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Interest in batteries other than lithium-ion has soared recently, and has included such developments as lithium-sulphur batteries, graphene batteries (or batteries employing a graphene electrode), CNT-electrode batteries and aluminium batteries.

Aluminium batteries are a relatively new development compared to other battery types and have gathered interest by employing aluminium anodes and graphite cathodes. However, until recently, aluminium electrode batteries have suffered from relatively poor efficiencies and inefficient cycle-discharge cycles.

A traditional battery, or cell, employs a standard set-up which is still present in more complex electrochemical systems. A rechargeable battery, otherwise known as a secondary battery, consists of two electrodes- an anode and a cathode. Separating the two electrodes is an electrolyte solution, commonly an ionic liquid.

The main difference between primary (non-rechargeable) and secondary batteries is that the electrolyte solution allows the ions to travel in both directions towards the electrodes (whereas primary batteries are unidirectional) - when in use, ions migrate to the cathode; when charging, ions move to the anode; and when there is no applied charge, the ions desorb into the electrolyte solution.

By understanding the intercalation mechanism at the electrodes, an international group of researchers has managed to effectively improve the efficiency of rechargeable aluminium batteries.

Despite their initial flaws, aluminium batteries are still gathering attention due to increasing technological demands. Aluminium batteries were initially found to possess a high gravimetric capacity compared to sodium and lithium-based batteries. This has been attributed to three electrons being involved in the redox coupling reaction at the electrode (per ion), rather than one electron in group 1 metal batteries.

Similar batteries in the past have relied on modified electrodes, such as fluorinated carbon, to perform reversible electrochemical deposition-dissolution reactions using ionic liquid electrolytes. Even though these have worked in the past, the cathodic material has been the issue. The aluminium anode has been proved to be more effective than any of the cathodes it has been paired with, and the overall efficiency of the batteries has not been great.

These inefficiencies prompted the researchers to not only find a new cathodic material to test, but to also employ a completely different mechanism. Previous cathodic mechanisms have been rendered inefficient and have been attributed to the low-specific cathode capacity.

The researchers incorporated a cathode composed of graphite flakes and a PVDF polymer binder. Due to the pore size on the cathode, there was only a certain amount of electrolyte species that could be tested. As the mechanism hadn’t been documented before, the researchers used the intercalation of chloroaluminate electrolyte ions to test for a high intercalation efficiency, and minimal side reactions.

The results obtained showed that whilst there were some Cl-Cl bonds that formed on the edge/surface defect sites surface of the cathode, it was only responsible for a small drop in the Coulombic efficiency. More importantly, under pressure within the cell, the AlCl 4 - electrolyte ions distorted from their ideal tetrahedron structure. This reduces the size of the ion allowing for a better fit in the cathodic holes- and ultimately a more efficient mechanism.

This graphitic cathode shows much greater results than any other graphitic cathode previously used and the new intercalation mechanism has proved to be efficient. Whilst these cathodes (and batteries) have the potential to further improved upon, they currently exhibit a Coulombic efficiency, a current density, a charge-discharge cycling current density of 98-99%, 99 mA g-1 and 660 mA g-1, respectively. They have also found to have little decay after 6000 cycles and possess an exceptional stability.

Whilst the researchers are not saying that the battery is perfect, the understanding of the associated intercalation mechanisms has provided a great insight into the challenges ahead and how to further develop these batteries for commercial use. The internal working of aluminium batteries can now be more efficiently predicted when deciphering experimental procedures, which is only going to increase the potential for further enhancement of these batteries in the future.

Sources and Further Reading

Wang D-Y., Wei C-Y., Lin M-C., Pan C-J., Chou H-L., Chen H-A., Gong M., Wu Y., Yuan C., Angell M., Hsieh Y-J., Chen Y-H., Wen C-Y., Chen C-W., Hwang B-J., Chen C-C., Dai H., Advanced rechargeable aluminium ion battery with a high-quality natural graphite cathode, Nature Communications, 2017, 8, 14283

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