Rice University scientists use red phosphorus to detect lithium dendrites in Li metal batteries

Rice University scientists have developed a detection system capable of alerting for Li dendrite formation in a two‐electrode battery with a Li metal anode with no additional electrodes required. A paper on the work is published in the journal Advanced Materials.

The Rice lab of chemist James Tour made test cells with a coat of red phosphorus on the separator that keeps the anode and cathode electrodes apart. The phosphorus acts as a spy for management systems used to charge and monitor batteries by detecting the formation of dendrites.

Li metal batteries are considered the next-generation technology because of their high energy density and power density. But commercially, Li metal has only been employed in nonrechargeable primary batteries due to the safety issues associated with dendrite growth upon charging that can penetrate separators and lead to runaways, fires, and explosions. Dendrite growth also occurs in traditional Li-ion batteries upon overcharge, charging after overdischarge, and charging at high rates. There have been very few studies on Li dendrite detection, and the only known method is to use a Cu film in the separator connected to a third electrode to detect short circuits. The added manufacturing complexity of three-electrode batteries retards such a consideration for large-scale use.

… if a detection system could be deployed in combination with so-called dendrite-free Li metal anodes where the chance of growing dendrites is already low, the large-scale production of safe, rechargeable Li metal batteries can become a reality. We describe here the use of a red phosphorus (RP)-coated separator for Li dendrite detection in an ordinary two-electrode battery system. No additional electrode is needed, and the presence of Li dendrites can be detected simply based on the voltage profile.

RP was selected because a) it is non-conductive, which is essential for making or modifying separators since it will not increase the risk of forming internal short circuits; b) it is chemically inert at room temperature, which makes it easily processed and compatible with various electrolytes; c) it reacts with Li metal; d) it is inexpensive and widely available. We tried other insulating or semiconducting materials such as Si, SiO₂, and S, but they did not produce reliable dendrite-detection signals with a mass loading of ≤4 mg cm^–2.—Wang et al.

Lithium metal anodes charge much faster and hold about 10 times more energy by volume than common lithium-ion anodes; however, charging lithium-infused anodes forms dendrites that, if they reach the cathode, cause a short circuit and possibly a fire or explosion. When a dendrite reaches a red phosphorus-coated separator, the battery’s charging voltage changes. That tells the battery management system to stop charging.

Manufacturing batteries with a third electrode is very hard. We propose a static layer that gives a spike in the voltage while the battery is charging. That spike is a signal to shut it down.—James Tour

The red phosphorus layer had no significant effect on normal performance in experiments on test batteries by the Tour lab.

The researchers built a transparent test cell with an electrolyte known to accelerate aging of the cathode and encourage dendrite growth. That let them monitor the voltage while they watched dendrites grow.

With an ordinary separator, they saw the dendrites contact and penetrate the separator with no change in voltage, a situation that would lead a normal battery to fail. But with the red phosphorus layer, they observed a sharp drop in voltage when the dendrites contacted the separator.

Images of a half-cell lithium metal battery show dendrites approaching a red phosphorus separator. The separator delivers a signal to the battery’s electronics to shut down when dendrites threaten to create a short circuit. (Credit: Tour Group/Rice University)

Last year, the lab introduced carbon nanotube films that appear to completely halt dendrite growth from lithium metal anodes.

By combining the two recent advances, the growth of lithium dendrites can be mitigated, and there is an internal insurance policy that the battery will shut down in the unlikely event that even a single dendrite will start to grow toward the cathode, Tour said.

Literally, when you make a new battery, you’re making over a billion of them. Might a couple of those fail? It only takes a few fires for people to get really antsy. Our work provides a further guarantee for battery safety. We’re proposing another layer of protection that should be simple to implement.—James Tour

Rice graduate student Tuo Wang is lead author of the paper and postdoctoral researcher Rodrigo Salvatierra is co-author. Tour is the T.T. and W.F. Chao Chair in Chemistry as well as a professor of computer science and of materials science and nanoengineering at Rice.

The Air Force Office of Scientific Research supported the research.