In the quest for more reliable and economical energy storage solutions, the benefits of aqueous battery are emerging. Today’s commercial aqueous battery lack the energy density and cycle life needed to compete in the rapidly growing fields of transportation and grid storage, but this will change with the development of new materials and battery design strategies.
Many limitations of conventional aqueous battery have been alleviated by innovations such as selective membranes, dilute aqueous electrolytes, and novel electrode reactions.
The origin of aqueous battery
Nickel-iron batteries were the first alkaline rechargeable batteries invented in the early 20th century. Due to its fairly high specific capacity, high reversibility, and its redox potential close to the anodic stability limit of alkaline electrolytes, NiOOH/Ni(OH)2 redox has always been the cathode of choice for alkaline batteries.
The search for a better anode material to match the nickel cathode continues to this day. Although the iron anode has a high theoretical specific capacity, it is thermodynamically unstable in water and easily corroded. However, the high-pressure tanks required to store hydrogen, as well as the noble metal catalysts (such as platinum black) used in the hydrogen reaction, limit NiMH battery in terms of reliability and longevity more than cost.
Hydrogen storage alloys, or metal hydrides, were commercialized as a low-cost alternative to hydrogen anodes in 1989, only two years before the commercialization of lithium-ion batteries. The next candidate material for alkaline battery anodes is zinc metal, which offers higher energy while being cheaper than other materials.
Advantages and limitations of aqueous battery
Aqueous battery is generally considered safe, reliable and affordable. These obvious advantages over Li-ion batteries, which use flammable organic solvents in their electrolyte solutions. Conventional aqueous battery use electrolytes that typically contain more than 70 percent water and are therefore generally non-flammable.
The low cost of aqueous battery satisfies three reasons: cheap raw materials, minimum requirements for the manufacturing environment, and battery management and protection systems.
The water and solutes used in conventional aqueous electrolytes, such as sulfuric acid and KOH, are much cheaper than the organic solvents and fluoride salts (such as LiPF6) used in li-ion batteries. However, if expensive solutes and organic co-solvents are used as electrolytes, the cost may also increase dramatically.
Besides the electrolyte, the active electrode materials used in conventional water batteries are also mostly low-cost, but this is more of a coincidence than a feature, as exceptions do exist: for example, electrode materials such as misch metals and vanadium do not necessarily cheaper than the active material used in lithium-ion batteries.
Additionally, aqueous battery manufacturing typically does not require the expensive and energy-intensive controlled environmental conditions required for lithium-ion batteries. Finally, aqueous battery require minimal reliance on battery management system and protection systems because of their safety and reliability, not just their non-flammability. All three factors contribute to the low capital cost of conventional water batteries.
Safety and reliability
While water batteries may not catch fire, they are vulnerable to mishandling and potentially catastrophic events such as explosions. Overcharging is an improper operation that irreversibly decomposes the electrolyte and degrades the battery cells.
But aqueous battery can largely tolerate overcharging through oxygen cycling. Reactions with electrolytes are of particular concern for emerging batteries using neutral electrolytes, which lack the inherent pH buffering capabilities of acidic and alkaline batteries.
Under the same conditions, aqueous electrolyte solutions are usually more conductive than non-aqueous electrolyte solutions because of the high ion dissociation and low viscosity of the solution. Higher conductivity could facilitate faster charging and discharging and potentially allow for thicker electrodes, which would increase energy density.
Narrow electrochemical stability window
In addition to all the advantages of aqueous battery, they also have significant limitations. First, water has a narrow thermodynamic and electrochemical stability window of 1.23 V. Some materials have high overpotentials for these reactions, effectively extending the stability window to 2.3 V.
Among conventional aqueous battery, lead-acid cells take full advantage of the expanded stability window, with a nominal voltage of ~2V. All other commercially available rechargeable water batteries operate above 1V. These values are much lower than the 3.3-3.9 V of Li-ion batteries. The insufficient voltage greatly limits the energy density of aqueous battery, and it is difficult to compensate with higher specific capacity.
Aggressiveness of aqueous solutions
Water has high polarity and strong coordination ability, so it is an excellent solvent for dissolution and dissociation. These properties make water an excellent component of electrolytes but a problem for many electrodes. The strong solvency of water is responsible for the recrystallization of PbSO4 and the premature failure of lead-acid batteries.
