No ‘One Size Fits All’ Approach to Thermal Runaway in Electric Vehicle Batteries

Alysha Liebscher, Business Development Manager, and Gary Gayman, Senior R&D Developer, at the Thermal Ceramics business of Morgan Advanced Materials explain why protecting against thermal runaway in electric vehicles is critical and discuss the options available to manufacturers.

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. Cultures are changing, with people willing to do their bit to safeguard the future generation, including considering the switch from conventional internal combustion engine cars to greener choices. Not only do electric vehicles help the environment, they are becoming more viable from a cost perspective.

The Guardian in the UK published in 2017, a Nissan Leaf could be leased for about £240 a month with a deposit of £2,000, just £70 a month more than the larger, petrol-engine Nissan Juke. It also found that the cost of an overnight charge that delivers a typical 100 miles of driving is about £3-£4 depending on the electricity tariff rate. To go the same distance in a petrol car would typically cost £15.

Governments are also introducing various initiatives to clamp down on pollution. France has initiated 'clean air' windscreen stickers as a legal requirement in some of its cities, to identify a vehicle’s emissions levels. Meanwhile, the UK is working towards five Clean Air Zones (CAZs), to be operational from 2020. Owners with vehicles that do not meet Euro 6 emissions standards will be charged a fee for entering a CAZ.

However, a common factor preventing consumers from purchasing fully electric vehicles is the range available. Typically known as ‘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.

The Volkswagen E-Golf, for example, had a theoretical range of 118 miles in its first generation 2014-2016 version, as cited by leading motoring journal Top Gear. Now, the E-Golf can do up to 144 miles in its current model released in 2017, as claimed on Volkswagen’s own website. The Nissan Leaf has steadily increased its battery capacity too, taking it from the 24 kWH battery first released in 2012 that offered 100 miles (as quoted by Nissan), up to a 40 kWH battery capable of delivering 168 miles. The Japanese manufacturer is rumoured to bring an even longer-range version of its popular car in 2019, with active thermal management.

Those last two words are key as well to electric vehicles and their success. While range is important for use, 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 electric vehicle can be done in multiple ways. This includes having larger battery packs with more modules and cells, through 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. Temperatures at the ruptured cell can increase to above 1,000°C (1,832°F). This rapid and extreme rise in temperature is termed thermal runaway 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 want to 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.

Morgan Advanced Materials 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 is 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. Namely, these are 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 is 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.

An Insight into Passive Thermal Management

Automotive manufacturers have plenty of choice when specifying what level of thermal protection they want to use. These methods of thermal management are split between two categories - Active Management, and Passive Management.

Active thermal management denotes cooling technologies that must introduce or remove energy using a substance to augment the heat transfer process. In electric vehicles, this includes air cooling, liquid cooling and refrigerant cooling, and involves an external device that helps with heat dissipation. These are generally more expensive and complex in comparison to passive thermal management techniques.

Passive thermal management techniques on the other hand, are technologies that rely upon thermo-dynamics of conduction, convection and radiation to transfer heat. Passive battery cooling technologies includes metal heat sinks, phase change materials (PCMs), and specialised heat shields. These are typically cheaper than active thermal management technologies and are easier to implement.

As cell-to-cell protection is generally the highest level of protection to achieve, Morgan’s Thermal Ceramics business has been testing and experimenting with different passive thermal management materials to learn how each one reacts in a thermal runaway situation.

Namely, the materials tested were:

• Foam (control) – This material maintains constant cell pressure, and is the protection used within the module.
• Insulation – Insulation materials thermally insulate upon exposure to heat
• Intumescent – Upon exposure to heat, intumescent materials volumetrically expand
• Endothermic – Endothermic materials, e.g. PCMs, absorb energy upon exposure to heat

To illustrate how each material performed, the parameters for the test were:

• A battery module with 24 prismatic Lithium Nickel Manganese Cobalt Oxide (NMC) cells
• Pairs of cells grouped together to form elements, with each element separated by material sheets
• One element within the module was overcharged until thermal runaway occurred
• 100 per cent State of Charge (SoC) for all cells within the module
• All inherent safety features were disabled.

 What was witnessed was an extraordinary set of results (Figure 1):

Figure 1:

The test was then repeated, but with active cooling management from a cold plate for the foam and endothermic materials (Figure 2):

Figure 2:

From the test results, the indication is that endothermic materials are the best performing from those tested, whether that is with or without active cooling management. On the other hand, foam materials do not perform well in thermal runaway situations. However, it should be noted that the foam designed into this module was not designed to prevent thermal runaway, and there are other foam solutions that could potentially perform better in a thermal runaway situation.

Constant Evolution and Discovery – How Morgan Helps

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.

Morgan’s Thermal Ceramics business can work with manufacturers from start to finish. This includes partnering at the design stage, prototyping, developing thermal management materials, testing and evaluating different solutions. 

Research and development into thermal protection is part of Morgan’s overall strategy. Morgan’s Global Fibre Centre of Excellence is devoted to this task, with researchers and scientists developing solutions not just for automotive but also for other industries such as petrochemical, aerospace and iron and steel.