The next generation of energy storage – Aluminium-Ion-Batteries
Concept of the Al/X-PVMPT battery. (a) Redox processes in the phenothiazine-based polymer X-PVMPT with oxidation states (A, B, C and D). (b) Schematic setup of the Al/X-PVMPT battery (the colours of the redox states of the PT units were chosen similarly to those experimentally observed).
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Gauthier Studer's research team, led by Prof. Dr. Birgit Esser from the University of Ulm and Prof. Dr. Ingo Krossing and Prof. Dr. Anna Fischer from the University of Freiburg, has developed an organic redox polymer as a positive electrode material for aluminium-ion batteries (AIB). The use of aluminium-ion batteries is considered a promising option to replace conventional batteries that rely on scarce and difficult-to-recycle raw materials such as lithium. After all, aluminium is one of the most abundant elements in the earth's crust, is easier to recycle and offers an advantage of safety and cheaper production compared to lithium. However, the development of aluminium-ion batteries is still at an early stage due to the lack of suitable electrode materials that allow sufficient storage capacity.
Climate change and the increasing demand for electrical energy require the development of new types of renewable energy storage devices. While classic lithium-ion batteries could benefit from engineered electrode materials, next-generation batteries should draw on abundant elements, be safe and cost-effective, use non-toxic materials and be easy to recycle.
Aluminium-ion batteries with improved storage capacity
Aluminium is the most abundant metal in the earth's crust and is easily recyclable. Its high volumetric capacity of 8040 mA h cm-3 as a negative electrode material even surpasses that of lithium with 2046 mA h cm-3. Unlike the latter, it can be reversibly detached and deposited without forming dendrites, which prevents short circuits. It is also advantageous that the ionic liquid electrolytes currently used in Al batteries are non-flammable.
Rechargeable AIB therefore promise great potential as next-generation energy storage devices. However, the positive electrode materials currently used suffer from low specific capacity, which limits the specific energies of these AIBs.
An electrode material that intercalates complex aluminium anions
The research project developed a positive electrode material consisting of an organic redox polymer based on phenothiazine. The team thus introduces organic redox polymer with two well-defined redox processes as a positive electrode material that overcomes these shortcomings. In the experiment, the aluminium batteries with this electrode material stored a previously unattained charge of 167 milliampere hours per gram (mAh/g). The organic redox polymer thus surpasses the capacity of graphite.
With its high discharge voltage and specific capacity, as well as its good 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.
Birgit Esser, Institute for Organic Chemistry II and New Materials at the University of Ulm
The electrode material is oxidised when the battery is charged, thereby depositing complex aluminate anions. In this way, the organic redox polymer poly(3-vinyl-N-methylphenothiazine) manages to intercalate two [AlCl4]¬- anions reversibly during charging. As an electrolyte, the researchers used ethylmethylimidazolium chloride as an ionic liquid with the addition of aluminium chloride. The focus of the research team is to develop new organic redox-active materials that have high performance and reversible properties. By investigating the redox properties of poly(3-vinyl-N-methylphenothiazine) in the chloroaluminate-based ionic liquid, a significant breakthrough was achieved by demonstrating for the first time a reversible two-electron redox process for a phenothiazine-based electrode material. The research results are promising and are likely to spur further investigation of organic redox polymers as positive electrode materials, paving the way for more sustainable energy storage devices.
Original source University of Freiburg
Original publication: G. Studer, A. Schmidt, J. Büttner, M. Schmidt, A. Fischer, I. Krossing and B. Esser, Energy Environ. Sci., 2023
