Li+cation coordination by acetonitrile—insights from crystallography

Abstract

Solvation is a critical factor for determining the properties of electrolytes and lithium reagents, but only limited information is available about the coordination number for Li⁺ cations in solution with different solvents. The present manuscript examines the manner in which acetonitrile (AN) fully solvates Li⁺ cations. The results are also likely pertinent to other nitrile and dinitrile solvents. In particular, the crystal structure for a (AN)₆:LiPF₆ solvate is reported—this is the first 6/1 AN/Li solvate structure to be determined. The structure consists of Li⁺ cations fully solvated by four AN molecules (i.e., [(AN)₄Li]⁺ species), uncoordinated PF₆⁻ anions and uncoordinated AN molecules (two per Li⁺ cation). This structure validates, in part, density functional theory (DFT) calculations which predict that there is little to no energetic benefit to coordinating Li⁺ cations with more than four AN solvent molecules.