In a study published in Nature Energy, Stanford researchers demonstrated how their novel electrolyte design boosts the performance of lithium metal batteries.
“Most electric cars run on lithium-ion batteries, which are rapidly approaching their theoretical limit on energy density,” said study co-author Professor Yi Cui. “Our study focused on lithium metal batteries, which are lighter than lithium-ion batteries and can potentially deliver more energy per unit weight and volume.”
A lithium metal battery can hold about twice as much electricity per kilogram as today’s conventional lithium-ion battery. Lithium metal batteries do this by replacing the graphite anode with lithium metal, which can store significantly more energy.
“Lithium metal batteries are very promising for electric vehicles, where weight and volume are a big concern,” said co-author Zhenan Bao. “But during operation, the lithium metal anode reacts with the liquid electrolyte. This causes the growth of lithium microstructures called dendrites on the surface of the anode, which can cause the battery to catch fire and fail.”
“The electrolyte has been the Achilles’ heel of lithium metal batteries,” said co-lead author Zhiao Yu, a graduate student in chemistry. “In our study, we use organic chemistry to rationally design and create new, stable electrolytes for these batteries.”
Yu and his colleagues explored whether they could address the stability issues with a common, commercially available liquid electrolyte.
“We hypothesized that adding fluorine atoms onto the electrolyte molecule would make the liquid more stable,” Yu said. “Fluorine is a widely used element in electrolytes for lithium batteries. We used its ability to attract electrons to create a new molecule that allows the lithium metal anode to function well in the electrolyte.”
The result was a novel synthetic compound, abbreviated FDMB, that can be readily produced in bulk.
“Electrolyte designs are getting very exotic,” Bao said. “Some have shown good promise but are very expensive to produce. The FDMB molecule that Zhiao came up with is easy to make in large quantity, and quite cheap.”
The experimental battery retained 90 percent of its initial charge after 420 cycles of charging and discharging. In laboratories, typical lithium metal batteries stop working after about 30 cycles.
“The anode-free battery in our lab achieved about 325 watt-hours per kilogram specific energy, a respectable number,” Cui said. “Our next step could be to work collaboratively with other researchers in Battery500 to build cells that approach the consortium’s goal of 500 watt-hours per kilogram.”
In addition to longer cycle life and better stability, the FDMB electrolyte is also far less flammable than conventional electrolytes, as the researchers demonstrated in this video:
Source: Stanford University