Paul Christensen, Professor of Pure and Applied Electrochemistry at Newcastle University talks thermal runaway

Professor Paul Christensen’s professional career in Electrochemistry started at the University of Oxford before moving to Newcastle University. He is currently involved in lithium-ion research projects including the ongoing research programmes concerning the ignition of large lithium ion battery modules, stacks of modules and battery packs in collaboration with a number of fire and rescue services, first responders, Kings College London and battery manufacturers and assemblers.

IFSJ Editor Iain Hoey caught up with Paul to get to grips with the risks surrounding lithium-ion batteries, what exactly thermal runaway is, and what can be done to stop it.

What are lithium-ion battery a fire risk?

Lithium-ion batteries shouldn’t exist. They’re what is called ‘thermodynamically unstable’. You’ve got two electrodes, one is a mixed metal oxide, the other one is mainly graphite. When you charge the lithium-ion battery, the lithium ions are initially in the mix with metal oxide, they come out, move through the solvent, through the separator and go into the graphite. That lithiated graphite is at a very high energy.

If you’ve ever dropped sodium or lithium into water and washed it fizz you’ll see it’s very energetic. It immediately reacts with the solvent to generate heat, hydrogen, methane, and all sorts of other gases. The problem is that heat speeds up these chemical processes, producing more heat and more gas. If that continues, you get a bang.

Fortunately, at the same time, a thin protective layer forms on the graphite particles called the solid electrolyte interface, and it acts like a prophylactic. It stops the particles touching the solvent whist allowing free passage of the lithium ions.

If that solvent electrolyte gets damaged for any reason, those reactions can start again. The problem is that the heat is produced exponentially but heat only dissipates through the surfaces of an object which is a linear process, not exponential. When these two lines cross, you will see a measurable temperature rise.

What is thermal runwaway?

Thermal runaway is when exothermic, heat producing processes become self-sustaining and then you just can’t stop it. You are in uncontrolled positive feedback, producing more heat and more gas until it pops.

Lithium batteries operate a little bit warm just by their nature, but as long as the battery is allowed to vent its heat then it dissipates nicely. For example, a mobile phone has a very high surface area to volume ratio. But if you charge a mobile phone under your pillow that can send it into thermal runaway and people have been injured, and I believe killed doing this.

As you get the bigger and bigger lithium-ion batteries, the surface area to volume ratio gets worse and worse. The heat that should be escaping and dissipating stays inside the battery.

Lithium ion batteries are being used widely in the electrification of vehicles – what is the scale of the problem here?

Electric vehicles cover everything from light electric vehicles right the way up to HGVs and trains. Large electric vehicles pose the lowest risk. Disregarding crashes, no large electric vehicles have killed anybody.

The immediate and present danger is from light electric vehicles such as bikes and scooters. The problem is that the time between an e-scooter showing the first signs of thermal runaway and fire or explosion can be 10 seconds or less. You’ve got no time to get out.

We’ve had many people die or be injured in house fires caused by light electric vehicles because they develop so fast that by the time the fire service arrives, it’s a house or a flat fire. These tragic incidents could easily be avoided just by some simple guidelines, but people continue to die, homes and lives continue to be wrecked because of such fires and as far as we can see, nothing is being done.

What are the simple guidelines that people should be following?

First of all, don’t charge your e-bike, e-scooter, e-skateboard etc indoors. Don’t. That is it. Full stop. If you have to – if you absolutely have to – do not charge them overnight. Do not charge them if you’re out. Charge them somewhere where anything that can burn is minimised. Don’t charge them in an exit or near an exit route, you might only have seconds to get out.

A significant problem is that people are modifying ordinary bikes or assembling spare batteries for e-bikeswith batteries bought online. The unregulated trade of lithium-ion batteries online should be stopped.

Is there a future in which we are able to have safe lithium ion-powered vehicles?

Yes. As a species were very good at managing risk. Lithium-ion batteries have simply taken us by surprise. The first lithium battery was discovered in in the mid-80s. They were commercialised in 1991 for camcorders, etc. The large lithium ion batteries, like full electric vehicles and grid-scale battery energy storage systems only really started off in 2008, so we haven’t had much time to learn about the problems.

Is there a way to neutralise the threat of thermal runaway?

Every single battery energy storage system designs for safety. Grid scale systems are also designing for failure. If we accept there’s going to be a failure the key thing is to stop this thermal propagation where heat gets passed from cell to cell and the thermal runaway propagates. There are ways and means of addressing that and they are being explored in grid scale systems all the time.  The same is happening with electric vehicles.

The bottom line is that if you cram a large amount of energy into a very small space and it gets released in an uncontrolled fashion and does so rapidly, then doesn’t matter what the chemistry is whether it’s a lithium-ion battery or something else – you could be in trouble.

What should the fire service be aware of when it comes to putting out these kind of fires?

I’ve been working with Fire Rescue Services in the UK and abroad for a few years and I think the general understanding has permeated. Our UK Fire Rescue Service has a very cautionary and staged approach to fires and a lot of it is risk assessment.

The challenge with electric vehicles is, first of all, the possibility of a vapour cloud explosion, the venting of the gas outside, and that gas igniting and causing a vapour cloud explosion – which has happened with electric vehicles. If the gas ignites as soon as it is vented you get rocket-like flames. You’ve got to be aware of them and that makes fighting the fire very difficult.

Where the gases vent and where those flames come from will be a weak spot – but they’re coming out with very high pressure, you’re not going to get any water back through that tiny orifice. Putting water on the car is rather like having a fire in your kitchen and putting the water onto the roof of your house. The fire services are well aware of this as well as the toxicity risks which is why they use breathing apparatus.

Finally, if you think you’ve put the fire out, then the chances are you actually haven’t – it’s likely just gone into the dormant phase. Electric vehicles are known to have reignited hours, days or even weeks after the initial incident when they were apparently extinguished. What you don’t want is to sign off the vehicle to a recovery firm and it then re-ignites on the firm’s premises and causes damage or, worse, injury or death.

This article was originally published in the April edition of IFSJ. To read your FREE digital copy, click here.