Lithium Ion Transport in Water-Containing Li(SCN) over a Wide Compositional Range: from Water Doping to Hydration

This work explores the Li+ cation transport mechanisms in the system Li(SCN) center dot x H2O, complementary to our previous studies on crystal structures and hydration properties of lithium thiocyanate Li(SCN). Anhydrous Li(SCN) is a Schottky defective material with lithium vacancies V'Li being the dominant mobile defect. Two water incorporation mechanisms are identified: (i) At very low concentrations H2O acts as a dopant and forms H2O center dot SCN defects, constituting a substitutive incorporation mechanism. Doping with water not only increases the concentration of V-Li' in Li(SCN), but also the vacancy mobility, making H2O-doping a special case. A complete quantitative defect chemical model is derived, including enthalpies and entropies of defect formation, association, and migration. (ii) Higher water activities induce chemical and structural changes in Li(SCN) center dot x H2O, i.e. hydration by reactive incorporation occurs. Single-phases as well as composites are investigated for their Li+ ion transport mechanisms. In case of x <= 1, defect associates determine the conductivity resulting in exceptionally high activation enthalpies. Further hydration to 1 < x < 2 creates composites of solid mono- and dihydrates, in which the low-melting diyhdrate can form percolating pathways and increase the conductivity to values as high as 7 x 10(-4) S/cm at room temperature. Further increase of x leads to complete liquefaction. The entire range of x in the Li(SCN) - H2O system was studied regarding ion transport properties, identifying the individual mechanisms and temperature dependences. The results of this work deepen the understanding of hydration effects on solid ion conductors in solid state ionics, and help to elucidate transport phenomena in salt - solvent systems.