Researchers from the University of Cambridge have demonstrated how many of the obstacles standing in the way of developing the “ultimate” battery can be overcome.
What is the “ultimate” battery?
Lithium-oxygen, or lithium-air batteries have been referred to as the “ultimate” battery due to their theoretical energy density, which is 10 times that of a lithium-ion (Li-ion) battery. Such a high-energy density would be comparable to that of gasoline—and would enable an electric car with a battery that is a fifth the cost and a fifth the weight of those currently on the market to drive about 400 miles on a single charge.
As with many other next-generation batteries, there are several challenges that need to be addressed before lithium-air batteries become a viable alternative to gasoline.
Previous attempts at working demonstrators have had low efficiency, poor rate performance, unwanted chemical reactions and can only be cycled in pure oxygen.
Creating the “ultimate” battery
What the researchers created is a working laboratory demonstrator of a lithium-oxygen battery that has very high energy density, is more than 90% efficient and can even be recharged more than 2000 times, showing how several of the problems holding back the development of these devices could be solved.
The demonstrator has a higher capacity, increased energy efficiency and improved stability over previous attempts.
It relies on a highly porous, ‘fluffy’ carbon electrode made from graphene and additives that alter the chemical reactions at work in the battery, making it more stable and more efficient.
“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,” says Professor Clare Grey of Cambridge’s Department of Chemistry.
There is a constant effort to achieve a smaller, more efficient battery among researchers. Apart from the possibility of a smartphone, which lasts for days without needing to be charged, the challenges associated with making a better battery could be damaging the take-off of electric cars and grid-scale solar power.
“In their simplest form, batteries are made of three components: a positive electrode, a negative electrode and an electrolyte,’’ says Dr Tao Liu, also from the Department of Chemistry, and the paper’s first author.
In the traditional Li-ion batteries we use in our laptops and electronics, the negative electrode is composed of graphite, the positive electrode is made of a metal oxide, such as lithium cobalt oxide, while 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.
The team’s method uses a very different chemistry than the earlier attempts at a non-aqueous lithium-air battery. It relies on lithium hydroxide instead of lithium peroxide. With the addition of water and the use of lithium iodide as a ‘mediator’, the battery showed less of the chemical reactions, which can cause cells to die, making it more stable after multiple charge and discharge cycles.
The researchers’ method reduced the ‘voltage gap’ between charge and discharge to 0.2 V. A small voltage gap means a more efficient battery.
Other issues that still have to be addressed include finding a way to protect the metal electrode so that it does not form spindly lithium metal fibers, as this can cause batteries to explode if they grow too much and short-circuit the battery.
Additionally, the demonstrator can only be cycled in pure oxygen, while the air around us also contains carbon dioxide, nitrogen and moisture, all of which are generally harmful to the metal electrode.
“There’s still a lot of work to do,” says 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,” says Grey.
The results were reported in the journal Science, and although they are making strides in the right direction, the researchers caution that a practical lithium-air battery still remains at least a decade away.
For more information, visit the University of Cambridge website.
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