The aluminium-ion battery is one of several promising post-lithium technologies under development to combat issues with lithium-ion batteries such as insecure lithium supplies, environmental impact and safety concerns associated with thermal runaway. ^[1,2] Aluminium is an abundant material with an energy density of 1060 Wh.kg^-1.^[3] The limiting factor for the battery performance is the capacity of the cathode towards the intercalation of [AlCl₄]^- or Al³⁺ ions. Although several different cathode materials have been studied throughout the past few years, there have been few studies that compare the capacities of different cathode morphologies. Graphitic carbon materials have many features that make them ideal for aluminium-ion intercalation cathodes: they are electrically conductive, easily expandable, low density and low-cost, and are commercially available in a wide variety of morphologies.

In this work, the capacity of four forms of graphitised carbon are compared as aluminium-ion cathodes: pyrolytic graphite, carbon paper, carbon cloth and carbon felt. The materials differ in terms of their physical properties, and thus have different properties as aluminium-intercalating agents. Of all the materials examined, carbon paper had the highest energy density at 122 Wh.kg^-1, and had an [AlCl₄]^- capacity of 70 mAh.g^-1. Carbon paper had superior stability compared to pyrolytic graphite, particularly as the C-rate of cycling was increased. It undergoes no change in crystallographic structure even after cycling up to the 20C rate.

Additionally, the optimum distance between electrodes and electrolyte volume in an aluminium-ion cell was studied, and was found to be 1-2 mm in order to avoid shorting caused by eventual growth of aluminium nodules. In ionic liquid, the cell could only be charged to 2.45 V for repeated cycling, but experiments in a separator-less sealed cell showed that if the cell could be charged to 2.75 V, the capacity could be tripled.

References

1. M. Lin, M. Gong, B. Yu, Y. Wu, D.Y.Wang, M. Guan, M. Angell, C. Chen, J. Yang, B.J. Hwang, H. Dai, Nature, 520, 2015, 324-328.
2. N. Jayaprakash, S.K. Das, L.A. Archer, Chem. Commun., 47, 2011, 12610-12612.
3. National Laboratory, Oak Ridge, “Aluminiun-Ion Battery to Transform 21^st Century Energy Storage” (pdf), retrieved 30 October 2014.