University of Freiburg Leads Breakthrough in Aluminum-Ion Battery Research

A team led by the University of Freiburg has made remarkable strides in aluminium ion battery chemistry, bringing us closer to a viable alternative to traditional batteries that rely on rare and challenging-to-recycle resources like lithium. Aluminium, being abundantly available in the Earth’s crust, offers a more sustainable and cost-effective solution, along with improved safety and recyclability.

Although the development of aluminium-ion batteries is still in its early stages, the main hurdle has been finding suitable electrode materials that can provide sufficient storage capacity. Addressing this challenge, a research team led by Prof. Dr Birgit Esser from the University of Ulm, in collaboration with Prof. Dr Ingo Krossing and Prof. Dr Anna Fischer from the University of Freiburg, has successfully created a positive electrode material using an organic redox polymer based on phenothiazine.

In their experiment, the team discovered that an aluminium-ion battery equipped with this innovative electrode material achieved an unprecedented storage capacity of 167 milliampere hours per gram (mAh/g). This surpasses the capacity of graphite, which has predominantly been used as electrode material in batteries thus far.

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The chemistry behind it…

During the charging process of the battery, the electrode material undergoes oxidation and incorporates complex aluminate anions. This enables the organic redox polymer poly(3-vinyl-N-methylphenothiazine) to reversibly insert two [AlCl4]− anions while charging. To create this system, the researchers employed an electrolyte consisting of the ionic liquid ethylmethylimidazolium chloride with added aluminium chloride.

Gauthier Studer remarked, “Aluminum batteries present an exciting area of study with immense potential for future energy storage systems. Our focus centres on developing novel organic redox-active materials that exhibit excellent performance and reversible properties. Through our investigation of the redox characteristics of poly(3-vinyl-N-methylphenothiazine) in a chloroaluminate-based ionic liquid, we have achieved a significant breakthrough by demonstrating, for the first time, a reversible two-electron redox process for an electrode material based on phenothiazine.”

After undergoing 5,000 charge cycles at 10°C, the battery retains 88 per cent of its initial capacity. Poly(3-vinyl-N-methylphenothiazine) deposits the [AlCl4]− anions at potentials of 0.81 and 1.65 volts, providing specific capacities of up to 167 mAh/g. In comparison, graphite as an electrode material in aluminium batteries has a discharge capacity of 120 mAh/g. The battery presented by the research team maintains 88 per cent of its capacity after 5,000 charge cycles at a rate of 10 C, corresponding to a charge and discharge time of 6 minutes. At a lower C rate, indicating an extended charge and discharge period, the battery regains its original capacities without any degradation. Achieving 5,000 cycles is a remarkable accomplishment, particularly at such a rapid charge and discharge rate.

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While the operating voltage and capacities may not be exceptionally remarkable, this technology holds promise for numerous applications and could alleviate the demand for lithium in the market. The breakthrough represents a significant step forward in the pursuit of sustainable energy storage solutions. Aluminium ion batteries show great promise in reducing our reliance on limited resources while offering enhanced recyclability and affordability. As further research and development continue, we can expect even more advancements in the field, bringing us closer to a greener and more efficient future.