The not-so-humble battery holds the key to the development of electric vehicles – and to the energy revolution, says Dr Greg Offer.

Interview: Becky Allen / Illustrations: Anthony Calvert

Batteries are ubiquitous. They’ve been around for more than 200 years. Even the latest lithium-ion technology, commercialised in the 1990s, can now be picked up for just a pound or two online. But while they might seem like the most ordinary of technologies, the fact is that batteries could well hold the key to an energy revolution, a new economy and minimising climate change. As a researcher working at the interface between science and engineering, it’s finding solutions to real-world problems like this that’s the real prize.

What do we need to do to change the world? Solve the tricky science and engineering problems posed by electric vehicle batteries. After all, using cleaner fuels, decarbonising the economy and radically redesigning transport systems  don’t only depend on personal choice and public policy. There are major scientific questions that we still need to answer.

Lithium-ion batteries – the last revolution in battery technology – were designed to be used in small, portable electronic devices, and putting them to new uses like powering electric vehicles generates new challenges. As part of the funding from the Faraday Challenge, we are developing the modelling tools that industry will need to bridge the gap between the fundamental science and the engineering of battery systems, while researching ways to extend battery life.

For instance, two of the most crucial questions concern diagnostics and prognostics. Batteries have finite lifespans. Each time they’re used they lose a little capacity, and as they  age they also produce less power, so the battery eventually needs replacing. But while this is a minor inconvenience for a mobile phone or laptop, it's a major issue for an electric vehicle that’s made worse because we lack the technology to diagnose  a battery pack’s state of health.

One of our papers showed that two similar batteries aged in different ways can show the same capacity and power fade, but one can remain safe to use for a further 1,000 cycles, while the other could explode the next time it’s used. We concluded that how each battery ages makes the difference, and our aim over the next ten years is to turn this science into better battery management systems for the electric vehicle industry.

The flip side is prognostics, because alongside better diagnostics, the industry also needs to know how long a cell will last, something that’s currently impossible to predict. Designers today rely on empirical models – collecting past data to predict what will happen in the future, but this is expensive as well as inaccurate. What we really need are models based on science, which means understanding the degradation mechanisms at  play and using these to make more accurate models.

We’re also involved in multiple projects focusing on translating our fundamental knowledge of battery life and thermal management into real-world applications that involve more than a dozen industrial partners. These include Innovate UK projects involving the likes of Rolls-Royce, Williams Advanced Engineering, Caterpillar, Delta Motorsport and Aston Martin.

We know, for instance, that the way batteries behave – and how they degrade – depends heavily on temperature.

Batteries perform best and most efficiently the hotter they get, because most desirable electrochemical processes occur faster at higher temperatures. But the same is true of processes that you don’t want to happen. Degradation mechanisms get faster and more aggressive at higher temperatures, resulting in an inevitable trade-off between how a battery performs and how long it lasts.

Like the porridge-eating, fairytale character, there’s a Goldilocks region. It depends on application, but for most battery cells this sweet spot is around 30°C. The trade-offs differ but the science remains the same. Whereas Formula 1 car batteries operate above 100°C and only last for a couple of races, the electric vehicle companies we work with want batteries to last ten years, so how do we keep cells at the optimum temperature?

The solution may sound simple, but it’s not. Tesla cars, for example, are powered by cells roughly the size of an AA battery. Of course, they don’t use just a couple, there are 5,000 to 8,000 cells packed together in the bottom of the car, and keeping thousands of small cells at the right temperature is extremely difficult.

That’s not the only challenge: temperature gradients are even more important. Our experiments and models show that operating most batteries at higher temperatures with smaller temperature differences between individual cells  is better than lower temperatures with higher thermal gradients.

Across the world, the drive towards electric vehicles is picking up pace. In 2018, electric vehicles accounted for two per cent of new vehicle registrations in the UK. In Norway, that figure was 48 per cent. And China’s massive market saw 142,000 electric vehicles sold in the first quarter of 2018 – an increase of 154 per cent over the previous year.

In the UK last year, sales of electric vehicles rose by 11 per cent. The government plans to end sales of new conventional diesel and petrol cars and vans by 2040, halve the number of people exposed to levels of particulate matter that breach WHO guidelines by 2025, and it’s setting new long-term air quality targets for particulates.

But at the heart of everything lies the humble battery, and it’s hard to overstate the excitement of being in battery research today. I always wanted to be a scientist and I always wanted to make a difference. As an undergraduate chemistry student at Imperial in the 1990s, I was fascinated by electrochemistry and started working on fuel cells before discovering batteries.

