Cell Thermal Runaway Propagation Resistance Using Dual Intumescent Material Layers - Patent application
The key feature is that after pack assembly, the cells and entire interior of the pack are sprayed with 2 layers of intumescent material. For those not familiar with it, its basically a material that when exposed to a heat source will absorb that heat, and then undergo a chemical reaction causing it to expand.
An earlier patent by this research team used a single layer, and didn't describe how it was to be integrated into the manufacturing process. In that patent, the single layer would suck heat out of the battery (significantly delaying or stopping thermal runaway) and then expand, keeping the battery thermally separated from other components. Once it got hot enough, it would char and harden. Once this char formed it created a hard thermally resistant cylinder which would direct any heat which managed to burst through the battery shell vertically through the cylinder, and away from surrounding batteries.
In the "finished" patent this charring layer is the second layer, while the first layer is able to absorb heat quicker (thus increasing the chance of the battery not bursting) and then transfer its heat to the second layer, which would provide the final barrier effect described above.
In addition, they describe a manufacturing process which would coat the interior surfaces of the cell with their own intumescent barriers, which would hopefully halt thermal runaway before it ever gets to the exterior of the battery. Again, because this just adds a quick (and cheap) step to the manufacturing process (where the battery is dipped or sprayed) it seems highly likely to be in use.
The pack assembly process is detailed, and basically the individual cells are integrated into the pack, and then the two layers of intumescent material are sprayed on the whole assembly, coating batteries and all of the interior surfaces of the pack. This leaves the metal shell of the batteries still connected to the active cooling elements, while the rest are coated. Any thermal runaway is thus shunted directly into the cooling system, while every other surface is protected by 4 layers of intumescent material (the two touching the battery, and the two on any opposing surface). And that isn't counting the intumescent layers that might be inside of the battery as well.
It seems simple, cheap and entirely fireproof, considering the small size of each individual battery. I am extremely impressed with the way that Tesla is using simple chemical reactions to snuff out thermal events before they occur, which then also create a mechanical barrier to reactions that still manage to get out of hand. This is aside from all of the active controls and safety features built into the macro-pack, including other mechanical barriers, cooling systems and power electronics.