The Aluminum-Ion Battery: A Sustainable and Seminal Concept?

Abstract

The expansion of renewable energy and the growing number of electric vehicles and mobile devices are demanding improved and low-cost electrochemical energy storage. In order to meet the future needs for energy storage, novel material systems with high energy densities, readily available raw materials, and safety are required. Currently, lithium and lead mainly dominate the battery market, but apart from cobalt and phosphorous, lithium may show substantial supply challenges prospectively, as well. Therefore, the search for new chemistries will become increasingly important in the future, to diversify battery technologies. But which materials seem promising? Using a selection algorithm for the evaluation of suitable materials, the concept of a rechargeable, high-valent all-solid-state aluminum-ion battery appears promising, in which metallic aluminum is used as the negative electrode. On the one hand, this offers the advantage of a volumetric capacity four times higher (theoretically) compared to lithium analog. On the other hand, aluminum is the most abundant metal in the earth's crust. There is a mature industry and recycling infrastructure, making aluminum very cost efficient. This would make the aluminum-ion battery an important contribution to the energy transition process, which has already started globally. So far, it has not been possible to exploit this technological potential, as suitable positive electrodes and electrolyte materials are still lacking. The discovery of inorganic materials with high aluminum-ion mobility-usable as solid electrolytes or intercalation electrodes-is an innovative and required leap forward in the field of rechargeable high-valent ion batteries. In this review article, the constraints for a sustainable and seminal battery chemistry are described, and we present an assessment of the chemical elements in terms of negative electrodes, comprehensively motivate utilizing aluminum, categorize the aluminum battery field, critically review the existing positive electrodes and solid electrolytes, present a promising path for the accelerated development of novel materials and address problems of scientific communication in this field.

Keywords: aluminum-ion battery; cathode; electrolyte; post-lithium; resources.

Figure 1

Result of the assessment of the elements up to the number 94. The highest possible value is 22. Light colors are related to economic and ecologic, whereas dark colors are related to electrochemical aspects. Aluminum is ranked first as negative electrode for an all-solid-sate aluminum-ion battery.

Figure 2

Comparison between gravimetric and volumetric capacities, standard reduction potential and earth's crust abundance of metal negative electrodes used or proposed for application in electrochemical storage systems (Fleischer, ; U. S. Geological Survey, 2015). Figure reproduced with permission from Elia et al. (2016) © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 3

Pourbaix diagram of aluminum in water at 25°C showing its corrosion behavior. It depicts the basic oxidation/reduction reactions for aluminum in aqueous systems. Outside the yellow region, water breaks down, not the metal. It can be seen that a secondary aluminum-ion battery with an aluminum metal as negative electrode based on an aqueous system will not be possible since the aluminum cannot be plated both at low and high pH. It cannot be solved in a medium pH, as well. Therefore, just primary battery systems can be realized (cf. section Aqueous or Primary Aluminum Battery). Redrawn from Deltombe and Pourbaix (1958), Vargel (2004), and Ashby and Jones (2012).

Figure 4

Advantages of utilizing aluminum as battery material (negative electrode, current collector, housing).

Figure 5

Categorization of aluminum batteries in regard to their operating scheme and their used type of electrolyte. Other battery types are dual-ion batteries (Zhao et al., 2018). Below, different conceivable secondary aluminum-ion battery designs are depicted. (A) This design does not make use of the full potential of aluminum since the negative electrode is either an alloy or a non-metal. The designs in (B,C) use aluminum metal as negative electrode, (B) uses a liquid and (C) a solid electrolyte.

Figure 6

Bond valence energy landscape (orange) of Al³⁺ in Na⁺-β”-alumina with Na being removed and the Al kept for the calculation (Al_10.35Mg_0.65Na_1.65O₁₇, ICSD #6326592) drawn at the energy threshold of 1.07 eV. Blue polyhedra denote AlO₆/AlO₄/MgO₄ octa- or tetrahedra. The calculations were performed according to Nestler et al. (2019b) using the program softBV (Adams and Rao, 2011).

Figure 7

Bond valence energy landscape (blue) for Al³⁺ intercalated chevrel Mo₆S₈ (ICSD #158986 with cell parameters from Levi et al., 2007). They are drawn at energy thresholds of (A) 0.33 eV to show the whole path and (B) 0.23 eV to show the match of the sites suggested by the bond valence method and Al sites (partially blue) reported in literature. S anions forming cubic cages are marked orange, while the enclosed Mo₆ octahedra and the unit cell are gray. In (B) the six atomic sites of the inner and the six of the outer rings are illustrated. The calculations were performed using the program softBV (Adams and Rao, 2011).

Figure 8

Capacity vs. voltage for reported positive electrodes for non-aqueous aluminum batteries. The dots present the practical average discharge voltages and the corresponding available specific capacities of each material. It has to be noted that these materials have not necessarily been proven to intercalate Al³⁺ but chloroaluminate ions, such as ${\text{AlCl}}_4^{-}$, instead. Figure reproduced with permission from Zhang et al. (2018) © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 9

Scheme of the suggested approach for identifying crystalline materials with fast ionic conduction for aluminum-ion battery materials: Voronoi-Dirichlet partitioning, bond-valence site-energies, density-functional theory (NEB) analysis. Simulation methods with different accuracy levels and thus computational effort, are performed in succession. Right: Al³⁺ pathways investigated by DFT-NEB analysis in the 1 × 1 × 3 AlVO₃ supercell (Nestler et al., 2019b). All pathways are connected via a nearest-neighbor, unoccupied aluminum site. Blue octahedra represent AlO₆, while gray octahedra are VO₆. Blue balls represent the start and end positions in the investigated paths.

Figure 10

Using crystallography, crystallographic databases, and data mining with crystallochemical methods for the identification of novel aluminum ion conductors (positive electrodes and solid electrolytes).

Figure 11

Bond valence energy landscape (orange) for Al (light blue) at 0.52 eV in AlVO₃ (ICSD #49645). (A) Dark blue polyhedra denote the 16d site, which is occupied by V (2/3) and Al (1/3) and coordinated by eight oxygen atoms. Half of all Al in the structure occupies the 8a site and is suggested to be part of a 3D pathway. This network coincides with the hopping positions calculated by VDP (gray). (B) For aluminum migration, an octahedral site (gray) has to be passed during migration. Reprinted with permission from Nestler et al. (2019b). Copyright 2019 American Chemical Society.