I've been trying to model the battery for the Model S, mainly to get an idea of what it costs Tesla to manufacture it. As a part of that I've been doing a deep dive into Tesla battery IP. I actually considered putting this into the investors section because of how impressed I am, but the thread is intended to explore the batteries from a technical standpoint (assuming anyone else finds it interesting, lol).
At first blush, I am extremely impressed by the scope of the patents that Tesla has devoted to the 18650 cells they use (though the IP itself is not limited to the 18650 form factor).
Core Design Changes
Here are the patents that likely describe the core changes Tesla made to the battery to make it less expensive to manufacture, and more suitable for use in EV's -
Cell cap assembly with recessed terminal and enlarged insulating gasket - Patent application
Battery cell with a partial dielectric barrier for improved battery pack mechanical and thermal performance - Patent application
Fundamentally, these patents dramatically simplify manufacturing, making the batter lighter, cheaper and more suited to use in a battery pack using massive numbers of these like Tesla does. I can't say that other manufacturers are blocked from using small li-ion cells, but I suspect these patents (if enforced) could easily trip them up.
Keep in mind that batteries with this design will probably explode in any other application than a highly controlled pack like Tesla uses, so there might not be competing IP, and it might not be considered "obvious". Patenting an otherwise non-functional design might be a literal bar for anyone attempting something similar, beyond the Roadster level (which probably has it's own IP to trip folks up) which just used conventional batteries. These batteries can probably only function in a macro-pack like Tesla uses (or with additional modifications). (Edit: removed reference to air cooling)
I'd be interested in hearing from anyone who knows of another auto-maker bringing commodity format cells to market so we can see possible strategies to get around this setup. Or maybe super basic, exploding batteries are an obvious improvement? Or maybe there is related IP from the olden days?
Also, I haven't finished researching the macro-level pack patents, but I suspect Tesla has made similar efforts to keep automakers from doing massive arrays of commodity batteries.
Regardless, the primary patent (the first one linked) basically removes most (all?) of the safety features in the commercial 18650. Here are diagrams of the interiors -
First (conventional commercial battery design) -
Cell cap assembly with recessed terminal and enlarged insulating gasket - diagram, schematic, and image 02
Second (Tesla's amazing, exploding, non-functional battery design) -
Cell cap assembly with recessed terminal and enlarged insulating gasket - diagram, schematic, and image 01
It should be obvious how much this simplifies the manufacturing process. Many complicated features and manufacturing steps are just deleted, and the safety systems are handled by the central battery control system. The battery is also considerably lighter because of the ability to use aluminum (the cost difference between that and steel should be negligible at this scale). Besides making it cheaper and lighter, these changes also facilitate Tesla's manufacturing process and pack integration. I'm impressed.
The second patent deletes the plastic film on the exterior of the battery, leaving bare metal. This cuts both weight, and improves the ability of Tesla to do thermal management on the battery (a plastic film is bad for preventing fires apparently). This change also makes the battery explode (technically, it will short circuit). Non-obvious improvement.
The National Geographic special showing a blurred interior with green batteries (which is the color of the commercial versions) is a complete fake. L.O.L.
There is a related patent that adds a gasket (basically a thin film at the cylinder edges) which is enough to keep the batteries from shorting out (which is one of the purposes of the plastic cover) in the very controlled environment that Tesla has inside of its pack. I didn't link it because its inclusion is almost immaterial in terms of cost, weight or functionality, though it does make the deletion of the cover possible. It also doesn't prevent the battery from exploding. Tesla's pack and manufacturing process does that. Non-obvious.
Tesla's Amazing Cell Level Thermal Management - Battery Fire All But Impossible
Here is the critical thermal management patent that is almost certainly being implemented, that also includes key information on how the cells are integrated into the pack, as well as the changes to the interior and exterior of the cell to prevent thermal runaway -
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.
In addition, two related pieces of IP which might or might not be implemented -
Battery Cell with a Center Pin Comprised of a Low Melting Point Material - Patent application
Battery Cell with Center Pin Comprised of an Intumescent Material - Patent application
In these patents, the stock central core is replaced with a core that either melts (to deform inwards) and/or contains an intumescent material. In a stock battery the configuration of the central core makes it more likely that the battery will burst during a thermal event. These patents cause the material to be sucked inwards, and/or helps cool down the reaction in the first place.
Seems smart, and I see no reason either would substantially increase costs over a stock core, because its basically just a different material choice. But the method might not be necessary since it looks like they are dipping the entire innards of the battery in intumescent material, and otherwise containing any thermal events with the exterior application.
Here is a basic kitchen sink patent which Tesla threw at the patent office, with a huge menu of ideas that are then fleshed out in other individual patents, and which gives an idea of alternative ways to customize small format cells. You can read it to get an idea of how Tesla went about trying to ensure that nobody else will be able to use replicate their thermal management breakthroughs, by basically patenting a large number of alternative methods -
Method and apparatus for maintaining cell wall integrity during thermal runaway using an outer layer of intumescent material - Patent application
I don't want to link and describe all of the alternate methods of preventing thermal runaway. But they involve wrapping the batteries, both inside and out with all kinds of different materials, (including intumescent materials applied differently than in the core patents) whether steel, titanium, kevlar or whatever.
There are multiple patents and ideas, all of which seem workable, but inferior to what I've decided is likely the core thermal protection methodology. It's possible that Tesla actually used some of those methods instead of what I think they did, but I really doubt it. The core patent I identified has research notes in it that the other patents lack, showing real effort being put into that program. The rest appear to be set up to make it impossible for automakers (or anyone really) to make changes to small li-ion batteries that will help prevent them from blowing up.
Maybe it will work, or maybe it wont. But Tesla clearly put a lot of thought into the alternatives and seem to have a nice defensive portfolio to keep others from following the path that Tesla is following.
EDIT: Just to be clear the cells are only unsafe if you attempt to use them individually. As a part of the pack the entirety of the system is incredibly safe!