Supramolecular complex liberates lithium

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.