[Arct1k]

FYI:

Lithium-Ion batteries are constructed with coated anode and cathode foils separated by thin layers of polymer material.

It looks like a jelly roll. You get a high surface area with thin layers. The thinner they go with the separators, the more room there is for the active material.

The coated layers are wound up on machines to create the individual Li-ion cell, and it’s at that stage that contaminants, such as metallic particles, can get embedded in the battery cell. The metallic particles such as happened to Sony may have been cast off by those commercial machines.

Generally, the polymer separator is very thin — less than 25 micron (one millionth of a meter) thick. If that is punctured by an electrically conductive material, like a metal particle, the battery cell’s anode and cathode short circuit.

There is nothing you can do to control this. In contrast, manufacturers have a variety of measures to guard the battery contents from external threats, like ambient heat.

Sony strengthened and reinforced the protective barriers and lining of their battery cells to address the danger of metal particles piercing the lining of the cell.