Aluminum-ion batteries represent a promising alternative to lithium-based systems due to the abundance of aluminum, its high theoretical capacity, and the trivalent charge carrier that enables multi-electron transfer. The anode plays a critical role in these batteries, where aluminum undergoes electroplating during charging and stripping during discharging. The efficiency and reversibility of these processes determine the battery's cycle life, energy density, and safety.

The electroplating process involves the reduction of Al³⁺ ions to metallic aluminum, which deposits onto the anode substrate. During discharge, the reverse reaction occurs as aluminum oxidizes back into Al³⁺ ions. A major challenge lies in achieving uniform deposition to prevent dendrite formation, which can penetrate separators, cause short circuits, and degrade Coulombic efficiency. Non-uniform plating often results from uneven ion flux, inhomogeneous nucleation sites, or unstable solid-electrolyte interphases (SEI).

Dendrite formation is exacerbated by high current densities, poor electrolyte stability, and inadequate substrate properties. Unlike lithium, aluminum tends to form rough, mossy deposits rather than needle-like dendrites, but these still lead to capacity fade and increased impedance. Strategies to mitigate this include optimizing electrolyte composition, modifying substrate surfaces, and introducing alloying elements.

Substrate materials must exhibit high electrical conductivity, chemical stability, and strong adhesion to deposited aluminum. Common choices include stainless steel, carbon-based materials, and nickel foils. Stainless steel provides mechanical robustness but may suffer from poor wetting behavior. Carbon substrates, such as graphene or carbon cloth, offer high surface area and improved nucleation uniformity. Nickel foils facilitate smooth deposition due to their favorable lattice matching with aluminum.

Surface modifications enhance plating behavior by creating uniform nucleation sites. Techniques like plasma treatment, chemical etching, or coating with seed layers (e.g., gold or zinc) reduce nucleation overpotential. A study demonstrated that a graphene-coated copper substrate improved aluminum deposition uniformity, achieving a Coulombic efficiency of 98% over 200 cycles. Another approach involves patterning the substrate with microstructures to guide ion flux and promote even plating.

Alloying aluminum with other metals can suppress dendrite growth and improve cycling stability. For example, aluminum-magnesium alloys exhibit reduced grain size and enhanced mechanical strength, leading to more compact deposits. Aluminum-tin alloys have shown improved reversibility due to tin's ability to facilitate homogeneous nucleation. However, alloying may introduce tradeoffs in capacity or voltage hysteresis, requiring careful optimization.

Electrolyte formulation significantly impacts anode performance. Chloroaluminate ionic liquids, such as AlCl₃/[EMIm]Cl, are widely used due to their high ionic conductivity and stability. The ratio of AlCl₃ to organic chloride affects plating behavior, with excess AlCl₃ promoting smoother deposits. Additives like urea or cesium chloride have been shown to refine grain structure and suppress side reactions. Non-corrosive electrolytes are also under investigation to mitigate current collector degradation.

Coulombic efficiency remains a key metric for evaluating anode performance. Losses occur due to incomplete stripping, parasitic reactions, or dead aluminum formation. Strategies to improve efficiency include pre-treating the anode to form a stable SEI, using pulse plating to control deposition kinetics, and incorporating protective coatings. For instance, an alumina nanolayer on the anode surface has been shown to reduce side reactions and improve cycling stability.

Mechanical confinement is another approach to mitigate volume changes during plating and stripping. Porous scaffolds or conductive polymers can accommodate expansion while maintaining electrical contact. A study using a 3D porous carbon host demonstrated stable aluminum deposition with minimal capacity decay over 500 cycles.

Despite progress, challenges remain in scaling aluminum-ion battery technology. Long-term stability under high current densities, compatibility with low-cost materials, and integration into full-cell configurations require further research. Advances in operando characterization techniques, such as X-ray tomography or atomic force microscopy, are providing deeper insights into deposition mechanisms.

In summary, optimizing anode design for aluminum-ion batteries involves a multifaceted approach addressing substrate selection, surface engineering, alloying strategies, and electrolyte interactions. By improving deposition uniformity and suppressing degradation mechanisms, researchers aim to unlock the full potential of this emerging energy storage technology. Future developments will likely focus on scalable manufacturing processes and system-level integration to enable commercial viability.