Stanford researchers create inexpensive electrolyte for lithium metal batteries

As lithium-ion batteries reach their limits for improvements, Stanford researchers looked to new electrolyte design to improve lithium metal battery performance with some success.

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

The researchers published a study in Nature Energy that used lithium metal batteries, which have the benefit of being lighter than lithium ion and may delivery more energy by volume and weight.

Two electrodes are in lithium-ion batteries: the anode, negatively charged and often made of graphite, and the cathode that is positively charged and contains lithium. Lithium ions can move between the electrodes in an electrolyte solution when the battery is recharging or in use, Stanford News reported

Lithium metal batteries use lithium metal instead of graphite, allowing it to store more energy. Reaction of the liquid electrolyte with the lithium metal anode creates the growth of dendrites on its surface, which may cause battery failure and fire.

“The electrolyte has been the Achilles’ heel of lithium metal batteries,” co-lead author Zhiao Yu, a graduate student in chemistry, told Stanford News. “In our study, we use organic chemistry to rationally design and create new, stable electrolytes for these batteries.”

The researches added fluorine to the electrolyte molecule to address this problem, which resulted in a synthetic compound that can be produced in bulk. This FDMB molecule is unlike other exotic molecules that are costly to produce.

Tests of FDMB in a lithium metal battery showed it greatly outperformed typical lithium metal batteries. After 420 charging and discharging cycles, it still held 90% of its initial charge, compared to typical batteries that stopped working after 30 cycles, Stanford News reported.

The efficiency of the lithium ions transfer also was measured, with a measure of 99.52% in half cells and 99.98% in full cells. A commercially viable batter needs this coulombic efficiency to reach at least 99.9%.

The researchers tested the new electrolyte in anode-free lithium metal pouch cells for potential consumer electronics use.

“The idea is to only use lithium on the cathode side to reduce weight,” co-lead author Hansen Wang, a graduate student in materials science and engineering, told Stanford News. “The anode-free battery ran 100 cycles before its capacity dropped to 80% – 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.”

This study was part of Battery500, a research consortium funded by the U.S. Department of Energy with the goal of making lithium metal batteries viable for the manufacture of lighter electric vehicles.

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