Surface coating is a simple yet effective strategy to prevent direct contact between water and the metal surface, thereby suppressing undesirable side reactions. Polymers are widely used as protective coatings on metal anode surfaces in aqueous electrolytes. An early study on aqueous Zn batteries reported that highly polar β-phase poly(vinylidene difluoride) (PVDF) serves as a bifunctional coating material. The hydrophobic PVDF keeps water molecules away from the Zn surface, and PVDF molecules aligned in a trans-configuration on the surface guide the selective transport of Zn²⁺ for uniform deposition [71].

Taking advantage of the PVDF polymeric protective coating in aqueous Zn batteries, the Li group applied the β-PVDF coating to an Al foil anode (PVDF–Al) (Fig. 4a) [72]. HER potential of PVDF–Al was recorded at 0.08 V (vs. Al/Al³⁺), whereas HER begins at 0.51 V (vs. Al/Al³⁺) for pristine Al foil, indicating the effectiveness of the PVDF coating in suppressing HER (Fig. 4b). Fig. 4c depicts the comparison of corrosion current measurements using linear polarization curves. PVDF–Al showed a lower corrosion potential (1.02 V vs. Al/Al³⁺) and corrosion current (1.32 mA cm^-2) than bare Al (1.15 V vs. Al/Al³⁺ and 1.78 mA cm^-2, respectively).

Stabilization of Al anode with PVDF was further evaluated in full-cell set up with K₂CoFe(CN)₆ cathode. Decent average discharge voltage of ca. 1.0 V and the initial reversible capacity of 60 mAh g^–1 was observed from galvanostatic charge/discharge (GCD) curves (Fig. 4e). With reduced side reactions on the anode surface, K₂CoFe(CN)₆ ‖ PVDF–Al full-cell demonstrated stable cycling over 400 cycles, where K₂CoFe(CN)₆ ‖ bare Al cell failed after 250 cycles.

MXene, a 2D material with various functional groups, has been widely adopted for protective coatings in energy storage due to its chemical and mechanical integrity as well as high electrical conductivity [73–75]. Yu et al. reported the self-assembly of a MXene thin film on the surface through charge exchange interactions between Al³⁺ and the oxygen-containing functional groups of MXene, with the thickness of the MXene layer adjustable by regulating the reaction time (Fig. 5a) [76]. Reduced voltage hysteresis between plating and stripping of Al was observed in cyclic voltammetry (CV) with the optimized thickness of the MXene film (MAl-80), indicating improved kinetics via the conductive coating (Fig. 5b). Fig. 5c further illustrates the reduced nucleation overpotential of 49 mV for MXene-coated Al foil, compared to 112 mV for bare Al foil. The MXene coating improved interfacial kinetics with low overpotential during the plating/stripping of Al, along with stable protection, as demonstrated by the long-term cycling performance of MAl‖ MAl symmetric cells (Fig. 5d).

With its capability to accept four electrons via sequential reduction, the organic cathode, pyrene-4,5,9,10-tetraone (PTO) was chosen as the cathode in this study for evaluating full-cell performance. Even with a high-mass loading cathode (~15 mg), the PTO ‖ MAl-80 full-cell achieved reversible capacity close to 300 mAh g^–1 with a low overpotential (0.45 V), attributed to the improved conductivity of MAl-80 anode (Fig. 5e). Moreover, the prevention of side reactions via MXene coating was illustrated by the superior long-term cycling performance of MAl-80 compared to the full-cell with a bare Al anode (Fig. 5f).

Exposed to atmospheric oxygen and water, a passivating Al₂O₃ layer readily forms on the surface of Al due to the high charge density and strong oxide bonding energy of trivalent aluminum ions. In aqueous electrolytes, the nucleation of Al involves the electrochemical decomposition of water; hence, the continuous formation of the aluminum oxide layer becomes inevitable, rendering the surface electrochemically inert. IL-based electrolytes have achieved satisfactory cycle life with Al₂O₃-passivated Al anodes, so a profound understanding of the interfacial reaction mechanisms in ILs could provide valuable insights into how to keep Al “active” for reversible electrodeposition in aqueous media.

