A radically new form of lithium-oxygen batteries avoids many of the problems that have prevented the uptake of what is, in theory, the ultimate transportation battery. If the work can be scaled up, it could mark the end of gasoline-powered cars.
The cost, weight, and insufficient lifespan of batteries represents a major obstacle to electric cars replacing internal combustion engines on our roads. There are two paths to address this: One, like Aesop's tortoise, involves slow incremental improvements in existing lithium-ion batteries, collectively bringing down the cost and extending the range of electric vehicles.
The other path involves a shift to a radically better technology, of which the one with the greatest potential is lithium-oxygen, also known as lithium-air. The announcement in Nature Energy of a very different way of making lithium-oxygen batteries indicates it is not time to write off the hare in this race.
Lithium-oxygen batteries can, in theory, be five times lighter for the same amount of power than lithium-ion versions. However, multiple technical problems have prevented the use of this tantalizing potential. One of the most important of these is their low efficiency. Senior author Ju Li, a professor from MIT, explained in a statement: "You waste 30 percent of the electrical energy as heat in charging... It can actually burn if you charge it too fast."
This inefficiency is a function of these batteries requiring 1.2 more volts to charge than they release when discharging, but Li has found a way to get this down to 0.24 volts, slashing energy wastage.
Rather than taking in oxygen from the air while discharging and releasing it again when being charged, Li stored the oxygen in lithium-oxygen compounds. As they charge and discharge, the batteries shift between Li2O, Li2O2, and LiO2, all of them solids.
Having to hold the oxygen in solid form, rather than take it in from the air, reduces some of the theoretical weight advantage of lithium-air batteries, but they are still about four times lighter than the best existing batteries with the same storage capacity, and Li is confident the benefits outweigh this cost. Besides the energy savings, Li says the change “means faster charging for cars, as heat removal from the battery pack is less of a safety concern.”
There are other benefits as well. Previous prototypes, Li said, “really can't handle moisture or carbon dioxide," so these must be blocked with a membrane. These versions also experience huge volume changes that shorten their lifespans. Both problems don’t apply to Li’s work.
"With a typical battery, if you overcharge it, it can cause irreversible structural damage or even explode," Li added. “We have overcharged [our] battery for 15 days, to a hundred times its capacity, but there was no damage at all." Testing also showed minimal loss of performance over 120 charging cycles.
The key to Li’s work is nanometer-sized lithium-oxygen particles that can change between the three compound forms. These particles make a glass embedded in a cobalt-oxide matrix, which Li calls nanolithia. Future plans include replacing the cobalt with the more common nickle, and increasing the contact area between the particles and the matrix.
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