K<sup>+</sup> Single Cation Ionic Liquids Electrolytes with Low Melting Asymmetric Salt
Luke Schkeryantz, Phu Nguyen, William D. McCulloch, Curtis E. Moore, Kah Chun Lau, Yiying Wu
Abstract
A K+ single cation ionic liquid (K-SCIL) contains only K+ cations. When used as an electrolyte in potassium batteries, it not only possesses the properties of conventional ionic liquids such as nonflammability, negligible vapor pressure, and good electrochemical stability but also increases the K+ transport number, which reduces concentration polarization of the electrolyte and improves the battery power performance. In this study, we report K-SCILs based on the low melting (Tm ∼ 50 °C) potassium salt of (3-methoxypropyl)((trifluoromethyl)sulfonyl) amide (MPSA–). Combined with potassium bis(fluorosulfonyl)imide (KFSI), the mixture of the two salts exists as a liquid at as low as −13 °C, generating for the first time a room-temperature K-SCIL. The phase diagram is established, and the ionic conductivities of various compositions are measured. Although the room-temperature ionic conductivity is low, mild heating to 55 °C enhances the conductivity to >1 mS/cm. Evidence for a strong resistance to K+ concentration gradients as well as a near unity K transport number were also found in this K-SCIL. This report hopes to serve as inspiration for the discovery of even lower melting K salts to enable low temperature K-SCIL electrolytes and to explore the benefits of SCILs in practical cells.