A University of Freiburg led team can now offer great progress on aluminum ion battery chemistry. Aluminum ion batteries are seen as a promising alternative to conventional batteries that use scarce and difficult-to-recycle raw materials such as lithium. This is because aluminum is one of the most common elements in the Earth’s crust, is easier to recycle, and is also safer and less expensive than lithium.
The schematic diagram of the battery shows the redox process in which the electrode material is oxidized and aluminate anions are deposited. Image Credit: Birgit Esser, University of Freiburg. Click the study report link for a link to a large image download file.
However, the development of such aluminum-ion batteries is still in its infancy, as suitable electrode materials that provide sufficient storage capacity are still lacking. A research team headed by Gauthier Studer and led by Prof. Dr. Birgit Esser of the University of Ulm and Prof. Dr. Ingo Krossing as well as Prof. Dr. Anna Fischer of the University of Freiburg has now developed a positive electrode material consisting of an organic redox polymer based on phenothiazine.
In the experiment, aluminum batteries with this electrode material stored a previously unattained capacity of 167 milliampere hours per gram (mAh/g). The organic redox polymer thus surpasses the capacity of graphite, which has mostly been used as an electrode material in batteries to date.
Electrode material inserts complex aluminum anions
The electrode material is oxidized 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 aluminum chloride.
Gauthier Studer noted, “The study of aluminum batteries is an exciting field of research with 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.”
After 5,000 charge cycles at 10 C, the battery retains 88 percent of its capacity
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 electrode material in aluminum batteries is 120 mAh/g. After 5,000 charge cycles, the battery presented by the research team still has 88 percent of its capacity at 10 C, i.e. at a charge and discharge rate of 6 minutes. At a lower C rate, i.e. a longer charge and discharge time, the battery returns unchanged to its original capacities.
Five thousand cycles is an amazing number, and that’s at what seems like a really fast charge and discharge rate.
The operating voltage and capacities are nothing to raise high excitement, but lots of applications could use this technology and take the pressure from the lithium market.
Other than carry along power needs like cell phones, this technology deserves lots more research.
By Brian Westenhaus via New Energy and Fuel
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