25 September 2012
Lithium air batteries could give EVs the range expected of a fuelled car
After watching an episode of Top Gear in which Jeremy Clarkson pushed a Nissan LEAF along a road just outside Lincoln after it ran out of charge, many viewers may not have been convinced that electric cars were the future. Indeed, range anxiety is cited as one of the major barriers between electric vehicles and their widespread public adoption. Where petrol powered cars may travel for up to 500 miles before they run out of fuel, electric vehicles generally only manage 100.
Yet in the early 1900s, electric vehicles were more common than their petrol powered counterparts – and so were steam powered versions, although these allegedly took 45minutes to start. By the 1930s, electric vehicles had all but disappeared as mass production made gasoline powered cars with longer range more widely available and affordable.
However, there could soon be up to 2billion of these fossil fuel powered cars on the road. The question is: can the world sustain this number of vehicles? People want more cars, but there's a strong possibility that conventionally power cars won't be sustainable. Although Clarkson might not be convinced about the future of electric vehicles, researchers from IBM certainly are.
IBM started the Battery 500 project in 2009 to explore the science of lithium air batteries, which hold the possibility of powering an electric car for 500 miles on a single charge. The project was developed out of the Almaden Institute, a forum that brings together thinkers from academia, government, industry, research labs and the media.
Lithium air technology was chosen because the lithium ion batteries used in today's electric vehicles do not have the energy density required to give the 500 mile range that is seen to be important. Batteries in electric cars have an energy density of roughly 15Whr/kg. The Battery 500 project is looking at ten times that – 150 to 200Whr/kg, which would provide the equivalent of a 'tank of gas.'
"We knew that, amongst all the different battery technologies, it was the only one that could guarantee the energy density that we need to solve the problem; namely, to be able to drive a car for several hundred miles with a single charge," explained Dr Alessandro Curioni, manager of the computational sciences group at IBM Research in Zurich.
Lithium air batteries 'borrow' oxygen from the air as the vehicle is being driven, creating an 'air breathing' battery. As the main component is air, the battery would also be lighter as it would eliminate the heavy metal oxides currently used. The technology could potentially create a battery 87% lighter than a lithium ion battery, with much greater energy density, thereby solving all range and even weight issues.
During discharge, or when driving, oxygen molecules from the air react with lithium ions, forming lithium oxide on a lightweight cathode. The electric energy from this reaction powers the car. When charging, the reverse reaction takes place, with the previously borrowed oxygen returned to the atmosphere and the lithium going back to the anode. Essentially, it 'inhales' while driving and 'exhales' while being charged.
Since 2009, the researchers have made several major advances. Dr Curioni said the team originally aimed to understand the chemistry of the battery and to overcome the hurdles facing rechargability and reversibility, by looking at what was happening inside the cell while also using advanced simulation models to study the reaction at the cathode. "The two efforts together were able to do two things: to understand why all the previous implementations of this battery were not working; and to identify the major factor which was hampering this rechargability and reversibility."
According to Dr Curioni, it was generally thought for many years that the biggest hindrance was with the catalyst at the cathode. "Through these combined experimental and simulation activities, we saw that one of the major problems was the stability of the solvent used in this battery."
Previously, this battery technology generally used carbonates, but the team demonstrated that the widely used propylene carbonate – which is stable elsewhere – was not suitable for lithium air batteries. The team found that lithium peroxide caused degradation of the solvent, damaging it irreversibly and leading to the production of alkyl carbonates, which hindered the recharging process.
Understanding the chemistry
Using advanced simulation to understand the chemistry that was damaging the solvents, the team looked for alternatives, using their results to lead them in the right direction and finding other solvents which provided up to 99% reversibility. "That has been the major accomplishment in recent years," said Dr Curioni. "Once you have the right solvent in the battery, you take away the major problems and other minor problems appear. But you understand the basic chemistry behind the battery much better," he added.
The team now understands that a catalyst is not needed for the chemical reaction. Dr Curioni said that if a catalyst is added to the cathode, the reversibility of the batteries (once you have the right solvent) becomes worse; rather than enabling the right reaction, the catalyst stabilises side reactions. Dr Curioni suggested this was a positive discovery because it reduces both the cost and complexity of the battery.
