Molten molecules could provide basis for safer lithium-ion batteries

By merging two contorted molecular structures, Cornell researchers have created a porous crystal capable of absorbing lithium-ion electrolytes and transporting them smoothly through one-dimensional nanochannels, a design that could lead to semi-conductive lithium-ion batteries. safer drivers.

The team’s paper, “Supramolecular assembly of fused macrocycle-cage molecules for rapid lithium-ion transport,” is published in the Journal of the American Chemical Society. The main author is Yuzhe Wang.

The project was led by Yu Zhong, assistant professor of materials science and engineering at Cornell Engineering and lead author of the paper, whose lab specializes in synthesizing soft and nanoscale materials that can make advancing energy storage and sustainability technologies.

Zhong had just joined the Cornell faculty two years ago when he was contacted by Wang, an undergraduate transfer student starting his first year, who was enthusiastic about taking on a research project .

At the top of Zhong’s list of potential topics was finding a way to make a safer lithium-ion battery. In conventional lithium-ion batteries, ions are transported via liquid electrolytes. But liquid electrolytes can form spiky dendrites between the battery’s anode and cathode, which short circuit the battery or, in rare cases, explode.

A solid-state battery would be safer, but that comes with its own challenges. Ions move more slowly through solids because they face more resistance. Zhong wanted to design a new crystal that was porous enough that the ions could take some sort of path. This path should be smooth, with weak interactions between the lithium ions and the crystal, so that the ions do not stick together. And the crystal should contain enough ions to ensure a high ion concentration.

Wang set to work and developed a method for merging two eccentric molecular structures with complementary shapes: macrocycles and molecular cages. Macrocycles are molecules with rings of 12 or more atoms, and molecular cages are multi-ring compounds that more or less resemble their name.

“Macrocycles and molecular cages have intrinsic pores where ions can sit and pass through,” Wang said. “By using them as building blocks for porous crystals, the crystal would have large spaces for storing ions and interconnected channels for transporting the ions.”

Wang fused the components together, with a molecular cage in the center and three macrocycles attached radially, like wings or arms. These macrocyclic cage molecules use hydrogen bonds and their interlocking shapes to self-assemble into larger, more complex, three-dimensional, nanoporous crystals with one-dimensional channels – “the ideal pathway for ion transport,” according to Zhong . -which achieve an ionic conductivity of up to 8.3 × 10^-4 Siemens per centimeter.

“This conductivity is the record for these molecule-based solid-state lithium ion-conducting electrolytes,” Zhong said.

Once the researchers had their crystal, they needed to better understand its composition. So they collaborated with Judy Cha, Ph.D., professor of materials science and engineering, who used scanning transmission electron microscopy to explore its structure, and Jingjie Yeo, an assistant. professor of mechanical and aerospace engineering, whose simulations clarified the interactions between molecules and lithium ions.

“So by putting all the pieces together, we finally managed to get a good understanding of why this structure is really good for ion transport and why we get such high conductivity with this material,” Zhong said.

In addition to making safer lithium-ion batteries, this material could also potentially be used to separate ions and molecules in water purification and to create mixed ion- and electron-conducting structures for circuits. and bioelectronic sensors.

“This caged molecule of the macrocycle is definitely something new in this community,” Zhong said. “The molecular cage and the macrocycle have been known for some time, but how you can actually exploit the unique geometry of these two molecules to guide the self-assembly of new, more complex structures is somewhat of an unexplored area.

“Now in our group we are working on the synthesis of different molecules, how we can put them together and create a molecule with a different geometry, in order to expand all the possibilities for making new nanoporous materials. That’s perhaps for lithium-ion conductivity or perhaps even for many other different applications.

Co-authors include doctoral student Kaiyang Wang; Ashutosh Garudapalli, master’s student; postdoctoral researchers Stephen Funni and Qiyi Fang; and researchers from Rice University, the University of Chicago and Columbia University.