A Biomolecular Eco-Friendly Catalyst for Lithium-Oxygen Batteries

In recent years lithium-oxygen (Li-O₂) batteries have drawn much attention due to their exceptionally high energy density, which could exceed the capable value of conventional Li-ion batteries.^{1, 2} Li-O₂ batteries operate via surface reactions that form (discharging) and evolve (charging) solid oxide products.^{3, 4} To facilitate reaction reversibility, the oxygen electrode for Li-O₂ cells often requires an efficient catalyst. Yet insulating solid products formed during discharging often deactivate the catalyst surface, making product evolution difficult.⁵ Soluble catalysts have recently been shown to improve charge transfer with isolated or poorly conductive products near the electrode/electrolyte interface.⁶ Choosing proper catalytic molecules is essential to reduce the barrier to oxygen evolution in the Li-O₂ cell. The redox molecules directly transport the generated electrons to/from the electrode substrate, consequently lowering overpotential. While a few redox molecules (e.g. Li iodide (LiI), tetrathiafulvalene (TTF), iron phthalocyanine (FePc), and 2,2,6,6-tetramethylpinperdinyloxyl (TEMPO)) have been investigated so far,^6-9 seeking a low cost and environmentally friendly alternative is desirable.

In this presentation, we report the use of a common biomolecule as a soluble, eco-friendly catalyst to promote Li-O₂ reactions with reduced overpotentials. We also elucidate the chemical reaction mechanism of its operation during oxide formation and evolution. In situ observations of chemical structure in the redox molecule are essential to establish the catalytic function and further design molecules for high performance Li-O₂ battery systems. Here, we discuss the catalytic effects of the redox biomolecule on the significantly improved electrochemical characteristics of a practical Li-O₂ battery.

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