Note 7 design "unsalvageable" due to battery compression: QC engineering firm
Analysis by Engineering QC firm Instrumental found that the Samsung Note 7's design could actually put pressure on the battery during normal use, according to a release on their website. The engineers found that the clearance for the battery was simply not sufficient enough to account for natural swelling of the battery. According to Instrumental, Lithium-Ion batteries will swell during use, especially as the battery ages. The typical clearance given is usually +10%, but their 2-month-old unit had literally no free space at all, resulting in pressure on the battery.
Pressure on Li-ion batteries is an issue because of their layered structure. The layers are alternatively positive and negative, with a buffer of electrolytes that allow energy to pass though without letting the positive and negative layers touch. If they do touch, it will likely lead to a runaway reaction—i.e. an explosion.
Instrumental believes that the Note 7 couldn't be made safe without reducing the battery to a size that would give it worse battery life than its competitors, thus Samsung pulled the product. Still, it is important to note that this is the analysis of a 3rd party firm and the results of Samsung's official investigation have not yet been released.