Thermal runaway has become the most visible, renown means of li-ion battery failure. This occurs when a battery cell is unable to dissipate its heat effectively, typically because the cell’s battery pack has become delaminated.
Each time the EV charges or discharges, the cells inside the battery pack undergo a chemistry change that causes the battery pack to swell, if ever so slightly. The thousands charge/discharge cycles an EV battery will experience, in addition to the shock and vibration of thousands of miles traveled, put considerable strain on the batteries electrical and thermal connections.
Ultimately, one battery cell that shorts or overheats is prone to fire, which if it occurs, will inevitably spread to neighboring cells and the dreaded thermal runaway.
However, even if a runaway doesn’t occur, there are practical reasons to ensure consistent electrical and thermal connections within the battery pack. Elevated temperatures and sub-optimal connections will reduce the capacitance of the battery pack and hasten battery charge degradation.
One notable area of research and development has been EV quick recharging. One of the factors complicating more widespread adoption of EVs has been the slow nature of the process. As quick chargers, which can charger most EV battery packs in two hours or less, become more common, this too is creating a challenge for the batteries, which now must handle a higher amperage, and the additional heat along with it. Some companies, such as StoreDot, claim that with its Extremely Fast Charging battery technology that is capable of charging 100 miles of range in 5 minutes, can deliver 750 Wh/L and 330 Wh/kg volumetric and gravimetric energy densities respectively, in production-ready cells.
All of this points to a highly-important technical challenge for EV batteries – the need to maintain optimal thermal and electric connections within the battery pack. This challenge is often met with cutting edge thermal interface materials and polyurethan or silicone foams.
Effective cooling for high-performance li-ion batteries
The challenge the EV industry has always faced is to keep cell temperatures within the optimal range of between 70 and 90° F to ensure peak performance throughout the life of the battery.
The introduction of liquid-cooling – initially water-glycol and more recently dielectric fluids – has greatly improved the heat dissipation and thermal management of the battery pack. Immersion cooling with a dielectric fluid has the potential of increasing the rate of heat transfer by 10,000 times relative to passive air-cooling. However, as batteries increase in energy density and get smaller these systems are being stretched to their limit.
To optimize the dissipation of the heat, manufacturers are turning to flexible and conductive materials as a means of further improving heat dissipation and thermal management of high energy density li-ion battery packs. The limiting factor for heat transfer by conduction is the interface between components such as the battery cells and the cold plate. The component surfaces, although appearing smooth and flat to the naked eye, are usually rough at the microscopic level.
To ensure a better interface, manufacturers use a thermal interface material (TIM) to connect the battery cells to thermal conduits or the cold plate.
The TIM promotes heat transfer and dissipation by displacing any air trapped within the large gaps and microscopic rough surfaces that exist between the substrates. TIM solutions range from simple greases and gels to high-performance gap fillers, thermal adhesive tapes, and thermal pads, all of which are common in EV battery packs.
These adhesives, greases, gels, tapes or pads are at least in part constituted of a highly thermal conductive material, typically a metallic or carbon powder or fiber with low specific heat.
Selecting TIMs
Although the thermal and electrical properties of may be the decisive factors in selection, there are other parameters that need to be considered. TIMs are also designed to provide additional electrical insulation to further safeguard against any high voltage breakdown occurring between the energized battery and the metallic cooling plate. Among those are viscosity, curing time, manufacturing integration and more.
High up on the list of these considerations is the TIM form, be it a viscous liquid or pre-shaped or cut-to-size solid material. There is no "one-size-fits-all" solution when it comes to the TIM’s form, and the choice ultimately comes down to the battery’s design configuration.
TIM forms are highly dependent on the manufacturing process. Liquid TIMs can be highly effective for automated machinery, as they mix, meter, dispense and apply TIMs in one station. Pre-shaped forms, which can be peeled like tape, might be more practical for manual assembly steps. Cut-to-size options are better for custom thermal management needs.
Gap fillers have been widely adopted in EV battery applications, due to the ability to be dispensed at high volumes. Cure-in-place, liquid-dispense gap fillers are applied using a two-part system; first, by applying the filler on one of the two substrates, and then pressing the two surfaces together to a specified thickness. The material then cures to form a solid, but compliant interface.
Thermal pads, on the other hand, are pre-cut to a desired shape, applied to one substrate, compressed and the adhesive is left to cure in place. The applied compressive load forces the solid, yet compliant pad into intimate contact with the rough surfaces. In this system the amount of pressure applied to the pad has a direct impact on the thermal resistance.
Evolving EV designs are also changing the equation. To increase energy density and extend driving range, EV batteries have been shifting from modular to cell-to-pack designs. Current battery designs consist of multiple, individual battery modules connected to form a battery pack, with each module having a separate casing, with a TIM to transfer heat.
In contrast, the cell-to-pack design combines the battery cells into a single, large battery module, eliminating the need for separate module housings and TIMs. This transition could reduce future TIM usage per vehicle, but increased the need for a reliable, practical TIM solution that is suitable for mass production. The elimination of module housings also brings the cells directly into contact with the cooling plate which requires an increased level of adhesion as the TIMs bond the cells to the cold plates.
Battery manufacturers need to keep this in mind, as they roadmap manufacturing in the future.
Ensuring electrical connections
The miniscule dimensional changes of a battery under charge or discharging strains the electrical connections, which could cause an arc or short, another key risk for battery packs.
Some manufacturers have addressed this by implementing mechanical compression springs that provide a pressure against the battery or terminal. However, springs are almost always metal, which means the are thermally and electrically conductive. In addition, they can create hard and soft spots in a battery, since the force is localized. The pressure of the spring also changes based on deflection, which can actually damage the components if the deflection and pressure is too high.
Instead, manufacturers are increasingly turning to silicone or polyurethane sheets of dielectric foam. The foams provide a consistent compression force deflection – that is, the return pressure of the foam under compression remains consistent, no matter the degree of deflection. This provides consistent, engineered return pressure, evenly across the battery.
Foams are also resistant to common challenges of automotive environment – extreme temperatures, vibration and shock loads and chemical interactions. These foams are always dielectric to help prevent unwanted arcing between cells, and fire resilient to help quell runaway issues.
Summary
Wider EV adoption arguably hinges on TIMs. Batteries remain the most expensive and critical part of the electric automobile. Supply chain issues mean manufacturers need their in-field batteries to last longer. Consumers want to ensure the longest possible service life for their car’s most critical subsystem. Neither manufacturers nor consumers want to see their car caught up in a thermal runaway scandal.
Although often an afterthought, the passive materials in a battery are mission critical components -not just to a car, but to the e-mobility revolution.
It is clear that the battery technologies underpinning the electric vehicle are developing at breakneck speed. As a result, these smaller, faster charging and more energy-dense battery packs will continue to challenge heat dissipation and thermal management systems for many years to come.
Even though air-cooled systems have largely been replaced by liquid-cooling, optimized by the use of thermal interface material, the search continues to reduce the cost, weight, and complexity of these systems, which might just see the return of air cooling enabled by newly developed TIMs.
