Breakthrough lithium-CO2 battery solution

24 September 2018

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MIT researches have produced a battery that could be made partly from power plants’ carbon dioxide emissions, potentially reducing greenhouse gas emissions while also offering a reliable power output.

Rather than the challenging conversion of CO2 to specialised chemicals using metal catalysts, this battery could continuously convert the gas into a solid mineral carbonate as it discharges.

While still based on early-stage research, and far from commercial deployment, the new battery formulation could open up new avenues for tailoring electrochemical carbon dioxide conversion reactions.

The battery is made from lithium metal, carbon, and an electrolyte that the researchers designed. The findings are described in the journal Joule, in a paper by assistant professor of mechanical engineering Betar Gallant, doctoral student Aliza Khurram, and postdoc Mingfu He.

Currently, power plants equipped with carbon capture systems generally use up to 30 percent of the electrical power that they generate just to power the capture, release, and storage of carbon dioxide. Anything that can reduce the cost of that capture process, or that can result in an end product that has value, could significantly change the economics of such systems, the researchers say.

However, as Gallant explains, "Carbon dioxide is not very reactive … [so] trying to find new reaction pathways is important." Generally, the only way to get carbon dioxide to exhibit significant activity under electrochemical conditions is with large energy inputs in the form of high voltages, which can be an expensive and inefficient process. Ideally, the gas would undergo reactions that produce something worthwhile, such as a useful chemical or a fuel. But efforts at electrochemical conversion, usually conducted in water, remain hindered by high energy inputs and poor selectivity of the chemicals produced.

Gallant and her colleagues, whose expertise involves non-aqueous (non-water-based) electrochemical reactions (such as those that underlie lithium-based batteries), researched whether carbon dioxide-capture chemistry could be put to use to make carbon-dioxide-loaded electrolytes (namely the latter one of the three essential parts of a battery) – where the captured gas could then be used during the discharge of the battery to provide a power output.

This approach is different from releasing the carbon dioxide back to the gas phase for long-term storage, as is now used in carbon capture and sequestration (or CCS: the process of removing a chemical from the environment and harnessing it in an organic or physical structure).

Such a field generally looks at ways of capturing carbon dioxide from a power plant through a chemical absorption process, before either storing it in underground formations or chemically altering it into a fuel or a chemical feedstock (raw, or unprocessed, material used for energy).

In this case, however, the MIT team developed a new approach that could potentially be used right in the power plant waste stream to make material for one of the main components of a battery.

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While interest has grown recently in the development of lithium-carbon dioxide batteries, which use the gas as a reactant during discharge, the low reactivity of carbon dioxide has typically required the use of metal catalysts. Not only are these expensive, but their function remains poorly understood, and reactions are difficult to control.

By the said incorporation of the gas in a liquid state, however, Gallant and her colleagues found a way to achieve electrochemical carbon dioxide conversion through only a carbon electrode. The key is to pre-activate the carbon dioxide by incorporating it into an amine solution.

"What we've shown for the first time is that this technique activates the carbon dioxide for more facile electrochemistry," Gallant says. "These two chemistries – aqueous amines and non-aqueous battery electrolytes – are not normally used together, but we found that their combination imparts new and interesting behaviours that can increase the discharge voltage and allow for sustained conversion of carbon dioxide."

They showed through a series of experiments that this approach does work, and can produce a lithium-carbon dioxide battery with voltage and capacity that are competitive with that of state-of-the-art lithium-gas batteries. Moreover, the amine acts as a molecular promoter that is not consumed in the reaction.

The key was developing the right electrolyte system, Khurram explains. In this initial proof-of-concept study, they decided to use a non-aqueous electrolyte because it would limit the available reaction pathways and therefore make it easier to characterise the reaction and determine its viability. The amine material they chose is currently used for CCS applications, but had not previously been applied to batteries.

This early system has not yet been optimised and will require further development, the researchers say. For one thing, the cycle life of the battery is limited to 10 charge-discharge cycles, so more research is needed to improve rechargeability and prevent degradation of the cell components. As a viable product, "lithium-carbon dioxide batteries are years away", Gallant says, as this research covers just one of several needed advances to make them practical.

Nevertheless, the concept offers great potential, according to Gallant. Carbon capture is widely considered essential to the realisation of reduced greenhouse gas emissions worldwide; but there are not yet proven, long-term ways of disposing of, or using, all the resulting carbon dioxide. While underground geological disposal is still the leading contender, this approach remains somewhat unproven and may be limited in how much it can accommodate. It also requires extra energy for drilling and pumping.

The researchers are also investigating the possibility of developing a continuous-operation version of the process: this would use a steady stream of carbon dioxide under pressure with the amine material, rather than a preloaded supply of the material – allowing it to deliver a steady power output as long as the battery is supplied with carbon dioxide.

Ultimately, the scientists hope to make this into an integrated system that will carry out, not only the capture of carbon dioxide from a power plant's emissions stream, but its conversion into an electrochemical material that could then be used in batteries. "It's one way to sequester it as a useful product," Gallant says.

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