Rechargeable Organic Molecule-Air Battery
Authors: Kitajima Showa , Kasai Hitoshi , Oka Kouki 
Published: December 2024

[Introduction]

The development of environmentally friendly batteries, such as rechargeable aqueous metal-air batteries, has been required for sustainable energy supply. Aqueous zinc-air batteries, composed of zinc as the anode, O2 in the air as the cathode, and a very base aqueous solution (e.g., 6 M KOH aqueous solution) as the electrolyte, are one of the representative environmentally friendly batteries, because of their high energy density (1353 W h kg−1 excluding oxygen) compared to conventional lithium-ion batteries (limited to be <350 W h kg−1 based on the intercalation chemistry).[1] However, these batteries usually have extremely low cyclability because of dendrite formation on the anode and carbonate salt clogging (i.e., the reaction between CO2 in the air and the base) in the electrolyte during charging and discharging.
To improve their cyclability, we previously reported a rechargeable organic polymer-air battery using a redox polymer with an anthraquinone derivative (which has a one-step two-electron redox capability in an acidic aqueous solution).[2] This rechargeable organic polymer-air battery exhibits a very high Coulombic efficiency of 99% because of the distorted structure of the anthraquinone derivative. However, organic redox polymers with linear polymer structures are not sufficiently robust for long-term use as an anode-active material in aqueous air batteries.
On the other hand, Prof. Yu et al. have recently fabricated a rechargeable organic polymer-air battery using a networked polymer based on anthraquinone as an anode-active material and demonstrated very high cyclability of presumably 99% capacity retention even after 60,000 cycles.[3] This is presumably because the networked polymer forms a robust three-dimensional network that prevents it from decomposing or dissociating from the electrode. However, the Coulombic efficiency was lower than that of organic polymer-air batteries with organic linear polymers, and was only 95%.
To achieve both the sufficiently high cyclability and high Coulombic efficiency as an aqueous air battery, the anode-active material must possess high hydrophilicity, exhibit high diffusivity of compensating ions in the material, and have a robust structure that is not decomposed or dissociated from the electrode during charging and discharging.
In this work, as the anode-active material, we focused on the anthraquinone molecule itself, which has a reversible redox capacity at the very negative potential close to the potential window in aqueous electrolytes and whose redox properties can be modulated by facile organic synthesis. To improve anthraquinone’s hydrophilicity and inhibit its aggregation, we synthesized 2-propoxyethyl anthraquinone-2-carboxylate. 2-propoxyethyl anthraquinone-2-carboxylate had a reversible redox potential in an acidic aqueous solution, and we established a novel rechargeable organic molecule-air battery using2-propoxyethyl anthraquinone-2-carboxylate as the anode-active material and an acidic aqueous solution as the electrolyte.

[Results & Discussions]

The battery was chargeable and dischargeable, with the very high Coulombic efficiency of >99% at 15 C. The discharge capacity was almost full capacity, indicating that almost all the molecules contributed to charging and discharging. The discharge capacity remained >99% even after 100 cycles, indicating that the battery had a very high cyclability. Even at 60 C, the discharge capacity of the battery was almost the same as that at 15 C, indicating a high-rate capability. Moreover, the results confirm that this novel rechargeable organic molecule-air battery exhibits the highest cyclability and Coulombic efficiency in rechargeable organic-based aqueous air batteries. Using 2-propoxyethyl anthraquinone-2-carboxylate as an anode-active material in aqueous air batteries will potentially achieve almost the same energy density and increase power density by several times or more, compared to the polymer-air batteries demonstrated. Therefore, the energy density of the aqueous air battery using 2-propoxyethyl anthraquinone-2-carboxylatewill be almost the same as that of metal-air batteries (e.g., zinc-air batteries), and its power density is expected to be higher than that of lithium-ion batteries.
We want to discuss this in more detail at the poster session.

[Reference]

[1] J. Zhang et al., Chem. Sci.
2019, 10, 8924-8929.
[2] K. Oka et al., Macromolecules
2021, 54, 4854-4859.
[3] L. Zhong et al., Angew. Chem. Int. Ed.
2021, 60, 10164-10171.

Figure 1