VCU study marks potential breakthrough in search for better lithium and magnesium ion batteries

New Virginia Commonwealth University research demonstrates how to stabilize a multiply charged anion (negative ion), a fundamental research question and one that could help in the development of better lithium and magnesium ion batteries.

The paper by VCU physics researchers has received “Very Important Paper” status. “Stability of B₁₂(CN)₁₂^2-: Implications for Lithium and Magnesium Ion Batteries” was published last month in Angewandte Chemie International Edition, a prestigious chemistry journal. Fewer than five percent of papers published in this journal receive the “Very Important Paper” distinction.

Multiply charged particles are generally not stable in the gas phase. If you dump a large number of electrons into a particle, the electrons will repel each other. This unstable environment can lead to two outcomes: Electrons will be ejected from the particle or the particle will break apart.

The VCU research team figured out a way to produce an unusually stable, multiply charged molecule by substituting ligands. Ligands are atoms or molecules that bind to a particle with a high energy. In the process, they also produced a molecule that acts as both a monoanion and dianion.

“This taught us that we can change ligands to effectively modulate chemistry,” said lead author Puru Jena, Ph.D., distinguished professor in the Department of Physics of the College of Humanities and Sciences. “This gives us a very powerful tool to design a whole new class of complexes.” Also on the project are Hongmin Zhao, Ph.D., visiting professor from Beijing Jiantong University in China, and postdoctoral researcher Jian Zhou, Ph.D.

The team started with a stable dianion, dodecaborate B₁₂H₁₂^2-. They substituted the hydrogen atoms with CN molecules. CN contains one carbon and one nitrogen atom. When C and N are placed together, they require just one extra electron to create a closed-shell species that does not give away or take in extra electrons.

This nonreactivity is essential to stability, and Jena likens the closed-shell state to being happily married. “If we are happy, we have no reason to do anything. But if we are an unhappy, the reactions start. Once you close those shells, there’s no need for them to react by giving away or taking in new electrons because they are happy.”

The new, multiply charged molecule, dodecacyanododecaborate B₁₂(CN)₁₂^2-, is an extremely stable anion. “It holds the world’s record on stability, and there’s no other molecule that has been found that’s as stable as this one in a vacuum,” Jena said.

The research team continued to make substitutions, replacing one of the boron (B) atoms with carbon (C) to create CB₁₁(CN)₁₂^2-, another stable anion. This was expected. However, they were surprised that it is also stable as a dianion. “If you replace boron with carbon, you only need one electron to close the shell,” Jena said. “If you have two electrons, the shell is not closed. The molecule should be reactive and not take the extra electron. But it did.”

Researchers used this new complex to design a halogen-free electrolyte for lithium and magnesium ion batteries, expanding on a previous studyfor safer, halogen-free batteries. The new research could lead to better, efficient, longer-lasting and lighter-weight batteries, according to Jena.

The researchers’ work was funded by the Fundamental Research Funds for Central University, the U.S. Department of Energy and CIT CRCF.