As promising as lithium-ion batteries have been, they haven’t always had the safest track record. In 2016, there were quite a few reports of batteries igniting in devices such as hoverboards (followed by a large number of hoverboard recalls) and the ever-popular Samsung Galaxy S7 and S7 Note. There are even reports that some phones exploded.
Why Li-Ion Batteries Fail
It's worth highlighting here that the Samsung Galaxy Note 7 caught fire for several reasons, none of which are inherent to Li-ion batteries in-and-of themselves. One conclusion, reached independently by Instrumental, was that the battery was placed in too small of a space to allow for regular expansion. But Samsung's investigation into the matter suggests that issues with the separator and damage to the electrodes within the batteries were at fault. For the full, Samsung-official list of problems that caused these incidents (as there were several), see this infographic.
The bottom line, however, is that lithium-ion batteries fail mainly due to manufacturing defects—but the underlying issue is that these batteries, while efficient, are packing a large energy density into a very small space. Failure is caused by an effect known as thermal runaway, where the separator between the cathode and anode is breached, causing the chemicals contained within the battery to heat up and ignite the flammable electrolytes inside. While this issue has been relatively overlooked by consumers, it becomes especially relevant as we require more and more energy from batteries in compact areas.
There has been a myriad of proposed solutions to the problem, though most efforts involve re-engineering the devices, themselves, to prevent shorts in their circuitry or attempting to fireproof batteries without losing efficiency. Unfortunately, there hasn’t been a satisfactory solution to these problems in regards to device performance, though new efforts have potentially resolved the current issues.
Image courtesy of Kārlis Dambrāns. [CC BY 2.0]
Building Flame Retardants Right into the Battery
Recently, researchers at Stanford University have come up way to deal with the problem by creating a new separator made with flame retardant materials. The battery design encapsulates a common flame retardant named triphenyl phosphate via electrospinning in a polymer fiber sheath that releases when the battery hits 160 degrees Celsius, the melting point of the plastic.
The idea, itself, isn’t necessarily original as researchers and scientists have been working on using flame retardant materials inside lithium-ion batteries for quite some time.
A basic representation of the battery designed by the Stanford team. Credit: Liu et al. Sci Adv. 2017 Jan;3(1):e1601978.
The uniqueness of the design is that the flame retardant is not actually contacting any of the chemicals inside the battery, unlike prior research efforts—that is, until it reaches 160 degrees Celsuis.
When the battery is operating under ordinary conditions, the retardant remains sealed within the polymer sheath, preventing it from degrading the batteries performance. This is the key component of the design and, while it doesn't prevent the battery from igniting, the new separator does extinguish flames within .4 seconds.
This gif shows the triphenyl phosphate inside the polymer shell extinguishing the flammable electrolyte in a period of .4 seconds. Credit: Liu et al. Sci. Adv. 2017;3:e1601978
The design has been tested in a coin cell battery with a graphite anode and lithium counter and reference electrode. Results concluded that the TPP was capable of extinguishing the flame successfully. Along with this conclusion, it was speculated that the new separator could be used in other high-energy storage devices. Further testing is needed to determine whether or not the separator will remain effective when faults such as short circuiting, piercing, and compression in larger cells occur.
The findings from the Stanford team's research could prove proprietary in the field of battery development. Thus far, it appears as if the separator does not affect the battery’s long-term efficiency, giving it promise for use in lithium-powered devices pending further testing.
The original article published in Journal Science advances can be found here.