Study identifies halogen-free superhalogen electrolytes for Li-ion batteries

25 October 2014

Most electrolytes currently used in Li-ion batteries contain halogens, which are toxic. An in-depth study based on first-principles calculations by researchers at Virginia Commonwealth University has shown that the anions of commercially available electrolytes for Li-ion batteries are all superhalogens. With this knowledge, they identified several halogen-free less-toxic superhalogen electrolytes with comparable performance characteristics. Their paper is published in the journal Angewandte Chemie International Edition.

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In the search for halogen-free electrolytes, we studied the electronic structure of the current electrolytes using first-principles theory. The results showed that all current electrolytes are based on superhalogens, i.e., the vertical electron detachment energies of the moieties that make up the negative ions are larger than those of any halogen atom. Realizing that several superhalogens exist that do not contain a single halogen atom, we studied their potential as effective electrolytes by calculating not only the energy needed to remove a Li⁺ ion but also their affinity towards H₂O. Several halogen-free electrolytes are identified among which Li(CB₁₁H₁₂) is shown to have the greatest potential.

The researchers hope that the findings will lead to production of safer, less toxic batteries. They also found that the procedure outlined for Li-ion batteries is equally valid for other metal-ion batteries, such as sodium-ion or magnesium-ion batteries.

The significance [of our findings] is that one can have a safer battery without compromising its performance. The implication of our research is that similar strategies can also be used to design cathode materials in Li-ion batteries.

Jena became interested in the topic several months ago when he saw a flyer on Li-ion batteries that mentioned the need for halogen-free electrolytes. Jena had already been working for more than five years on superhalogens, a class of molecules that mimic the chemistry of halogens but have electron affinities that are much larger than that of the halogen atoms.

I had not done any work on Li-ion batteries at the time, but I was curious to see what the current electrolytes are. I found that the negative ions that make up the electrolytes are large and complex in nature and they contain one less electron than what is needed for electronic shell closure.

I knew of many superhalogen molecules that do not contain a single halogen atom. My immediate thought was first to see if the anionic components of the current electrolytes are indeed superhalogens. And, if so, do the halogen-free superhalogens that we knew serve the purpose as halogen-free electrolytes? Our research proved that to be the case.

The researchers’ work was funded partially by a $615,000 grant from the Department of Energy to study “Atomic Clusters – Bare, Coated and Supported,” (ER45579) as well as by the VCU Presidential Research Quest Fund.

Resources

• Santanab Giri, Swayamprabha Behera and Puru Jena (2014) “Superhalogens as Building Blocks of Halogen-Free Electrolytes in Lithium-Ion Batteries” doi: 10.1002/ange.201408648