New battery electrolyte can improve the performance of electric vehicles

The new lithium-based electrolyte, invented by scientists at Stanford University, could pave the way for next-generation battery-powered electric vehicles.

In a study published in Nature Energy on June 22, Stanford researchers show how new electrolyte designs improve the performance of lithium metal batteries, a promising technology to power electric vehicles, laptops and other devices.

“Most electric cars work with lithium-ion batteries that are rapidly approaching their theoretical limits on energy density,” said Yi Cui, professor of material science and engineering and photon science at the SLAC National Accelerator Laboratory. Our work has focused on lithium metal batteries, which are lighter than lithium-ion batteries and can potentially provide more energy per unit weight and volume. ” said.

Lithium-ion and lithium metal

Lithium ion batteries 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 move back and forth between the anode and cathode when the battery is used and charged.

A lithium metal battery can hold about twice as much electricity per kilogram than today’s traditional 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 major concern,” says study co-author Zhenan Bao, K.K. Lee Professor of Engineering Faculty. “However, during operation, lithium metal anode reacts with liquid electrolyte. This causes the lithium microstructures called dendrites to grow on the surface of the anode, which can cause the battery to burn and fail. ”

Researchers have spent decades trying to solve the dendrite problem.
Chemistry graduate student lead author Zhiao Yu. “In our work, we use organic chemistry to design and manufacture new, stable electrolytes for these batteries.” said.

New electrolyte

For the study, Yu and his colleagues investigated whether they could address stability issues with a common liquid electrolyte commercially available.

“We assumed that adding fluorine atoms to the electrolyte molecule would make the liquid more stable,” Yu said. “Fluorine is an element commonly used in electrolytes for lithium batteries. We used the ability to attract electrons to create a new molecule that makes the lithium metal anode work well in the electrolyte. ”

The result is abbreviated as FDMB, a new synthetic compound that can be easily produced in bulk.

“Electrolyte designs are very exotic,” Bao said. “Some made a good promise, but their production is very expensive. The production of the FDMB molecule from Zhiao is huge and quite cheap. “

‘Incredible performance’

The Stanford team tested the new electrolyte with a lithium metal battery.
The results were stunning. The experimental battery retained 90 percent of its initial charge after 420 charge and discharge cycles. In laboratories, typical lithium metal batteries stop working after about 30 cycles.

The researchers also measured how efficiently lithium ions are transferred between the anode and the cathode during charging and discharging, a feature known as “chlombic efficiency”.

“If you charge 1,000 lithium ions, how much you get back after charging?” Cui said. “Ideally, 1,000 out of 1,000 would be required for a 100 percent coulombic efficiency. To present the product commercially, a battery cell needs at least 99.9 percent chulombic efficiency. In our study, 99.52 percent in half cells and 99.98 percent in whole cells; an incredible performance. ”

Anode-free battery

For potential use in consumer electronics, the Stanford team also tested the FDMB electrolyte in anode-free lithium metal battery cells.

“The idea is to use lithium only on the cathode side to reduce weight,” says Hans Hansen, co-author of the MSc. “The anode-free battery ran 100 cycles before its capacity dropped to 80 percent – one of the best-performing anode-free cells, although not as good as an equivalent lithium-ion battery from 500 to 1,000 cycles.”

“These results are promising for a wide variety of devices,” said Bao. Lightweight, anode-free batteries will be an attractive feature for drones and many other consumer electronics. ” said.

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