Aqueous Batteries: Challenges and Future Potential

Aqueous batteries have gained attention as a safer and more environmentally friendly alternative to conventional non-aqueous systems. Their use of water-based electrolytes significantly reduces fire hazards and enhances sustainability, making them an attractive option for large-scale energy storage. However, challenges such as limited voltage windows, electrochemical stability, and material compatibility must be addressed before they can compete with traditional lithium-ion technology.

To fully understand their potential, it is essential to examine key chemistries, electrolyte properties, electrode reactions, and operational limitations.

Major Aqueous Battery Chemistries

Aqueous batteries rely on various metal-ion chemistries, each offering distinct advantages and limitations in energy density, cycle life, and application. Three of the most studied systems—zinc-ion, sodium-ion, and lithium-ion—demonstrate unique electrochemical behaviors in water-based electrolytes, influencing their feasibility for large-scale storage.

Zinc-Ion

Zinc-ion batteries (ZIBs) have attracted interest due to the abundance, low cost, and high theoretical capacity of zinc (820 mAh/g). Unlike lithium-based systems, ZIBs operate with a Zn metal anode and typically use manganese dioxide (MnO₂) or vanadium-based compounds as cathodes. The primary advantage of this chemistry is the non-flammability of the aqueous electrolyte, eliminating the risk of thermal runaway. However, challenges such as zinc dendrite formation and cathode dissolution hinder long-term stability.

Recent studies, such as one published in Nature Communications (2021), have explored electrolyte modifications to suppress dendrite growth, including zinc sulfate solutions with organic additives. Additionally, the reversibility of Zn²⁺ intercalation remains a concern, as repeated cycling can degrade the cathode material. Despite these issues, advancements in electrolyte engineering and electrode design continue to improve the viability of ZIBs for grid storage.

Sodium-Ion

Sodium-ion aqueous batteries (SIBs) offer a promising alternative due to sodium’s abundance and similar electrochemical characteristics to lithium. Unlike non-aqueous sodium-ion batteries, aqueous variants benefit from faster ion transport and reduced safety concerns. Typical cathode materials include Prussian blue analogs and layered oxides, while anodes range from carbon-based materials to titanium compounds.

A significant limitation is the relatively low energy density, often constrained by the narrow electrochemical window of water-based electrolytes. Research in Advanced Energy Materials (2022) has highlighted the role of concentrated electrolytes, such as water-in-salt solutions, in expanding voltage stability and improving cycling performance. Additionally, sodium’s larger ionic radius compared to lithium can lead to sluggish diffusion kinetics, impacting charge/discharge rates. Despite these hurdles, SIBs are being explored for stationary energy storage where cost-effectiveness and safety outweigh energy density concerns.

Lithium-Ion

Aqueous lithium-ion batteries (ALIBs) aim to combine the energy density of lithium chemistry with the safety of water-based electrolytes. These systems typically employ LiMn₂O₄ or LiFePO₄ cathodes and lithium titanium oxide (LTO) anodes to ensure stability. The primary challenge is the limited electrochemical window of water, which restricts voltage output and overall energy density.

Research published in Joule (2023) has explored hybrid electrolytes containing lithium salts in high concentrations to suppress side reactions and enhance stability. Another issue is the dissolution of active materials, particularly for manganese-based cathodes, which leads to capacity fading. While ALIBs are not yet competitive with non-aqueous counterparts in portable electronics, their potential for safe, low-cost energy storage in stationary applications continues to drive research.

Electrolyte Composition And Ionic Conductivity

The performance of aqueous batteries is heavily influenced by electrolyte composition, which governs ionic conductivity, electrochemical stability, and efficiency. These electrolytes typically consist of dissolved metal salts in water, forming an ionic medium that facilitates charge transport. The choice of salt and its concentration play a significant role in determining conductivity. Common options include zinc sulfate (ZnSO₄) for zinc-ion systems, sodium perchlorate (NaClO₄) for sodium-ion configurations, and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) for aqueous lithium-ion variants.

Conductivity is dictated by ion dissociation and mobility, both influenced by solvation dynamics and intermolecular interactions. Highly dissociative salts, such as lithium nitrate (LiNO₃), provide a greater concentration of free charge carriers, improving conductivity. However, excessive salt concentrations can lead to ion pairing or clustering, reducing the number of mobile species and increasing viscosity, hindering ion diffusion.

