"What we found was surprising: the design can compress the battery even during normal operation."Why does this matter? The Note 7’s lithium-polymer battery is a flattened “jelly-roll” consisting of a positive layer made of lithium cobalt oxide, a negative layer made of graphite, and two electrolyte-soaked separator layers made of polymer. The separator layers allow ions (and energy) to flow between the positive and negative layers, without allowing those layers to touch. If the positive and negative layers ever do touch, the energy flowing goes directly into the electrolyte, heating it. This causes more energy to flow and more heat, typically resulting in an explosion. Compressing the battery puts pressure on those critical polymer separator layers that keep the battery safe. Samsung stated that these separator layers may have been thin to start with due to aggressive manufacturing parameters. Add some pressure, due to normal mechanical swell from the battery or accumulated stress through the back cover (e.g.: from being sat on in a back pocket), and that pressure could be enough to squeeze the thin polymer separator to a point where the positive and negative layers can touch, causing the battery to explode.
"Looking at the design, Samsung engineers were clearly trying to balance the risk of a super-aggressive manufacturing process to maximize capacity, while attempting to protect it internally."The report concludes by saying that there was no competitive salvageable design. Which means that Samsung would either have had to put a smaller battery in the device, and therefore have a high-end device without a battery suitable to get it through a day's worth of use, or find another way to compromise the phone.