Potassium Superionic Conduction Mediated by Water Molecules in the Crystal Lattice

Abstract

While many superionic conductors for light alkali metal ions (Li⁺ or Na⁺) have been developed based on ceramics and metal-organic composite materials, fast ion conduction of larger K⁺ ions in ambient conditions has long been a challenging target because of the large ionic radius of K⁺ ion. Recently, our research group reported a unique supramolecular framework of the nanometer-sized spherical metal complex (K-NCIS), which shows excellent K⁺ conduction of 13 mS/cm at room temperature. Although the presence of many solvated water molecules in the crystals accelerated the K⁺ conductivity in K-NCIS, the mobility of K⁺ was not frozen even below 0 ºC owing to the anomalous behavior of water molecules filled in the crystal lattice. The high transport number (t) of K⁺ for K-NCIS was proven by the NMR spectroscopy and ion-diffusion experiment in the solid-state. An ongoing study on constructing all-solid-state K⁺ batteries using K-NCIS as solid-electrolyte will also be described.