Electric vehicles (EVs) are increasingly common, but still only account for a small proportion of total car sales in the United Kingdom. Although sales of EVs in the UK have reached a record high, representing one in every 12 new cars purchased in 2018, the Society of Motor Manufacturers and Traders points out that hybrid, plug-in hybrid and pure electric cars make up just eight per cent of the overall market.
Richard Fields, an electrochemical engineer at the University of Manchester, says “a big upfront cost” and “a distinct lack of infrastructure” are the main reasons for the slow uptake of EVs. “Even if people have the purchasing power to get an EV, without convenient charging points, they may be put off.”
There are also concerns about EV batteries’ capacity, range, and the time it takes to charge them. As Lee Cronin, regius professor of chemistry at the University of Glasgow, explains, “most normal EVs have a solid battery, and when it runs out of charge you have to recharge it by plugging it in. If you use a rapid charger at a service station, it could be between 30 and 40 minutes. Some people charge their EVs overnight at home, which is hardly convenient.”
Broadly speaking, batteries are packages of chemicals that contain two electrodes (an anode and a cathode) made from different chemicals. These are immersed in a third material, called an electrolyte. When the two electrodes are linked, chemical reactions within the battery release electrons from one substance to another, creating a charge. Lithium-ion batteries, widely known from their use in laptops, cameras, smartphones and other electronic devices, are the most common type of battery used in EVs. They are produced in huge volume and, thanks to economies of scale, have become cheap to produce.
EVs use electricity stored in battery packs to power the electric motors that turn their wheels. When depleted, the batteries are recharged using electricity from the grid. However, lithium-ion batteries can lose capacity for every full charge they receive, known as a cycle. The more cycles a battery completes, the more it will degrade and lose capacity. In a car, this results in a decrease in range.
Range is an EV battery’s energy capacity divided by its energy consumption. A car with a 24 kiloWatt-hour battery pack – the size used most commonly in EVs – might diminish to 20 kiloWatt-hours, over time. A fully charged range of 84 miles, then, would become 67 miles maximum range, if the pack diminishes to 80 per cent of its original capacity.
Capacity loss is expected, and at the normal rate of loss it would take several years to reach 8o per cent. A 2018 study featured on academic journal website PrePrints found that the Nissan Leaf – the UK’s most popular EV – dropped to 80 per cent capacity after five years. For comparison, according to market researcher Polk Automotive, the average petrol car, if maintained well, will lastt 11 years at peak performance.
Flow batteries, developed by NASA in the 1970s, are rechargeable batteries where rechargeability is provided by two chemical components dissolved in liquids contained within the battery unit. A liquid, rather than solid, electrolyte of metallic salts is pumped through a core that consists of a positive and negative electrode, separated by a membrane. The main difference between conventional batteries and flow batteries is that energy is stored not in the electrode material, but in the electrolyte liquid.
Cronin says this gives flow battery technology some major advantages and could make them, with improvements, as practical as petrol. He says: “The flow battery we developed is made of a liquid. So if you run out of charge, you could, in principle, pump out the depleted liquid and –as you would with a normal car – refill it with liquid, in just a few minutes.”
Cronin’s technology uses a metal oxide that can be charged with electricity when added to water. “The part of a battery containing the charge is known as the electrolyte and when this is made of a solid, it is resting between two electrodes. When you use the battery, a reaction takes place inside the electrolyte, and charge passes from one electrode to the other until the electrolyte runs out. Then you charge the battery up, by forcing charge in the opposite direction. In a flow battery, the electrolyte is made of a liquid, so it can be stored in a tank, and pumped past the electrodes.”
While flow batteries have technical advantages over conventional rechargeables, such as potentially separable liquid tanks, current uses of flow batteries are comparatively less powerful and require far more sophisticated electronics. Cronin continues: “We had to work out a way of improving the energy density, so we could store more in the liquid. We were playing around different salt solutions and found a way of getting more salt into the water and keeping it stable.” Should this technology take off, the ambition, Cronin says, “is to have EVs perform on a level with petrol cars.” Drivers would remove the “spent” liquid, he says, “using a withdrawal nozzle at a refuelling station”. They would then use a second nozzle to refill the flow battery with fresh liquid from another pump.
While Cronin’s approach seeks to make more of the liquid electrolyte, another type of battery, a solid-state battery, goes the other way. Solid-state batteries ditch that liquid electrolyte entirely in favour of a solid conductive material. In Southampton, a company called Ilika was awarded £4.2m of funding from Innovate UK’s Faraday Battery Challenge to develop them. The challenge that exists is in finding a solid material that is conductive enough use in large batteries. Some of the materials being explored for use as solid electrolytes in solid-state batteries include ceramics and glass.
The major benefits of solid-state batteries, according to Ilika’s product commercialisation manager Denis Pasero, derive from their use of non-flammable solid electrolyte as opposed to the organic solvent used in current lithium-ion batteries, which is both flammable and has a relatively short useful life. “All parts of solid-state batteries are solid ceramic films, which are safer [because they are not flammable]; and also because they are dense, thin films, they may provide more energy for given volume available in the car; and they can also be charged faster.”
He adds: “The PowerDriveLine project [for which we received funding] will establish a pre-pilot line for solid-state battery cell technology and develop processes for a solid-state materials supply chain. The technology also has useful application for stationary energy storage, that is large size batteries that can store the energy harvested by solar panels on roofs of buildings, allowing consumers to more efficiently use energy provided by the grid. An important part of the PowerDriveLine project is ensuring availability of the full supply chain, particularly in the UK, for this new innovative technology.”
Richard Fields, who works for the University of Manchester’s Graphene Engineering Innovation Centre, says that advanced materials have a significant part to play in improving EV batteries. What role can graphene – a super-thin and super-strong material derived from graphite – have in making them better? Fields says that shaping the carbon atoms of graphene into hexagonal lattice sheets to “coat” the battery’s anode, will improve conductivity.
Keith Pullen, professor of energy systems at City, University of London, says that splitting EV battery packs could help to improve their performance. “Even if there were a battery with 400 miles’ range at reasonable cost, size and weight, the charging issue remains as a major barrier,” he explains. Splitting the battery from one large unit into a number of smaller ones, according to Pullen, offers advantages such as reducing voltage and energy losses.“This can facilitate a method of swapping out discharged with pre-charged batteries.”
Pullen says that splitting the battery, and using effective cooling and packaging could offer a bridge between lithium-ion batteries and the new types which take longer to develop. “Despite a lot of hype, lithium-ion cell development is now on a plateau and bringing new chemistries to production is not easy and takes a lot of time. The success of lithium-ion in terms of cost reduction is, strangely, a barrier to bringing in new technologies which must compete on cost as well as offer better performance.”
Cronin, who himself drives an Audi TT Sport, agrees that the immediate challenge in developing battery technology is in attracting the investment to do it. “We’ve got to convince people that it’s economically viable. In terms of rolling it out, then obviously the infrastructure is a challenge as well.” To solve that, Cronin suggests “retrofitting” or converting the existing infrastructure – petrol stations – to accommodate new battery charges.
“The government has a responsibility,” Cronin says, “to make EV investment attractive. There are things that can be done: tax credits, a duty on fuel… a green levy could encourage more manufacturers to shift over to EV technologies more quickly. EVs are part of a wider aim to reduce climate change and the UK should lead the conversation.”