LiFePO4, a cathode material known for its durability in aqueous Li-ion batteries, exhibits dissolution-induced degradation in neutral aqueous solutions. Even Ni(OH)2, which is known for its stability, is susceptible to Ostwald ripening in alkaline electrolytes. Water, being an amphoteric compound, is also corrosive, especially at extreme potentials where electrolysis creates additional acids and bases.
Innovation of aqueous battery
The above advantages and disadvantages determine the performance and application limitations of traditional aqueous battery. New materials and battery designs should overcome one or more of these limitations, making aqueous battery good partners for lithium ion battery storage and transpartations.
Selective membranes and coatings
Ion-selective layers, which allow some ions and molecules to pass while blocking others, prevent some battery components from being exposed to poor electrolyte conditions. Although the electrochemical stability window of water remained constant at all pH values, the electrolysis potential varied linearly with a slope of 0.059 V per pH value.
Ion-selective membranes prevent the diffusion of hydrogen and hydroxide ions, allowing acidic and alkaline electrolytes to be used in the same cell. Coupling the anodic limit of an acidic electrolyte (e.g., pH = 1) with the cathode of an alkaline electrolyte (e.g., pH = 15) yields a thermodynamic stability window of ~2.17 V.
The reactivity of water molecules varies according to their coordination state, and the stability window can be manipulated by driving water molecules in solution from a free state to a coordinated state. This operation is first accomplished by dramatically increasing the amount of salt, the “water-in-salt” electrolyte.
Electrolyte formulations that do not contain high concentrations of salt have also received attention. Organic compounds with a high atomic ratio of O and N have been introduced as “co-solvents” for aqueous electrolyte solutions. These chemicals disrupt the hydrogen-bonding structure in water so that the characteristic vibrations of water cannot be detected.
Both overconcentration and organic co-solvents work by reducing the water content of the aqueous electrolyte solution, sometimes to the level of 5 wt%. Therefore, many water-poor electrolytes have conductivity in the range of 0.001–0.01 S cm−1, which is similar to that of nonaqueous electrolytes.
The cost of the electrolyte increases due to the use of organic and fluorine-containing components. But despite their low water content, most of these electrolytes are non-flammable.
New electrode reaction
Besides traditional aqueous battery, many novel electrode reactions have been established. Among them, the intercalation reactions established by Li-ion batteries provide a variety of structurally stable electrode materials. New electrolytes and advanced surface modifications enhance deposition reactions, leading to structurally simple compounds that serve as high-capacity electrode material candidates.
With the aid of selective membranes, capacity-power decoupling and potentially good scalability can be achieved. Air is also being integrated into rechargeable water batteries, and air gas is becoming a low-cost and high-capacity electrode material due to new battery designs.
Summary and outlook
Electrolytes, membranes, and electrodes all require continuous improvement to produce modern aqueous battery with commercial impact. In addition to the previously outlined strategies, the following urgent but untapped research directions are highlighted.
Electrolyte related issues. The compromise with water-poor electrolytes offers great room for improvement, especially in terms of ionic conductivity, cost, and non-flammability. Understanding the ion conductivity, flammability and (electro)chemical stability of solutions with moderate water content (5-70 wt%) can open new research areas in materials science.
The oxygen cycle in these new electrolytes remains unknown, raising uncertainty about the robustness of many modern aqueous battery. The realization of the oxygen cycle will reduce the requirements on the range of electrode reaction potentials and safety mechanisms in the battery, resulting in greater freedom in the design of materials and batteries.
To avoid the expected failure of the inactive parts, current collectors and binders must be designed together with the electrolyte. In modern designs, the SEI phenomenon in aqueous battery is rapidly becoming mainstream and will become a driving force for innovation.
Diaphragm customization. Most of the selective membranes studied in modern aqueous battery come from other industries, with widely varying criteria in terms of mechanical stability, chemical stability, selectivity, and size. Membranes for neutral electrolytes may not require the chemical resistance offered by Nafion.
In high energy density batteries, thinner membranes may be better than stronger ones. Reports on membranes specialized for specific electrode reactions are still scarce, but this is a growing field for performance optimization and integration into energy-dense batteries.
While it is important to address these issues independently, it is equally important to focus on strategies for integrating different solutions. Integration should be easily scalable so that battery production does not become an insurmountable obstacle to manufacturing. The characteristics of modern water batteries are very different from traditional water batteries, and the research topics and methods should also be different.