Electrolyte formulations play a critical role in the performance and viability of aluminum-ion batteries, which are emerging as a promising alternative to lithium-ion systems due to the abundance of aluminum, its high theoretical capacity, and inherent safety. The electrolyte must facilitate efficient aluminum ion transport while mitigating challenges such as corrosion, passivation layer formation, and poor reversibility. This article examines aqueous and non-aqueous electrolytes, ionic liquids, and recent advancements in additive engineering to enhance stability and conductivity.

Aqueous electrolytes are attractive for aluminum-ion batteries due to their low cost, high ionic conductivity, and environmental friendliness. Common formulations include aluminum salts such as aluminum chloride (AlCl₃), aluminum sulfate (Al₂(SO₄)₃), or aluminum nitrate (Al(NO₃)₃) dissolved in water. These systems benefit from fast ion transport, enabling high power density. However, aqueous electrolytes face severe limitations, primarily due to the hydrogen evolution reaction at the anode, which competes with aluminum deposition and reduces Coulombic efficiency. Additionally, the formation of a passive oxide layer (Al₂O₃) on the electrode surface inhibits ion transport and increases overpotential. Acidic electrolytes can partially mitigate passivation but accelerate corrosion of current collectors and battery components.

Non-aqueous electrolytes address some of the limitations of aqueous systems by eliminating water-induced side reactions. These typically consist of aluminum salts dissolved in organic solvents such as ethylene carbonate (EC), propylene carbonate (PC), or dimethyl sulfoxide (DMSO). Aluminum chloride is often paired with organic chlorides like 1-ethyl-3-methylimidazolium chloride (EMImCl) to form a Lewis acidic ionic liquid that enhances aluminum ion solubility. Non-aqueous systems exhibit wider electrochemical stability windows, enabling higher operating voltages. However, they suffer from lower ionic conductivity compared to aqueous electrolytes, and the high viscosity of many organic solvents limits ion mobility. Furthermore, aluminum deposition in non-aqueous media often results in dendritic growth, which poses safety risks and reduces cycle life.

Ionic liquids have emerged as a superior alternative due to their non-volatility, non-flammability, and wide electrochemical stability. Chloroaluminate-based ionic liquids, such as those derived from AlCl₃ and EMImCl, are particularly effective for aluminum-ion batteries. These liquids provide high ionic conductivity and enable reversible aluminum plating and stripping with high Coulombic efficiency. The ratio of AlCl₃ to organic chloride is critical; a molar ratio greater than 1.0 creates a Lewis acidic environment favorable for Al³⁺ solvation, while ratios below 1.0 lead to a basic regime with diminished performance. Ionic liquids also suppress dendrite formation more effectively than conventional organic electrolytes, though their high cost remains a barrier to large-scale adoption.

A major challenge in aluminum-ion batteries is the formation of a passivation layer on the aluminum anode, which increases interfacial resistance and reduces cycle life. This layer consists of oxides or hydroxides that form spontaneously in the presence of trace water or oxygen. Electrolyte additives such as urea, hydrochloric acid, or fluorinated compounds have been explored to disrupt passivation. For instance, adding small amounts of HCl to an ionic liquid electrolyte can etch the oxide layer, improving aluminum deposition kinetics. Similarly, fluorinated solvents like bis(trifluoromethanesulfonyl)imide (TFSI) anions enhance stability by forming a protective fluoride-rich interphase.

Another critical issue is aluminum corrosion, particularly in acidic or chloride-rich environments. Corrosion not only degrades the anode but also generates insoluble byproducts that clog the electrolyte and separator. To combat this, researchers have introduced corrosion inhibitors such as sodium molybdate or organic heterocyclic compounds. These additives adsorb onto the aluminum surface, forming a protective film that slows degradation without impeding ion transport. Recent studies have also shown that hybrid electrolytes combining ionic liquids with small amounts of organic solvents can balance corrosion inhibition with high conductivity.

Ion transport efficiency is heavily influenced by electrolyte composition. The solvation structure of Al³⁺ ions determines their mobility and deposition behavior. In aqueous systems, Al³⁺ forms a hexaaqua complex [Al(H₂O)₆]³⁺, which is bulky and slow-moving. In contrast, ionic liquids facilitate smaller solvation shells, such as [AlCl₄]⁻ or [Al₂Cl₇]⁻, enabling faster kinetics. Additives like crown ethers or glymes can further modulate solvation by selectively coordinating with Al³⁺, reducing desolvation energy at the electrode interface. Recent work has demonstrated that optimizing the anion size and solvent donor number can significantly enhance ion transport rates.

Recent advancements in electrolyte formulations focus on improving stability and reversibility. One approach involves using deep eutectic solvents (DES), which are mixtures of hydrogen bond donors and acceptors that exhibit ionic liquid-like properties at lower costs. For example, a DES composed of AlCl₃ and urea has shown promising results, offering high conductivity and stable aluminum deposition. Another innovation is the use of solid polymer electrolytes incorporating aluminum salts, which provide mechanical stability while maintaining reasonable ion transport. Hybrid systems combining polymers with ionic liquids or ceramic fillers have also been explored to enhance mechanical and electrochemical properties.

The development of high-performance electrolytes for aluminum-ion batteries remains an active area of research. Key metrics include Coulombic efficiency, cycle life, and voltage stability, all of which are heavily influenced by electrolyte design. While no single formulation yet meets all requirements for commercial viability, the combination of ionic liquids, advanced additives, and hybrid solvent systems represents a promising path forward. Future efforts will likely focus on reducing costs, improving environmental compatibility, and scaling up production processes to enable widespread adoption.