As illustrated in Fig. 6, Archer et al. proposed a simple strategy to construct an artificial SEI on Al foil by dipping it in an AlCl₃-[EMIm]Cl IL electrolyte (the treated Al is denoted as TAl) [77]. Fourier transform infrared spectroscopy (FTIR) results indicate the formation of an SEI enriched with functional groups such as C=O, C=C, and C=N, derived from the interaction between Al and the IL. Additionally, Cl- and N-rich SEI formation was detected using cross-sectional scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) (Fig. 6a,b). Participation of imidazolium in the SEI formation via ring-opening reaction was also evidenced from reduced signal in the vibration mode of C–H bond after the treatment. Due to the corrosion of the passivating oxide layer and improved ion-transport kinetics through the artificial SEI, low voltage hysteresis in the plating/stripping of TAl was observed in aqueous electrolytes (Fig. 6c).

Fig. 6d,e demonstrate α-MnO₂ ‖ TAl full-cell performance in 2 m Al(Otf)₃ electrolyte. GCD curves of a full-cell showed the energy density equivalent to 500 Wh kg^–1 with a specific capacity of 380 mAh g^–1 and an average discharge voltage of ~1.3 V (Fig. 6d). However, the stability of artificial-SEI on TAl remains questionable as witnessed with rapid capacity decay and limited cycle life of 50 cycles or less (Fig. 6e). Subsequent studies focused on enhancing the stability of the artificial SEI and reducing costs by replacing [EMIm]Cl with different precursors, including acetamide [78] and urea [79].

Manipulating the solvation structure of aluminum ions extends the ESW of aqueous electrolytes to some extent, but the nucleation potential of aluminum remains far below the HER threshold, even in saturated electrolytes [63]. Underpotential deposition (UPD) of metals describes the positive potential shift during the electrodeposition of a metal on other metal substrates through the reversible formation of intermetallic phases [80–82]. Consequently, investigations into substrates capable of forming Al-metal alloys have been actively conducted in recent years.

In 2020, the Yu group prepared an Al pre-intercalated MnO₂ cathode, with Zn serving as a substrate via the reversible formation of a Zn–Al alloy in 2 M Al(Otf)₃ aqueous electrolyte [83]. A continuous increase and abrupt fluctuations in the voltage profile are conspicuous in the Al‖ Al symmetric cell; meanwhile, polarization in the Zn–Al cell remains stable at under 25 mV for 1,000 hours of cycling (Fig. 7a). Since the reduction of Zn²⁺ occurs prior to Al³⁺, Al ions provide electrostatic shielding on the surface of the Zn substrate during the plating/stripping of Zn. Moreover, the co-deposition of Zn²⁺ and Al³⁺ via alloying significantly lowers the energy barrier for the nucleation of Al (Fig. 7b). The absence of metallic Al or an aluminum oxide layer in the X-ray diffraction (XRD) patterns, along with the homogeneous elemental distribution of Zn and Al (Fig. 7c), demonstrates that Al³⁺ is completely plated in the form of a Zn–Al alloy. A full-cell was demonstrated with Al preloaded in MnO₂ (Al_xMnO₂) as the cathode and Zn as the alloy-forming substrate in 2 M Al(Otf)₃ electrolyte, which showed a high discharge voltage plateau at 1.6 V, with the initial capacity exceeding 675 mAh g^–1 (Fig. 7d), and a reversible capacity of 460 mAh g^–1 was well maintained after 80 cycles (Fig. 7e).

