Hydration effects on defect chemistry of LiSCN and Li${}_2$Sn${}_2$S${}_5$

This thesis explores the impact of hydration on the electrochemical properties of two ionic materials with Li${}^{+}$ cations: lithium thiocyanate (LiSCN) and lithium tin sulfide (Li${}_2$Sn${}_2$S${}_5$). Both materials are hygroscopic, yet the impact on ion transport and defect chemistry upon hydration (incorporation of undissociated molecules) is very different. LiSCN is a very stable ionic salt composed of lithium cations Li${}^{+}$ and thiocyanate SCN${}^{-}$ anions. The anion has a linear structure where Li${}^{+}$ cations directly bond to either the sulphur or nitrogen atom, and represents a molecular ligand. Anhydrous LiSCN forms Schottky defects with lithium vacancies as the dominant mobile charge carriers. The vacancy concentration and mobility are low, making this material a poor ionic conductor at 25 °C (10${}^{-13}$ S/cm) . This low conductivity is attributed to the SCN${}^{-}$ anion, coordinating to lithium both via the nitrogen and sulphur atom, but with very different bonding strengths. Thus a Li${}^{+}$ jump requires substantial structural relaxation of the environment, which slows down long-range transport. Near the melting point defect-defect interactions come into play, and the concentration of vacancies as well as the conductivity are fairly high (10${}^{-5}$ S/cm). In case of LiSCN · xH${}_2$O, water contents of x ≈ 1000 ppm suffice to raise the conductivity by ∼3 orders of magnitude. In these substoichiometric amounts water acts as a donor dopant (H${}_2$O${}_{SCN}^{\cdot }$) and increases the concentration of lithium vacancies. In contrast to other doping agents, the incorporation of H${}_2$O also increases the mobility of lithium vacancies even at very low water concentrations. At low temperatures the conductivity in H${}_2$O-doped LiSCN is impeded by the association of incorporated water with lithium vacancies, which become free carriers only at higher temperatures. A comprehensive defect chemical model is derived, including defect formation enthalpies and migration barriers. When the hydration level is increased to stoichiometric values, the mono- and dihydrate form. Within the two-phase regime of dry LiSCN plus the monohydrate as well as for the pure monohydrate, ion transport is determined by defect associates, causing an exceptionally high activation enthalpy. Given the low melting point of the dihydrate (38 °C) and the possibility of supercooling, the conductivity of two-phase mixtures of mono- and dihydrate is determined by percolating molten dihydrate, giving rise to conductivities as high as 7 × 10${}^{-4}$ S/cm at 25 °C.