Fuelling the cars of tomorrow

Solid oxide fuel cells and test bench. Researchers have succeeded in lowering the operating temperature of these batteries by 100°C. © CNRS Photothèque/François Jannin
Solid oxide fuel cells and test bench. Researchers have succeeded in lowering the operating temperature of these batteries by 100°C. © CNRS Photothèque/François Jannin
Toyota Prius as presented at the 2006 London Motor Show. © Paul Nieuwenhuis
Toyota Prius as presented at the 2006 London Motor Show. © Paul Nieuwenhuis
Th!nk, the Norwegian electric car, can be equipped with a lithium-ion battery as an option.© Paul Nieuwenhuis
Th!nk, the Norwegian electric car, can be equipped with a lithium-ion battery as an option.
© Paul Nieuwenhuis
Assembling a plastic lithium-ion battery at the LRCS at Amiens.© Hubert Raguet/CNRS
Assembling a plastic lithium-ion battery at the LRCS at Amiens.
© Hubert Raguet/CNRS
Mathieu Morcrette, director of the Laboratoire de réactivité et chimie des solides (LRCS) at Amiens (FR) assembling a lithium-ion button cell in a glove box..© Hubert Raguet/CNRS
Mathieu Morcrette, director of the Laboratoire de réactivité et chimie des solides (LRCS) at Amiens (FR) assembling a lithium-ion button cell in a glove box.
© Hubert Raguet/CNRS

With the resounding launch of the hybrid Toyota Prius in 2001 and the market debut of Tesla’s first electric sports cars, one small detail is being overlooked: the battery technology on which these cars are based still needs serious improvement to withstand the constraints faced by vehicles in everyday use.

The fact is that the lithium-ion batteries (Li-ion) of the best laptops allow them to be run for an hour and a half before needing to be recharged for two hours or more. And a laptop is a stationary application while a car is designed to be mobile! In other words, today’s batteries are inadequate for automotive applications.

Much work remains to make lithium batteries capable of powering urban cars at reasonable prices. As Daimler AG spokesman Matthias Brock is keen to point out, “the question of cost is paramount and the battery is an important part of the price of the car. To be competitive, we must reduce the price of batteries, but this will take another few years.”

According to Paul Nieuwenhuis, automotive industry expert at Cardiff University Business School (UK), the battery for a standard hybrid car costs approximately €17 000, the same amount as is required to build the rest of the car. “One can assume that, by 2020 and with mass production, the cost of the batteries will have halved. This mass production will start with the ‘plug-in’ hybrids – hybrid cars rechar geable via an electrical socket (connected hybrids) – but ‘pure’ battery electric vehicles will also benefit”, he says.

A race for performance

Before that, these cars must gain speed, power and range. Right now, few vehicles are able to travel more than 60 km on a single charge. So far, many models operate on nickel-metal hydride bat teries (NiMH). “These are conventional batteries for electric cars and they are perfectly functional”, insists Saiful Islam of the University of Bath (UK), a member of the Alistore European Network of Excellence. This is a fact confirmed by the emergence in small numbers, mainly in urban areas, of hybrid and electric cars like the Mercedes-Benz Smart Car or the Toyota Prius.

Right now, NiMH batteries are more reliable and less expensive than lithium-ion batteries. However, as Saiful Islam explains, “lithium-ion batteries offer other benefits, in particular in terms of energy density, which is much greater for the same mass.” This property can have a major impact on battery weight and on the storage capacity of the little cells comprising it. According to Peter Bruce, an expert in energy storage at the Scottish University of St Andrews (UK), a Li-ion battery produces three to four volts per cell against a little over two volts per cell for other types of batteries. This makes it possible to reduce the number of battery cells and increase energy density. But adapting this potential to mass use means improving the performance of several components of the batteries.

Today’s Li-ion batteries have one major drawback: unreliability. Some manufacturers saw their products explode when laptop manu facturing was in its infancy. Such scenarios have to be avoided at all costs in the case of a moving vehicle. “New materials are the key to progress in this area,” explains Saiful Islam.

Reliable, effective materials

German chemicals company Evonik Degussa GmbH is trying to solve this problem through the Li-Tec project, the outcome of a commercial partnership with Daimler AG.

Evonik has developed a new material called SEPARION ® for producing the separator film (or simply ‘separator’) which is a major component of batteries. As its name suggests, it separates the two electrodes, the anode (+) and cathode (-), through which circulates the flow of lithium ions, and hence the current. One role of the separator is to prevent shortcircuits while being sufficiently permeable and porous to allow the passage of moving ions.

Separators are usually composed of semipermeable polymer membranes based on polyethylene or polypropylene. But these materials are flammable and are stable only up to 140 °C. In case of overcharging, the separator can overheat, melt and trigger a short circuit, which may cause an explosion.