In a later study, Jiang et al. introduced the lamella-nanostructured eutectic Al₈₂Cu₁₈ alloy (E-Al₈₂Cu₁₈) electrode for aqueous Al-ion batteries [84]. The eutectic alloying during the synthesis created phase separation, composed of Al₂Cu lamellas and α-Al lamellas. The alternating α-Al and Al₂Cu nano-lamellar structure was confirmed through EDX elemental mapping, as shown in Fig. 8a. The more-noble Al₂Cu nano-lamellas facilitate the transport of electrons and promote homogeneous plating/stripping, while Al-ions are supplied by the electroactive α-Al. The phase-segregated Al–Cu alloy exhibited stable long-term plating/stripping performance at various current densities (Fig. 8b). Fig. 8c illustrates the electrochemical impedance spectroscopy (EIS) spectra; a significantly low charge transfer resistance of E-Al₈₂Cu₁₈ was measured (~160 Ω) due to suppressed formation of thick oxide layers and the synergistic effect of α-Al/Al₂Cu galvanic couple.

Eutectic nano-lamellas enabled selective redox of Al with improved kinetics, as observed from the significantly reduced overpotential in Al_xMnO₂ ‖ E-Al₈₂Cu₁₈ full-cell (Fig. 8d). The improvement of reversible Al plating/stripping capability in the full-cell demonstration contributes to a remarkable capacity retention of 83% after 400 cycles (Fig. 8e).

Amorphous or glassy metals, unlike crystalline metals, lack long-range atomic arrangements in their structure [85,86]. The disruption in the interstitial sites of glassy metals imparts exceptional properties, such as excellent oxidation resistance and a low diffusion rate of oxygen within the structure. Another interesting feature of glassy metals is the positive shift in the electrodeposition potential, as described in UPD, due to their higher reactivity compared to crystalline metals [87–89]. To investigate whether structural amorphization could stabilize the Al anode in aqueous electrolytes, Yu and co-workers conducted the amorphization of Al metal (Al@a-Al) via (de)alloying with Li ions (Fig. 9a) and examined its performance in aqueous Al batteries [90].

Once electrochemically plated Li is stripped (Al@a-Al), a distinct boundary between the amorphous and the crystalline phases is created (Fig. 9b). Restrained corrosion in glassy Al@a-Al was confirmed by a normalized corrosion current density approximately 400 times lower than that of pristine Al foil, indicating sluggish HER kinetics (Fig. 9c). As shown in the cyclability comparison of symmetric cells in Fig. 9d, Al@a-Al outperforms the bare Al anode in terms of durability and voltage hysteresis. It is also noteworthy that this study employed an aqueous electrolyte with low-cost Al₂(SO₄)₃ salt instead of expensive Al(Otf)₃-based electrolytes, highlighting the potential for structural modification of Al metal in large-scale applications.

Potassium nickel hexacyanoferrate (KNHCF) is one of the promising cathode candidates for hosting Al-ions due to its structural integrity for long-term cycling. Disordered orientation in amorphous region enhanced the kinetics of Al plating/stripping, as shown by high open-circuit voltage and the average discharge potential in KNHCF ‖ Al@a-Al full-cell (Fig. 9e) with dramatic enhancement in cyclability (Fig. 9f).

The presence of a native Al₂O₃ layer significantly limits the electrochemical activity of Al-metal anodes in aqueous batteries. Specifically, the continuous accumulation of a thick, electrically and ionically insulating oxide layer undermines the reversibility of Al plating/stripping. To maintain the electrochemical activity of Almetal anodes in aqueous environments, future studies should focus on strategies to minimize the formation of Al₂O₃ on the surface.

The application of hydrophobic coating layers is one potential approach. While extended cycle life and improved electrochemical reversibility have been demonstrated for Al anodes, the underlying reaction mechanisms remain poorly understood. It is important to note that protective membranes with complex functional groups can impede the investigation of ion transport and diffusion mechanisms through the coating. Although surface coatings can stabilize the Al-electrolyte interface by physically separating Al from the aqueous electrolyte and mitigating the HER, questions remain about their impact on the kinetics of Al-ion transport. Future research should prioritize the exploration of hydrophobic, yet aluminophilic, compounds as candidates for surface coatings.

