Electric cars: under the bonnet

How does an electric vehicle battery actually work?
22 May 2020


electric car parked


How many devices do you have in your house that are powered by a rechargeable battery...

Laptops, phones, smart watches, games consoles: the list is long, getting longer, and the UK Government hopes that, soon, we’ll all be adding one more, even larger device to this list – an electric vehicle, or EV. And despite the huge difference in size, these devices are all powered using lithium-ion batteries. These batteries use structures called cells to convert chemical energy to electrical energy. But how do these cells generate power? And why is a lithium-ion battery rechargeable, but some other batteries aren’t?

Electrochemical cells

To produce electricity, we need a flow of electrons. A flow of electrons is not that hard to generate if you wire up the right two materials. If we connect together two conducting materials, one that is not too fussed about holding on to its electrons and another that is desperately seeking electrons from anywhere, two chemical reactions will happen. The material not overly attached to its electrons - the negative electrode - will release electrons to the wire, simultaneously generating positively-charged particles known as ions. These electrons are transferred, via the wire, to the electrode the positive electrode that desperately wants electrons. There they are incorporated into the material by a different reaction. This flow of electrons, from the negative to the positive electrode, is what generates electrical power.

For the electrochemical cell to function, the flow of these negatively-charged electrons needs to be balanced so that charge doesn’t build up in the system. This is achieved by allowing the positively-charged ions that are generated at the negative electrode to flow to the positive electrode as well. This doesn’t happen through the wire, but instead through a material known as the electrolyte, which can be a liquid or paste sandwiched between the two electrodes. In most rechargeable devices, the positive ions that flows are lithium ions, which is why these batteries are known as lithium-ion. The flow of both electrons and positively charged ions must happen for the battery to work.

From cells to batteries

At its most basic level, the cell described above can consist of just two electrodes and an electrolyte. The number of cells linked together in a battery can vary widely, from one or two for a phone battery up to several thousand for an electric vehicle. In some batteries, the trip the electrons make from the negative electrode to the positive electrode is firmly one-way, either because the electrode is physically consumed during the battery discharge, or because it becomes covered during the process in other materials that subsequently prevent the reverse flow of electrons and ions.

But, in rechargeable batteries such as lithium-ion, the flow of electrons and ions can be reversed by connecting the battery to an external electricity source. This reverses the discharge process, sending ions and electrons back to the negative electrode and getting the battery ready to provide power all over again.

Lithium-ion batteries are rechargeable because the carefully designed electrode materials are not extensively damaged by the movement of the lithium ions as the battery charges and discharges. The negative electrode is typically made of graphite, and the positive electrode is usually a lithium metal oxide. Both structures have layers that allow the lithium ions to enter and leave the structure with ease, so they can pass into the electrolyte, and the battery can charge or discharge.

Electric vehicle batteries have only recently become viable because of changes in modern battery chemistry. “The main concept of lithium-ion batteries hasn’t really changed,” explains Dr David Hall from the University of Cambridge, “but the electrode materials are significantly different from the original lithium battery materials that won the Nobel Prize. Now you can hold something like two or three times as much energy in the same space.”

It’s this ability to hold lots of energy in a small space that means lithium-ion batteries are so widespread. But, as anyone who has owned a mobile phone for more than a couple of years will know, they don’t last forever. “Ideally, all of these changes, moving lithium from one side to the other, are completely reversible,” Hall says. “But, unfortunately, there are side reactions. It slowly degrades those materials, and they break down.” So, like a car rusting over time, the battery’s performance and capacity will degrade with use.

However, users can adopt better battery habits to improve the battery life of devices, from cars to phones. “For phones and laptops, the best thing you can do is make sure they don’t get too hot,” advises Hall.

A giant battery on your driveway

As we transition away from petrol cars, more and more of us will be the proud owners of what is essentially a very large battery on wheels. And that transition can open the door to new technologies. For example, it’s possible for electric vehicles to put electricity back onto the grid via a process known, creatively, as "vehicle-to-grid".

This process currently only works with the handful of vehicles that are equipped to work with a special charger that can remove energy from the car battery as well as put it back into the car. But for those with access to the right car and the right charger, vehicle-to-grid enables tailored charging and discharging, so that the car can provide energy to the grid at times of high demand, and then charge overnight when energy is cheap and there is often a surplus of certain renewable forms of energy, like wind.

“It gives access to sources of energy beyond burning gas, which in the UK we do to meet demand very quickly,” explains Claire Miller from Octopus Electric Vehicles. “Vehicle-to-grid is just one way in which we’ll be able to control and decide where we use energy, and where we put energy in the future.”

What effect do these additional charge and discharge cycles have on the battery life? Some initial studies have found that vehicle-to-grid might actually extend long term battery life because of the controlled way in which the battery is charged and discharged in a vehicle-to-grid cycle.

When you drive your car around town, on the other hand, there is a lot of stopping and starting, accelerating and slowing down quickly in traffic. These are all actions that demand energy in short sharp bursts from the battery, which, over time, can cause damage to the electrodes. A vehicle-to-grid charge and discharge cycle, in contrast, charges and discharges the battery in a smooth, controlled way that can be designed to cause minimal damage.

Reusing and recycling EV batteries

Vehicle-to-grid or not, over time the battery life will degrade to the point where the car battery needs to be replaced. It’s possible the battery may be initially reused, finding a second life as a static battery. A 60-mile range may not be of much use in a car, but stacks of these second-life car batteries are still useful for storing renewable energy. At the home of the Ajax football team in Amsterdam, old electric vehicle batteries are used to store solar energy generated during the day to power the stadium lights during night-time events.

Once the capacity has degraded further, the battery needs to be recycled. This is important, as industry will struggle to source all the materials needed to make new batteries if we don’t have a way to supplement supplies of raw materials.

One material of particular importance is cobalt. Batteries with high cobalt content in the positive electrode can give electric vehicles a good range, but the element is difficult to source. “Cobalt largely comes from the Democratic Republic of Congo,” explains Dr Gavin Harper of the University of Birmingham. He highlights that the conditions in which cobalt is mined are undesirable. “You’ve got children that are working in mining and an unregulated industry that runs alongside the mainstream mining industry.”

This means that there is a drive to reduce the reliance on cobalt by replacing the element with other metals, such as nickel. If this work is successful, it could lead to a push to recycle the electric vehicle batteries of today much more quickly, with one high-cobalt content battery used to produce multiple low-cobalt content batteries.

Driving into the future

Under the bonnet of an electric car is chemistry that has been forty years in the making. This chemistry is very similar to that which powers the mobile devices that are ubiquitous in modern life, albeit on a larger scale. Having a high-capacity battery on wheels in every home will undoubtedly provide infrastructure challenges from a charging perspective, but the ability to develop grid-balancing and new recycling technologies means that electric vehicles have potential to take us beyond just the local supermarket.


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