Batteries that absorb carbon emissions move a step closer to reality – new study

What if there were a battery that could release energy while trapping carbon dioxide? This isn’t science fiction; it’s the promise of lithium-carbon dioxide (Li-CO2) batteries, which are currently a hot research topic.

Lithium-carbon dioxide (Li-CO2) batteries could be a two-in-one solution to the current problems of storing renewable energy and taking carbon emissions out of the air. They absorb carbon dioxide and convert it into a white powder called lithium carbonate while discharging energy.

These batteries could have profound implications for cutting emissions from vehicles and industry – and might even enable long-duration missions on Mars, where the atmosphere is 95% CO2.

To make these batteries commercially viable, researchers have mainly been wrestling with problems related to recharging them. Now, our team at the University of Surrey has come up with a promising way forward. So how close are these “CO2-breathing” batteries to becoming a practical reality?

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Like many great scientific breakthroughs, Li-CO2 batteries were a happy accident. Slightly over a decade ago, a US-French team of researchers were trying to address problems with lithium air batteries, another frontier energy-storage technology. Whereas today’s lithium-ion batteries generate power by moving and storing lithium ions within electrodes, lithium air batteries work by creating a chemical reaction between lithium and oxygen.

The problem has been the “air” part, since even the tiny (0.04%) volume of CO2 found in air is enough to disrupt this careful chemistry, producing unwanted lithium carbonate (Li2CO3). As many battery scientists will tell you, the presence of Li2CO3 can also be a real pain in regular lithium-ion batteries, causing unhelpful side reactions and electrical resistance.

Nonetheless the scientists noticed something interesting about this CO2 contamination: it improved the battery’s amount of charge. From this point on, work began on intentionally adding CO2 gas to batteries to take advantage of this, and the lithium-CO2 battery was born.

How it works

Their great potential relates to the chemical reaction at the positive side of the battery, where small holes are cut in the casing to allow CO2 gas in. There it dissolves in the liquid electrolyte (which allows the charge to move between the two electrodes) and reacts with lithium that has already been dissolved there. During this reaction, it’s believed that four electrons are exchanged between lithium ions and carbon dioxide.

This electron transfer determines the theoretical charge that can be stored in the battery. In a normal lithium-ion battery, the positive electrode exchanges just one electron per reaction (in lithium air batteries, it’s two to four electrons). The greater exchange of electrons in the lithium-carbon dioxide battery, combined with the high voltage of the reaction, explains their potential to greatly outperform today’s lithium-ion batteries.

In terms of the benefits to carbon emissions, by our rough calculations, 1kg of catalyst could absorb around 18.5kg of CO2. Since a car driving 100 miles emits around 18kg-20kg of CO2, that means such a battery could potentially offset a day’s drive.

However, the technology has a few issues. The batteries don’t last very long. Commercial lithium-ion packs routinely survive 1,000–10,000 charging cycles; most LiCO2 prototypes fade after fewer than 100.

They’re also difficult to recharge. This requires breaking down the lithium carbonate to release lithium and CO2, which can be energy intensive. This energy requirement is a little like a hill that must be cycled up before the reaction can coast, and is known as overpotential.

You can reduce this requirement by printing the right catalyst material on the porous positive electrode. Yet these catalysts are typically expensive and rare noble metals, such as ruthenium and platinum, which is a significant barrier to commercial viability.

Our team has found an alternative catalyst, caesium phosphomolybdate, which is far cheaper and easy to manufacture at room temperature. This material made the batteries stable for 107 cycles, while also storing 2.5 times as much charge as a lithium-ion. And we significantly reduced the energy cost involved in breaking down lithium carbonate, for an overpotential of 0.67 volts, which is only about double what would be necessary in a commercial product.

Our research team is now working to further reduce the cost of this technology by developing a catalyst that replaces caesium, since it’s the phosphomolybdate that is key. This could make the system more economically viable and scalable for widespread deployment.

We also plan to study how the battery charges and discharges in real time. This will provide a clearer understanding of the internal mechanisms at work, helping to optimise performance and durability.

A major focus of upcoming tests will be to evaluate how the battery performs under different CO2 pressures. So far, the system has only been tested under idealised conditions (1 bar). If it can work at 0.1 bar of pressure, it will be feasible for car exhausts and gas boiler flues, meaning you could capture CO2 while you drive or heat your home. Demonstrating that this works will be an important confirmation of commercial viability, albeit we would expect the battery’s charge capacity to reduce at this pressure.

If the batteries work at 0.006 bar, the pressure on the Martian atmosphere, they could power anything from an exploration rover to a colony. At 0.0004 bar, Earth’s ambient air pressure, they could capture CO2 from our atmosphere and store power anywhere. In all cases, the key question will be how it affects the battery’s charge capacity.

Meanwhile, to improve the battery’s number of recharge cycles, we need to address the fact that the electrolyte dries out. We’re currently investigating solutions, which probably involve developing casings that only CO2 can move into. As for reducing the energy required for the catalyst to work, it’s likely to require optimising the battery’s geometry to maximise the reaction rate – and to introduce a flow of CO2, comparable to how fuel cells work (typically by feeding in hydrogen and oxygen).

If this continued work can push the battery’s cycle life above 1,000 cycles, cut overpotential below 0.3 V, and replace scarce elements entirely, commercial Li-CO2 packs could become reality. Our experiments will determine just how versatile and far-reaching the battery’s applications might be, from carbon capture on Earth to powering missions on Mars.

Daniel Commandeur receives funding from the Royal Society. He is a member of the Royal Society of Chemistry and Christians in Science.

Siddharth Gadkari receives funding from UKRI.

Mahsa Masoudi does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.