A group of researchers from the University of Ulm and the University of Freiburg have developed a positive electrode material made of an organic redox polymer based on phenothiazine for aluminum-ion batteries to help enhance its storage capacity.

Given that aluminum is the most abundant metal in the Earth’s crust and its recycling is much easier compared to the largely used lithium, the experiment targets developing new organic redox-active materials that show high performance and reversible properties.

Headed by Professor Gauthier Studer and led by Professor Birgit Esser from the University of Ulm and Professor Ingo Krossing along with Professor Anna Fischer from the University of Freiburg, the researchers used the new redox-electrode material and achieved a storage capacity of 167 milliampere hours per gram (mAh/g) in the aluminum-ion batteries.

The storage capacity attained by the scientists surpassed the capacity of graphite, which is largely used as the electrode material in lithium-ion batteries, among others, to date.

Published in the scientific journal Energy and Environmental Science, the work showed that aluminum’s higher volumetric capacity of 8,040 mAh cm−3 as a negative electrode material exceeded the capacity of lithium, i.e., 2,046 mAh cm−3.

The researchers found that compared to lithium, aluminum’s reverse stripping method helps avoid dendrites that usually degrade the batteries over time.

Moreover, the ionic liquid electrolytes currently used in aluminum-ion batteries are non-flammable, making rechargeable aluminum-ion batteries a potential storage device.

Methodology

The only challenge that poses a hurdle for the wider use of aluminum batteries is developing suitable positive electrode materials to help maintain an enhanced storage capacity.

For this, the researchers oxidized the redox-polymer poly (3-vinyl-N-methylphenothiazine) material while the battery was charging and inserted two aluminum chloride anions in the battery reversibly. Ionic liquid ethyl methylimidazolium chloride and added aluminum chloride were included as an electrolyte.

“By studying the redox properties of poly(3-vinyl-N-methylphenothiazine) in chloroaluminate-based ionic liquid, we have made a significant breakthrough by demonstrating for the first time a reversible two-electron redox process for a phenothiazine-based electrode material,” explained Professor Studer.

The anions deposited after the incorporation of phenothiazine helped attain the energy storage capacity of 167 mAh/g in the battery that retained 88% of its capacity even after 5,000 charge cycles at 10 degrees Celsius, i.e., at a charge and discharge rate of six minutes, per the observations of the researchers.

Professor Esser states, “With its high discharge voltage and specific capacity, as well as its excellent capacity retention at fast C rates, the electrode material represents a major advance in developing rechargeable aluminum batteries and thus of advanced and affordable energy storage solutions.”

In a similar study, researchers at the Massachusetts Institute of Technology developed a new battery made from aluminum and sulfur, two of the nature-abundant and cost-friendly materials.

A recent study by the Karlsruhe Institute of Technology showed that the formation of the solid electrolyte interphase, which is crucial for the functioning of lithium-ion batteries, occurs through the aggregation of solutions rather than directly at the electrode.