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12th October 2009
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Powering the way ahead

Anthony King, 12 October 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.

Li-ion batteries

Li-ion stays ahead
Why will lithium stay ahead? ‘Lithium is one of the most electropositive metals and so gives you one of the highest voltages. So it’s hard to do better – it’s basic chemistry,’ explains Ceder, whose group is searching for new electrodes via a computationally driven strategy that evaluates thousands ofmaterials.

There is room for improvement within the lithium-ion family, and its all about moving lithiumions. With lithium cobalt oxide, you move just half a lithium ion for each cobalt ion. If you replace the cobalt with a mixture of ions, you could increase that number and perhaps double the amount of storage in a possible electrode, explains Bruce. Such a juiced up lithium-ion battery could revolutionise electric vehicles. Bruce notes that factors of even 20% are a big deal when it comes to energy storage.

Kirill Bramnik of BASF believes lithium-sulphur batteries could offer such a breakthrough; they have much higher energy densities because each sulphur atom at the electrode can hold two lithium ions. The technology was developed by Sion Power in Arizona,US, which has joined forces with BASF to develop these batteries; it could double or treble driving distances, says Bramnik.

Another avenue of innovation is nanotech. Ifyou reduce the distance travelled by ions andelectrons in a battery you can get much faster charge and discharge. The ‘nano’ word has been greatly overused, but offers genuine advantages in batteries, says Bruce.

Nevertheless most experts predict stepwise improvement in lithium-ion batteries. Magnus Thomassen of Norway’s research organisation Sintef believes we will not see a drastic increase in performance over the next ten years, although the price of batteries in the automotive sector will probably drop by half due to increased production volumes and materials development.

Safety concerns
The failure rate of laptop batteries worries car makers. The issue arises because the electrolytes are flammable organic solvents that can cause runaway reactions. Bruce’s group is trying to replace these with a polymer that would keep the electrodes apart and carry the ions. This solid state system would be inherently much safer, an important issue for high energy car batteries.

Another approach is electrolyte additives. ‘Small amounts of the proper electrolyte additives can dramatically stabilise the electrode/electrolyte interfaces by forming very stable protective films on electrode surfaces,’ notes Gary Henriksen of Argonne National Laboratory, US, who has worked on such additives. His lab has also developed NMC (nickel, manganese, cobalt) materials, which he says have the same crystal structure as lithium cobalt oxide but are cheaper and less demanding in terms of electronic safety components.

Most experts agree safety will not put the brakes on an electric car future, even though carmaker sare wary of new battery technology. The Tesla Roadster got around such unease by deploying over 6000 laptop-type batteries, thus using an expensive though proven lithium-cobalt-oxide technology. The Toyota Prius, a hybrid vehicle, relies on nickel-metal hydride batteries – three times heavier than lithiumion – and was road tested for four years in Japan.

The road ahead
Electric vehicles currently have a range of about 100-120 miles, but this should extend to 150 miles by 2015, says Roshan Devadoss, an industry analyst with Frost & Sullivan. The market for electric cars is presently miniscule though, and Devadoss predicts a global volume of just over a million electric vehicles by 2015. ‘Post 2020 is when we expect the electric vehicle to occupy a significant part of the car market. ’Renault Nissan, Mitsubishi, BMW and Daimler are some of the manufacturers leading the way.

Considerable gains in cathode capacity and durability have already been pocketed, says Devadoss: ‘Research is now focusing on other constituent parts of the battery, such as anodes, electrolytes and separators.’ He says the capacity of lithium-ion batteries can be expected to reach ten times the current capacity through such efforts.

With lithium-ion so dominant, some have expressed concerns about lithium reserves. The consensus though is that there is enough lithiumin the world to meet demand, especially since advanced technologies will require less lithium/cell. However, experts say lithium deposits are confined to certain regions and that the bargaining power of countries holding reserves may give rise to price fluctuations similar to that seen for oil. Bolivia, not exactly the US’s best friend, has the largest known reserves.

Crystal structure

Air batteries and zebras
Battery advances over the next decade will be lithium-ion technology, but beyond that it could be air batteries. Bruce, who researches lithium–air batteries, explains that these substitute lithium cobalt oxide with space for oxygen to enter the electrode and combine with lithium ions and electrons to form lithium oxide. Though air breathing batteries are futuristic, they offer fantastic possibilities well within the laws of physics. They could store between five and 10 times the energy of today’s lithium-ion batteries, says Bruce. However many challenges remain, including finding the right electrolyte, protective membranes, porous carbon structures and anodes.

Zinc-air batteries also have revolutionary potential. These use zinc as the anode and oxygen from the air as a cathode, so you need only supply one reactant, cutting down on weight. ReVolt Technology recently joined with BASF to advance a rechargeable zinc–air battery it had developed. Henriksen cautions that such metal air batteries are far less mature than lithium–ion batteries, but says they do have a place. ‘They offer potential for very high specific energies for use in electric vehicles and should be supported with longer range R&D funds.’

Another candidate is the Zebra from Mes-Deain Switzerland, a well developed technology that has sodium and nickel as reagents. The Think car company offers the Zebra battery as one option. These high temperature molten salt batteries can suffer from energy losses during extended standby periods, but operate more efficiently in areas with very hot and very cold climates than their lithiumion competitors.

Industry trends
The US is handing out some $2.4bn in stimulus grants to help battery manufacturing and research for hybrid and electric vehicles. Devadoss believes this may give the US an edge over Europe. Nonetheless, Europe has real prominence in the science behind battery advances, argues Bruce. For example, Sony’s investment transformed lithium-ion battery technology into a commercial reality in the early 1990s, but the lithium-cobalt-oxide cathode was developed in Europe. Manganese spinel was something first developed in Oxford, UK, addsBruce.

However, MIT’s Ceder warns against underestimating the amount of activity in China, both in small and large companies. ‘It’s the Wild West, but sometimes something emerges from the Wild West,’ he says. His point is that the Chinese are more willing to experiment with battery technologies, while US and European car manufacturers are held back by liability and safety concerns. Ceder views the recent strategic partnership between Samsung and Bosch as something to watch, along with companies like Korea’s LGChemicals and China’s BYD.

Renault Z E electric vehicle

Component makers BASF, 3M and DuPont have been taking an extra long look at battery technologies. ‘A lot of them are making materials or components,’ says Bruce, ‘but that is where a lot of the innovation needs to be.’ It’s important that Europe gets the kind of support that the US is now giving to the area, because it is going to be a key technology for clean energy for the future and one in which Europe has a stake.

For now, transitional technologies, such as hybrid vehicles like the Prius and plug-ins like the Volt, may race ahead of pure electric vehicles. But this should boost safety and reliability, improve cycle life and reduce costs in batteries, helping to usher in an age of electric vehicles and allowing the world to ease off that emissions accelerator.

Anthony King is a freelance writer based in Dublin, Ireland.

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