Batteries that eat carbondioxide
The trends and technologies that are the shape of things to come
Lithium-carbon dioxide batteries can store renewable energy while absorbing carbon. How do we move from theory to building,
That is exactly what researchers at the University of Surrey are working to create: a lithium-carbon dioxide (Li-CO2) battery. If successful, such batteries could not only help suck carbon dioxide out of the air but also offer better solutions to power electric vehicles, the energy grid and even missions to Mars.
But don’t breathe a sigh of relief just yet: not only are these designs merely batteries in gas-filled jars in a lab, and at least a decade off in the future in terms of commercial viability, they also come with plenty of caveats. The question is, how do we get from here to smartphones that inhale CO2 and exhale energy?
Building a better battery
Cars and laptops alike are powered by lithium-ion batteries, which move lithium ions back and forth to either power a device or recharge, using a lithium metal oxide such as lithium cobalt oxide as the cathode and a graphite as the anode.
While these work well enough to power gadgets around the world, this design comes with challenges, including costs, the use of conflict minerals, difficulty in recycling, and less charge capacity every time the battery is recharged. What’s more, lithium-ion batteries have an annoying tendency to catch fire if damaged or overheated – you may remember Samsung having to recall the Galaxy Note 7 after it kept exploding back in 2016.
So this leaves us on the hunt for a better battery. In particular, researchers such as Daniel Commandeur, a Surrey Future Fellow at the University of Surrey; Siddharth Gadkari, lecturer at the same institution; and PhD student Mahsa Masoudi. The aim is to produce a battery that is cheaper, provides better performance and produces fewer environmental or ethical impacts. How nice if it’s also less likely to explode. There are plenty of options, as we explore in “New batteries on the horizon” on p128. U
Accidental improvement
A little over a decade ago, researchers in the United Stated and France were attempting to create a lithium-air battery; instead of moving ions, these batteries would be powered by a chemical reaction between lithium and oxygen.
Commandeur told PC Pro that there has been a lot of interest in lithium-oxygen or lithium-air batteries, as well as zinc-air. “It’s a misnomer,” he said. “They’re working with oxygen rather than air, and often actually for them to work well you need pure oxygen, rather than just drawing it in from the air.”
Normally, he says, a battery would be made up of two solid electrodes inside a liquid, but this style of battery uses a solid electrode, liquid electrolyte (though sometimes solid state) and a gas inlet. “They poke all these holes in the batteries to allow the gases to flow in,” Commandeur explained. “These are quite an unusual kind of battery. Metal gas batteries are in their infancy – it’s quite a weird thing to do.”
However, alongside oxygen, our air naturally has carbon dioxide, and when mixed with lithium that creates lithium carbonate. In a battery, such contamination causes issues including electrical resistance.
“There was a theory that these batteries weren’t working as well as they could because there was too much carbon dioxide in there, and these liquid parts of the batteries preferentially absorb carbon dioxide – they’re better at absorbing it than oxygen,” said Commandeur. “So even with a little bit of carbon dioxide in the atmosphere, they found it was spoiling their batteries, so people were starting to look into how much carbon dioxide is too much.”
As a result, researchers sought to reduce or eliminate carbon dioxide in lithium-air battery experiments – until a team of French and American scientists noticed that this so-called contamination had benefits. “They found something quite exciting, which was you put in a bit more carbon dioxide and the capacity of the battery, how much charge it can store... shoots up by quite a lot,” said Commandeur.
In a paper in 2012, the scientists noted that it actually improved the battery’s charge. And that sparked a new line of research into creating lithium-carbon dioxide batteries, with the first demonstration of a proof-of-concept design in 2015 followed by a fully rechargeable version in 2019. Since then, efforts have continued to improve the design, boost the performance and make it commercially viable.
How it works
The lithium-carbon dioxide system works by creating a permeable battery that can allow carbon dioxide access to the liquid electrolyte, where it can react with dissolved lithium.
The reaction between lithium in the battery and carbon dioxide in the air forms lithium carbonate. “It’s a four-electron process,” said Commandeur. “In simple terms, that means that rather than changing over one electron per reaction, it changes over four – effectively, you’re doing four times the amount of charge storage per chemical reaction.”
In a standard lithium-ion battery, only one electron is exchanged per reaction. By exchanging four, this design could massively expand performance. Commandeur adds: “This is the reason why those batteries theoretically have a much higher energy density than normal.”
Existing lithium-ion batteries have an energy density of 250 or 300 watt-hours per kilogram, while the theoretical energy density for lithium-carbon dioxide is as much as 1,800Wh/kg. But, the researchers stress, that remains unproven. “Even though the theoretical energy density is quite high, even in lab scale studies people have not reached that; they are very much closer to lithium-ion or slightly higher,” said Gadkari. “It will take a few years before it can actually reach that.”
So far, the batteries have been tested under ideal lab conditions. “We take a jar, fill it with carbon dioxide, and put our battery in the jar,” Commandeur explained. “Very rarely would you find circumstances with that amount of carbon dioxide. “But what we want to look into now is what if we were to reduce the pressure of that carbon dioxide and make it a little bit more like realistic conditions,” he added, perhaps more like the levels seen in a car exhaust or natural gas boiler — or even just in the levels seen in the regular atmosphere. “They’re not going to work quite as well obviously as in ideal conditions,” Commandeur said, possibly reducing the capacity of the battery.