Studies on the pre-treatment of Al foil have demonstrated effective erosion of Al₂O₃ and the formation of an artificial SEI. However, ongoing debates exist regarding several aspects: i) the cyclability of Al with an artificial SEI showed minimal improvements, raising questions about the long-term stability of the SEI in aqueous media; ii) circumventing oxide layer passivation for Al redox has only been reported in Cl-rich environments, without a clear understanding of the underlying mechanisms. Even with a Cl-rich SEI, Al anode in aqueous electrolytes exhibited inferior performance, whereas the plating/stripping efficiency exceeded 99% for 7,500 cycles in an AlCl₃-[EMIm]Cl ionic liquid electrolyte. This leads to the question: What is the nature of the interaction between Cl⁻ and the surface oxide? Is the artificial SEI stable enough to provide long-term protection?

Since the introduction of the AlCl₃-[EMIm]Cl IL electrolyte for reversible Al plating/stripping, numerous studies have sought to elucidate the underlying reaction mechanisms, which may provide answers to the aforementioned questions. In 2014, Grady et al. investigated the interaction of Cl⁻ with the passive aluminum oxide film in ILs, reporting that Cl⁻ initially adsorbs and incorporates into the bulk oxide. Cl⁻ then migrates toward the metal-oxide interface through oxygen vacancies, thinning the oxide layer and inducing localized dissolution of the underlying Al [91]. Later studies revealed that Cl⁻ enhances the electrochemical performance of the Al anode in aqueous electrolytes primarily through a corrosion process rather than by promoting Al redox through SEI formation, as previously hypothesized [92]. For example, the electrochemical performance of pristine Al foil in 2 m Al(Otf)₃ electrolytes containing a small amount of chloride salt (0.15 m NaCl) was found to be comparable to that of pre-treated Al foil. The SEI is unstable and dissolves rapidly in water, rendering it incapable of preventing the continuous decomposition of water molecules in aqueous electrolytes [64]. Thus, it must be recognized that aggressive chloride ions are responsible for dissolving Al beneath the oxide layer, leading to low overpotentials for the anode. Furthermore, Cl⁻ is continuously consumed during repetitive corrosion, indicating that neither the formation of an “artificial SEI” nor the mere addition of a chloride-containing source can achieve long-term stabilization of the Al anode.

While a pre-formed SEI generated through IL treatment lacks long-term stability, it has shown promise in enhancing the electrochemical activity of aluminum metal. To address the instability of artificial SEIs in aqueous electrolytes, applying additional protective layers offers a potential solution. Hydrophobic coatings, such as PVDF, can improve the chemical stability of the SEI in aqueous environments, preserving the aluminum anode’s electrochemical performance.

In parallel, structural modifications like UPD offer another approach, allowing the use of alloy or glassy metal anodes by shifting the nucleation potential of aluminum. This method effectively suppresses HER, but at the expense of reduced energy density due to the mass of the alloying elements. Recent studies have also highlighted significant gas evolution during aluminum dissolution in Al–Cu and Al–Zn alloy anodes in aqueous batteries, raising concerns about the previously reported performance improvements of these alloyed systems [93].

To balance HER suppression with energy density, future research should focus on optimizing alloy compositions and structures. For instance, using lightweight alloying elements or dopants that enhance nucleation potential without adding substantial mass could address energy density concerns. Further, understanding the mechanisms behind gas evolution during aluminum dissolution is essential. Advanced characterization techniques, such as in-situ XPS or operando spectroscopy, could shed light on the Al-alloy interface, guiding the development of gas-suppressing coatings or additives.

Additionally, composite materials that combine UPD-modified alloys with electrochemical stabilization strategies, like hydrophobic or multifunctional coatings, could enhance both stability and energy density. The exploration of hybrid electrolytes, which balance the characteristics of aqueous and ionic liquid systems, may also offer promising advancements in the performance of aluminum alloy anodes.

Investigations into the effect of microstructural properties, such as grain size and morphology, can further refine the performance of these anodes. By tailoring the microstructure, researchers can minimize defects that lead to inefficiencies and enhance both the mechanical and chemical integrity of the anode during cycling. Through a combination of these strategies, it is possible to develop alloy and glassy metal anodes that optimize performance while maintaining the essential metrics of energy density and stability in aqueous aluminum batteries.

