Thursday, 27 September 2012

Lithium air batteries could give EVs the range expected of a fuelled car

Jezza of of Top Gear 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 petrol.'

"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.

No comments:

Post a Comment