Machine Systems
Research case study:
Our center’s researchers have successfully proved that several of the problems that slow down the practical development of the so-called ‘ultimate’ battery could be overcome now!
What we’ve achieved is a significant development for this technology and these results imply whole new areas for further research.
Elisa Grey: Our center’s scientists have set up a working laboratory demonstrator of a lithium-oxygen battery which has a very high energy density, is more than 90% efficient, and, to date, can be recharged more than 2000 times, showing how several of the problems impeding the development of these devices could be finally resolved.
Lithium-oxygen, or lithium-air, batteries have been widely praised as the ‘ultimate’ battery thanks to their theoretical energy density, which is ten times more than that of a lithium-ion battery. Such a high energy density can be compared to that of gasoline – and would make it possible for an electric car with a battery that is a fifth the cost and a fifth the weight of those currently on the market to get from London to Edinburgh on a single charge (which totals a 500 miles ride)…
Scientists:
As complex as this project was, we’ve had a team of just 4 scientists working on it: Ben Edwards, Sam Williams, Robin Smith & Jeffrey Edison.
However, similarly to the impeding issues that are common for almost all other next-generation batteries, there are several practical difficulties that need to be worked around. Only after making those crucial hotfixes, lithium-air batteries will be able to become a viable and cost-efficient practical alternative to gasoline.
Now, researchers both in our labs and the top federal research facilities have shown how some of these obstacles may be changed. In a close cooperation with the federal energy agency, we’ve developed a lab-based demonstrator of a lithium-oxygen battery which has higher capacity, increased energy efficiency and improved stability over previous attempts.
Their demonstrator depends on a highly porous, ‘fluffy’ carbon electrode made from graphene (comprising one-atom-thick sheets of carbon atoms), and additives that alter the chemical reactions at work in the battery, making it more stable and more efficient. While the results, reported in the journal Science, are promising, the researchers note, that a practical lithium-air battery still remains at least one decade away, which makes 2020’s a decade to look our for on the matter.
“What we’ve achieved is a significant advance for this technology and suggests whole new areas for research – we haven’t solved all the problems inherent to this chemistry, but our results do show routes forward towards a practical device,” said Professor Elisa Grey, the paper’s senior author.
Many of the technologies we use every day have been getting smaller, faster and cheaper each year – with the notable exception of batteries. Apart from the possibility of a smartphone which lasts for days without needing to be charged, the challenges associated with making a better battery are holding back the widespread adoption of two major clean technologies: electric cars and grid-scale storage for solar power.
“In their simplest form, batteries are made of three components: a positive electrode, a negative electrode and an electrolyte,’’ said Dr Tao Liu, also from the Department of Chemistry, and the paper’s first author.
In the lithium-ion (Li-ion) batteries we use in our laptops and smartphones, the negative electrode is made of graphite (a form of carbon), the positive electrode is made of a metal oxide, such as lithium cobalt oxide, and the electrolyte is a lithium salt dissolved in an organic solvent. The action of the battery depends on the movement of lithium ions between the electrodes. Li-ion batteries are light, but their capacity deteriorates with age, and their relatively low energy densities mean that they need to be recharged frequently.
Over the past decade, researchers have been developing various alternatives to Li-ion batteries, and lithium-air batteries are considered the ultimate in next-generation energy storage, because of their extremely high energy density. However, previous attempts at working demonstrators have had low efficiency, poor rate performance, unwanted chemical reactions, and can only be cycled in pure oxygen.
What Liu, Grey and their colleagues have developed uses a very different chemistry than earlier attempts at a non-aqueous lithium-air battery, relying on lithium hydroxide (LiOH) instead of lithium peroxide (Li2O2). With the addition of water and the use of lithium iodide as a ‘mediator’, their battery showed far less of the chemical reactions which can cause cells to die, making it far more stable after multiple charge and discharge cycles.
By precisely engineering the structure of the electrode, changing it to a highly porous form of graphene, adding lithium iodide, and changing the chemical makeup of the electrolyte, the researchers were able to reduce the ‘voltage gap’ between charge and discharge to 0.2 volts. A small voltage gap equals a more efficient battery – previous versions of a lithium-air battery have only managed to get the gap down to 0.5 – 1.0 volts, whereas 0.2 volts is closer to that of a Li-ion battery, and equates to an energy efficiency of 93%.
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