Taking Advantage of a Highly “Charged” Situation

Tiny lithium ions enable the chemical reactions within lithium batteries, continuously traversing the cell from one electrode to the other in order to sustain a steady supply of current.  

Water is a desirable medium for these lithium ions to flow through because it is inexpensive, nontoxic, and nonflammable (compared to organic liquid solvents that are conventionally used in batteries), but it is also not very electrochemically stable. This typically limits choice of electrodes to those yielding low voltages.

E Ink Professor of Engineering Matt Panzer, Assistant Professor Graham Leverick, and Tarkan Ayata, E26, recently contributed to a study that investigated how adding net neutral, naturally-occurring molecules called “zwitterions” to water-based lithium salt electrolytes–a class of ionically conducting liquid materials that could be used in lithium batteries–could help address this challenge. Tarkan, a co-first author of the study, also completed a Senior Honors Thesis under Panzer’s guidance on this work. 

Their team found that—although adding zwitterions increases the viscosity, or “thickness,” of the liquid mixture, slowing overall ion motion down—the addition of zwitterions did not reduce the speed of lithium ions as much as might have been anticipated based on the larger mixture viscosity. Their work was published in the Journal of the American Chemical Society. 

“We’re learning that we can use zwitterions to manipulate how lithium ions move in viscous, highly ion-dense salt-water mixtures that are more electrochemically stable than dilute solutions,” explained Panzer. “This could help us understand how to make water a more practical electrolyte option for use in high voltage lithium batteries.” 

Water-in-salt electrolytes: the pros and cons

Because of water’s low electrochemical stability, researchers have been exploring the potential of water-in-salt electrolytes: mixtures with a very large concentration of salt that reduces the number of bulk-like, unstable water molecules that can be easily split with too large a voltage. But the tradeoff is that the electrolyte—now highly concentrated with salt—is more viscous, and ions can get stuck to each other. This limits the ability of lithium ions to move and conduct sufficient current. 

“We want batteries to charge in minutes, not days,” Panzer said. “The faster lithium ions can move, the better.” 

Panzer and his team tested the effect of three different zwitterions, anticipating that adding another charge-dense “player” into the mix could disrupt the sticking behavior of the ions without compromising what makes water-in-salt electrolytes more electrochemically stable in the first place.  

The team measured several physiochemical properties of the water-in-salt solution after adding zwitterions. As expected, they found that the mixture was more viscous and ion movement slowed down overall with zwitterions in the mix. This lowered the overall conductivity, as slower ions were unable to carry a current as quickly as before.  

“But conductivity isn’t the whole story, here,” Panzer explained. “Ions moved more slowly overall, but they actually moved unusually rapidly relative to how viscous the zwitterion-containing mixture was.” 

To better understand the possible reasons for their unexpected experimental observations, the team turned to molecular dynamics simulations. Led by collaborators at the University of Texas at Austin, the researchers modeled water molecules, lithium salt ions, and zwitterions in proportions matching the actual experiments. The simulation results revealed that zwitterions can act to reorganize the local anion environment inside the mixtures, weakening ion pairing without excessively slowing down lithium ion motion.

Towards future devices for more efficient energy storage

The team’s findings demonstrate a potential way to reprogram how ions move and are coordinated in water-in-salt electrolytes, which could change the transport behavior of lithium ions to allow them to better sustain higher currents in viscous media.  

This work contributes to Panzer’s continued efforts to study how zwitterions can enhance new electrolyte materials, a theme which his lab—The Green Energy and Novel Electrolytes Lab—has been researching for the past 10 years. His team’s research focuses on how novel electrolytes can be used to advance safer energy storage using electrochemical devices, including batteries and supercapacitors.

Leverick's lab studies questions closely tied to this work, seeking to understand how the arrangement of molecules and the interactions between them affect how ions move in both liquid and solid electrolytes. By building fundamental understanding at the molecular level, his group aims to develop the electrolytes that will enable next-generation electrochemical energy conversion and storage devices.

Learn more about Panzer’s research and Leverick’s research.