“The attraction of a lithium-ion battery is that lithium is the least dense metallic element, which means that weight-for-weight it can pack more power than other types of battery. But lithium is also a highly reactive substance; it belongs to the alkali metal group, which contains sodium and potassium, the high reactivity of which will be familiar from school chemistry classes.
“Like all batteries, lithium ones consist of two electrodes separated by an electrolyte. Typically for a lithium cell the electrolyte is a solution of lithium salts and organic solvents. When the battery is charged, lithium ions are driven from the electrolyte into a carbon anode. When the battery is discharged they flow back, creating a balancing flow of electrons in a circuit that powers the device.
“The trouble comes about if there is a small fault or damage is caused to the extremely thin separators that keep the elements of the battery apart. This can lead to an internal short-circuit and a subsequent build-up of heat. This can trigger what is known as a “thermal runaway” in which the battery overheats and can burst into flame. That can cause adjacent battery cells to overheat, which is why groups of cells in some battery packs (such as those used in Tesla cars) are kept in separate protective compartments.
“Lithium batteries can also be damaged by using them in hot environments, and by excessive discharging and charging—which is why most lithium batteries contain special circuits to prevent this. Catching fire if something goes wrong, then, is in their nature.”
http://www.economist.com/blogs/economist-explains/2014/01/economist-explains-19
Thank You Pelham! This was this jist of my earlier post #9