Charged EVs | Thermal runaway in EV battery packs: designing a mitigation technique
Aspen Aerogels’ cutting-edge supplies shield in opposition to thermal runaway
Thermal runaway happens inside a battery cell by a fault, a crash situation or another defect that causes the cell to launch thermal vitality by chemical reactions. That thermal vitality will increase the temperature of the cell, the elevated temperature drives quicker kinetics, and extra warmth is launched, additional driving up the temperature.
Finally, you may get to a tipping level and, primarily, the cell catches on fireplace. Thermal propagation happens when that first cell in thermal runaway causes its neighbors to additionally tip over into thermal runaway, which then spills over into the cells subsequent to them.
Talking on the latest Charged Digital Convention on EV Engineering, John Williams, Vice President of Technical Companies at Aspen Aerogels, gave a wonderful overview of cutting-edge strategies to delay or forestall thermal propagation situations.
MORE: Don’t miss John Williams’ subsequent webinar on April 19, 2023:
Balancing Inner Forces Inside An EV Battery Module Or Pack
Security requirements
First, just a little background. In China, the usual for thermal propagation delay is 5 minutes, defined Williams. What meaning is that, from the second of detection of a hazardous scenario within the battery pack, you have to have a five-minute delay earlier than there’s a harmful scenario within the passenger compartment. These 5 minutes permits the driving force to discover a place to park, exit the automobile, and get a protected distance away.
“A five-minute thermal propagation delay is basically the desk stakes now for producers of electrical automobiles.”
“A five-minute thermal propagation delay is basically the desk stakes now for producers of electrical automobiles,” stated Williams. “As electrical automobiles turn into extra widespread, that 5 minutes will virtually definitely ratchet as much as longer durations till, ultimately, we’d attain the promised land of getting no thermal propagation in any respect.”
The final word objective is that one cell can go into thermal runaway and that’s it. It is going to be a upkeep occasion and never a security occasion. However we’re not there but, and Williams defined why.
The physics of thermal propagation
The physics of thermal propagation contain a number of pathways for warmth to switch from a cell in runaway to its neighbors, Determine 1. The primary and most necessary is cell-to-cell conduction, and Williams is an knowledgeable on this explicit pathway. Aspen Aerogels manufactures a barrier materials, known as PyroThin, designed to forestall cell-to-cell conduction. However that isn’t the one thermal pathway.
There’s additionally gasoline launch, each main combustion, by which the reactants come from throughout the cell, and secondary combustion, by which the gasoline contained in the cell reacts with the exterior atmosphere. These gasoline flows and reactions drive a number of the behaviors inside a pack throughout a thermal propagation occasion. There are additionally sizzling particulates being ejected throughout the preliminary part of the thermal runaway. There are secondary conductive pathways, resembling busbars, cooling plates and structural interconnects between cells. These are primarily thermal bridges—conductive pathways round a cell-to-cell barrier. Additionally, warmth could also be transferred by pure convection throughout air gaps that permit sizzling gases to unfold all through the battery pack.
“Lively cooling may also help draw down the vitality of a cell in thermal runaway,” defined Williams, “though many occasions you possibly can’t depend on it, particularly after a crash situation.”
Thermal and mechanical boundaries
A thermal barrier materials between the cells can cut back warmth switch by cell-to-cell conduction. Limitations in prismatic and pouch packages serve two functions: mechanical and thermal. The best barrier acts as each a mechanical compression pad and a thermal insulator, however there’s a trade-off: supplies that make superb compression pads usually present little thermal resistance, and those who act nearly as good thermal and gasoline boundaries are typically too stiff to function compression pads.
The best barrier acts as each a mechanical compression pad and a thermal insulator, however there’s a trade-off: supplies that make superb compression pads usually present little thermal resistance, and vice versa.
Cells will swell as they age, and breathe as they undergo charge-discharge cycles. Compression pads take up the ensuing cell motion. Swelling with age happens over prolonged time scales, however the respiratory that happens over a short while interval needs to be absorbed by the cell-to-cell barrier as a way to preserve constant strain. Polyurethane and silicone foams are very efficient compression pads, however, as low-temperature supplies, they merely can’t stand up to thermal runaway conditions. They might delay thermal propagation a bit, however not often sufficient to attain even the five-minute commonplace, and definitely no more than that. barrier is a versatile materials that’s additionally able to withstanding excessive temperatures. These two attributes—flexibility and high-temperature tolerance—are sometimes mutually unique.
Determine 2 reveals potential cell-to-cell barrier supplies, with the Y-axis representing the utmost publicity temperature {that a} materials can stand up to and the X-axis indicating how a lot pressure vitality a cloth can take up. The precise-hand aspect of the graph reveals that supplies generally used as compression pads, resembling silicone foams and polyurethane, can take up a number of pressure vitality, however their temperature tolerances max out round 100-300° C. On the left aspect of the graph, we see that metals and mica sheets are very proof against excessive temperatures, however aren’t good at absorbing mechanical vitality. The Holy Grail of barrier supplies would reside within the upper-right nook of the graph, offering warmth resistance 5 occasions better than silicone foam and mechanical absorption 4 orders of magnitude above that of mica sheets. In response to Williams, Aspen Aerogels’ PyroThin suits the invoice.
