Volkswagen Magazine

THINK AHEAD

dance of the molecules.

Electromobility as a possible mass phenomenon depends on the availability of powerful, safe and affordable batteries. Volkswagen researches new materials and is developing next-generation batteries.

Text  Sabrina Künz
Illustration KircherBurkhardt Infografik

In 1972, Volkswagen's first electric vehicle, the Transporter T2 Electric, weighed in at a stately two-and-a-half tonnes, including an 850-kilogram lead-acid battery. This electric pioneer was no speedster – it topped out at 70 km/h and 33 kW (45 hp). Electric Volkswagens have since slimmed down considerably. The battery of the e-up!, for example, weighs 230 kilograms and achieves a top speed of 130 km/h and an output of 60 kW (82 hp).

 

And yet many wonder why the technology can’t shed bulk even more quickly. If today we have more computing capacity in our mobile phones than there used to be in huge mainframe computers, why can't someone develop a battery that fits in the glove compartment and has a range of 600 kilometres? The answer is that battery technology involves a chemical process, so the developments are incomparible.

How does an e-battery work?

A look inside the battery of the e-up!. In the cathode, lithium ions (red) are embedded between metal oxide layers.

Volkswagen uses lithium-ion batteries in its electric vehicles. How can they be improved? Well over a hundred employees are working on the issue in Volkswagen’s research and development department. Over the next four years, the goal is to develop prototypes of cells with an energy density of 280 watt-hours per kilogram (W-h/kg); the best batteries today achieve just 180 W-h/kg. To reach the goal we've set ourselves, researchers are analysing systems and materials already in use, testing alternatives, building research vehicles and evaluating the prototypes of other manufacturers.
The Volkswagen VARTA Microbattery research company in Ellwangen has been working onthe issue since 2009. The joint venture has a staff of 40 employees working on the development of lithium-ion batteries for electric vehicles. Together with Volkswagen, they test whether research results can be successfully utilised in series production. Unfortunately, many materials do not deliver on the promise they show in the lab. Even the strongest of candidates often fail to make the grade for series production – for instance, if they cannot be sufficiently condensed. This is important, because different particle sizes in the active layer ensure optimal utilisation of the limited space. Other materials are highly energetic, but rupture easily and are thus unsuitable for series production. The success of a battery depends on five criteria: lifetime, energy, output, safety and cost.

What makes the e-up! dynamic?

During the charging procedure, the lithium ions (red) migrate from the cathode (right) through the separator to the anode and embed themselves in the graphite structure.

What's inside the car's battery?

Structure Strictly speaking, a car battery is not a battery but a rechargeable cell. It is comprised of numerous current-producing elements – in the e-up!, for example, some 200 cells. They have a cathode (positive electrode) of lithium metal oxide and an anode (negative electrode) made of graphite. Cathodes and anodes are divided by a separator, a thin synthetic membrane that isolates the electrodes from each other but is permeable for ions. The batteries also contain an electrically conducting liquid called electrolyte.

Process In the cathode, freely moving lithium ions are enclosed between metal oxide layers. During charging, they migrate through the electrolyte between the graphite layers of the anode. Here they take on electrons. An important factor for electric vehicles is the energy density of the battery, which is measured in watt-hours per kilogram (W-h/kg). The more free lithium ions there are in the cathode, the more there are to embed themselves in the anode during the charging process, and the further the electric vehicle will be able to drive. When discharging, the ions migrate from the graphite through the electrolyte back to the metal oxide of the positive electrode.

Lifetime.

The lifetime is measured in charging cycles. A cell is ready for series production when it lasts for more than 1,600 cycles. As soon as a battery is charged, it endeavours to restore its thermodynamic rest state. It discharges itself continuously even when not in use. This effect is known as self-discharge. Moreover, a battery ages according to when, how and for how long a user recharges and uses the battery. Ambient factors affect both states. Self-discharge increases with higher temperatures. Batteries stay fresh longer when not used and kept cool. But if a car is used, the battery ages more quickly in cooler temperatures because the ions in the cells are sluggish and moving them requires more force. One potential consequence is “lithium plating”, in which lithium is deposited on the anode and forms a hard layer impervious to other ions. Researchers are looking for ways to increase the lifetime of batteries – for example by testing materials that are less temperature-sensitive.

Energy & power.

The second major theme is the energy itself – that is, the availability, electric range, charging time and necessary infrastructure. To increase the range, batteries have to become lighter. The cells account for roughly 62 percent of the overall weight of the battery in an electric vehicle. Researchers are therefore working on lightweight construction methods for the structure, or non-active parts – and with some success. First generation battery elements were screwed together; second generation ones were welded. This saves weight. Researchers also want to utilise space more effectively. The goal for the next generation is to double capacity in the same amount of space. Material research is also a core aspect in increasing the range. Volkswagen is advancing the development of the lithium-ion cell and researching new material combinations such as lithium-sulphur batteries with an energy density of 600 W-h/kg and lithium-oxygen batteries with 1,000 W-h/kg. Endurance tests will determine whether they fulfil the criteria for series production. Closely associated with that is power, as dynamic driving, acceleration and braking in the various fields of use such as plug-in hybrids and electric vehicles place massive demands on the batteries.

What happens during braking?

Welcome to the anode! This is where the lithium ions (red) take on electrons (yellow). The more free ions there are in the cathode, the more there are to embed themselves in the anode during the charging process, and the further the electric vehicle will be able to drive.

Safety.

Safety is of paramount importance. The high energy density in a small space can lead to short circuits and fires. Batteries must be secured in such a way that they do not endanger anyone in case of accident. In the electric vehicles from Volkswagen, the batteries are positioned so that they cannot be damaged in accidents. Moreover, Volkswagen continues to research materials that do not trigger unintended chemical reactions. In normal operation of the vehicle, the batteries must also be safe and convenient to maintain and repair. After all, the costs should be comparable to conventional engines.

Cost.

 

That leaves the final, and decisive, factor: cost. The battery for an electric car costs a few thousand euros more; otherwise the mass manufacture of an electric car would cost roughly the same as a conventional vehicle. Research and development is therefore committed to making electric cars affordable for more customers. Efforts are also focused on developing a balanced cycle of materials. Every component of a battery should be recyclable. This protects the environment and also lowers the price. And it will remain a challenge well into the future. Many people are interested in trying out new drive technologies. A representative survey in August 2013 by the high-tech association BITKOM revealed that two-thirds of Germans would be willing to buy battery-powered cars. However, 40 percent would only do so if the overall costs were not higher than for a conventional vehicle.

 

Through its research efforts, Volkswagen is actively shaping the future of mobility. Although there won't be any overnight revolution to rival the miniaturisation of computer chips, much will change in the years to come. Whether it’s fully electric, hybrid or modern combustion engines, the results of Volkswagen research and development will be evident in every future Volkswagen vehicle.