Quantum Calculations lead to Li Battery Prototype that Withstands 2000 Cycles

July 23, 2019 by Scott McMahan

Researchers from Florida State University and Cornell University found that batteries constructed out of inexpensive and safe components can deliver three to four times the punch of batteries made with state-of-the-art lithium-ion technology.

The researchers' work is published in Nature Communications.

A. Nijamudheen, a postdoctoral researcher at the FAMU-FSU College of Engineering, and Snehashis Choudhury, a doctoral student at Cornell University, along with faculty members at both institutions embarked on an investigation into what hampers current battery design and how to improve it.

"If one looks at the cost of batteries over time, it is unsurprising to see that the vector is consistently pointed upward," Choudhury said. However, "Broad-based adoption of technologies that require batteries demand lower costs," Choudhury said.

The researchers tackled a few specific problems related to electrolytes, with the hope of bringing those costs down. Electrolytes promote the movement of ions from one electrode to the other in batteries.

The teams set out to understand the chemical pathways by which electrolytes degraded at the battery electrodes. The researchers not only identified the mechanisms for how the electrolytes degrade, but they also found multiple strategies to remedy the problem.

"We discovered that controlling the ionic properties of the interphases formed at the negative electrode is the key," Nijamudheen said.

Using quantum calculations, Nijamudheen and his adviser, Jose Mendoza-Cortes FAMU-FSU Assistant Professor of Chemical Engineering for the Energy and Materials Initiative at FSU, discovered that the problem originates from the way a component of the electrolytes called diglyme goes through polymerization. Polymerization is a process where molecules chemically combine into long chain-like molecules called polymers

In batteries, after prolonged contact with both the negative and positive electrodes of a battery, electrolytes frequently break apart and re-form to make much larger molecules.

"While the degradation process itself is harmless, its byproducts block ions from accessing the battery electrodes, which over time reduces the amount of stored energy than can be recovered from a battery," said Lynden Archer, a Cornell University professor and Choudhury's adviser.

However, while some types of polymers that emerge from this process would block ions from reaching the electrodes, others were proven to prolong battery life.

With their polymerization calculations in hand, the researchers began examining other types of electrolytes where the polymerization process wouldn't impede the battery's performance.

Usually, lithium batteries are made with organic carbonate electrolytes, but these electrolytes are highly flammable. An expensive thermal regulation infrastructure that offers to cool overheated battery cells is therefore required to reduce the risk of thermal runaway and battery fires. Instead, the researchers tested a lithium-nitrate electrolyte, a stable electrolyte that wasn't flammable.

Using that electrolyte, the researchers began experiments on the solid electrolyte interphase (SEI), a protective layer formed on the negative electrode that results from electrolyte decomposition, most often during a battery's first cycle.

"Once you have a good SEI, you have a good battery," said Mendoza-Cortes, who is also an assistant professor at the FAMU-FSU College of Engineering. "The idea is to find an electrolyte and solvent that can form an SEI that can be stable and plays in your favor."

The new type of SEI that the researchers developed forms spontaneously in a battery cell that uses sacrificial salt or molecular species introduced via the electrolytes. They also introduced a string of molecules known as chain transfer agents that interacted with the diglyme to make a shield that protects the negatively charged electrode from degrading.

A series of experiments was conducted in order to evaluate the design and its ability to be used and then recharged. They found it could undergo about 2,000 cycles, well above the usual numbe of 300 to 500 charge cycles associated with most lithium-ion batteries.

"With this process, we could get an efficiency that is unprecedented for this kind of system," Mendoza-Cortes said. "The bottom line is we improved the SEI. That would mean more power that lasts longer. There is much potential there."

Funding for this research came from the National Science Foundation, the FSU Research Computing Center, the Energy and Materials Initiative at FSU, and the FSU-headquartered National High Magnetic Field Laboratory.