By working on batteries, I thought that I could be part of one of the most exciting periods of change in transport and the automotive industry. What I didn’t predict was the Faraday Challenge – it’s a game changer, not just for me, but for the world.

By focusing on battery packs for electric vehicles, Imperial can support the UK’s growing battery industry, bringing jobs and investment. It’s the kind of opportunity that comes along once in every ten careers – having the chance to make this much impact at this stage in my career is amazing. Without it, the alternative  is breathing polluted air and suffering the more extreme consequences of climate change, so it’s essential for us all.

Dr Greg Offer (MSci Chemistry 2001, PhD 2006) is a Reader in Mechanical Engineering at Imperial.

Inside the thermal management rigs

The tab cooling thermal management rig

Increasing the power from a battery pack of a given physical size can be achieved by demanding more current from each cell. As a result, the cells' temperatures increase to levels that shorten the battery life and, in the worst case, cause them to catch fire. This rig explores ways that cell-surface cooling can maintain cells at a safe operating temperature during high power operation.

The future of batteries: Inside the tab cooling thermal management rig

This illustration of the rig shows the:

Copper plate

Conducts heat from peltier elements to the water cooling block.

Battery cell

Lithium titanate cell used for high power applications with a long cycle life.

Water tubes

Cold water is pumped through the water cooling block, carrying away the unwanted heat from the battery cell and peltier elements.

Coolant outlet and inlet

Connects water tubes to the cooling block.

Water cooling block

A repurposed computer CPU cooler that transfers heat from the copper plate into the cooling water.

Peltier elements x 8

Thermoelectric coolers made of semiconductor material that generate a heat flux based on an applied voltage. One surface becomes cold and is used to hold the cell at a fixed temperature. The other becomes hot as a result and must be cooled by the water block.

The passive cooling thermal management rig

Batteries generate plenty of heat during the charging and discharging process. If the heat is not dissipated quickly, it will accelerate the battery degradation, shorten its life and even cause the thermal runway.  This rig employs phase-change materials of RT42 to absorb the heat with its high latent heat capacity, thus maintaining an ideal working temperature environment for the battery cell.

Inside the passive cooling thermal management rig

This illustration of the rig shows the:

Battery charging cables

Connect the battery cylinder to the battery testing system, and supply and measure the voltage/current during charging and discharging.

Thermal paste

High-conductive paste used to keep the thermocouple wires in close contact with the battery surface and monitor the temperatures accurately.

Green and white thermocouple wires x 2

One to the centre of the battery, one to the side, to monitor temperature changes during charging and discharging.

Cylinder battery

Standard 18650 cylindrical lithium ion battery used in many electrical products.

3D printing battery test case

Houses the cylinder battery with phase-change materials and holds them in a vertical position.

Battery revolution: The Faraday Challenge

The Faraday Challenge, named after Michael Faraday, who discovered electromagnetism, benzene and electrolysis in the 1820s-30s, is a £246 million government investment in the UK research, innovation and scale-up of battery technology, part of the UK government’s industrial strategy.

The aim is to ensure that batteries – whether in cars, aircraft or grid storage – will be a cornerstone of a low-carbon economy, through investment in three initiatives: a new UK Battery Industrialisation Centre in Coventry; a series of collaborative research and development projects led by Innovate UK; and the Faraday Institution, a new virtual research institute based in Harwell, Oxford.

Imperial was a founding institution: in 2018, it was announced that the College would lead one of the Faraday Institution’s first four flagship projects, and work on a second led by the University of Cambridge researching ways to extend battery life.

The team at Imperial plays a significant role in the effort going into the Faraday Challenge. The Imperial-led Multi-Scale Modelling Fast Start project, worth £10m, brings together scientists, engineers and mathematicians – 72 researchers across eight UK universities – to make the modelling tools that industry needs to bridge the gap between the fundamental science and the engineering of battery systems.

Imperial also won a significant chunk of the research and innovation project funding, led by Innovate UK, as part of the Faraday Challenge.

The College's Electrochemical Science and Engineering group won funding for nine of the Innovate UK battery research projects, and has just won two more, in total worth more than £13.4 million, that focus on battery life and thermal management, and which involve more than a dozen different industrial partners. Its ultimate aim is simple: to make better batteries.

Learn more about The Faraday Challenge