Several obstacles have been overcome in the quest for the perfect transportation battery, one that will carry as much charge for its weight as possible. The announcement comes just a day after another paper revealed a big step forward for the lithium-ion batteries currently favored in electric cars and phones.
Lithium-air batteries represent the holy grail of electric cars. In theory, they are capable of being almost as energy dense as a full tank of gasoline, something other battery types will never approach. While other uses, such as home energy storage, are more focused on cost, the ratio of charge to weight is of vital importance to displacing petrol-powered cars from the roads.
Professor Clare Grey of Cambridge University has announced in Science a series of modifications that bring lithium-air batteries far closer to practicality, although she acknowledged to IFLScience that there are still some big steps left to go.
Theoretically, lithium-air batteries can store 12 kilowatt hours per kilogram (Kwh/kg), compared to 0.18 Kwh/kg for lead-acid batteries and 13 Kwh/kg for petrol. As Grey pointed out at a press conference, the potential of lithium-air comes from the fact that it uses, “two very light elements, lithium and oxygen (mass 7 and 16) that react to form the product lithium peroxide.”
However, existing lithium-air batteries come nowhere near their potential. “The reversible capacity... is determined by the pore volume of the porous electrode,” the paper notes. Existing electrodes not only have pore volumes far below the theoretical maximum, but get clogged easily, preventing lithium and oxygen diffusing through the electrode and creating considerable inefficiency.
Grey's team used one-atom-thick sheets of graphene to produce a highly porous electrode. This “is the lightest carbon you could possibly use,” Grey told IFLScience, and her team use “The cheapest form of graphene. We also use very little of it.”
A further modification was to replace lithium peroxide (Li2O2) as a proton source with lithium hydroxide (LiOH), reducing chemical damage and eliminating the need to keep batteries dry. This meant performance was maintained even after 2,000 charge-discharge cycles.
Schematic of the formation of lithium hydroxide on the graphene electrodes. Credit: Tao Liu, Gabriella Bocchetti and Clare P. Grey
The result was a battery with 93.2% energy efficiency. Grey and her colleagues acknowledge commercial versions would likely be lower, but this compares well with efficiencies of around 85% for lead acid batteries, and in the 70s for alternative lithium-air versions, while still some way short of lithium-ion batteries. In keeping with this efficiency, the gap between input and output voltage is just 0.2 Volts.
LiOH forms on the graphene electrode on discharging (top). On charging, the iodide ions are oxidized to iodine, removing the LiOH and reforming the bare graphene electrode. Credit: Tao Liu, Gabriella Bocchetti and Clare P. Grey
These advantages don't mean lithium-air-powered cars will hit the roads any time soon, Grey told IFLScience, “The rate is still very slow, so we need to find ways to increase oxygen solubility and design electrode structures that allow higher rate cycling.” Moreover, while the lab version works well in pure oxygen, Grey says the lithium anode “Reacts with N2 and CO2, both of which are found in air! We need to also reduce CO2 intolerance on the cathode.” Nevertheless, she said, “Given the extremely high energy density, its worth the challenge.”
Grey said at a press conference that before lithium-air batteries are used for cars, they are likely to have smaller applications, such as hearing aids, which would provide opportunities for refinement.
While lithium-air batteries may be the eventual winners in the battery race, the lithium-ion is likely to dominate for a while to come. Besides lacking the theoretical potential of lithium-air, the ion batteries are hamstrung by the cost and short lifespan of graphite anodes.
In Nature Communications, Professor Zhongwei Chen of the University of Waterloo, Canada, announced that silicon anodes allow lithium-ion batteries to store 40-60% more energy at the same size, potentially greatly extending the range of electric cars without adding to the weight. A modification like this is likely able to be brought to market far more quickly than Grey's work. “As batteries improve, graphite is slowly becoming a performance bottleneck because of the limited amount of energy that it can store," Chen said in a statement.