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.

That resilience changes expectations for rechargeable battery lifespans and for how safely they could be disposed of after several decades in service.

Benefits of neutral chemistry

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

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

The electrodes retained both their structure and performance throughout intensive cycling instead of breaking down under chemical strain.

This durability is the study’s key finding, although it still raises the question of how a neutral system can provide both stability and practical power.

Minerals used in tofu

In tofu production, brine contains mineral coagulants such as magnesium chloride and calcium sulfate, which transform soya milk into curds.

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

Keeping the solution at 7.0 on the acidity scale ensured it remained neutral and non-corrosive.

While this gentle chemistry limited internal degradation, 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 using a covalent organic polymer, a carbon network formed from bonded molecules.

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

Having assessed three versions, the team chose a material named Hex TADD, a covalent organic polymer made from linked carbon-based units. Its electron-donating bonds help electrons travel more readily through the material’s structure.

However, even a durable polymer negative electrode requires a compatible positive electrode that can exchange ions without sacrificing its structure.

Prussian blue counterpart

For the positive electrode, the cell used a Prussian blue analogue, a crystal capable of moving ions in and out of its structure.

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

Although recognised as a blue paint pigment, the material remained stable in water while undergoing repeated ion exchanges.

Using this positive electrode, the complete cell achieved a 2.2-volt range, though water still places a ceiling on possible voltage.

Testing battery lifespan

Under stress testing, the water battery remained stable through 120,000 charge cycles, substantially exceeding the performance achieved by many laboratory cells.

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

Such extended service life is particularly valuable where battery replacement is difficult, including remote sensors and grid-storage cabinets.

Battery energy capacity

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 stored as ions entered the polymer structure and released when the circuit direction was reversed.

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

However, water-based batteries generally offer less compact energy storage than lithium packs, restricting their use to larger and heavier systems.

Disposal of battery waste

The safety argument relied on chemistry that was neither highly acidic nor highly alkaline, meaning that a leak would resemble salty water.

Under the EPA’s classification, many discarded lithium-ion packs are considered 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 hazardous-waste law.

Scaling the water battery

Converting a laboratory cell into a commercial battery will require fitting more energy into a smaller space while retaining its safety advantages.

Increasing electrode thickness and tightening packaging can usually boost energy storage, but these changes also impede ion movement and retain heat.

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

These scaling requirements will determine whether the neutral-salt method remains specialised or becomes part of everyday energy storage.

Practical applications for water batteries

For many applications, batteries fail because their liquids gradually corrode 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 through relentless cycling.

A longer operating life could reduce both maintenance spending and waste, especially in fixed infrastructure intended 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 raise energy density and manufacture the polymers consistently, this chemistry could reduce the waste batteries commonly leave behind.