Managing heat more efficiently and effectively is the new ‘must have’ of next generation hybrid and electric vehicles
The growing demand for hybrid and electric vehicles requires new engineering solutions and one of the greatest challenges is how to manage heat. Morgan Advanced Materials is taking industry-leading products and expertise in developing thermal management systems (which have transformed gas and diesel-powered trains to electrified trains) and is now applying that capability to the automotive sector.
In an internal combustion engine for example, temperatures in the order of 600-900°C are generated while the engine is in operation; this heat needs to be contained from the rest of the other temperature-sensitive components located in the vehicle. Hybrid and battery electric vehicles that use lithium-ion cells require that these cells are maintained at 20-25°C, with any exposure to significantly hotter or colder temperatures likely to have an adverse effect on the performance of the battery pack.
In order to keep the battery pack at its designated temperature, active or passive thermal management can be incorporated. In active thermal management a material such as air, liquid, or refrigerant, along with an external device, such as a fan or a heat pump, to maintain movement of the coolant through the battery pack is used. Passive thermal management does not use external devices and instead relies on thermodynamics to move heat. Typical examples of passive thermal management are aluminum heat sinks and phase change materials (PCMs).
As the temperature difference through the pack should be no larger than 2-3°C from the coldest to warmest cell, the integration of an appropriate thermal management system is crucial. Automotive manufacturers often opt for a combination of both active and passive systems in their final designs.
Compared to other commercial batteries, which comprise of different chemistries, lithium-ion batteries have a very high energy density, much higher voltage and low self-discharge. To achieve these features, the electrolytes within the cells must be non-aqueous. Essentially all are based on liquid carbonates which are highly flammable at relatively low temperatures.
There are various possible failure mechanisms for lithium-ion batteries, however failure in a cell can result in a rapid rise in temperature known as ‘thermal runaway’, which would cause the electrolyte within the cell to ignite. Unless additional safety systems are employed, the thermal runaway of one cell will cause adjacent cells also to enter thermal runaway and the chain reaction could result in a violent explosion of the battery pack. The transmission of thermal runaway from one cell to another is termed ’thermal runaway propagation’.
Challenges in preventing or mitigating thermal runaway propagation
In order to help prevent or mitigate thermal runaway propagation, different levels of protection can be used. These primarily include cell-cell, module-module, and pack level.
Cell-cell protection is the highest system level of protection and involves designing a material to go between individual cells. As space is a premium in a battery pack, the spacing between cells is minimal and the material included must be able to handle the heat and energy released by an individual cell.
Solutions for cell-cell protection vary depending on the type of cell format used in the design. For example, pouch cells need a material, such as Morgan’s Superwool® EST Compression Paper, that can maintain constant pressure as the cells expand and contract in normal operation. Since each module design is still unique, it’s important to work with the company developing the module to understand what their specific design constraints are.
Module-module protection is when a material is incorporated to stop or delay the thermal event between adjacent modules. There is generally more spacing between modules than cells, but the amount of exothermic energy being released is also significantly higher than on the cell-cell level.
One of Morgan’s most popular products, Superwool® EST Paper M, is commonly used as module-module protection. Its laminated design allows for the mica to help spread the heat on the hot face in the x and y directions while its insulating Superwool® Paper prevents the heat from spreading in the z direction to the cold face.
Pack level protection is the term given when a solution is placed between the battery pack and the occupants. This solution often sits between the modules and the lid of the battery pack. The primary purpose of this type of protection is to provide the occupants extra time to exit the vehicle in the event of a catastrophic thermal incident.
The additional time added is greatly dependent on the pack design and the lid material composition. Pack level solutions are commonly used in China, as they can often meet the existing GB/T 31467.3 standard. Morgan’s Superwool® EST Paper G is one example of this type of solution.
Just as every electric vehicle is unique, so are the battery packs they employ. For vehicles and packs to be saleable within multiple geographies, it is essential that they incorporate thermal runaway prevention or mitigation systems which meet current and emerging safety standards in each region. The most effective way to do this is to engage a specialist like Morgan Advanced Materials in the earliest stages of design.
As electric vehicle manufacturers continue to design packs with increasing energy densities, identifying thermal management options and continuing to research novel strategies will help to ensure battery technology remains fit for the future.
Morgan’s Thermal Ceramics business works with manufacturers from start to finish, partnering at the design stage, prototyping, testing, and evaluating different solutions, with the assurance of being able to manufacture to the agreed specifications on a global basis.