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Energy Storage

Supramolecular complex liberates lithium

Cation gets a conductivity boost by capturing its counterion

by Bethany Halford
July 21, 2018 | APPEARED IN VOLUME 96, ISSUE 30

09630-scicon10-scheme.jpg
Credit: J. Am. Chem. Soc.

By sequestering the common lithium counterion PF6 in a supramolecular complex, chemists have found they can enhance the cation’s conductivity and how much current it generates—a finding that could lead to better lithium-ion batteries. MIT’s Jeremiah A. Johnson, Yang Shao-Horn, and Bo Qiao came up with the idea after a talk Qiao gave during his first week as a postdoc at MIT. Qiao spoke about his doctoral work with cyanostar, a macrocycle that forms a supramolecular complex with PF6 in a 2:1 ratio (shown). “LiPF6 is one of the most common salts used as a Li+ source in Li batteries,” Johnson says. The chemists realized that if they could bind the anion of that salt with cyanostar, it could dramatically affect the properties of LiPF6. They reasoned that capturing the counterion would spur the salt to break up into free ions and that the large supramolecular complex would diffuse slowly compared with free Li+. Experiments bore out this hypothesis (J. Am. Chem. Soc. 2018, DOI: 10.1021/jacs.8b05915). The researchers say using cyanostar in batteries is still far off, but they believe that the mechanistic insights they’ve gained could guide the design of practical systems.

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