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- May 17, 2018 -

The lithium-ion battery is the main power source in many such as cell phones, laptops and electric vehicles. Yet, much about the basic science taking place at the atomic and molecular levels during charge and discharge remains a mystery.

It has been reported in a new study a breakthrough in understanding the chemistry of the microscopically thin layer that forms at the interface between the liquid electrolyte and solid electrode. Battery researchers commonly refer to this layer as the "solid-electrolyte interphase" called SEI.

A lot of research over the past decades has been devoted to understanding the SEI in the lithium-ion battery. Scientists know that the SEI forms on the graphite negative electrode, is extremely thin, and primarily takes shape during the first charge of the battery. Also, the SEI prevents detrimental reactions from occurring at the interface, while at the same time allowing the important lithium ions free rein to move between the electrolyte and electrode.

All good lithium-ion batteries have well-functioning SEIs. Battery performance is highly dependent on the quality of the SEI. If we manage to understand it, the SEI could be tuned to improve battery performance.

Researchers deciphered the chemistry behind one of the more common components of the SEI in typical lithium-ion batteries. Based on both experimental and computational results, their findings showed that this phase forms during battery charge by the electrochemical reaction of hydrogen fluoride, producing hydrogen gas and solid lithium fluoride.

This reaction depends highly on the electrode material, which could be a metal, graphene or graphitic material, and thus demonstrates the importance of catalysis in battery operation. The team discovered a new method for monitoring the concentration of the hydrogen fluoride, a highly detrimental impurity that forms from a reaction between trace amounts of moisture and the salt (LiPF6) in the electrolyte. This monitoring capability should prove vital to future basic science studies of the SEI.

The results are already having a commercial impact and  will also open new opportunities for the improvement of existing, and the design of new, lithium-ion technologies."