Powering the way ahead

C&I Issue 19, 2009

Researchers are gearing up to make batteries for electric vehicles that are cheaper, safer, lighter and store more energy, reports Anthony King

UN secretary general Ban-Ki-moon warns that our foot is stuck on the accelerator and we are hurtling towards the abyss. Time is running out for the cuts in emissions needed to avert climate change and economic disaster. Meanwhile, 600m cars around the world continue to guzzle oil, with their inefficient internal combustion engines converting just 25% of their fuel into motion. And their numbers are rising.

The dream solution is an electric vehicle that runs on battery energy obtained from renewable sources, powering new lean, green machines. With no tailpipe emissions, our cities would have cleaner air. We could eliminate traffic noise. What stands in the way of this utopia is the battery, but researchers and chemical companies are gearing up for this challenge.

The arrival of lithium batteries in the early 1990s injected renewed belief into electric vehicles due to their high energy capacities and longer cycle lives. Lithium-ion batteries weigh one-sixth that of their lead-acid counterparts, and most experts agree they are the only batteries with enough energy/kg for electric vehicles. But there is a downside.

Battery cost is the major issue with electric vehicles, says Gerbrand Ceder, professor of materials science and engineering at Massachusetts Institute of Technology, US. Typically an electric car drives between one and four miles/kilowatt-hour (kWh), so you need a minimum battery size of 50kWh to drive 200 miles. Today such a lithium-ion battery weighs 500kg and costs between $25,000 and $50,000. ‘You can make it, but you have an extremely expensive component in the car and, what is worse, one that you don’t have proven reliability for,’ says Ceder.

Electronic devices have shrunk in size in recent years as the energy/unit weight of their batteries has doubled, but this was achieved through engineering: reduced packaging, thinner separators between electrodes and tighter packing in batteries. Future gains in terms of weight and cost will demand new materials and chemistries. Fortunately, lithiumion is a family of batteries composed of a number of chemistries, offering plenty of scope for future improvements.

Laptop batteries rely on the movement of ions and electrons between a graphite anode and a cathode made of lithium cobalt oxide, converting chemical energy into electricity. But these materials are not cheap. ‘Cost is much more sensitive for transport than for consumer electronics,’ says Peter Bruce, professor of chemistry at the University of St Andrews in Scotland. ‘So you really need to get rid of the cobalt and that is why the focus is on iron and manganese materials.’

Lithium manganese spinel is a new cathode that is less energy dense, but more stable than cobalt oxide. This was included in the new Chevrolet Volt from GM, which has a 16 kWh battery that lasts up to 40 miles. GM favours lithium battery technologies but says this doesn’t mean lithium is the only answer for future autos. Lithium iron phosphate is another alternative cathode that has been used in some vehicles.

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