Water-Based Battery Endures 120,000 Charge Cycles in Neutral Saltwater

A water-based battery has withstood 120,000 charge cycles while running in a neutral salt solution comparable to the brine used in tofu production.

This level of stability changes expectations around rechargeable battery life and the safety of disposing of batteries after decades in service.

Neutral chemistry payoff

During continuous laboratory cycling, the prototype repeatedly charged and discharged in ordinary water without the corrosive deterioration that constrains many conventional cells.

Dr. Chunyi Zhi of City University of Hong Kong (CityUHK), who monitored the tests, linked the water battery’s record lifetime directly to its neutral, non-corrosive chemistry.

Despite sustained cycling, its electrodes retained both their physical structure and operating performance instead of breaking down under chemical strain.

This resilience supports the study’s central finding, although it also raises the question of how a neutral system can provide both long-term stability and practical power.

Minerals from tofu

In tofu making, brine contains mineral coagulants, including magnesium chloride and calcium sulfate, which transform soya milk into curds.

For this battery, the same salts were used as the electrolyte: the liquid responsible for carrying electrical charge between the electrodes.

Maintaining the solution at 7.0 on the acidity scale kept it neutral and prevented the liquid from becoming corrosive.

While this gentler chemistry limited internal degradation, it also required the researchers to reconsider the design of the negative electrode.

New negative electrode design

Instead of a metal negative electrode, the researchers created one using a covalent organic polymer, a carbon network formed from connected molecules.

Its porous channels provided spaces for ions to sit, enabling the electrode to store charge without creating metallic deposits.

The team assessed three versions before choosing Hex TADD, a covalent organic polymer composed of linked carbon-based units. The material contains electron-donating bonds that help electrons travel more readily through its structure.

However, even a robust polymer negative electrode requires a compatible positive electrode that can exchange ions while preserving its structure.

Prussian blue partner

For the positive side, the cell used a Prussian blue analogue, a crystal structure able to take in and release ions.

Its open framework stored charge by altering the metal state within the lattice, then reversed that process when recharged.

Although it is recognised as a blue pigment used in paints, the material remained stable in water throughout repeated ion exchanges.

Combined with this positive electrode, the complete cell achieved a 2.2-volt range, although water still places a limit on maximum voltage.

Testing the water battery lifespan

Under stress testing, the water battery remained stable through 120,000 charge cycles, a figure well beyond that achieved by many laboratory cells.

Every cycle required ions to move into and out of the electrodes, meaning weak bonds would have failed much earlier. In theory, a phone-sized pack using this design and charged daily could operate for more than 300 years.

Such exceptional lifespan is especially relevant where battery replacement is difficult, including remote sensors and grid-storage cabinets.

Energy capacity of the battery

As well as its long lifespan, the device stored approximately 3,200 milliamp-hours per ounce of active material, equivalent to 112.8 milliamp-hours per gram.

This charge storage resulted from ions entering the polymer structure and then leaving it when the circuit direction was reversed.

At full-cell level, its specific energy - the energy stored for each unit of weight - was close to 22 watt-hours per pound (48.3 per kilogram).

Water-based batteries generally provide less compact energy storage than lithium battery packs, however, which restricts them to larger and heavier systems.

Disposing of battery waste

The safety case depended on chemistry that was neither highly acidic nor highly alkaline, meaning that any leak would resemble salty water.

Under the EPA’s view, many discarded lithium-ion battery packs are considered hazardous waste because they can catch fire.

“Compared to current aqueous battery systems, the new system offers exceptional long-term cycling stability and respect for the environment under neutral conditions,” wrote Zhi.

The paper described the cell as non-toxic and disposable under several standards, including the Resource Conservation and Recovery Act, a US law covering hazardous waste.

Scaling the water battery

Converting a laboratory cell into a commercial battery will require more energy to be packed into a smaller space without compromising safety.

Increasing electrode thickness and making packaging more compact usually improves energy storage, but it can also impede ion movement and retain heat.

Manufacturing the polymer negative electrode at scale will depend on producing consistent pore structures; otherwise, performance could differ between batches.

These scale-up challenges will determine whether the neutral-salt method remains a specialist technology or becomes part of everyday energy storage.

Practical uses for water batteries

For numerous applications, batteries fail because their liquids gradually corrode the electrodes rather than because their initial charge capacity is inadequate.

By using neutral salts and organic electrodes, CityUHK’s cell reduced these side reactions and continued operating after intensive cycling.

A longer operating life could reduce maintenance costs and waste, particularly for infrastructure designed to remain in place for decades.

Practical battery packs will still require seals, current collectors and control systems, so the neutral liquid represents only one part of the overall design.

Future of neutral salt batteries

Neutral saltwater, an organic negative electrode and a Prussian blue positive material combined to make durability the defining feature of this water-based battery.

If engineers can increase energy density and manufacture the polymers consistently, this chemistry could reduce the waste and complications batteries commonly leave behind.