Water-Based Battery Lasts 120,000 Charge Cycles in Neutral Salt Solution

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.

That level of stability changes expectations for rechargeable battery longevity and for how safely they could be disposed of after decades in service.

Benefits of neutral chemistry

During repeated laboratory cycling, the prototype kept charging and discharging in ordinary water without the corrosive deterioration that restricts many conventional cells.

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

The electrodes retained both their structure and performance even after intensive cycling, rather than breaking down under chemical strain.

This resilience supports the study’s main claim, while raising the question of how a neutral system can provide both stability and practical power.

Minerals used in tofu

In tofu making, brine contains mineral coagulants such as magnesium chloride and calcium sulphate, which transform soy milk into curds.

For this battery, those salts acted as the electrolyte: the liquid that transports electric charge between the electrodes.

Keeping the solution at 7.0 on the acidity scale maintained a neutral, non-corrosive liquid.

Although this gentle chemistry reduced internal deterioration, it also required the researchers to reconsider the negative electrode.

Water battery negative electrode design

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

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

The team assessed three versions before choosing Hex TADD, a covalent organic polymer made from linked carbon-based units. The material contains electron-donating bonds that enable electrons to travel more freely through its structure.

However robust the negative polymer is, it still requires a compatible positive electrode that can exchange ions while retaining its structure.

Prussian blue counterpart

For the positive electrode, the cell used a Prussian blue analogue, a crystal able to move ions in and out.

Its open structure stored charge by altering the metal state within the lattice, before reversing this process during recharging.

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

Combined with this positive electrode, the complete cell achieved a 2.2-volt range, although water still imposes a limit on the voltage that can be reached.

Battery lifespan testing

During stress testing, the water battery remained stable through 120,000 charge cycles, substantially more than many laboratory cells manage.

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 based on this design and charged daily could operate for more than 300 years.

This type of longevity is especially valuable where battery replacement is difficult, including remote sensors and grid-storage cabinets.

Energy capacity of the battery

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

This charge was produced as ions entered the polymer structure and then returned when the circuit direction was reversed.

At whole-cell level, specific energy – the energy stored for each unit of weight – approached 22 watt-hours per pound (48.3 per kilogram).

Water-based batteries generally remain less energy-dense than lithium packs, however, which confines them to larger and heavier systems.

Battery waste disposal

The safety case relied on chemistry that was neither highly acidic nor highly alkaline, meaning that any leaks would simply resemble salt water.

Under the EPA’s classification, many discarded lithium-ion packs are regarded as hazardous waste because they may 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 fitted into a smaller space while preserving safety.

Increasing electrode thickness and making packaging more compact generally boosts energy, but can also slow ion movement and retain heat.

Manufacturing the polymer negative electrode at scale will demand consistent pore structures; otherwise, results will differ between production batches.

These scale-up requirements will determine whether the neutral-salt method remains specialised or enters everyday energy storage.

Practical applications for water batteries

In many applications, batteries fail because their liquids gradually erode the electrodes, rather than because their initial charge is insufficient.

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

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

Working battery packs will still require seals, current collectors and controls, so 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 came together in this water-based battery to make durability its defining feature.

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