The Achilles’ heel of the electric car remains its limited energy density of the batteries, which will only sustain short drives.
That is where Lithium-Air (Li-air) batteries come into play.
“If we succeed in developing this technology, we are facing the ultimate break-through for electric cars, because in practice, the energy density of Li-air batteries will be comparable to that of petrol and diesel, if you take into account that a combustion engine only has an efficiency of around 30 percent,” said Tejs Vegge, senior scientist in the Materials Research Division at Risø DTU.
Now – 110 years after the first electric car, – battery technology, combined with the effect electronics and the electric engine, have come so far in performance, size and price the electric car is on the verge of becoming a true automotive force. The electric car does not pollute locally and it can, if used cleverly, introduce more renewable energy into the electricity supply.
The advantages of the electric car are first and foremost they can integrate into the electricity system and potentially serve as a buffer in the electricity system of tomorrow, where most of our electricity originates from fluctuating renewable energy. Where there is excess electricity from e.g. wind turbines, the electric cars can get a charge. When there is a shortage of electricity, some of the power can return to the electric grid. The other major advantage is that, if mass-produced, the electric car could be cheaper to produce than current cars.
Gaining more power
Today, battery packs are expensive and are only able to store a relatively low amount of energy. Researchers all over the world are working to change that. For electric cars to become the consumers’ preferred mode of transport, the battery capacity must significantly increase.
The most promising electric car batteries focus on metal lithium (Li). Lithium is a soft, silver-white metal – the lightest of all metals. Lithium is extremely reactive and corrodes quickly in a humid atmosphere. That is why lithium typically stores in a kerosene environment to avoid contact with oxygen and water. The lightness is one of the strengths of lithium. Traditional car batteries use lead (Pb), which is one of the heaviest metals in existence. To reduce the weight of batteries, lithium is the way to go. Just look at the prominence of rechargeable Li-ion batteries in e.g. mobile phones, cameras and MP3 and MP4 players. These batteries have the highest energy density among rechargeable batteries.
The lithium battery market is going to grow exponentially, and a discussion has already emerged whether there is going to be enough lithium to electrify the entire world’s car park. Lithium is naturally occurring with approx. 65g per metric ton in top soil and approx. 0.1g per metric ton of water and can extract from soil as well as water, but if the lithium content is small, extraction ends up being costly.
New age battery
That is where Li-air batteries come in. They could end up having the same efficient energy density as petrol
“If we succeed in developing this technology, we are facing the ultimate break-through for electric cars, because in practice, the energy density of Li-air batteries will be comparable to that of petrol and diesel, if you take into account that a combustion engine only has an efficiency of around 30 per cent,” said Vegge.
If batteries with an energy density this great become a reality, one could easily imagine electrically powered trucks. Li-air batteries are thus a promising research area, but there are many research challenges to overcome before the batteries find their way to the electric cars.
The Li-air battery uses a lithium electrode (the anode), and electrolyte and a porous carbon electrode (the cathode), which attracts oxygen from the air when the battery is in operation. The battery is therefore open at one end, or it has an oxygen supply of its own. During discharge, oxygen reacts with lithium to form lithium peroxide (Li2O2), and during charging, this process reverses to release oxygen. Both reactions take place on the surface of the porous carbon electrode.
The interaction with air requires the electrode to have a very large surface area. The prototypes now have a current density of 1 milliamp per square centimeter surface area, and this has to increase by at least one order before the batteries are ready for real life action.
The fact the battery absorbs oxygen atoms from the air means the battery gains weight as it discharges. Theoretically, the battery can more than double its weight.
At the same time, the electrode could become short of breath. The oxygen absorbed by the battery reacts with lithium to form lithium peroxide, which may cause clogging of aggregates in the battery’s channels, causing them to become blocked and preventing the supply of further oxygen. “In our trials, we use pure oxygen, so we are okay, but the problems accumulate when the oxygen has to be extracted from ordinary air,” said Søren Højgaard Jensen from the Fuel Cells and Solid State Chemistry Division at Risø DTU. Ordinary air also contains moisture, which is a problem for lithium.
An extremely high overvoltage will need to recharge the battery again after a discharge. The equilibrium voltage for the Li-air battery is 3 volts. When the battery discharges, the voltage drops to 2.6-2.7 volts. But when you want to recharge the battery, the voltage must increase to 4.5 volts. In comparison, a Li-ion battery can recharge at an overvoltage of only 10 percent.
“The discharge process is proceeding really well. Our problem is that the reverse process has a very high energy loss,” said senior scientist Poul Norby, Materials Research Division at Risø DTU. “The high overvoltage for recharging is hard going for the current battery components, which limits the number of times the battery can be recharged.”
The cyclic energy loss in charging/recharging is about 40 percent in Li-air batteries. The challenge is to reduce this number to 10 percent, corresponding to Li-ion batteries.
In order to solve this issue, Vegge performed extensive DFT calculations (Density Functional Theory), on the Li-air batteries. Using this method, it is possible – at atom level applying an approximation to the famous Schrödinger equation, to calculate how the lithium and oxygen atoms interact. “In this way, we hope to find an explanation of the high overvoltage and a solution to what we can do to reduce it, e.g. by adding an appropriate catalyst,” said Vegge.
In addition to the computer calculations, the batteries undergo examination using X-ray and neutron rays. These techniques allow the scientists to study how ions and electrons move in the electrode-electrolyte interfaces when the battery charges and discharges. “We focus particularly on solid-state electrolytes because they offer safety and transport advantages. Large lithium batteries with liquid electrolytes could pose a safety risk in the event of accidents,” Vegge said.
Testing of large lithium batteries takes place in a converted chest freezer in the laboratories of the Fuel Cells and Solid State Chemistry Division. “The batteries have to be able to withstand heavy frost and extreme heat, and we can subject them to that in our converted chest freezer, which is able to cool objects down to -60°C and heat them to around 50°C,” Højgaard Jensen said.
Today, metal-air batteries act as disposable batteries for special purposes with high energy density requirements, e.g. for military equipment, and zinc-air batteries are disposable batteries in e.g. hearing aids.
If the battery is to withstand a car running e.g. 250,000 kilometers during its lifetime, and the battery is able to deliver approx. 800 kilometers from one charge, it must be able to handle full charging and discharging at least 300 times. Li-air battery prototypes can currently handle 50 charges, so the researchers face other scientific challenges.
In addition to the number of charges the battery must be able to withstand, it must also be possible to charge it quickly.
“Think about the volume of energy transferred when you put petrol into your car. It takes a couple of minutes, and then you can go another 800-1000 kilometers. This is a true challenge for the Li-air batteries, because they may potentially be able to contain the same amount of energy as petrol, but it takes considerably longer to refuel,” Vegge said.