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| Fig. 1: Lithium-ion battery example. (Source: Wikimedia Commons) |
Lithium batteries (Fig. 1) have crept into our lives without us noticing. Recently, however, they have been announcing their presence in a rather explosive manner. Reports about the batteries overheating surfaced as early as 2006, when Sony had to replace millions of their lithium-ion battery packs used in computers. Issues surfaced in more serious sectors such as Tesla's automobiles and Boeing's aircraft. [1] Yet the most famous incident happens to be the most recent one: Samsung's complete recall of their Galaxy Note 7 due to several of them exploding. This report seeks to investigate the factors resulting in lithium-ion battery explosions.
To understand what causes the failure of the lithium ion batteries, it is necessary to first understand what makes it function. A typical battery consists of a series of cells that produces electricity. Each cell contains three essential parts: the anode, the cathode and the electrolyte. A conductor allows electrons to flow from the anode to the cathode while the electrolyte allows positive ions to flow in the opposite direction, generating a current. The battery is depleted once there are no more ions left to flow, and is recharged by using electricity to move the ions back against their natural gradient. In a lithium-ion battery, the positive ions are lithium ions which is easily made from lithium. The small size of lithium ions allows the positive charge to be carried in a smaller amount of space, making lithium-ion batteries more than twice as effective as regular alkaline batteries. Their energy density is what makes them so widely used today. [2]
Yet the very thing that gives lithium-ion batteries an advantage is also what causes them to be volatile. The lithium electrolyte is extremely flammable and can overheat in circumstances such as a short circuit, where a breach in cell compartments allow the current to flow unimpeded through an unintended path. If the resulting temperature increase happens fast enough, the resulting increase in pressure may be sufficient to cause an explosion. [3,4] Short circuits have been the main culprit behind lithium-ion battery explosions since the early days of cell phone explosions in 2004. However, these were mostly associated with defective or counterfeit batteries since authorized batteries include vents and temperature-activated shutoff switches as fail-safes. Furthermore, cases where authorized batteries failed usually involved drop damage or sub-optimal conditions such as heat vent blockage. [3]
Samsung's case is different in that the fault seems to be intrinsic within their design. As many as 112 phones have caught fire after only a month on sale. While Samsung has yet to provide an official explanation at the time this report was written, several possible causes have been proposed. An unpublished preliminary report to Korea's Agency for Technology and Standards suggests Samsung's sub-optimized assembly process caused variations in tensions that placed pressure upon the battery when contributing factors occurred simultaneously. Since the system was already tightly packed to maximize battery capacity, the additional pressure may have resulted in the short circuit situation described above. Of course, the over-packing scenario may also have been caused by an overzealous attempt at weight reduction at the expense of insulation integrity. Alternatively, the pressure may have deformed the separator between anode and cathode, leading to an increase in resistance and thus additional heat being generated. However, cell phones nowadays are designed to charge faster (and get hotter) when first plugged in, then trickle charging the last few percent to full charge. This implies that any overheating should occur in the beginning of the charging cycle whereas actual reports indicate fires and explosions occurring after the phone has been left charging for a while. One theory in line with this observation is that only part of the battery is under pressure so that the phone could not recognize when it was fully charged, allowing trickle charging to occur indefinitely. This causes lithium ions to plate on the surface, and under the right conditions, could form spikes that penetrate the separator between compartments, causing the short circuit. [4] Of course, these are merely the most plausible explanations in the absence of an official statement from Samsung.
The lithium-ion battery explosions may have caused some setbacks, but will ultimately pave the way for further innovation. Companies are driven to improve battery safety while also increasing capacity, and decreasing size and charge time. We may even move away from the current paradigm altogether. One possible next generation battery is the solid-state battery, where the liquid electrolyte is replaced with a solid substance, which significantly improves safety. [2] Trying times are often accompanied by a boom in innovation. So keep an open mind as Samsung and other companies around the world fight to overcome this challenge, and maybe they will end up surprising us all.
© Wayne Sheu. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] P. Cohan, "Boeing, Dell Technologies, Samsung, Tesla And Burning Lithium Ion Batteries," Forbes, 12 Sep 16.
[2] S. Gibbs, "Charged Issue: How Phone Batteries Work - and Why Some Explode," The Guardian, 10 Oct 16.
[3] B. Charny, "Cell Phones: Too Hot to Handle?" CNet News, 25 Oct 04.
[4] S. Hollister, "Here's Why Samsung Note 7 Phones Are Catching Fire," CNet News, 10 Oct 16.