Overall, substantial knowledge gaps and ongoing debates persist concerning the strategies for stabilizing aluminum metal anodes in aqueous electrolytes and the underlying reaction mechanisms. A thorough and critical investigation is necessary to gain deeper insights, which are essential for enabling the practical implementation of modified aluminum metal anodes in aqueous battery systems.

Batteries can be conceptualized as reservoirs of interconnected chemical reactions; thus, an integrated and holistic approach is essential when designing advanced battery systems. In the case of aqueous Al batteries, the stability of the anode-electrolyte interface poses significant challenges. However, it is equally critical to recognize the importance of electrolytes and cathodes, both of which play pivotal roles in determining the overall electrochemical performance of the system.

Aqueous electrolytes, particularly those with low pH values, present unique challenges by increasing proton activity, which in turn enhances HER during aluminum nucleation [57]. Moreover, the high energy barrier associated with breaking H₂O–Al³⁺ bonds in the hexa-aqua complex results in sluggish Al deposition kinetrics. To address these issues, advancements in aqueous electrolyte formulations must focus on several key areas. First, achieving a neutral or weakly acidic pH is crucial to mitigate HER and promote stable aluminum redox reaction. Second, altering the solvation structure of the H₂O–Al³⁺ complex through the incorporation of suitable salts, additives, or co-solvents can significantly enhance electrochemical stability. Tailoring the electrolyte composition to optimize ion transport and reduce the energy barriers for Al deposition is essential for improving the overall efficiency and longevity of aqueous Al batteries.

The selection of cathode materials is equally critical in the development of practical, grid-scale ESSs. Ideal cathode materials must not only possess high energy density and excellent conductivity, but also demonstrate chemical and structural compatibility with the aqueous electrolyte and aluminum anode. In addition, cathodes should exhibit resilience in the aqueous environment, maintaining their structural integrity over extended cycling. Current research has primarily focused on metal oxides, conducting polymers, and organic compounds; however, the discovery and development of novel cathode materials are necessary to further enhance battery performance. Cathode materials that provide a high average discharge voltage, specific capacity, and long-term cycling stability are essential to achieving the energy density and durability required for grid-scale applications.

Ultimately, future research should pursue an integrated approach, balancing improvements in anode stability, electrolyte formulation, and cathode material development to realize the full potential of aqueous aluminum batteries for large-scale energy storage solutions.

As previously discussed, the primary objective of aqueous batteries is to enable large-scale energy storage, ranging from GW- to TW-levels, at a low cost while ensuring enhanced safety. To achieve this, it is imperative that the performance of aqueous aluminum full-cells be rigorously assessed under realistic operating conditions that mirror practical applications. However, most studies to date have been limited to lab-scale demonstrations utilizing milliwatt-hour (mWh)-level coin cells, typically featuring an excess aluminum anode. While these setups are valuable for evaluating the fundamental electrochemical characteristics of individual battery components, they do not accurately reflect the complexities of full-scale systems. The reduced scale often minimizes the occurrence and detection of side reactions, which can be more pronounced in larger systems.

In order to advance aqueous Al-metal batteries toward real-world applications, a comprehensive evaluation of key factors essential for scaling up the technology is required. One of the most critical aspects is the N:P ratio (anode to cathode capacity ratio) of the full-cell, as an optimized ratio is necessary to achieve balanced and efficient charge/discharge cycles. Additionally, scaling up the cell design itself—from small coin cells to larger pouch or prismatic cells—presents challenges related to uniformity in material distribution, heat management, and mechanical integrity. Another essential consideration is the operating temperature, as elevated temperatures may exacerbate unwanted side reactions or degradation, while lower temperatures may slow reaction kinetics. Furthermore, practical applications require testing at higher current densities to ensure that the cells can deliver high power output while maintaining stable operation over extended cycling.

Beyond these factors, considerations such as electrolyte composition, pressure tolerance, and long-term cycling performance must be critically assessed as part of the scaling-up process. Only by addressing these scaling challenges can aqueous aluminum batteries transition from promising lab-scale systems to practical solutions capable of meeting the demands of grid-scale energy storage.