Electric Vehicle Batteries: Managing Thermal Runaway vital to raising safety

Alysha Liebscher and Gary Gayman, of Morgan Advanced Materials, explain why protecting against thermal runaway in electric vehicles is critical and discuss the options available to manufacturers.

Automotive charging points electric and hybrid vehicles batteries thermal runaway – shutterstock image.
It’s no secret that the electric and hybrid vehicle market is experiencing a high level of growth.

It’s no secret that the electric and hybrid vehicle market is experiencing a high level of growth, particularly as consumers become more aware of global sustainability issues. Not only do electric vehicles help the environment, they are becoming more viable from a cost perspective.

However, a common factor preventing consumers from purchasing fully-electric vehicles (EVs) is ‘range anxiety’, barriers to entry include concerns about not being able to do long journeys, lengthy charging times and a lack of charging points and infrastructure.

To this end, automotive battery OEMs and manufacturers are pouring huge efforts into developing lithium-ion battery packs that can carry cars further and further, increasing the range to new heights.

While range is important, thermal management of batteries is vital to the actual safety of the battery, vehicle and its occupants. This is due to the phenomenon of thermal runaway, a dangerous reaction that can occur in lithium-ion batteries.

What is Thermal Runaway?

Increasing the range of an EVs can be done in multiple ways, from having larger battery packs with more modules and cells, to putting in higher energy density cells with higher capacity. However, all systems are susceptible to thermal runaway, some more so than others.

Each cell in a lithium-ion battery contains a flammable liquid electrolyte. If the cell short-circuits, the electrolyte can combust and the pressure within the cell will rapidly increase until the cell vents the flammable electrolyte.

Nexeon and its partners have been awarded £7m in Innovate UK funding for a project to develop better materials for Li-ion batteries - image courtesy of Depositphotos.
Each cell in a lithium-ion battery contains a flammable liquid electrolyte – image courtesy of Depositphotos.

Temperatures at the ruptured cell can increase to above 1,000°C (1,832°F). Thermal runaway is the rapid and extreme rise in temperature and when it initiates the same reaction in adjacent cells it is known as ‘thermal runaway propagation’.

When thermal runaway happens, it can produce smoke, fire and even explosions. Occupants need to have as much time as possible to escape the vehicle if it does occur. Since 2015, when the electric vehicle market really became mainstream, there have been many battery-related accidents that have been recorded.

This includes an electric bus that caught fire after heavy rain in Nanjing, China, with water immersion cited as the possible reason for the short circuit.

Although thermal runaway is clearly life-threatening, to date there is yet to be global regulation in place.

Whereas China has implemented the GB/T 31485 standard, the UN has only proposed legislation. This leaves automotive manufacturers with the choice of whether they design their battery packs with systems designed to deal with thermal runaway incidents. It’s up to their own risk assessment programmes to determine how likely thermal runaway is to occur.

Putting any protection in is likely to hinder the range capacity of the vehicle though – naturally, more protective materials equals less space for cells in a finite space.

Reaching, and going beyond, the middle ground

Seemingly, there is no middle ground between the two. However, it does not need to be the case that battery manufacturers compromise safety for range, or vice versa.

Windsor-based Morgan Advanced Materials is a global materials engineering company which designs and manufactures a wide range of high specification products with extraordinary properties, across multiple sectors and geographies.

Automotive charging points electric and hybrid vehicles batteries thermal runaway EV (electric vehicle) Li-Ion battery concept – image courtesy of Depositphotos.
Every battery design is different, and so the protection method must be unique – image courtesy of Depositphotos.

The business has been significantly researching and developing a range of thermal management protection materials and methods over many years.

These can provide more time for occupants to exit a vehicle, while the dissipation of heat lessens the chance of thermal runaway spreading uncontrollably.

It’s not a ‘one-size-fits-all’ approach though. Every battery design is different, and so the protection method must be unique.

There are three levels of protection that engineers can design into their systems to significantly reduce the impact of thermal runaway in electric vehicles: cell-to-cell, module-to-module, and battery pack level.

Cell-to-Cell

Cell-to-cell protection involves designing a material to go between individual cells. It is the highest level of protection, but also the most challenging due to space constraints. If an individual cell experiences thermal runaway, the absorption of heat and deflection of flame from the protective materials minimise the thermal affects to adjacent cells.

One of the most effective methods of protection at cell level is by using phase change materials (PCMs), such as Morgan’s thermal insulation EST (Energy Storage Technology) Superwool® Block, a solution that can be used for certain cell formats.

PCMs absorb the heat of ruptured cells, as when the temperature of the cells gets too high, they turn the insulation material from either solid to liquid, or liquid to gas.

During the phase change, the heat can be dissipated throughout the body of the material. If the phase change is from solid to gas, this offers additional protection as the gas from the insulation material pushes the cell’s gases out through vents of the module, helping to lower the temperature quicker.

It is important to consider the cell’s shape when specifying cell protection, as different cells have different insulation needs. Cells are split into three main types: cylindrical, prismatic and pouch.

With cylindrical batteries, the insulation material can be solid shapes, but with pouch cells, they expand and contract, so you cannot use a rigid insulation to protect them. Prismatic cells can use either solid or flexible insulation materials.

Module-to-Module

There are several materials designed to go between modules depending on the module size and design. Thermal runaway within the module can occur but can be contained to stop spread to adjacent modules.

With module-to-module protection, protection can come in a paper format. Notably, module-to-module protection offers significant weight savings compared to cell-to-cell protection. Lighter batteries in turn increase the range and allows the battery to be more easily accommodated in the vehicle’s design.

Pack Level Protection

Pack level protection is the simplest and most affordable type of protection. This is aimed towards improving safety to the vehicle’s occupants by giving them additional time to exit the vehicle, but provides little protection for the battery pack itself. That said, it’s still a far better option than no protection at all.

Standard insulating paper is a common form of pack level protection, such as Superwool® Plus Paper.

The EV market is only likely to increase, as costs of traditional petrochemical fuels become progressively more expensive with time.

With a wealth of choice in how thermal management is achieved, it is evident that automotive manufacturers need to work with materials engineers. Only by doing so can commercially viable solutions be achieved, and the electric vehicle market be improved.