http://www.flightglobal.com/news/articles/elon-musk-boeing-7...
"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."
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.
I wonder as well if the honeycomb type structure formed is inherently less prone to structural deformation.