Low-cost ‘breathing’ battery could solve renewable energy storage problem

A new type of battery made from sulphur, air, water and salt has been developed by Massachusetts Institute of Technology (MIT) scientists. It is nearly 100 times less expensive to produce than batteries currently on the market and can store twice as much energy as a lead-acid battery.

The research team believes the battery is ideal for storing electricity produced from wind and solar sources so that the grid can still be powered when they are not generating.

The ‘air-breathing’ battery is capable of storing electricity for very long durations for about one-fifth of the cost of current technologies and could make sporadic renewable power a more reliable source.

For its anode, the rechargeable flow battery uses cheap, abundant sulphur dissolved in water. An aerated liquid salt solution in the cathode continuously takes in and releases oxygen that balances charge as ions shuttle between the electrodes.

Oxygen flowing into the cathode causes the anode to discharge electrons to an external circuit. Oxygen flowing out sends electrons back to the anode, recharging the battery.

“This battery literally inhales and exhales air, but it doesn’t exhale carbon dioxide, like humans - it exhales oxygen,” said MIT professor Yet-Ming Chiang.

The battery’s total chemical cost - the combined price of the cathode, anode, and electrolyte materials - is about 1/30th the cost of competing batteries, such as lithium-ion batteries. Scaled-up systems could be used to store electricity from wind or solar power, for multiple days to entire seasons, for about $20 to $30 (£15-£23) per kilowatt hour.

A major issue with batteries over the past several decades, Chiang says, has been a focus on synthesising materials that offer greater energy density but are very expensive.

The most widely used materials in lithium-ion batteries for cellphones, for instance, have a cost of about $100 for each kilowatt hour of energy stored.

“This meant maybe we weren’t focusing on the right thing, with an ever-increasing chemical cost in pursuit of high energy-density,” Chiang said. “We said, ‘If we want energy storage at the terawatt scale, we have to use truly abundant materials.’”

Using that concept, the team of researchers created a type of flow battery, where electrolytes are continuously pumped through electrodes and travel through a reaction cell to create charge or discharge.

The battery consists of a liquid anode (anolyte) of polysulphide that contains lithium or sodium ions and a liquid cathode (catholyte) that consists of an oxygenated dissolved salt, separated by a membrane.

Upon discharging, the anolyte releases electrons into an external circuit and the lithium or sodium ions travel to the cathode.

At the same time, to maintain electroneutrality, the catholyte draws in oxygen, creating negatively charged hydroxide ions. When charging, the process is simply reversed. Oxygen is expelled from the catholyte, increasing hydrogen ions, which donate electrons back to the anolyte through the external circuit.

“What this does is create a charge balance by taking oxygen in and out of the system,” Chiang says.

As the battery uses ultra-low-cost materials, its chemical cost is one of the lowest - if not the lowest - of any rechargeable battery to enable cost-effective long-duration discharge. Its energy density is slightly lower than today’s lithium-ion batteries.

Although the prototype is currently about the size of a coffee cup, flow batteries are highly scalable and cells can be combined into larger systems.

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