At the moment, imagining a modern society without lithium-ion batteries seems pretty much impossible. These unique inventions are used in pretty much everything from phones, tablets, laptops, digital cameras, and flashlights to portable game consoles, power tools, electronic cigarettes, and electric cars. Given their omnipresence and the fact that we rarely buy them individually, we can sometimes take them for granted. Well, at least until they start exploding. That's pretty much what happened with the recently launched and promptly discontinued Samsung Galaxy Note 7. The phone which many were touting as the potentially best 2016 Android flagship turned out to be a fire hazard despite the fact that Samsung spent enormous amounts of money on research and development prior to its launch, and then spent comparably huge sums on trying to figure out what went wrong with the first batch of its devices. Unfortunately for the South Korean tech giant, Galaxy Note 7 just wasn't meant to be. The first worldwide recall of the phablet didn't amount to much and replacement units proved to be just as dangerous as their predecessors.
So, what went wrong with the Galaxy Note 7? To answer that question, we must also answer a much more general one - why do lithium-ion batteries explode? Going down that rabbit hole, let's first see how lithium-ion batteries work. Simply speaking, these devices operate by moving lithium ions through a membrane. To be more precise, they move lithium ions from a negative electrode called an anode to a positive electrode called a cathode. In this way, the lithium-ion batteries discharge and deliver power to a connected device while the process is reversed for charging. Between these two electrodes is a vast sea of electrolytes. Electrolytes are electrically conducting solutions which facilitate movement of lithium ions described above. However, they don't flow completely freely as they have to pass through a membrane which is more commonly called a separator. Separator's primary function is keeping the two battery electrodes from touching. Why can't they be allowed to touch? Well, because that scenario results in lithium ions being redirected back towards electrolytes. And you've surely already heard how a situation in which ionic charge carriers aren't reaching their intended destinations is called. Yes, you do, it's called a short circuit. Granted, a physical contact between a cathode and an anode rarely leads to explosions directly. However, it does lead to fires and fires don't tend to mix very well with sensitive battery chemicals. In other words, batteries explode when lit on fire due to their very contents.
A modern lithium-ion battery is currently almost as capable as it can possibly be. Cornell University material scientist Lynden Archer claims that the industry has already achieved close to 90% of theoretical maximum battery life as far as the existing lithium-ion technology is concerned. That isn't to say further advancements are impossible, just that we probably need better logic circuitry which would solve issues like lithium ion congestion. Until that happens, manufacturers are pretty limited in terms of options they have for significantly improving lithium-ion battery life and charging time. Well, at least if they want to make batteries that aren't fire hazards which they obviously do.
And that brings us back to the Galaxy Note 7. According to most industry experts and several statements from Samsung itself, the company's latest flagship had a tendency to catch fire in certain scenarios due to the fact that its battery was pushed beyond its safe limits in an effort to squeeze more performance out of it. In this particular case, the number one reason for Galaxy Note 7 fires was the aforementioned contact between the cathode and the anode. However, there are several more ways lithium-ion batteries can become unstable and turn into fire hazards. One of them is known as overcharging and happens when a cathode ends up sending too much lithium into an anode. Pretty much all modern smartphones have mechanisms which prevent overcharging but a circuitry defect can obviously cause those mechanisms to fail.
Another issue which can lead to unstable batteries is when a charging process is simply sending lithium ions between electrodes too fast. This usually happens when batteries are charged with incompatible chargers which can easily short them out. The problem occurs when lithium ions are traveling between electrodes at such a high speed that they're unable to be situated correctly and end up forming piles called dendrites which tend to short out a battery after multiple such charging cycles. Last but not least, batteries can also turn into fire hazards when their voltage is increased beyond safe limits. Voltage is an electromotive force, i.e. it measures the number of charge carriers—lithium ions, in this case—which pass between two electrodes in a given unit of time. In other words, lithium-ion batteries can not only short out if they try to send ions between electrodes too fast but also if they try to send too much of them at once regardless of their actual speed of movement. This leads to a scenario that's similar to the one involving dendrites mentioned above.