Traction batteries for electric cars still have potential for improvement: charging times are getting shorter, the range is increasing, and battery life is going to increase. Researchers are working on the future of the cells, and therefore on the future of electric mobility as a whole.
The success of electric cars stands and falls with battery technology. Batteries essentially convert chemical energy into electrical energy. The process is reversible: electrical energy is applied, regenerating the original chemical reactants. “The great thing is that you can design batteries that are highly efficient and that have high energy density as well as a high capacity for reversibility. This is why batteries have also become interesting for vehicle propulsion” says Prof. Martin Winter, head of the MEET Battery Research Center at Münster University.
What will follow lithium-ion technology?
Currently there is no technology that can compete with lithium-ion in electric vehicle batteries. “Battery life, quality, performance, and cost: no other battery technology is satisfactory in all four respects” says Andreas Docter, head of battery development at Daimler AG. “Continuing development and enhancement of lithium-ion technology has placed ranges of about 500 km within reach, and there is the possibility of reducing charging times thanks to quick-charging technology.”
He believes battery size to be one of the greatest challenges facing future vehicle generations. Yet lithium-ion is far ahead of other technologies in volumetric energy density as well.
Let’s not confuse being able to produce the technology with suitability for series production
New milestones appear again and again on the path toward the super battery of the future; one such milestone was reached by a University of Texas research team working with physicist John Goodenough, who is considered one of the fathers of the lithium-ion technology. His approach is to replace the liquid electrolyte (which transports ions between the plus and minus pole) with a solid; in this case, it is glass. Replacing lithium with the more abundant sodium would give the battery a higher energy density and make it less expensive, more durable, as well as safer. Liquid electrolytes are, after all, the primary risk factor for fires in modern electric cars.
However, technical feasibility must not be confused with suitability for series production. “Getting the technology out of the lab and on the road takes years” Prof. Winter points out. Other technologies, such as magnesium batteries, could offer additional benefits; but like metal-air technology, these are still in the basic research stage.
Lithium-metal technology a beacon of hope
Experiments with silicon as an electrode material are being conducted to advance lithium-ion technology. The metalloid can significantly increase current power storage capacity and, with it, the range of electric cars. The disadvantage is battery life: with silicon, the volume of the negative electrode expands substantially during charging and shrinks again during discharging. “In the long term, this leads to damage and cracks in the electrode,” says Professor Ulrich Schubert, battery expert from the Center for Energy and Environmental Chemistry Jena.
Lithium-metal technology, which uses metallic instead of ionic lithium, is also promising. Its greatest advantage is higher energy density. If the technology is combined with a solid electrolyte, the risk of fire can be reduced as well. Yet series production of lithium-sulfur batteries for electric vehicles will also be possible only in the next 15 to 20 years, according to Prof. Schubert.
On the way to a better ecological balance
The degree of improvement we are seeing in traction batteries will make electric cars more suitable for widespread use due to the optimization of their practical benefits. “Customer acceptance is decisive for the breakthrough and success of electric mobility”, says Daimler employee Docter. In addition to costs, the ecological aspect also plays an important role. “Battery production emits 150 to 200 kg of carbon dioxide per kWh” says Prof. Schubert. When this is taken into consideration, an electric car has to drive 70,000 to 90,000 kilometers before it has a more positive CO2 balance than a gasoline engine, depending on the current technology and electricity mix.
Yet, given the growing share of renewable electricity, good available recycling options, the so-called “second-life option” (e.g. second use as a basement storage battery for photovoltaic systems), as well as the optimization of the battery technology itself, his colleague Winter remains convinced of the future of battery-driven cars: “If we get this right, the future speaks clearly for the ecological superiority of the battery-powered drive system.”