This trade-off has led to the development of “water-in-salt” electrolytes, where the salt-to-water ratio is significantly increased to suppress water activity and expand the electrochemical stability window. A study in Science (2018) demonstrated that lithium bis(fluorosulfonyl)imide (LiFSI) in highly concentrated solutions enhances voltage stability by forming a protective solvation sheath around lithium cations, reducing parasitic side reactions.

Beyond salt selection, pH and buffer systems influence ionic conductivity by affecting ion speciation and electrode stability. For example, in zinc-ion batteries, acidic electrolytes (pH < 4) can lead to hydrogen evolution and zinc corrosion, while highly alkaline conditions (pH > 10) promote the formation of zinc hydroxide species that precipitate and reduce ionic mobility. Buffered electrolytes, such as phosphate or borate-based systems, help maintain a stable pH, mitigating unwanted side reactions while preserving high conductivity.

Cathode And Anode Reactions

The electrochemical behavior of aqueous batteries is governed by the redox reactions at the cathode and anode, which influence energy efficiency, capacity retention, and cycle life. These reactions involve the reversible insertion and extraction of metal ions within electrode materials.

At the cathode, transition metal oxides, Prussian blue analogs, and polyanionic compounds serve as hosts for cation intercalation. In zinc-ion systems, manganese dioxide undergoes a multi-step redox process where Zn²⁺ ions insert into the structure. This reaction is often accompanied by cathode dissolution, leading to capacity fading. Researchers have explored surface coatings and dopants to reinforce structural stability.

The anode, responsible for metal deposition and stripping, faces challenges in zinc and lithium-based aqueous batteries. Zinc metal anodes suffer from dendritic growth, which decreases efficiency and compromises safety. Various strategies, including electrolyte modifications and artificial interfacial layers, have been proposed to suppress dendrite formation. In aqueous lithium-ion systems, lithium titanium oxide has emerged as a preferred anode material due to its zero-strain characteristics, preventing volumetric expansion and enhancing stability.

pH Range And Electrochemical Stability

The stability of aqueous batteries depends on the pH of the electrolyte, which influences electrode integrity and parasitic reactions. Since water-based systems operate within a restricted electrochemical window, maintaining a controlled pH environment is essential to preventing side reactions such as hydrogen evolution and metal dissolution.

The electrochemical window of water, typically around 1.23 V under standard conditions, shifts with pH due to changes in hydrogen and oxygen evolution potentials. Lower pH values increase the likelihood of hydrogen gas formation, reducing coulombic efficiency and posing safety concerns. In contrast, higher pH levels extend anodic stability but risk forming unwanted precipitates that disrupt electrode performance. Buffered electrolytes have been explored to stabilize pH fluctuations, with phosphate- and borate-based systems improving cycling stability by minimizing electrode corrosion.

Temperature Effects

Temperature fluctuations influence ion mobility, electrolyte stability, and electrode kinetics. High temperatures accelerate solvent evaporation and side reactions, leading to faster capacity degradation. They also exacerbate cathode dissolution, particularly in manganese-based systems, where Mn²⁺ leaching increases. Zinc-ion batteries suffer from aggravated dendrite growth under thermal stress, increasing the risk of short circuits.

Conversely, low temperatures reduce ionic conductivity and slow charge transfer kinetics. In aqueous lithium-ion batteries, sluggish lithium-ion diffusion at subzero temperatures leads to higher internal resistance and lower discharge capacities. Researchers have explored electrolyte modifications, such as antifreeze additives, to enhance low-temperature operability while maintaining stability.

Gas Evolution In Water-Based Systems

Aqueous batteries face challenges from unintended hydrogen and oxygen gas evolution due to water electrolysis. Hydrogen evolution at the anode, particularly in zinc-based systems, depletes electrolyte volume and reduces efficiency. Oxygen evolution at the cathode accelerates electrode degradation, leading to structural breakdown and capacity fading.

Gas accumulation within a sealed battery environment increases internal pressure and safety risks. Researchers have explored electrolyte engineering strategies, such as proton-buffering additives and highly concentrated “water-in-salt” electrolytes, to suppress water decomposition and reduce gas evolution.

Electrolyte Additives In Water-Based Systems

To address aqueous electrolyte limitations, researchers have introduced additives that enhance stability and suppress side reactions. Fluorinated salts, such as LiTFSI and Zn(OTf)₂, expand the electrochemical stability window by reducing water activity. Organic and inorganic inhibitors mitigate dendrite formation and electrode degradation.

While electrolyte additives have significantly improved aqueous battery performance, their widespread adoption depends on balancing cost-effectiveness with scalability.