Polymer to enhance storage capacity of aluminium-ion batteries

Aluminium-ion batteries are a promising alternative to conventional batteries that use scarce and difficult-to-recycle raw materials such as lithium. This is because aluminium is easier to recycle, safer and less expensive than lithium. However, the development of aluminium-ion batteries requires further research, as suitable electrode materials that provide sufficient storage capacity are still lacking.

A research team led by Gauthier Studer and Professor Dr Birgit Esser of the University of Ulm and Professor Dr Ingo Krossing of the University of Freiburg has developed a positive electrode material consisting of an organic redox polymer based on phenothiazine. In the experiment, aluminium batteries with this electrode material stored a capacity of 167 milliampere hours per gram (mAh/g). The organic redox polymer reportedly surpasses the capacity of graphite, which has mostly been used as an electrode material in batteries to date. The research findings were published in the journal Energy & Environmental Science.

The electrode material is oxidised during charging of the battery, thereby taking up complex aluminate anions. In this way, the organic redox polymer poly (3-vinyl-N-methylphenothiazine) manages to insert two [AlCl₄]^– anions reversibly during charging. The researchers used the ionic liquid ethylmethylimidazolium chloride as electrolyte with added aluminium chloride. Studer said that the study of aluminium batteries has great potential for future energy storage systems.

“Our focus lies on developing new organic redox-active materials that exhibit high performance and reversible properties. 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,” Studer said.

Poly (3-vinyl-N-methylphenothiazine) deposits the [AlCl₄]⁻ anions at potentials of 0.81 and 1.65 volts and provides specific capacities of up to 167 mAh/g. In contrast, the discharge capacity of graphite as an electrode material in aluminium batteries is 120 mAh/g. After 5000 charge cycles, the battery presented by the research team still has 88% of its capacity at 10 C, at a charge and discharge rate of six minutes. At a lower C rate (a longer charge and discharge time), the battery returns unchanged to its original capacities.

“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 the development of rechargeable aluminium batteries and thus of advanced and affordable energy storage solutions,” Esser said.

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