It now appears the team possesses a rechargeable battery, capable of a large formation of lithium peroxide on the cathode and therefore the ability to store a lot of energy. "This step forward has been very positive and gave credibility to the project," observed Dr Curioni. "So, on top of the initial partners, who were more academic, a couple of commercial partners have joined the consortium."
The latest companies to join the consortium are chemical manufacturer Asahi Kasei and electrolyte manufacturer Central Glass, both of which have experience with lithium ion batteries. "You can expect to see more partners in the future, because to be successful, we need to have the full spectrum," noted Dr Curioni.
Now the Battery 500 project has more commercial partners and the reversibility and energy density issues have been solved, the team is focusing on some of the other problems that have appeared. One is that lithium peroxide is an insulator, so the large deposit on the cathode means electrical conductivity decreases. This causes the power density – the amount of current that can flow in the battery – to be reduced as well.
Dr Curioni noted how the team has solved the problem of how much energy can be put in and how many cycles it can do. "The next step is to solve the rate at which we can put energy into the battery and take it out," he said. "That is the next challenge." This is important because, as useful as it would be having a battery with a range of 500 miles, for it to be commercially viable, users would want to be able to fully charge the battery overnight and be ready to hit the road in the morning or to charge it quickly during the day.
Dr Curioni is confident that a laboratory prototype, with all the characteristics necessary for application in the real world, will be ready in 2013. By 2015, a scaled up prototype capable of powering a car should be ready. If these go well, he envisages the technology could be implemented in the next decade or so.
Even so, issues such as packaging, safety and cost will have to be addressed for the battery to function in a real environment. Dr Curioni said the biggest problem is the fact that the battery breathes air, not pure oxygen, so will have to handle nitrogen, pollutants such as CO, NO, SO2, and water. Therefore, at this future stage, it will need to include some sort of membrane at the cathode that allows oxygen to enter, but which blocks gases and vapours that would poison the device. "If you look from the lab to the real environment, this additional step of building a membrane that has these properties is a must," said Dr Curioni. "It's something we're not concentrating on now, but it is something that will have to happen in the future."
To ensure safety, the team believes it will have to make the battery more complex. This will involve more weight and cost, so a balance will have to be found. Dr Curioni explained certain design considerations have been made for cost and safety since the beginning of the project. Moving forward, decisions are always made with these factors in mind, even though it is difficult to ascertain their impact on the battery. "We are not pushing the technology, trying to get something that works as soon as possible, while forgetting the safety and cost issues," he said. "We are trying to embed this consideration in each of the steps we do."
Alongside the ability to engineer membranes that filter out pollutants in a way that can keep manufacturing costs down, two main concerns are the stability of the lithium and packaging it safely. What are the implications when these are solved and the technology becomes commercialised? According to Dr Curioni: "If we can push this so it becomes a reality with the characteristics and the costs that we are considering, electric mobility will become a reality."
How about electronic applications outside the automotive world? Dr Curioni can imagine the technology being used in other environments and devices, but the real competitive advantages would be in electric mobility. However, conventional batteries may be cheaper when weight is not an issue. It depends how the technology evolves.
To put all of this in perspective, Dr Curioni noted the project is not assuming that lithium air batteries will provide a complete substitute for oil. In the future, there will be more emissions from emerging countries, but this technology will most likely allow emissions from mobility to be kept at today's levels and stop the exponential growth seen over the past few years.
Either way, Dr Curioni and his team believe the technology will have a strong impact on the way we move. Even if it isn't the complete solution, the lithium ion battery will at least be the start of the solution. As Dr Curioni concluded: "The world will be different if we succeed."
Safer lithium ion batteries
Over the next three years in Germany, 15 partners from science and the automotive industry will conduct research as part of a €36million project into how to improve the safety of lithium ion batteries.
The SafeBatt project will explore how to optimise the cell chemistry to increase the safety of the cathode material and electrolytes, as well as investigating semiconductor sensors made of new materials such as graphene.