Water-in-salt (WiS) electrolytes represent a significant advancement in aqueous battery technology, addressing the limitations of conventional aqueous electrolytes while enabling high-voltage operation. Unlike traditional dilute aqueous electrolytes, WiS systems feature a high concentration of salts dissolved in water, creating a unique electrochemical environment. This approach expands the voltage window, enhances stability, and reduces water activity, making it suitable for next-generation lithium-ion and sodium-ion batteries.

The defining characteristic of WiS electrolytes is their unusually high salt concentration, typically exceeding the conventional solubility limits. For lithium-based systems, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) is a commonly used salt due to its high solubility and electrochemical stability. In sodium-ion systems, sodium equivalents such as NaTFSI are employed. The high salt-to-water ratio alters the electrolyte structure, reducing free water molecules and forming a quasi-solid-state interfacial layer on electrode surfaces. This layer suppresses hydrogen and oxygen evolution reactions, effectively widening the electrochemical stability window to around 3 volts, compared to the 1.23-volt limit of traditional aqueous electrolytes.

One of the key advantages of WiS electrolytes is their ability to stabilize high-energy electrode materials that would otherwise decompose in water. For instance, lithium manganese oxide (LiMn2O4) and lithium nickel manganese cobalt oxide (NMC) cathodes can operate in WiS electrolytes without significant degradation. Similarly, anode materials like lithium titanate (LTO) and even metallic lithium (in certain configurations) exhibit improved stability. The expanded voltage window allows aqueous batteries to achieve energy densities closer to those of non-aqueous systems while maintaining inherent safety advantages.

Salt selection plays a critical role in determining the performance of WiS electrolytes. LiTFSI is favored for its high solubility, low lattice energy, and ability to form a stable solid-electrolyte interphase (SEI). However, its high cost and hygroscopic nature present challenges. Alternative salts, such as lithium triflate (LiOTf) or lithium bis(fluorosulfonyl)imide (LiFSI), are being explored to reduce expenses while maintaining performance. In sodium-ion systems, NaTFSI and sodium triflate (NaOTf) are commonly used, though their solubility limits and interfacial stability require further optimization.

Corrosion inhibition is another critical aspect of WiS electrolytes. The high salt concentration inherently reduces corrosion by minimizing free water molecules that participate in parasitic reactions. Additionally, certain additives, such as lithium nitrate (LiNO3) or sodium fluoride (NaF), can further enhance passivation layers on metal current collectors like aluminum and stainless steel. These additives mitigate dissolution and pitting corrosion, extending the lifespan of battery components.

Despite their advantages, WiS electrolytes face several challenges. The high salt concentration increases viscosity, reducing ionic conductivity compared to dilute electrolytes. This trade-off between voltage window and kinetics necessitates careful optimization for specific applications. Moreover, the cost of high-purity salts like LiTFSI remains a barrier to large-scale adoption. Researchers are investigating lower-cost alternatives and hybrid systems that balance performance and affordability.

Another challenge is the limited temperature range of WiS electrolytes. At low temperatures, the high viscosity can lead to poor ion transport, while at elevated temperatures, accelerated side reactions may occur. Strategies such as optimizing salt mixtures or incorporating organic co-solvents in controlled amounts are being explored to improve thermal stability.

WiS electrolytes have found applications in various battery systems, particularly where safety and environmental concerns are paramount. Aqueous lithium-ion batteries using WiS electrolytes are being developed for grid storage, where non-flammability and low toxicity are critical. Sodium-ion variants are also gaining attention for large-scale energy storage due to the abundance of sodium resources. Additionally, WiS systems are being explored for flexible and wearable electronics, where leakage and flammability risks must be minimized.

The environmental benefits of WiS electrolytes are notable. Unlike organic solvents, water is non-toxic and non-flammable, simplifying battery disposal and recycling. However, the high salt concentration necessitates recovery processes to prevent environmental contamination. Research into closed-loop recycling methods for WiS electrolytes is ongoing to ensure sustainability.

In summary, water-in-salt electrolytes offer a promising pathway for safer, high-voltage aqueous batteries. Their unique properties, including an expanded voltage window and reduced water activity, enable the use of high-energy electrode materials while maintaining the inherent safety of aqueous systems. Challenges such as cost, viscosity, and corrosion must be addressed to facilitate widespread adoption. With continued research into salt formulations and additive engineering, WiS electrolytes could play a pivotal role in the future of energy storage, particularly in applications where safety and sustainability are prioritized.

The development of WiS electrolytes underscores the importance of interdisciplinary research, combining electrochemistry, materials science, and engineering to overcome existing limitations. As the demand for safer and more sustainable energy storage grows, WiS technology is poised to bridge the gap between conventional aqueous batteries and their organic counterparts, offering a viable solution for next-generation energy storage systems.