Lithium-Ion batteries are constructed with coated anode and cathode foils separated by thin layers of polymer material, said Dan Doughty, manager of the Advanced Power Sources Research and Development Department at Sandia National Laboratory.

"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," Doughty said.

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

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, Doughty said.

He said an internal short circuit was "the worst scenario in battery design, because there's nothing you can do to control it," he said. In contrast, manufacturers have a variety of measures to guard the battery contents from external threats, like ambient heat.