Metal-free cathode rivals commercial battery performance

Mining metals for lithium-ion batteries carries environmental and ethical concerns. To make batteries more sustainable and to reduce costs, carmakers are trying new strategies, including cutting the amount of cobalt in cathodes and adopting chemistries like lithium iron phosphate (LFP). Metal-free organic cathodes would be an ideal alternative, but their performance has been lackluster. Now, researchers report an organic cathode material with performance that matches state-of-the-art lithium-ion cathodes. The material, reported on the preprint server ChemRxiv (2023, DOI: 10.26434/chemrxiv-2023-j91zf), can hold as much energy as commercial Co- and Ni-based cathodes, can charge in minutes, and boasts a lifetime of over 2,000 recharge cycles; today’s EV batteries typically last 1,000 to 1,500 cycles.

The new material, bis-tetraaminobenzoquinone, is also “super easy to make [using] organic precursors that are commodity chemicals made on a million-kilogram scale,” says Mircea Dincă, a chemist at MIT who led the work, which is due for publication in ACS Central Science. The raw materials for the cathode should cost $1-2 per kilogram, far cheaper than LFP.

Electric vehicle batteries today mostly use nickel-manganese-cobalt (NMC) cathodes with a layered crystal structure in which lithium ions quickly move in and out of the space between the layers during charge and discharge. But mining Co and Ni is linked to biodiversity loss, pollution, and human rights concerns. Many big carmakers are now switching to low-cost LFP cathodes even though the material has lower energy density.

For a more sustainable, cost-effective option, researchers have tried to make cathodes from organic small molecules and polymers. But these have typically had low conductivity and eventually dissolve in the electrolyte, killing the battery, Dincă says.

Dincă and colleagues got around those two key limitations by designing an organic material that is a stack of two-dimensional sheets. Each sheet is a lattice of closely packed tetraaminobenzoquinone (TAQ) molecules connected to each other via hydrogen bonds between their carbonyl and imine groups. Lithium ions move quickly between the flat sheets just like in NMC cathodes, he says, while “the superstrong lattice of hydrogen bonding makes it incredibly insoluble in any battery electrolyte.”

In laboratory tests, the material showed an energy density of 765 watt-hours/kg, higher than most cobalt-based cathodes, and had a charging time of six minutes. Small coin cells made with the cathode held 70% of their charge capacity after 2,000 charge cycles. The team has launched a startup to commercialize the technology.

Whether the large-scale synthesis of TAQ is possible and cost-effective remains to be seen, says Kostiantyn Kravchyk, a materials scientist at Empa, the Swiss Federal Laboratories for Materials Science and Technology. But the “material exhibits exceptional performance. Its sustainability and [abundant] constituent materials could position it as a competitive alternative to existing commercial cathodes.”

The material is also novel, says Alexandru Vlad, a chemist at Université catholique de Louvain, because unlike conventional cathode materials, it can be used as an electrode in its pure form without conductive carbon additives or binders, which improves the energy density and “could simplify the TAQ-based battery manufacture in the future.”