Evonik’s innovation has been to introduce separators consisting partially of ceramic compounds, which are harder but still flexible enough to allow the perforation of little pores through which the electrons can flow. The idea is not new, but Evonik has adapted it. “The ceramics were too fragile and it was therefore difficult to use a separator composed exclusively of this material”, says Li-Tec’s project director, Volker Hennige. Instead, Evonik has invented a composite material in which a nonwoven polymer serves as a support substrate and is mixed with ceramic powder. “In small cells like in a laptop, you can use 100 % polymer membranes as there is no major safety issue. This arises only with the larger cells that are essential for producing cost-efficient electric cars”, says Volker Hennige.

The current model of the new Roadster, the electric sports car by California car-builder Tesla, also contains thousands of little cells rather than a small number of larger cells, in particular in order to reduce the risk of explosion within one of the cells. This manufacturing precautionary measure is also partly reflected in the cost of this car, which comes out at over €120 000!

Working on the electrodes

“The materials used until now for the cathode prevent large-scale battery production”, says Saiful Islam. One research objective is to design cathodes capable of storing more energy by increasing their lithium content using new materials.

In a Li-ion battery, when both electrodes are connected to the circuit, chemical energy is released. The lithium ions flow from the cathode to the anode when the battery is charging, and from the anode to the cathode during discharge. While the anode is made of graphite, the cathode is mainly composed either of a layer of metallic oxide such as lithium cobalt oxide, or a polyanions-based material such as lithium iron phosphate or spinels of magnesium oxide and lithium. Of these materials, lithium cobalt oxide is the most common. However, as Saiful Islam points out, “cobalt raises issues of price and toxicity”.

To replace the cobalt oxide and allow largescale development of batteries for automotive applications, scientists have focused their research on oxides based on iron, nickel or manganese as well as on lithium iron phosphate (LiFePO4) cathodes. The latter show a greater resistance to heat and to high-intensity elec trical current.

Even more avant-garde research is seeking to get rid of the cobalt cathode altogether with a lithium-air battery in which lithium enters into the electrode and reacts with oxygen to form lithium oxide. Results suggest that this approach makes it possible to store more energy than with traditional lithium-ion batteries. Peter Bruce talks of up to 5 to 10 times more.

The necessary investments

The current research looks promising, and although it will take another decade before competing with the advantages of modern internal combustion engine technology, electric vehicle technology is well established on the EU agenda. In March 2009 the European Commission earmarked a billion euros for the development of green cars as part of the Green Cars Initiative, which is an integral part of its economic recovery plan. A portion of these funds has been earmarked for research into high-density batteries, electric motors, intelligent electricity distribution networks and vehicle recharging systems.

According to a study by bankers HSBC, govern ments worldwide have provided €12 billion of stimuli to low-carbon-emission vehicles. The major portion of this sum has been allocated to research and development of lighter batteries and plug-in hybrid cars as well as credits or tax refunds for consumers buying new, low-emission cars. But more is still needed. According to Lew Fulton, an expert from the International Energy Agency (IEA), if we succeed in reducing the cost of batteries to €380 per kilowatt hour, a connected hybrid with a range of 50 km would cost around €3 000 more than a conventional non-connected hybrid model (where the battery is recharged by the thermal engine and braking). “Putting on the road 2 million connected hybrids a year by 2020 would therefore cost an additional €8 billion per year. Research on batteries and electric vehicles in general would cost another several hundred million euros a year if it was also desired to develop purely electric vehicles”, says Lew Fulton.

Developing transmission and electricity distribution systems adapted to the era of electric cars and hybrids is another challenge. Will new energy production capacities be needed? Could the development of an intelligent power distribution network – using computer techno logy to communicate consumption information minute by minute – pave the way for the widespread use of electric vehicles?

Electrical U-turn ahead

Recharging battery-driven cars will certainly push up energy demand. But these cars could also be used to inject electricity back into the network. Since this is already achievable with lead batteries, it would be easy to establish an interconnection between the electricity grid and electric cars.

From whatever perspective one addresses it, the future development of electric vehicles is highly ambitious and will require, first and foremost all, large investments of money. A portion of the funding for the Green Cars Initiative is also dedicated to creating cleaner and more efficient combustion engines, which is undoubtedly an easier path to follow. Even so, many carmakers have fully embraced the concept of electric cars. Matthias Brock from Daimler AG predicts the emergence of three tracks: “electric cars could be used in town, given their more limited range. For longer distances, internal combustion engines will remain the most popular form of transportation. But we are also concentrating on fuel cells because of their total carbon emission neutrality.”

General Motors has also adopted the idea of electric cars. Despite the crisis it is planning to launch a new hybrid vehicle called the Opel Ampera in Europe, as early as 2011. “Production of the Ampera is going ahead whatever happens”, says Craig Cheetham, spokesman for the American auto giant. Increased sales and Toyota’s improved image since launching the Prius have almost certainly made GM’s mouth water. This innovative ingredient that is attracting attention at all car shows, combined with the long-term rise in oil prices, undoubtedly heralds further changes to come.

Elisabeth Jeffries