Development of Aluminum-Foil Anodes for Rechargeable Lithium Batteries
Authors: Li Hongyi , Yamaguchi Takitaro , Matsumoto Shingo , Hoshikawa Hiroaki , Kumagai Toshiaki , Ichitsubo Tetsu 
Published: December 2024

Alloy anode materials, such as silicon, tin, and aluminum, are expected to be used in rechargeable lithium batteries owing to high theoretical capacities, compared to conventional carbonaceous anodes. However, the high capacities inevitably cause drastic volume change (+100~300%) of the anode materials during battery reactions (i.e., lithium insertion and extraction), which usually leads to fatal structural degradation.[1,2] Although downsizing the anode materials into fine particles is an effective way to endure the huge volume change,[3] the complicated preparation process raise the production costs, and the indispensable supporting substance (e.g. polymer binder, current collector) for the active material particles decreases the practical capacity of the total anode. Therefore, easy-to-manufacture strategy is still strongly demanded for practical application of alloy anode materials towards high energy-density and low-cost rechargeable lithium batteries.
Recently, we have reported a metallurgical approach which stabilizes the structure change of Al foils during repeated lithiation and delithiation. By adjusting the composition and hardness of the Al matrix, a homogeneous in-plane lithiation and an out-of-plane (one-dimensional) volume expansion (+100%) are achieved during lithiation.[4] Because the LiAl phase has a certain tolerance to off-stoichiometry composition, a concentration gradient can be formed from the surface to the inside of Al foils, driving the interdiffusion of Al and Li. During delithiation, the Al matrix is recrystallized and self-organized to pillar/porous structure, which can tolerant the volume changes in subsequent cycles. The reversible structure changes allow the Al foils to work as self-contained anodes. The Al-foil anodes can be divided into a surface layer that alloys Li, and a base layer that maintains the structural stability and electric conductivity. However, during cycling, the electrolyte exhibits a significant impact on the structure stability of the Al-foil anodes. The decomposition products of the conventional carbonate-based electrolytes, such as 1 M LiPF6 in EC-DMC, easily passivate the surface of Al-foil anodes. As a result, the base layer is gradually consumed during cycling accompanied with the pulverization of the surface layer. The electrolyte permeates the fresh Al surface would further passivate the Al surface. Eventually, the capacity would be vanished, when the Al-foil anodes are completely pulverized, or the electrolytes are completely decomposed.
To extend the cycle life of Al-foil anodes, in this presentation, we summarize the degradation mechanism of the Al-foil anodes and introduce the improvement strategies from various perspectives, including alloy additions, electrolyte selection and the structural design. Appropriate alloy additions can moderate the surface formation of Al matrix during delithiation, which prevent the electrolyte permeation. In addition, salt concentrations, anion and solvent combinations markedly affects the electrolyte compatibility with the Al-foil anodes, where the cyclability can be significantly improved with electrolytes that are less susceptible to passivating the Al surface. Furthermore, introducing distinct thermodynamic and kinetic characteristics between the surface and base layers, such as cladding Al foils,[5] can effectively prevent the electrolyte permeation and Li penetration consume the base layer.
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