Stanford’s radical take on lithium metal battery shows promise for EV range

New research from Stanford university has demonstrated a promising new take on the lithium metal battery that could increase electric vehicle driving range.

Current electric vehicles, laptops and other rechargeable devices are powered by lithium-ion batteries that commonly use a graphite anode, a heavy and expensive but – until now – a necessary battery component.

Lithium metal batteries offer a lighter option as they replace the graphite with lithium metal which is able to store more energy, which for electric vehicles has the benefit of dramatically increasing driving range or reducing battery weight as less cells are required for the same range.

While lithium metal batteries can store about double the energy of standard lithium-ion batteries, they have inherent stability issues that mean they are unable to hold a charge after just 30 cycles, meaning they are currently only useful as non-rechargeable batteries.

The new research is significant because it uses a new electrolyte materials that allows increased stability and therefore longer cycling, which if made commercially available could mean lithium metal could be used as rechargeable batteries.

“Most electric cars run on lithium-ion batteries, which are rapidly approaching their theoretical limit on energy density,” said study co-author Yi Cui, professor of materials science and engineering and of photon science at the SLAC National Accelerator Laboratory in a statement.

“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.”

In a new study published in Nature Energy on Monday, Stanford University researchers describe a novel electrolyte design that radically increases the stability of the lithium metal material potentially making it a serious contender for powering electric vehicles and other devices.

“Lithium metal batteries are very promising for electric vehicles, where weight and volume are a big concern,” said study co-author Zhenan Bao, the K.K. Lee Professor in the School of Engineering in a statement.

“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.”

To solve this decades-old dendrite problem, the researchers hypothesised adding flourine atoms to the electrolyte molecule to increase its stability when used in the lithium metal battery.

“The electrolyte has been the Achilles’ heel of lithium metal batteries,” said co-lead author Zhiao Yu, a graduate student in chemistry.

“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,” said Yu.

Infused with flourine, the new liquid electrolyte molecule dubbed FDMB was tested in a lithium metal battery in both half cell and full cell conditions.

The results were dramatic: not only did the battery cycle 420 times in the lab environment instead of just 30 times, its “coulombic efficiency” – how much energy you get in return for energy stored – was extremely high. It is also far less flammable than traditional liquid electrolytes, say the researchers.

Flammability test of conventional carbonate electrolyte (left) and the novel FDMB electrolyte (right) developed at Stanford. The conventional carbonate electrolyte is flammable immediately after touching the flame, but the FDMB electrolyte can tolerate the direct flame for at least three seconds.

“If you charge 1,000 lithium ions, how many do you get back after you discharge?” Cui says.

“Ideally you want 1,000 out of 1,000 for a coulombic efficiency of 100 percent. To be commercially viable, a battery cell needs a coulombic efficiency of at least 99.9 percent. In our study we got 99.52 per cent in the half cells and 99.98 per cent in the full cells; an incredible performance.”

To put this figure in perspective, Tesla battery research Jeff Dahn said in a recent interview that lithium-ion batteries in current use (for example in a phone) are already at 99.8 per cent and last about 4-5 years, but that for an EV battery to last 20 years the coulombic efficiency must be more like 99.95 per cent.

“Coloumbic efficiency is a measure that tells you how perfect the lithium-ion battery is,” Dahn says. “You’d like the coulombic efficiency to be exactly 1 [100 per cent].”

“If you want a battery in an EV to last 20 years … the columbic efficiency has to be more like 0.9995 [99.95 per cent] in test conditions.”

The Stanford team have also experimented with using the FDMB electrolyte with anode-free lithium metal pouch cells – a commercially available battery that does away with the anode altogether, reducing how much lithium is needed and hence reducing weight.

While the results of this experiment still saw a drop in capacity after 100 cycles, the researchers say it is still one of the best demonstrations of anode-free technology to date.

“The idea is to only use lithium on the cathode side to reduce weight,” said co-lead author Hansen Wang, a graduate student in materials science and engineering.

“The anode-free battery ran 100 cycles before its capacity dropped to 80 percent – not as good as an equivalent lithium-ion battery, which can go for 500 to 1,000 cycles, but still one of the best performing anode-free cells.”

“These results show promise for a wide range of devices,” Bao says. “Lightweight, anode-free batteries will be an attractive feature for drones and many other consumer electronics.”

The researchers say they hope their demonstrations could lead to working with the Battery500 consortium, a US government-backed body that is seeking to achieve 500 watt-hours per kilogram energy density in batteries.

To put this in perspective, current Tesla batteries (as noted in this Tesla “lithium doping” battery patent explainer) have a 247 watt-hours per kilogram capacity.

According to the researchers, the anode-free battery using FDMB electrolyte has already achieved 325 watt-hours per kilogram.

“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,” says Cui.

“Our study basically provides a design principle that people can apply to come up with better electrolytes,” Bao added. “We just showed one example, but there are many other possibilities.”

Citation:
Molecular design for electrolyte solvents enabling energy-dense and long-cycling lithium metal batteries.
Yu, Z., Wang, H., Kong, X. et al.
Nature Energy (2020)
https://doi.org/10.1038/s41560-020-0634-5