinc-ion batteries have emerged as a promising technology for utility-scale energy storage systems (ESS) due to their non-flammable water-based electrolytes, low manufacturing costs, and minimal environmental footprint compared to lithium-ion alternatives. However, standard designs struggle to achieve high energy capacity alongside rapid charging capabilities.

The primary operational challenge stems from the chemical behavior of zinc ions. As divalent charge carriers, zinc ions migrate slowly through traditional porous electrode structures during high-rate charging cycles. While smaller protons move rapidly through electrolyte solutions, unregulated proton activity typically leads to byproduct formation on electrode surfaces, blocking zinc transport and degrading cell performance over time.

To overcome these limitations, a research group led by Sarah S. Park in KAIST's Department of Chemistry modified a two-dimensional conductive metal-organic framework (MOF) to manage ion capture by voltage tier, as first reported by TechXplore. Metal-organic frameworks feature structured crystalline networks containing uniform microscopic pores.

The research team incorporated amine functional groups into the micropores of a MOF compound designated as Cu₃(HHTATP)₂. The amine groups were tuned to trigger proton bonding only when internal cell potential drops to a specific lower voltage threshold. As a result, larger zinc ions insert into the electrode structure first at higher operating voltages, followed by proton absorption as the voltage decreases. The mechanism allows smaller protons to occupy remaining space inside the micropores without interfering with earlier zinc accumulation, similar to placing large stones into a vessel before filling the remaining cavities with fine sand.

According to research findings published in the journal Chem by lead author Geunchan Park and colleagues, the Cu₃(HHTATP)₂ electrode demonstrated a specific capacity of 368.7 mAh g⁻¹ at a current density of 0.5 A g⁻¹. When tested under a 16-fold increase in charge and discharge speed, the material preserved 46.9% of its base energy capacity.

Comprehensive X-ray analytical techniques confirmed the step-by-step uptake and release of both ion species during cycling. The structural design prevented parasitic surface reactions, allowing the test cells to sustain stable performance across more than 500 rapid charge and discharge cycles.

The molecular engineering approach offers a scalable pathway for designing high-rate, safe battery materials suitable for large-scale energy infrastructure. "This study demonstrates that protons, previously regarded as 'troublemakers' that could degrade battery performance, can instead be used to store more energy," Park said in a statement. "We expect that applying this principle to various electrode materials will lead to the development of batteries capable of rapidly storing larger amounts of energy."