There are more caveats, including seriously limited charging cycles. So far, researchers across the field have only managed 100 cycles with this style of design, well below the many thousands achieved by commercial batteries today. They believe that lithium-carbon batteries could be viable if they can last beyond 1,000 charges and prices can be cut by removing all scarce materials.
Alongside that, the researchers are developing a case that only lets in carbon dioxide. “It will be a lot more tricky than in a conventional battery, because we’re dealing with the liquid-gas interface there,” said Commandeur. “Inevitably, it will be about designing technology that allows the passage of carbon dioxide in while preventing the electrolyte from drying out – it’s a big, big engineering challenge there.”
You put in a bit more carbon dioxide and the capacity of the battery… shoots up by quite a lot
Recharging challenges
There are other challenges, notably what happens during recharging. First, breaking down the lithium carbonate to recharge requires a boost of energy; that can be reduced using a catalyst, but they are expensive and often require rare metals.
In a recent paper, the Surrey team revealed success using an alternative catalyst to those expensive and rare metals: caesium phosphomolybdate, which they simply call CPM.
Not only is it cheaper, the CPM material kept the batteries stable for 107 recharge cycles — above the 100 or so previously achieved, but nowhere near the thousand required for commercial success — while storing two-and-a-half times as much charge as a comparable lithium-ion design. Next up is ditching the caesium in favour of an even cheaper alternative.
Welcome back, carbon dioxide
There’s another problem raised by recharging these batteries: the process releases the carbon dioxide, meaning you’re not permanently removing any from the atmosphere.
The battery uses carbon dioxide, mixing it with lithium to create lithium carbonate and energy. That process removes carbon dioxide from the atmosphere – but if you want to use the battery more than once, the recharging process uses electrical energy to decompose the lithium carbonate and carbon back into the original lithium and carbon dioxide. So, the carbon dioxide doesn’t disappear.
“Basically, the battery will give out energy while drawing in carbon dioxide,” Commandeur explained. “Then you have the option, if you want to recharge the battery, of releasing the carbon dioxide.”
That means this isn’t yet a solution to eat up emissions and output them into non-dangerous materials while powering homes, as wonderful as that would be. “It’s not like 100% capture, because it’s letting go of that captured carbon dioxide again in the charging/ discharging process,” said Gadkari.
That said, these batteries could be used alongside carbon capture systems to use all the carbon dioxide that’s trapped and stored, as it would effectively be lab-like conditions in terms of gas purity. The lithium carbonate produced by the reaction is also a precursor material for making lithium-ion batteries. In other words, if you built the right system, you could use up carbon dioxide to store power and make more materials for more power.
“You can be quite imaginative with how these systems could be integrated,” said Commandeur.
But all of that is a long way off; at least a decade, suggests Gadkari.
The fantasy application is the Martian atmosphere, because it’s 95% carbon dioxide
Why it’s worth the work – and the wait
While the challenges are serious, this technology could be worth the wait. Though they’re unlikely to ever be small enough to power your phone or laptop, one particularly good use could be balancing grids, such as storing renewable energy gathered at peak times to be used when the wind isn’t blowing or the sun isn’t shining. “The most important application is definitely grid-scale energy storage,” Gadkari said.
“Because of its high-energy density, it’s attractive for large-scale grid storage, helping to stabilise our renewable energy storage.”
These batteries could work with cars, not least because there’s plenty of carbon dioxide available on road networks. “If we have a kilogram of catalyst in there, then we can basically absorb all of the carbon dioxide from a day’s driving,” said Commandeur.
Gadkari has even bigger goals. “The fantasy application is the Martian atmosphere, because it’s 95% carbon dioxide. And whenever there are any Mars shuttles or Mars programmes or any kind of settlements, they will need batteries,” he said. “These batteries can easily work there.”
A safer battery that could clean up our own atmosphere and power space travel? Not bad for jars in a lab.
New batteries on the horizon
Lithium-ion batteries have plenty of benefits – good energy density, fast charging and light weight among others – but there are enough downsides that researchers have long been seeking alternatives. Lithium is costly to extract, requiring huge amounts of water, and the batteries remain difficult to recycle. Plus, if damaged they have a tendency to catch fire and even explode. Here are a few potential replacements that are already in the works.
Lithium-air/lithium-oxygen
Like lithium-carbon dioxide, these batteries have a higher theoretical energy density than lithium-ion batteries. But they also face issues with degradation when recharging, complicated engineering, and the possibility of fires, especially if pure oxygen is used.
Lithium-sulphur
Proponents of a lithium-sulphur design claim it could deliver twice the energy density of lithium-ion designs while being much lighter. That makes them ideal for use cases where weight matters, so they’ve already been used in drones and satellites, and companies such as Lyten are working on versions for electric vehicles. In addition, sulphur is a cheap, easily accessible material.
Calcium-oxygen/air
This design has particular merit because calcium is widely available. A project based at a trio of universities in China has successfully created a rechargeable calcium-air battery that manages 700 recharge cycles using a gel electrolyte.
Calcium-sulphur designs are also being considered.
Sodium-ion
These batteries replace the lithium with sodium, a smart move as salt is rather easy to source. Sodium-ion batteries show potential with fast charge times and safety, but further development is required. One concept design from Germany’s Karlsruhe Institute of Technology uses seawater to carry a charge.
Glass electrolytes
One of the challenges of a lithium-ion battery is that the liquid electrolyte is flammable. Maria Helena Braga at the University of Porto has developed a solid-state electrolyte using glass; her work was backed by John Goodenough, the Nobel-winning creator of lithium-ion batteries, and that’s good enough for us.
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