Faster, further, longer
Removing heat would see a step change in EV battery technology

The future of cars is electric. The UK Government will ban the sales of internal combustion engine (ICE) vehicles in 2030, making it one of fourteen countries that have announced ICE restrictions. Most automotive manufacturers have pivoted to electric vehicles (EVs), with Audi expecting to be completely electric by the 2030s. According to BloombergNEF, half of global passenger-vehicle sales in 2035 will be electric.

This rise in EVs has been driven, in no small part, by constant, incremental improvements in EV battery technology. Better battery performance allowed EVs to travel farther and charge quicker, making them more tempting to the average driver. But potential converts still worry about range anxiety, long waits at charge points, and expensive battery replacements every few years.

Growth will likely continue steadily, but a step change in battery technology could provide the catalyst we need to allay concerns and deliver a giant leap in sales.

Too hot to handle?

Batteries have improved gradually through ever better designs. But all face an ongoing challenge that has not yet been properly addressed – heat management. If this could be addressed, we could see a step change in performance and lifetime.

Currently the battery packs that power most EVs are made up of Li ion cells – some use thousands of small cells, some use hundreds of larger ones. These come in one of three formats – cylindrical, prismatic or pouch cells. Each has benefits and drawbacks, but all create complex patterns of heat transfer, and all suffer due to too much heat in the wrong places.

EV battery packs have the best thermal stability and lifetime if – when the vehicle is being driven – they maintain a temperature between 15 to 35°C, with a maximum across cell temperature difference of 5°C. To make things more complicated, they should be charged at temperatures of around 50°C to minimise dendrite formation.

Li ion batteries generate large amounts of heat in use, especially during rapid charging and accelerating. And this heat is very unevenly distributed, meaning some cells, or parts of cells, degrade much quicker than others. A single cell’s immature deterioration can considerably reduce performance and efficiency of the whole battery pack.

The safety of the vehicle is also a serious consideration. Li-ion batteries are particularly susceptible to thermal runaway events – a repeating cycle in which excessive heat causes more heat until operation ceases or an explosion occurs. This is due to their propensity to self-heat once the electrolyte inside reaches a certain temperature.

Current battery cooling does not solve the problem

For the above reasons, EV battery packs employ battery thermal management systems (BTMS). A BTMS must provide the necessary heat transfer for optimal charging and discharging, in the confined space available in the battery pack, and be manufactured economically. This is no small ask.

There are two main types of BTMS: active and passive. Active systems depend on forced circulation of a coolant such as water or air. Passive systems use methods like heat pipes or hydrogels to remove heat from the pack. The complexity of these systems adds significantly to the cost of the battery pack – in the region of 10-20%.

Alongside cost, one of the biggest problems with current BTMS options is that they create thermal gradients. For example, a cold plate beneath the cells cools the bottom much faster than the top. Meanwhile, a liquid cooling loop will remove heat more effectively from cells at the start of the loop but by the end it can’t absorb any more heat.

Temperature gradients cause adverse voltage distributions and differential ageing between the cells. In other words, the cell does not age uniformly, an ageing gradient occurs inside the cell, reducing the efficiency and lifespan of the batteries. Strong thermal gradients can also lead to deformations in cylindrical cells.

If we could develop a cost-effective way to deliver isothermal heat management – ie maintaining a consistent and even temperature throughout the cells and the system – we could deliver a big change to battery performance.

A revolution in battery cooling technology

At Reaction Engines, we’re pretty good at thermal management. Our expertise has been developed through years of work on our revolutionary hypersonic air breathing rocket engines, which can cool passing air from 1,000oC to -150oC in 1/20th of a second.

Taking this expertise, we looked at how heat can be better managed within EV batteries.

The trick was to use the system itself to manage heat evenly. So, when one spot rises in temperature, we wanted to take that heat and dump it in a cooler place first. That is an efficient immediate solution, as the heat has less far to travel. Cooling heat pipes still take heat out of the whole system, but in the meantime, all the cells within it stay perfectly regulated. All components stay cooler, which ensures any degradation is slow and even.

A perfect heat management system might use materials with complex structures to manage this in a highly sophisticated way, and this was our first thought. But we needed a solution that was affordable to the average EV battery manufacturer. So, we went back to the drawing board – same idea, but with the starting point of making it affordable. We relooked at the concept and considered how we could create something cost-effective that still did the job.

After a few iterations with different materials and designs, we created a simple foil that could be attached to any battery cell. This creates a thermal ground plane which transfers heat evenly across the surface, then takes it out of the system, so you’re not left with hotspots.

The nature of the foil design means that the benefits are realised not just on a specific one-cell basis, but across the whole system. They equalize thermal gradients across the whole battery pack. So it all degrades at the same time, in the same way. Plus, unlike rigid cooling systems, which can suffer from the thermal resistance of air bubbles in the interface between cooling and the cells, our solution is malleable and conformable, so it adheres to the cells – even pouch cells which expand and compress during use.

The benefits of isothermal EV batteries

Improved cooling and isothermal performance means batteries last longer because parts are exposed to less heat and therefore degrade more slowly. And because all parts degrade uniformly, batteries don’t need to be replaced when only certain parts are degraded. The whole battery lasts for the whole of its useful life. This could easily add an extra two years to the battery’s lifetime.

There are other benefits too. Pushing a car to its limits of charging (eg using ultra-rapid chargers) or discharging (eg accelerating) creates heat. This creates risk of thermal runaway if just a single part crosses a certain temperature threshold, so BTMS’s limit performance to keep batteries at safe temperatures. But this control system is based on poor knowledge of heat within the cell, so must set cautious safety limits based on the potential hottest point, limiting efficiency.

Isothermal heat management would eliminate these dangerous hotspots and create an even, easily measurable temperature across the cell. That would give BTMS control systems much greater scope to enable faster charging and better vehicle performance, without the risk of overheating, unlocking the full potential of the battery.

Finally, we can reduce weight. Because they’re so lightweight, BTMS developed using our foils can be up to 35% lighter, making vehicles more ‘fuel’ efficient.

Altogether, this adds up to a step change in battery performance. We expect that deploying these foils would deliver a 20-30% improvement in battery performance.

How Reaction Engines can help battery manufacturers

At Reaction Engines, we are heat management experts. We believe our thermal management expertise to be a decade ahead of anyone else. But we are not large-scale manufacturers. We are therefore looking to partner with innovative battery companies or vehicle OEMs. We are looking for partners interested in licensing our design, or who we can work with to design new battery systems which build in isothermal heat management from the ground up.

We have a patented recyclable foil design, and patented techniques for fitting or retrofitting it to a range of EV fuel cells to ensure they adhere in ways that deliver near-perfect isothermal cooling. The secret is in how you deploy them into a complex battery design. But once you have them in, they are easy to scale and do not add to end-of-life costs.

The commercial opportunity for EV batteries

What does all this mean for the bottom line?

Taken together, the heat management innovation discussed in this article, will allow us to help battery manufacturers offer a 1.5 step change in performance, charge speed, and battery lifetime. This could benefit all battery manufacturers, but for a bold new player in particular – who is open to new approaches – this could be a significant opportunity to leapfrog the competition.

For the vehicle OEM they are selling to, this would mean they have better performing, longer lasting, cheaper vehicles, giving them a better proposition, which they can back with longer warranties – would you buy a car guaranteed for eight years of battery life, if there was a similar model that guaranteed ten?

Ultimately, this would be good for the manufacturers, good for the consumer, and good for a world where competition for battery materials is hotting up, and pressure on environmentally friendly battery disposal is rising.