Aspen Aerogels’ cutting-edge supplies defend towards thermal runaway
Thermal runaway happens inside a battery cell by way of a fault, a crash situation or another defect that causes the cell to launch thermal power by way of chemical reactions. That thermal power 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 will 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 current Charged Digital Convention on EV Engineering, John Williams, Vice President of Technical Companies at Aspen Aerogels, gave a superb overview of cutting-edge methods to delay or forestall thermal propagation eventualities.
MORE: Don’t miss John Williams’ subsequent webinar on April 19, 2023:
Balancing Inside Forces Inside An EV Battery Module Or Pack
Security requirements
First, slightly 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 state of affairs within the battery pack, you need to have a five-minute delay earlier than there’s a harmful state of affairs within the passenger compartment. These 5 minutes permits the driving force to discover a place to park, exit the automobile, and get a secure 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,” mentioned Williams. “As electrical automobiles grow to be extra widespread, that 5 minutes will virtually actually ratchet as much as longer durations till, finally, we’d attain the promised land of getting no thermal propagation in any respect.”
The last word aim is that one cell can go into thermal runaway and that’s it. Will probably 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 essential is cell-to-cell conduction, and Williams is an skilled on this specific pathway. Aspen Aerogels manufactures a barrier materials, referred to as PyroThin, designed to stop cell-to-cell conduction. However that’s not the one thermal pathway.
There’s additionally gasoline launch, each major 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 lots 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, corresponding to 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 way of pure convection throughout air gaps that enable sizzling gases to unfold all through the battery pack.
“Energetic cooling may help draw down the power of a cell in thermal runaway,” defined Williams, “though many occasions you’ll be able to’t depend on it, particularly after a crash situation.”
Thermal and mechanical obstacles
A thermal barrier materials between the cells can cut back warmth switch by way of cell-to-cell conduction. Boundaries in prismatic and pouch packages serve two functions: mechanical and thermal. The perfect 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 people who act nearly as good thermal and gasoline obstacles are typically too stiff to function compression pads.
The perfect 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 to be able to preserve constant stress. 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 could delay thermal propagation a bit, however not often sufficient to attain even the five-minute normal, and definitely no more than that. A superb 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 exhibits 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 power a cloth can take up. The appropriate-hand aspect of the graph exhibits that supplies generally used as compression pads, corresponding to silicone foams and polyurethane, can take up lots of pressure power, 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 immune to excessive temperatures, however aren’t good at absorbing mechanical power. 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 line with Williams, Aspen Aerogels’ PyroThin matches the invoice.
“The universe of attainable issues that may stay in that neighborhood may be very, very small,” mentioned Williams. “One among them is an aerogel corresponding to our PyroThin, which is engineered particularly to be a unitary materials inside a cell-to-cell barrier, offering each the mechanical operate of a compression pad and the thermal operate of a hearth barrier.”
Aspen Aerogels has been within the aerogel enterprise for greater than 20 years, primarily targeted on optimizing power effectivity and passive fireplace safety for industrial power amenities. The corporate is now transferring its expertise to the EV section, with the aim of growing the world’s finest cell-to-cell obstacles. Aspen’s testing exhibits {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 power from cell respiratory and swelling. Williams claims that Aspen’s aerogels are the thinnest insulating supplies accessible, and that they provide the absolute best thermal insulation properties.
It’s the chemistry
An aerogel is produced when complicated polymer chains of silica molecules kind by way of a sol-gel course of. This course of is just like making gelatin, however utilizing silica and ethanol as an alternative 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,” mentioned 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’ve a cloth that has a thermal conductivity decrease than nonetheless air, which signifies that while you compress it and squeeze out the air, the thermal conductivity can really get higher, [see Figure 3]. Beneath the identical situations, virtually each different insulation materials will worsen.”
Determine 4 exhibits the connection between compressive stress and pressure—the realm below the curve represents the quantity of pressure power the fabric can take up reversibly. Williams says that Aspen’s newer M-grade supplies will proceed to soak up pressure power 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 check setup in Determine 5 contains two off-the-shelf prismatic cells compressed within a jig, permitting the experimenters to regulate the cell-face stress. The stress on the finish of a cell’s life tends to be larger than at first of its life, so Aspen conducts a worst-case situation check by artificially creating larger stress contained in the jig. The graph on the appropriate exhibits that the elevated temperature of the set off cell (proper) is absorbed by the barrier materials, protecting the temperature of the adjoining cell (left) at a secure 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 check merely demonstrates that thermal runaway might be stopped on the cell-to-cell degree, permitting designers to concentrate on 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 obstacles, battery designers can pack extra cells into the identical area, rising 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 area is much more complicated, so different supplies could carry out higher than PyroThin as cell-to-cell obstacles. In these conditions, Williams says that PyroThin continues to be appropriate for pack covers, sidewall covers and different flat geometries.
A number 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 Normal Motors, which has chosen PyroThin because the cell-to-cell barrier for Ultium, the modular battery and drivetrain platform that can energy greater than 30 new EV fashions.
Partnerships
Aspen Aerogels presently affords consulting providers by way of its world engineering assist staff, which incorporates engineers in Asia, Europe and North America who will reply questions on battery pack and barrier designs. The corporate’s clients embrace Normal Motors, which has chosen PyroThin because the cell-to-cell barrier for Ultium, the modular battery and drivetrain platform that can energy greater than 30 new EV fashions.
To be taught extra about thermal obstacles, take a look at the recording of Aspen Aerogel’s presentation on the Charged Digital Convention.
Don’t miss John Williams’ subsequent webinar on April 19, 2023:
Balancing Inside Forces Inside An EV Battery Module Or Pack
This text appeared in Difficulty 63: Jan-Mar 2023 – Subscribe now.
