Synthetic Graphites as the Cathode Material for Aluminium-Carbon Batteries
Authors: Langli Helene Lillevestre , Birkeland Daniel Johan Gustafsson , Bjelland Eirik Brådland , Wang Zhaohui , Svensson Ann Mari 
Published: December 2024

Developments such as growth in electric vehicle production and increased use of renewable energy sources, has caused an exponential increase in the demand for batteries [1,2], with Li-ion batteries as the dominating technology. Various issues are associated with the materials of Li-ion batteries, like an uncertainty about the future access, environmental footprint and social impact related to raw materials extraction and production, as well as the challenge of recycling [3]. For this reason, there is a renewed interest in alternative battery chemistries, and one candidate technology is the Aluminium-Carbon (Al-C) battery. This is a so-called dual-ion concept, with an Al anode, a carbon (mostly graphite) cathode, and an electrolyte based on Al salts. During charging, anions intercalate into the graphite, while Al deposits on the anode [4]. Al has a high theoretical volumetric and gravimetric capacity, in addition to being abundant [5]. Like graphite, it is also a low-cost material [4,5], so using these together can provide an affordable battery technology. In addition, these batteries would be very convenient to recycle due to the mature, worldwide recycling infrastructure of Al [6] combined with the fact that graphite can withstand high temperature treatment [7]. The most commonly used electrolyte is AlCl3 in 1-Ethyl-3-methylimidazolium chloride (EMIMCl), which is a thermally stable ionic liquid [8]. It also has a high ionic conductivity [8], allowing high currents and subsequently high-power batteries, but it has the drawback of being highly corrosive [9]. Alternative electrolytes are aluminium trifluoromethanesulfonate (Al(OTF)3) or aluminium bis(triﬂuoromethanesulfonyl)imide (Al(TFSI)3) and LiAlH4 in tetrahydrofuran (THF), which eliminates the corrosion issue. With Al(OTF)3 as the main salt, it has been shown to reversibly plate Al [10]. However, to the best of our knowledge such electrolytes are not used in Al-C batteries to this date.

Work on Al-C batteries have been conducted with a wide range of carbons (e.g. natural and synthetic graphites or fibrous carbon materials), and it has been shown that a high degree of crystalline perfection and flake-shaped particles are preferred for high capacity [4,11]. Capacities obtained when using graphite powders are in the range between 60 and 142 mAh/g, depending on their characteristics [4]. Al-C batteries can also have excellent rate capabilities, as observed in the work of Wu et al. [12] where a 3D graphitic foam gave a stable capacity around 60 mAh/g for 4000 cycles at the high rate of 12 A/g. However, carbon materials for which outstanding results are reported, are not always commercially available. Al-C batteries are still at an early stage of development, and knowledge related to graphite properties and how these affect stability at high voltages, and intercalation of anions from the relevant electrolytes is still limited.

In this work, we have studied industrially relevant synthetic graphites as cathodes for Al-C batteries, with the purpose of investigating the anion intercalation and oxidation stability with respect to graphite properties, like surface area, surface structure (edge, basal and defect sites), and surface chemistry. Graphite powders have been investigated by nitrogen adsorption, LECO oxygen analysis and XRD measurements in order to determine the relevant properties. The electrolytes used are AlCl3 in EMIMCl, and Al(OTF)3 as well as Al(TFSI)3 with LiAlH4 in THF. To provide further information about the reactions happening during cycling, post-mortem characterization of cycled electrodes with SEM, AFM, FTIR and Raman spectroscopy is carried out. In this manner it can be investigated if the reactions occurring is the wanted reversible intercalation, or if irreversible reactions at the electrode surface also take place.

This project has received funding from The Research Council of Norway: Contract number 331964. Partners in the project are NTNU, SINTEF Industry, Vianode, Beyonder and Equinor ASA.

References

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