Anode materials for aluminium batteries

Influence of Composition and Surface Condition

The global transformation of energy systems requires innovative storage solutions that go beyond established lithium ion technology. Rechargeable aluminium batteries (RABs) are considered promising candidates for large, stationary applications in the context of renewable energy sources due to their exceptional theoretical capacity, high level of safety, and the excellent availability of aluminium. However, the development of high performance anodes has so far often been limited by the stable native oxide layer and the resulting interfacial reactions, which represent a major obstacle to stable and efficient aluminium plating and stripping in ionic electrolytes. Within the cooperation between AMAG and the Christian-Doppler-Laboratory for Deformation–Precipitation Interactions in Aluminium Alloys (DEPICT-Al), an experimental processing route has been developed in which precise alloy design and controlled surface modification are strategically combined and optimized to unlock new approaches for aluminium anodes.

Aluminium Anodes for Large Scale Applications

The search for new and complementary technologies for rechargeable batteries is driven by the need to develop systems with higher energy density, lower cost, and improved environmental performance. In this context, aluminium offers unique advantages as an anode material. With a theoretical gravimetric capacity of 2980 mAh/g and a theoretical volumetric capacity of 8046 mAh/cm³, aluminium far exceeds conventional graphite anodes. In addition, the three electron transfer mechanism (Al↔Al3++3e-) enables extremely high charge density. [1, 2]A critical obstacle, however, is the natural oxide layer in the form of Al₂O₃. This layer only a few nanometres thick acts as an electrical insulator and severely impedes ion transport, resulting in high overpotentials during charging and discharging. In electrolytes typically used for RABs, based on ionic liquids (e.g., AlCl₃ dissolved in 1 ethyl 3 methylimidazolium chloride, abbreviated AlCl₃/[EMIm]Cl), this barrier must be overcome to enable efficient metal deposition (plating) and dissolution (stripping). [3] Current research therefore focuses on modifying the anode surface to reduce charge transfer resistance and create favourable conditions for stable plating and stripping behaviour. [4]

AMAG and the CD Laboratory DEPICT-Al

Solving such complex materials science challenges requires close integration of fundamental research and industrial application.The Christian Doppler Laboratory for Deformation–Precipitation Interactions in Aluminium Alloys (DEPICT-Al) at Technical University of Leoben, headed by Dr. Irmgard Weißensteiner, is dedicated to uncovering fundamental mechanisms at the atomic scale.The cooperation with AMAG Rolling GmbH plays a central role. While the CD Laboratory provides theoretical models and high resolution analytics, AMAG contributes its extensive expertise in large scale alloy development and thermomechanical processing. A particular focus lies on enabling the use of secondary aluminium. The research investigates how accompanying elements typically regarded as impuritiescan be transformed into functional microstructural features through targeted heat treatments and deformation processes. This approach not only supports technical performance but also represents a key pillar of AMAG’s sustainability strategy.

Alloy Concepts for Tailored Microstructure Design

The results of the joint research show that the performance of an aluminium anode is strongly linked to its metallurgical history. Instead of relying on ultra pure primary aluminium, a concept based on deliberately alloyed systems was pursued. [5]

The Al-Mg-Sc-Zr Alloy ConceptThe research focused on the Al-Mg-Mn-Sc-Zr system. Each of these elements plays a specific role in determining the subsequent anode performance:

  • Magnesium (Mg): At concentrations of approx. 5 wt.%, magnesium acts as the primary former of the β phase (Al₃Mg₂). This intermetallic phase is key to later surface modification, as it exhibits lower chemical resistance in specific etchants than the aluminium matrix.
  • Scandium (Sc) and Zirkonium (Zr): These elements form finely dispersed, coherent Al₃(Sc,Zr) dispersoids. During thermomechanical processing, these particles act as Zener pinning centres, stabilising grain boundaries and preventing recrystallisation. This results in very high dislocation densities and an ultrafine grained microstructure. Grain boundaries can serve as preferred diffusion pathways and nucleation sites for β phase formation.

