A step closer to 'greener' recyclable batteries

A team of researchers has introduced a new organic electrode material (OEM) for aqueous organic high-capacity batteries that can be easily and cheaply recycled. Unlike modern rechargeable batteries, such as lithium-ion batteries, organic batteries with redox-organic electrode materials can be synthesised from natural ‘green’ materials. Traditional inorganic electrode materials in commercial batteries involve a spectrum of problems: limited resources, toxic elements, environmental problems, limited capacity, and high costs. Sustainable batteries cannot be developed on a large scale based on these electrodes, though they are needed for an energy transition.

Organic batteries with OEMs are still at the beginning of the road toward practical applications. A team led by Chengliang Wang at Huazhong University of Science and Technology has taken a significant step that could facilitate the use of OEMs in batteries with aqueous electrolytes. These are ‘greener’, more sustainable and less expensive than the conventional organic electrolytes in lithium-ion batteries.

The researchers used azobenzene, a material that can be produced inexpensively on a large scale and is insoluble in water while being highly soluble in organic solvents. Whereas most other functional groups can only transfer one electron, the azo group (–N=N–) in this molecule is able to reversibly transfer two electrons, which contributes to a high capacity.

Comprehensive analyses demonstrated that, during the discharge process, the azobenzene is converted to hydroazobenzene after absorbing two of the electrons — through the rapid, reversible binding of two protons (H+). Prototype coin cells and laminated pouch cells of various sizes with azobenzene OEMs and zinc counter-electrodes reached capacities on the scale of ampere hours, which were retained over 200 charge–discharge cycles.

In contrast to polymeric OEMs, the small azobenzene molecules can be recycled with a simple extraction using commercial organic solvents. The electrode material is air-stable in both its charged and discharged states and can be recycled in yields of over 90% in every state of charge. The recycled products could be directly reused as OEMs with no loss of capacity. The research findings were published in the journal Angewandte Chemie.

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