Water-based battery in tofu brine neutral salt solution lasts 120,000 charge cycles

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

That level of stability changes expectations for how long a rechargeable battery might realistically operate - and how safely it could be disposed of after decades in service.

Neutral chemistry payoff

During repeated cycling in the laboratory, the prototype kept charging and discharging in ordinary water, avoiding the corrosive failure modes that cap the life of many conventional cells.

Tracking that performance, Dr. Chunyi Zhi at City University of Hong Kong (CityUHK) linked the battery’s record-long lifespan directly to its neutral, non-corrosive chemistry.

Despite continuous cycling, the electrodes retained both their structure and their output, instead of breaking down under chemical strain.

This robustness underpins the paper’s central point, while still leaving the mechanism open to scrutiny: how a neutral system can deliver both long-term stability and practical power.

Minerals from tofu

In tofu production, the brine contains mineral coagulants - such as magnesium chloride and calcium sulfate - that help convert soya milk into curds.

In this battery, those salts were used as the electrolyte, the liquid medium that transports charge-carrying ions between electrodes.

Keeping the solution at 7.0 on the pH scale maintained neutrality, ensuring the liquid remained non-corrosive.

That gentle chemistry lowered internal degradation, but it also forced a rethink of what to use for the negative electrode.

New negative electrode design

Instead of choosing a metal for the negative electrode, the researchers constructed it from a covalent organic polymer - a carbon-based network formed from molecules linked together.

The material’s porous pathways provided spaces for ions to sit, allowing the electrode to hold charge without creating metal deposits.

After evaluating three candidates, the team settled on Hex TADD, a covalent organic polymer assembled from linked, carbon-based building blocks. The structure includes electron-donating bonds that help electrons move more readily through the framework.

However, even a durable polymer negative electrode still requires a compatible positive electrode that can exchange ions repeatedly without losing its own integrity.

Prussian blue partner

On the positive side, the device used a Prussian blue analogue, a crystalline material capable of moving ions in and out.

Its open lattice stored charge by shifting the metal state within the structure, then reversed that process during recharging.

Although widely known as a blue pigment used in paints, the compound remained stable in water while enduring repeated ion exchange.

With this pairing, the complete cell delivered a 2.2-volt range, even though water chemistry still places limits on achievable voltage.

Testing the battery lifespan

Under accelerated stress testing, the water battery maintained stable operation for 120,000 charge cycles - well beyond what many laboratory cells manage.

Because each cycle drives ions into and out of both electrodes, weak bonds would have failed quickly; instead, the system held up. On a once-per-day charging pattern, a phone-sized pack built on this design could, in theory, operate for more than 300 years.

This sort of longevity is most valuable where swaps are difficult or costly, including remote sensors and grid-storage enclosures.

Energy capacity of the battery

In addition to lifespan, the cell stored about 112.8 milliamp-hours per gram of active material, equivalent to 3,200 milliamp-hours per ounce.

That capacity came from ions entering the polymer framework during charging and leaving again when the current direction was reversed.

At the full-cell scale, specific energy - energy stored per unit mass - reached near 48.3 watt-hours per kilogram (22 per pound).

Even so, water-based batteries typically lag behind lithium packs in energy density, which tends to confine them to larger, heavier deployments.

Disposing of battery waste

The safety argument depended on chemistry that was neither strongly acidic nor strongly alkaline, meaning any leakage would essentially resemble salty water.

From the U.S. Environmental Protection Agency (EPA) perspective, many discarded lithium-ion packs are treated as hazardous waste because they can ignite.

“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 publication described the cell as non-toxic and disposable under several standards, including the Resource Conservation and Recovery Act, a U.S. hazardous waste law.

Scaling the water battery

Moving from a laboratory demonstration to a commercial battery requires fitting more energy into a smaller volume while maintaining safety.

In practice, thicker electrodes and denser packaging can boost energy, but they may also slow ion transport and increase heat retention.

Manufacturing the polymer negative electrode at scale will also need tightly controlled pore structures; otherwise, performance could vary between production batches.

These scale-up challenges will determine whether the neutral-salt design remains specialised or finds a place in everyday energy storage.

Practical uses for water batteries

In many applications, batteries fail because their liquids gradually attack the electrodes, rather than because the initial capacity is inadequate.

By combining neutral salts with organic electrode materials, CityUHK’s cell reduced those side reactions, enabling it to continue working through relentless cycling.

A longer service life could cut both maintenance demands and waste, particularly in fixed infrastructure intended to run for decades.

Even so, real-world battery packs still rely on seals, current collectors, and control systems, so the neutral electrolyte is only one part of a broader engineering package.

Future of neutral salt batteries

By uniting neutral saltwater, an organic negative electrode, and a Prussian blue positive material, this water-based battery puts durability at the centre of its appeal.

If engineers can raise energy packing and produce the polymers consistently, this chemistry may reduce the long-term burden that batteries often leave behind.