Experimental Validation with YttriumDue to the high cost of scandium, a substitution approach using yttrium (Y) was examined in parallel. The results show that yttrium, in combination with zirconium, can achieve similar microstructure stabilising effects. [6] Within the examined test window, this system also exhibited reduced initial charge transfer resistances and stable cycling behaviour. This marks an important step toward the economic scalability of the technology without compromising microstructural benefits.

walzen Kopie
Figure 1: Adapted laboratory rolling mill for thermomechanical processing

From Melt to Porous Anode

The developed production route deviates from conventional approaches (avoiding inhomogeneities, ensuring uniform microstructures) and instead makes targeted use of metallurgical inhomogeneities. The process consists of three essential stages:

Thermomechanical Processing:In an initial tailored heat treatment, the dispersoid structure required for later steps is established. The alloys are then hot rolled and subsequently subjected to extreme cold deformation by rolling (Figure 1). Reductions of over 90% are achieved, leading to pronounced grain elongation, fragmentation of primary intermetallic phases, and accumulation of lattice defects.

Controlled Age HardeningA subsequent heat treatment at moderate temperatures (~150 °C) promotes the formation of Mg rich precipitates in a favourable spatial distribution originating from the Al₃(Sc,Zr) dispersoids. This helps prevent dominant β phase precipitation along grain boundaries. The precision and execution of this heat treatment determine the spatial distribution and size of these precipitates.

Selective Subtractive EtchingThis is the decisive process step. The rolled foils are immersed in a 10% phosphoric acid solution (H₃PO₄).The acid selectively attacks Mg rich regions, dissolving them out of the aluminium matrix and creating channels several micrometers deep (measured 4.5-5.5 µm).

 

Influence of Surface Morphology on Electrochemistry and Charge Transfer Resistance

The result is an anode with a highly complex, three dimensional surface structure.Selective etching produces a sponge like morphology whose properties can be tuned via etching duration and prior precipitation kinetics. SEM imaging revealed substantial changes in the anode surface: While conventional rolled surfaces are smooth and defect minimised, the modified anode foils show a highly porous structure.BET gas adsorption measurements show an increase in accessible specific surface area by a factor of 210 (yttrium variant) up to 400 (scandium variant) compared to a flat rolled surface (Figures 2.1 and 2.2 a, b, c). Electrochemical impedance spectroscopy (EIS) indicated improved interfacial properties associated with this morphology: The (Rct), dominated by the oxide layer and the contact area with the electrolyte, decreased by up to 88% under the investigated conditions. The porous structure not only enlarges the reaction zone but also appears to interrupt the continuity of the passivating oxide layer, significantly facilitating Al³+ exchange. In symmetric cell tests, the modified anodes stabilised earlier than 99.99%-aluminium under the tested conditions. Despite the reduced initial charge transfer resistance, overpotentials remained higher.

Anodenmaterialien_2.1
Figure 2.1: SEM images of the scandium containing sample after 40 minutes of etching in a 10% H₃PO₄ solution; (a) the overview shows the homogeneity of the pore structure, (b) the pore channels in detail.
Anodenmaterialien_2.2
Figure 2.2: SEM images of the yttrium‑containing sample after 40 minutes of etching in a 10% H₃PO₄ solution; (a) homogeneous distribution of the pore structure, (b) slightly coarser pore structure compared to the Sc‑containing sample.

Identified Challenges and Future Research Directions

The findings indicate a promising materials science approach at laboratory scale and provide clear evidence of the influence of alloy design, process control, and surface morphology on electrochemical behaviour. At the same time, they reveal current limitations, leading to a clear future research roadmap:

  • High resolution phase analysis: The exact atomic structure of the precipitates is not yet fully clarified. Future TEM studies are needed to determine how the oxide layer reforms over the pores.
  • Electrolyte compatibility: RABs use highly reactive ionic liquids. The long term corrosion resistance of the porous surface structures must be validated in multi thousand hour tests.
  • Dendrite inhibition: Whether the porous surface also provides stabilising effects against dendrite growth under more demanding conditions is a crucial safety aspect, especially at higher current densities and in full cell configurations.
  • Scaling of Etching Processes: For industrial scale production, the discontinuous immersion etching process must be transferred into a continuous strip processing line. In this context, questions of bath stability and the environmentally compliant treatment and recycling of etching media play a central role.

