Cornell achieves record-high conductivity in lithium batteries, making them safer

Researchers at Cornell University used an innovative arrangement of macrocycle and cage molecules to achieve record-high conductivity in solid-state lithium-ion batteries. The research paves the way for much safer batteries in the near future.

Lithium-ion (Li-ion) batteries are critical to the transition to cleaner and greener sources of energy. In conventional Li-ion battery setups, ions that carry electric current move in a liquid electrolyte. However, liquid electrolytes can form spiky dendrites between the anode and the cathode, the two terminals of the battery.

A newer circuit thus formed can short out the battery, rendering it useless. Worse still, it can even cause the battery to explode. This has been the reason for fire incidents with Li-ion batteries.

An alternate option is to use solid-state batteries instead. However, ions move slower in a solid electrolyte since they face more resistance.

Porous crystal for better ion flow

A research team led by Yu Zhong, an assistant professor of material science and engineering at Cornell University, came up with the idea of making a porous crystal that would allow ions to move more smoothly inside the solid-state electrolyte.

For effective operation, the crystal would need to hold a high ion concentration but still have a weak interaction with lithium ions to facilitate their easy movement. Zhong’s student Yuzhe Wang devised a method to fuse two eccentric molecular structures, macrocycles, and molecular cages, which have complementary shapes.

“Both macrocycles and molecular cages have intrinsic pores where ions can sit and pass through,” said Wang in a press release. “By using them as the building blocks for porous crystals, the crystal would have large spaces to store ions and interconnected channels for ions to transport.”

Record high conductivity

Macrocycles are molecules with rings of 12 or more atoms, whereas molecular cages contain multi-ring compounds. The porous crystal made by the researchers consists of a molecular cage in the center and three macrocycles attached radially to it, much like wings.

The two components are held together by hydrogen bonds. Due to their interlocking shapes, they self-assemble into larger three-dimensional nanoporous crystals. The self-assembled structure has one-dimensional channels through which ions can flow.

According to a university press release, the researchers achieved ionic conductivity of up to 8.3 X 10-4 Siemens per centimeter. “That conductivity is the record high for these molecule-based, solid-state lithium-ion-conducting electrolytes,” Zhong added.

Due to the crystal’s self-assembling nature, the researchers had little clue how it was formed and how it worked. So, they teamed up with other researchers at the university to use scanning electron microscopy to explore the structure and simulations to determine the interaction with lithium ions.

The work, however, isn’t limited to making safer lithium-ion batteries alone. “The molecular cage and macrocycle have been known for a while, but how you can really leverage the unique geometry of these two molecules to guide the self-assembly of new, more complicated structures is kind of an unexplored area,” said Zhong in the press release .

The research team is now working on synthesizing new molecules and achieving new geometries with these molecules. Potential applications include separating ions to purify water and making mixed ion-electron structures for bioelectronic circuits and sensors.

The research findings were published in the Journal of the American Chemical Society .