“The universe of doable issues that may stay in that neighborhood may be very, very small,” stated Williams. “One among them is an aerogel resembling our PyroThin, which is engineered particularly to be a unitary materials inside a cell-to-cell barrier, offering each the mechanical perform of a compression pad and the thermal perform of a fireplace barrier.”
Aspen Aerogels has been within the aerogel enterprise for greater than 20 years, primarily targeted on optimizing vitality effectivity and passive fireplace safety for industrial vitality amenities. The corporate is now transferring its expertise to the EV phase, with the objective of growing the world’s greatest cell-to-cell boundaries. Aspen’s testing reveals {that a} two-millimeter sheet of PyroThin materials can stand up to publicity to a propane torch at 1,000° C, and take up the pressure vitality from cell respiratory and swelling. Williams claims that Aspen’s aerogels are the thinnest insulating supplies out there, and that they provide the very best thermal insulation properties.
It’s the chemistry
An aerogel is produced when complicated polymer chains of silica molecules kind by a sol-gel course of. This course of is just like making gelatin, however utilizing silica and ethanol as a substitute of sugar and water. A moist part of suspended silica begins out with the viscosity of water; in about two minutes, the suspension turns gelatinous. Within the course of, the silica self-assembles into lengthy polymeric chains, which home tiny nano-pores.
“An aerogel’s pores are 10,000 occasions smaller than that of another insulation materials. In that 4 orders-of-magnitude distinction, the physics fully modifications.”
“That nano-porosity is the key in an efficient thermal insulation materials,” stated Williams. “An aerogel’s pores are 10,000 occasions smaller than that of another insulation materials. In that 4 orders-of-magnitude distinction, the physics fully modifications. Now you might have a cloth that has a thermal conductivity decrease than nonetheless air, which signifies that whenever you compress it and squeeze out the air, the thermal conductivity can truly get higher, [see Figure 3]. Beneath the identical situations, virtually each different insulation materials will worsen.”
Determine 4 reveals the connection between compressive stress and pressure—the world beneath the curve represents the quantity of pressure vitality the fabric can take up reversibly. Williams says that Aspen’s newer M-grade supplies will proceed to soak up pressure vitality over the lifetime of a automobile with minimal compression set or fatigue.
Actual-world testing
To check how supplies behave throughout a thermal occasion, Aspen’s engineers topic the supplies to mini-module testing. For instance, they put a thermal barrier between two cells and triggered one cell with a heating pad to see if the barrier would forestall thermal propagation from one cell to the opposite.
The take a look at setup in Determine 5 consists of two off-the-shelf prismatic cells compressed within a jig, permitting the experimenters to manage the cell-face strain. The strain on the finish of a cell’s life tends to be greater than in the beginning of its life, so Aspen conducts a worst-case situation take a look at by artificially creating greater strain contained in the jig. The graph on the proper reveals that the elevated temperature of the set off cell (proper) is absorbed by the barrier materials, holding the temperature of the adjoining cell (left) at a protected degree.
This mini-module experiment has limitations, nevertheless. In an actual battery pack, different pathways—secondary conduction, sizzling particulates and convection—additionally have an effect on the system. This take a look at merely demonstrates that thermal runaway will be stopped on the cell-to-cell degree, permitting designers to deal with isolating the opposite mechanisms.
PyroThin has about the identical density as urethane foam, however the latter isn’t a thermal barrier. Urethane foam coupled with a thermal barrier will sometimes be heavier than a comparable half that’s made with PyroThin alone. As a result of PyroThin is each lighter and thinner than different thermal/mechanical boundaries, battery designers can pack extra cells into the identical house, growing automobile vary.
Different cell packages and battery chemistries
Aspen concentrates on Li-ion cells in prismatic and pouch packages as a result of the aerogel materials naturally lends itself to flat, rectangular objects. In cylindrical cells, the interstitial house is much more complicated, so different supplies might carry out higher than PyroThin as cell-to-cell boundaries. In these conditions, Williams says that PyroThin remains to be appropriate for pack covers, sidewall covers and different flat geometries.
A few of the firm’s clients have efficiently used PyroThin with lithium-metal batteries, which are likely to burn hotter than Li-ion cells, typically reaching 1,200 to 1,400° C. Lithium-metal batteries make use of totally different grades of PyroThin, relying on the temperature requirement.
Apsen’s clients embrace Common Motors, which has chosen PyroThin because the cell-to-cell barrier for Ultium, the modular battery and drivetrain platform that may energy greater than 30 new EV fashions.
Partnerships
Aspen Aerogels presently gives consulting companies by its world engineering help group, which incorporates engineers in Asia, Europe and North America who will reply questions on battery pack and barrier designs. The corporate’s clients embrace Common Motors, which has chosen PyroThin because the cell-to-cell barrier for Ultium, the modular battery and drivetrain platform that may energy greater than 30 new EV fashions.
To study extra about thermal boundaries, try the recording of Aspen Aerogel’s presentation on the Charged Digital Convention.
Don’t miss John Williams’ subsequent webinar on April 19, 2023:
Balancing Inner Forces Inside An EV Battery Module Or Pack
This text appeared in Subject 63: Jan-Mar 2023 – Subscribe now.
