A new lithium-based electrolyte invented by Stanford University scientists could pave the way for the next generation of battery-powered electric vehicles (EVs). Their electrolyte design boosts the performance of lithium metal batteries, a promising technology for powering not only EVs but also laptops and other devices.
“Most electric cars run on lithium-ion batteries, which are rapidly approaching their theoretical limit on energy density,” said Yi Cui, professor of materials science and engineering and of photon science at the SLAC National Accelerator Laboratory. “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.”
Lithium-ion batteries, which are used in everything from smartphones to electric cars, have two electrodes – a positively charged cathode containing lithium and a negatively charged anode usually made of graphite. An electrolyte solution allows lithium ions to shuttle back and forth between the anode and the cathode when the battery is used and when it recharges.
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 Zhenan Bao, the K.K. Lee Professor in the School of Engineering. “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.”
Researchers have spent decades trying to address the dendrite problem. “The electrolyte has been the Achilles’ heel of lithium metal batteries,” said 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. When the team tested the new electrolyte in a lithium metal battery, the results were dramatic. 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.