British scientists have developed a working laboratory demonstrator of a lithium-oxygen battery that has very high energy density, is more than 90% efficient, and, to date, can be recharged more than 2000 times.
Lithium-oxygen, or lithium-air, batteries have been touted as the 'ultimate' battery due to their theoretical energy density, which is 10 times that of a conventional lithium-ion (Li-ion) battery. Such a high energy density would be comparable to that of gasoline, allowing an electric car with a battery that is a fifth of the cost and a fifth of the weight of those currently on the market to drive from London to Edinburgh on a single charge.
As is the case with other next-generation batteries, however, several practical challenges need to be addressed before lithium-air batteries become a viable alternative to gasoline. Researchers from the University of Cambridge have now demonstrated how some of these obstacles may be overcome.
Their demonstrator relies 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 caution that a practical lithium-air battery still remains at least a decade away.
"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."Clare Grey, University of Cambridge
"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 Clare Grey, professor of chemistry and the paper's senior author.
"In their simplest form, batteries are made of three components: a positive electrode, a negative electrode and an electrolyte,'' explained Tao Liu, first author of the paper.
In the Li-ion batteries used in 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 they have relatively low energy densities, meaning 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 suffered from low efficiency, poor rate performance, and 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 unwanted chemical reactions that can cause cells to die, making it far more stable after multiple charge and discharge cycles.
When the researchers combined this different chemistry with a negative electrode made from a highly porous form of graphene, they 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%. The highly porous graphene electrode also greatly increases the capacity of the demonstrator, although only at certain rates of charge and discharge.
Other issues that still have to be addressed include finding a way to protect the metal electrode so that it doesn't form spindly lithium metal fibers known as dendrites, which can cause batteries to explode if they grow too much and short-circuit the battery. Additionally, the demonstrator still needs to be cycled in pure oxygen, because the carbon dioxide, nitrogen and moisture in air are generally harmful to the metal electrode.
"There's still a lot of work to do," said Liu. "But what we've seen here suggests that there are ways to solve these problems – maybe we've just got to look at things a little differently.
"While there are still plenty of fundamental studies that remain to be done, to iron out some of the mechanistic details, the current results are extremely exciting – we are still very much at the development stage, but we've shown that there are solutions to some of the tough problems associated with this technology," said Grey.
This story is adapted from material from the University of Cambridge, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.