When it comes to safety in an Electric Vehicle (EV), the most important aspect is controlling the massive amount of energy stored in the battery pack. To help with this, significant advances have been made in cell design and battery management systems (BMS). However, even with these advances, the possibility of a single-cell failure is always present.
In the worst-case scenario, the failure of a single cell will cause the failure of the cells near it through overheating. If this occurs, the cells can fail in a cascade of thermal events. In recognition of this remote but still plausible possibility, EV pack designers are beginning to design systems with features to limit or eliminate a single cell’s failure from propagating to the point where passenger safety is compromised.
Currently, the US and Europe only have limited regulations regarding the prevention of thermal propagation in EVs. This is expected to change over time. When it does, the OEMs must be ready with a proven solution.
Thermal Propagation Prevention Methods
There are currently four primary methods to prevent thermal propagation in prismatic and pouch cell packs. These propagation prevention methods all have significant consequences for cell cycle lifetime, the ability to fast charge, and driving range. These methods are used individually or combined, with each manufacturer adding their experience to produce the best results.
In the isolation method, the modules are placed within a heat-resistant pack housing. If there is a cell failure, there will be sufficient time for the EV to stop and for passengers to safely exit the vehicle. The most common escape-time requirement is five minutes.
Since the failure of an individual cell may generate a hot spot on the pack housing, various thermal insulation and heat-spreading materials are used inside the housing to spread the heat. Even if there is a cascading failure, the heat will be widely distributed on the housing surface, maintaining its thermal integrity throughout the failure event. These materials can include SpreaderShield flexible graphite, mica, aerogel, air gaps, and ceramics.
Isolation is often considered the least expensive way to meet the five-minute escape time requirement. These packs have high energy density and high specific energy. Construction is simple and cost-effective.
When isolation is the primary propagation prevention method, the individual cells often have inadequate thermal management. Cell temperature and cell temperature gradients generally are not tightly controlled. Fast charging and discharging can overheat and damage the cells, limiting cell cycle lifetimes. Thermal degradation of a single cell can significantly reduce the range of the overall battery pack.
In the immersion method, the individual cells are partially or entirely surrounded by a dielectric liquid circulated throughout the module by a mechanical pumping and cooling system. By surrounding each cell in a cooling fluid, immersion is often seen as the ultimate method of preventing propagation. If a cell fails, the liquid will carry away the heat and stop the fire from spreading to the adjacent cells.
The cell temperatures and temperature gradients can be tightly controlled using a cooling liquid. The ability to fast charge and cell cycle lifetimes are typically excellent. In immersion, the system’s overall size, weight, and complexity must be considered. There may be pumps, chillers, valves, baffles, and piping that are not part of other systems. Although the pack’s baseplate size may be significantly reduced, the system may have a higher overall weight due to the needed volume of cooling liquid.
To date, this method has not found widespread use in EVs. The cooling liquid itself may be considered a hazardous substance. Special maintenance, handling, and disposal rules may apply. Due to the system’s increased complexity, initial costs and maintenance may be higher than with other types of systems.
In the insulation method, thermal insulation materials are placed between cells. In a single-cell failure event, the heat will not spread to the adjacent cells before the failure event is over.
A combination of different insulating materials, such as aerogel, fiberglass, phase-change, mica, polyimide, and ceramics, are used to prevent heat from transferring to adjacent cells during a cell failure. Three to five millimeters of insulation material are typically needed between cells to stop propagation.
The insulation method benefits from low complexity and the use of lightweight materials. This typically results in a high overall specific energy.
When the individual cells are enclosed in thermal insulating material, it is harder for them to shed heat to the environment during day-to-day operations. Fast charging can result in overheating of the cells, leading to substandard cell cycle lifetimes. In addition, the added thickness between cells leads to a lower overall pack energy density, potentially making the pack too large to be practical.
To overcome these issues, a layer of SpreaderShield flexible graphite is added to the insulation. This results in a solution that is more effective than insulation alone — without an increase in thickness.
In the spreading method, heat from a cell failure will be spread across a thermally conductive material. The heat is then spread to a cold plate or heat sink to be shed to the environment. Sufficient heat is removed to prevent the failure of adjacent cells.
In day-to-day operations, the spreading method will allow the cells to be fast-charged without a heat buildup. The heat-spreading material will also maintain a low thermal gradient across the cells, giving extended cell cycle lifetimes.
SpreaderShield flexible graphite cooling fins (0.25 – 1.0mm) and aluminum plates (1.0 – 3.0mm) are the two most common heat-spreading materials. If graphite is used instead of aluminum, the pack energy density / specific energy will be significantly improved – resulting in smaller, lighter packs with a greater driving range.
In addition to the heat spreader, a layer of polyurethane foam and a layer of dielectric material are typically added between the cells to maintain physical contact of the heat spreader against the cell and add additional thermal and electrical insulation.
Flexible Graphite vs. Aluminum
In many lower-performance battery packs, aluminum has been seen as the primary heat-spreading material. It is often used for both mechanical structure and heat spreading.
For battery packs requiring propagation control, the amount of aluminum needed to achieve the thermal and propagation prevention goals results in an unacceptably heavy and bulky pack.
For spreading heat, graphite is considered a direct replacement product for aluminum. Where aluminum is dense and has poor thermal conductivity (~180W/m·K), SpreaderShield graphite is lightweight and has high thermal conductivity (400 – 1600 W/m·K).
Graphite is half the thickness and a third of the weight for the same heat spreading as aluminum. Substituting graphite for aluminum will typically result in a smaller EV pack that weighs approximately twenty-five to forty kilograms less.
There are four primary ways to prevent thermal propagation in pouch and prismatic cell battery packs. These methods are isolation, insulation, immersion, and spreading. Of these, spreading not only prevents propagation but enables extended cell cycle lifetime, fast charging, and produces a battery pack that is small and lightweight. Aluminum is popular for many lower-performance battery packs but is too thick and heavy to be practical in a high-performance battery. SpreaderShield flexible graphite heat spreaders are often used as the thermal management material in high-performance battery packs where a small and lightweight form factor is desirable.
Author: Bret Trimmer – NeoGraf Solutions Applications Engineering Manager