Asides from price, why not engineer a battery that did not rely on 18650 cylindrical cells? Seems like a bit of extra weight and lost capacity/space by using tons of 18650 batteries which each have their own casing instead of just making a 'brick' style battery, similar to those found in mobile phones (but obviously much larger).
"Large cells without enough space between them to isolate against the cell-to-cell thermal domino effect means it is simply a matter of time before there are more incidents of this nature," he adds.
"Moreover, when thermal runaway occurs with a big cell, a proportionately larger amount of energy is released and it is very difficult to prevent that energy from then heating up the neighboring cells and causing a domino effect that results in the entire pack catching fire," says Musk.
"They [Boeing] believe they have this under control, although I think there is aUnfortunately, the pack architecture supplied to Boeing is inherently unsafe," writes Musk in an email to Flightglobal.
"The fundamental safety issue with the architecture of a pack with large cells," writes Musk in an email. "It is much harder to maintain an even temperature in a large cell, as the distance from the center of the cell to the edge is much greater, which increases the risk of thermal runaway."
I'm pretty sure that the cell form factor was a very deliberate cost/benefit decision.
The 18650 cell is the most-produced Li-Ion battery cell in the world, and Tesla has been enjoying the benefits of low prices on these cells as Roadster/Model S/Model X battery pack development has coincided with the switch to laptop form factors where this battery cell doesn't fit. In fact, Panasonic has re-opened production lines for these cells that were shuttered due to falling demand, due to purchase agreements with Tesla. Tesla's incredible demand for batteries means that cell choice is a very important decision. It's worth noting that Tesla's engineering leadership has indicated they are both form-factor and chemistry agnostic, and will always go with the option that makes most economic sense.
In a video Musk addressed this. The 18650 batteries are produced on such a large scale that they provide the cheapest source of prefabricated batteries on the market. I would guess that this would change once the Tesla battery production facility opens, and they can tool for a battery that better fits their needs.
I wonder as well if the honeycomb type structure formed is inherently less prone to structural deformation.
The ability to fuse the cells individually is a huge benefit. That means that if a cell shorts out internally only that little bit of capacity will disappear.
Hopefully someone knowledgeable can elucidate properly but I believe cylinders are an inherently good shape for certain types of battery. The internal chemical process relies on two surfaces being in contact. You can roll up two surfaces and put them inside a cylinder which gives you lots of contact surface area in a small volume.
And a space in between to cool them. Charging and discharging a battery goes through Ri (the internal resistance of the battery) and as a result they get warm or even hot if not adequately cooled.