AMAG’s Objectives

AMAG’s investment as a leading aluminium producer in the development of innovative battery anodes follows a multifaceted strategy. A key aspect is positioning the company as a driver of innovation. By engaging in battery research, the company secures a strategic knowledge advantage in a highly dynamic future market.This expansion of expertise, however, reaches far beyond the field of energy storage. A significant increase in materials know how is achieved, as the insights gained into precise control of precipitation processes under extreme deformation conditions also provide valuable impulses or directly flow into the development of new high strength alloys for lightweight applications in the aerospace and automotive industries, including their associated surface treatments.In parallel, AMAG is strengthening the topic of sustainability through innovative alloy design. The current research impressively demonstrates that promising functional properties can also be achieved with alloy concepts that can be further developed with regard to more sustainable material solutions reinforcing AMAG’s pioneering role within the circular economy.Ultimately, this engagement aims at a strategic expansion of the product portfolio. Beyond classical structural components, functional materials for the global energy transition are gaining increasing importance, with R&D in aluminium anode technology serving as a forward looking lighthouse project.

Why Collaboration with AMAG Makes the Difference

For AMAG’s customers and partners, these research activities translate into concrete competitive advantages:

  • Deep Materials Understanding: Customers receive not only a product but scientifically grounded guidance. Knowledge of how trace elements influence surface reactivity enables tailored material solutions for specific chemical environments.
  • Joint Innovation in the CMI: At the Center for Material Innovation (CMI) in Ranshofen, customers can adapt the described approaches to their specific applications together with AMAG experts. A seamless chain is available from alloy variations in lab casting to rolling trials.
  • Future proof Materials: By focusing on recyclable alloys, AMAG ensures that customer materials meet future regulatory requirements (e.g., CO₂ footprint, circular economy).
  • Reliability Through Analytics: Access to state of the art diagnostics (SEM, TEM, AFM, electrochemical characterization) guarantees quality assurance at the highest level. The research work demonstrates that aluminium, through targeted metallurgical interventions, is far more than a structural material.

References:

[1]  M. Jiang, C. Fu, P. Meng, J. Ren, J. Wang, J. Bu, A. Dong, J. Zhang, W. Xiao, B. Sun, Challenges and Strategies of Low-Cost Aluminum Anodes for High-Performance Al-Based Batteries, Advanced Materials 34 (2022) 2102026. https://doi.org/10.1002/adma.202102026.[2]   K.L. Ng, B. Amrithraj, G. Azimi, Nonaqueous rechargeable aluminum batteries, Joule 6 (2022) 134–170. https://doi.org/10.1016/j.joule.2021.12.003.[3]  S. Choi, H. Go, G. Lee, Y. Tak, Electrochemical properties of an aluminum anode in an ionic liquid electrolyte for rechargeable aluminum-ion batteries, Phys. Chem. Chem. Phys. 19 (2017) 8653–8656. https://doi.org/10.1039/C6CP08776K.[4]  Y. Long, H. Li, M. Ye, Z. Chen, Z. Wang, Y. Tao, Z. Weng, S.-Z. Qiao, Q.-H. Yang, Suppressing Al dendrite growth towards a long-life Al-metal battery, Energy Storage Materials 34 (2021) 194–202. https://doi.org/10.1016/j.ensm.2020.09.013.[5]   G. Razaz, S. Arshadirastabi, N. Blomquist, J. Örtegren, T. Carlberg, M. Hummelgård, H. Olin, Aluminum Alloy Anode with Various Iron Content Influencing the Performance of Aluminum-Ion Batteries, Materials 16 (2023) 933. https://doi.org/10.3390/ma16030933.[6]   Y. Zhang, H. Gao, Y. Kuai, Y. Han, J. Wang, B. Sun, S. Gu, W. You, Effects of Y additions on the precipitation and recrystallization of Al–Zr alloys, Materials Characterization 86 (2013) 1–8. https://doi.org/10.1016/j.matchar.2013.09.004.

Back