(Invited) Cathode Active Materials for Aluminum Metal Anode Rechargeable Batteries
Authors: Tsuda Tetsuya 
Published: December 2024

Aluminum (Al) metal anode secondary batteries are a battery system that has been attracting attention in recent years. This is not only because Al resources are abundant, but also because Al metal can be used with a large charge-discharge capacity. The electrolytes include Lewis acidic AlCl3-based organic ionic liquids (AlCl3–1-ethyl-3-methylimidazolium chloride ([C2mim]Cl) etc.) and inorganic ones (AlCl3–NaCl, AlCl3–NaCl–KCl etc.), in which the aluminum metal anode reaction proceeds with low overvoltage and high coulombic efficiency. It is known that the Al metal anode reaction proceeds according to equation [1].
4[Al2Cl7]− + 3e− ⇌ Al + 7[AlCl4]− [1]
The ionic species that make up these electrolytes depend on the type of salt combined with AlCl3, but, in many cases, are [C2mim]+, alkali metal cations (Li(I), Na(I), K(I)), [AlCl4]−, and [Al2Cl7]−. The two chloroaluminate complex anion species are directly involved in the favorable metal anode reaction.1 However, as described below, the type of cation also would be indirectly involved in battery reactions.
Aluminum metal anode secondary battery began to attract attention after a paper by Lin et al. published in Nature.2 They reported that by using three-dimensional graphical foam as the cathode active material, an Al metal anode secondary battery system with excellent cycle and rate characteristics could be constructed. Since that report, Al metal anode secondary batteries with various graphite-based cathode active materials have been reported. Similarly, non-graphite materials are also being considered for application as cathode active materials of the Al batteries. In this lecture, mainly based on our research results, I will outline the characteristics of graphite-based and non-graphite-based cathode active materials for Al metal anode secondary batteries.

Graphite-based cathode active materials- The reaction of graphite-based positive electrodes used in Al metal anode secondary batteries is described by equation [2], and no matter what graphite material is used, the reaction itself remains the same.

nC (graphite) + [AlCl4]− ⇌ C
n
[AlCl4] + e− [2]
However, electrode performances such as charge-discharge capacity and rate characteristics vary greatly depending on not only the shape of the active material but also the presence or absence and type of conductive additive used when making the composite cathode. Figure 1 shows the charge-discharge behaviors of some composite cathodes when graphene nanoplatelets and polysulfone were used as the cathode active material and the binder, respectively, under different usage conditions of conductive additives.3 The performance alters depending on the type of conductive additive used, and we have succeeded in obtaining extremely good rate characteristics. On the other hand, when expanded graphite sheets are used as the cathode active material, the rate characteristics are not so great, but the capacity increases,4 and a value of about 130 mAh g−
1 can be obtained when the inorganic AlCl3–NaCl–KCl IL electrolyte is employed.5 We guess this unexpected result would be caused by the presence of Na(I) and K(I) in the electrolyte.

Non-graphite-based cathode active materials- Carbon materials other than graphite-based ones, e.g., activated carbon fiber cloth, can also be employed as cathode active materials. But, because charge-discharge reactions proceed through electrochemical adsorption/desorption reactions of anionic species (including anionic species newly generated in electrode reactions), no clear plateau voltage appears and energy storage performance is not enough, like EDLCs.6

In recent years, the exploration for cathode active materials for the purpose of increasing the cathode capacity has become active. Our laboratory is investigating cathode active materials with sulfur, focusing on the fact that it is a cheap and abundant resource. Among these, materials in which sulfur is doped into the carbon skeleton, such as sulfur-modified polyethylene glycol (SPEG)7 and sulfur-modified polyacrylonitrile (SPAN)8, are promising materials, because they can greatly increase charge-discharge capacity compared to conventional graphite-based materials. However, unfortunately this characteristic can only be exhibited by the inorganic IL electrolyte, and the reason for this is not clear at present.

References

T. Tsuda, G. R. Stafford, and C. L. Hussey, J. Electrochem. Soc., 164, H5007 (2017) and references therein.

M. C. Lin, M. Angell, H. J. Dai, et al., Nature, 520, 324 (2015).

T. Tsuda, Y. Uemura, et al., Electrochemistry, 86(2), 72 (2018).

Y. Uemura, T. Tsuda, et al., Electrochemistry, 92(4), 043012 (2024).

C.-Y. Chen, T. Tsuda, S. Kuwabata, and C. L. Hussey, Chem. Commun., 54, 4164 (2018).

T. Tsuda, M. Ishikawa, S. Kuwabata, et al., J. Electrochem. Soc., 161, A908 (2014).

T. Tsuda, H. Senoh, S. Kuwabata, et al., Chem. Commun., 58, 1518 (2022).

T. Tsuda, Y. Tsuji, S. Kuwabata, H. Senoh, et al., Meeting Abstracts of 242nd Meeting, L02-2126 (2022).

Figure 1