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Team identifies better lithium-ion electrolytes

Credit: Oak Ridge National Laboratory

By running hundreds of simulations using the Cray XK7 Titan supercomputer at the US Oak Ridge National Laboratory, a team from the California Institute of Technology has found new electrolytes which could enhance the performance of lithium-ion batteries.

Typically, lithium-ion batteries feature liquid electrolytes, but research is focusing on polymeric electrolytes, which are known to be more stable, less flammable, and less volatile.

Historically, the best polymer electrolyte for lithium-ion batteries is said to have been polyethylene oxide (PEO). The addition of lithium salts to polymers like PEO enables them to be used as solid polymer electrolytes. The salt contains lithium cations and some negatively charged anions to balance the charge.

The problem with PEO is that it conducts lithium ions poorly in comparison to liquid electrolytes. It also conducts the anions too quickly.

Using high performance computing, the researchers created a chemically specific dynamic bond percolation model – a coarse-grained simulation protocol – to screen electrolyte materials based on short molecular dynamics trajectories.

They found one group of polymers: lewis-acidic molecules, that hold a positive charge and strongly interact with anions.

"It was known that Lewis-acidic molecules slowed down anions," explained postdoctoral fellow Brett Savoie. "What was surprising here was that by using a purely Lewis-acidic system, we also sped up the lithium."

Because Lewis-acidic chemical groups' positive regions are contained in a small amount of space and their negative regions are spread over a large amount of space, they give positive lithium ions more opportunities to dissolve.

According to the researchers, the simulations showed that the polymers may be capable of producing an eight-fold increase in desired lithium conduction and a marked decrease in the unwanted anion conduction.

Author
Peggy Lee

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