A lithium metal battery, most commonly found in phones and laptops, comprises of a metallic lithium anode and cathode separated by a “separator.” The separator is usually an isolator with no electrical conductivity, it is moistened with an electrolyte to promote free movement of electrons from anode to cathode. Even though the separator is non-conductive, a small amount of electric current passes through the separator, called self-discharge. Self-discharge is present in all types of batteries in varying degrees and it determines the longevity of the battery during prolonged storage.
Lithium metal batteries are susceptible to failures due to the formation of internal short circuits, causing rapid heat build-up and fire. During repeated charging of a lithium metal battery, a needle-like structure called dendrites grow on the anode. These dendrites are tiny and rigid structures with needle-like projections called whiskers. These grow over time and can pierce through the separator layer of the battery creating an internal short circuit. This causes a sudden discharge of electrons from anode to cathode, overheating the battery and even causing a fire.
To overcome this problem, researchers led by NanoEngineering professor Ping Liu and his Ph.D. student Matthew Gonzalez at the University of California San Diego (UC San Diego) developed a safety feature by tweaking the separator to prevent the rapid discharge and overheating of the battery. The separator developed by the UC San Diego team is covered by a thin, partially conductive web of carbon nanotubes to intercept the dendrites. The addition of this thin layer doesn’t prevent the dendrites from piercing the separator, but since the web is partially conductive, it provides a pathway for the electrons to slowly drain instead of discharging all of a sudden to the cathode. The researchers’ work is detailed in a paper published in Advanced Materials.
According to Matthew Gonzalez, who is also the first author of the study, they are not preventing the battery failure from happening but making it much safer so that when it fails it doesn’t catastrophically catch fire or explode. Gonzalez compared the new separator to the dam’s spillways. Spillways in dams are used for the controlled release of water from a dam. They ensure water doesn’t overflow and destroy the dam. The separator, they developed works similar to the spillway by draining the charges much slower to prevent the flood of electron discharge to the cathode. When the growing dendrites reach their web of carbon nanotubes, the battery begins to self-discharge. Eventually, when the battery does short there is less energy left to cause a catastrophic failure.
In laboratory tests, lithium metal batteries equipped with their new separator showed gradual signs of failure over 20 to 30 cycles. On the other hand, regular lithium metal batteries without the web of carbon nanotubes experienced abrupt failure in a single cycle.
According to Gonzalez, many types of research focuses on building separators out of strong materials to block the dendrites from piercing them. These materials still need to have pores to let ions pass through, so eventually, when the dendrites make it through the separator it will cause an even worse short circuit. Hence the team decided to mitigate the effect than prevent it from happening. The real use case for their separator would be to provide an early warning about the battery’s degradation.
According to the researchers, the separator can also work with lithium-ion as well as other battery chemistries. The team would focus on optimizing their separator for commercial use.
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