Elon Musk explains that the risk is too high.
Electric vehicles are using very different battery types – both in terms of chemistry (although almost all belong to the lithium-ion family) and in terms of cell form factor.
Currently, there are three main types of cell form factors – cylindrical, prismatic and pouch – all in various sizes/capacities. Additionally, some are envisioned to be a structural part of the pack (without modules).
Tesla‘s CEO Elon Musk earlier this month revealed that his company strongly recommends against the use of large pouch cells, as their probability of thermal runaway is dangerously high (compared to other form factors).
“Generally agree, but probability of thermal runaway is dangerously high with large pouch cells. Tesla strongly recommends against their use.”
The debate about battery cell form factor is dumb.
Tell me the pack level performance and economics.
The ideal form factor varies with chemistry, use case, and pack design.
— 🔋 The Limiting Factor 🔋 (@LimitingThe) September 3, 2021
There are no details about how to understand it, but that’s very interesting, especially considering GM’s large battery recall (and a similar Hyundai’s battery recall), related to manufacturing defects.
We guess that Elon Musk wanted to say that even without any defects, the large pouch cells are the most challenging if something goes wrong inside the pack.
In a following tweet, that we found later, Elon Musk explains that the problem lies in the geometry of the cell (distance between the cooling loop and the center of the cell is too high) and thus thermal management is limited.
Additionally, the bag is too weak to prevent fire from spreading, unlike with a cylindrical or prismatic cell (although we would note that the pouch cells are usually installed in modules that have their own cases).
So smaller, reinforced, pressure protected prismatic cells for iron based cells (LFP) are good & safe, and steel cylindrical for nickel (and iron) are the overall design sweet spot?
— 𝗧𝗲𝘀𝗹𝗮 𝗙𝗮𝗰𝘁𝘀 🔋 (@truth_tesla) September 3, 2021
Since the beginning, Tesla opted for cylindrical battery cells – 1865, 2170 and 4680 in the future – but also uses the prismatic form factor (CATL’s LFP batteries). We know that at least initially (over 15 years ago), there was not much choice beyond 1865 cells, as they were reliable and produced in high volume.
Now there is a broad choice, which means that there must be other factors than availability to stick with cylindrical cells. This is a very interesting thing, because so many manufacturers introduced dozens of electric and plug-in hybrid models with large pouch cells (usually NCM chemistry). Is the safety issue mentioned by Elon Musk not an issue in their opinion? Or maybe other advantages are worth the risk?