The Ksp gap enabled precipitation transformation reactions from transition metal hydroxides to sulfides for alkali metal ion storage

Abstract

The solubility product constant (K_sp) is an important concept in inorganic chemistry for determining the solubility of sparingly soluble ionic compounds in aqueous medium. K_sp guided precipitation reactions are a popular route towards the acquiring of inorganic solid materials or the removal of unwanted ions from an aqueous solution. Furthermore, the K_sp gap of two solids sharing the same cation or anion triggers precipitation transformation reactions if one solid is treated with an aqueous solution containing the different ion of the other solid with a significantly lower K_sp value. Herein, it is found that the K_sp values of the sulfides of transition metals in groups 9–12 of the periodic table of elements are significantly lower than the corresponding hydroxides, and this finding is taken advantage of for the preparation of transition metal sulfides through the precipitation transformation from the corresponding hydroxides in the aqueous solution of sodium sulfide. As an example, Cu(OH)₂ nanowires are converted to nanostructured CuS with inherited morphology to a certain extent, which is applied for the electrochemical storage of alkali metal ions (Li⁺, Na⁺, K⁺); among them sodium ion batteries exhibits the most promising performances. The chemical equations from the hydroxides to sulfides are determined and the corresponding equilibrium constants are derived from the K_sp of the solids of hydroxides and sulfides without or with the hydrolysis equilibrium constants of the S²⁻ ions. Accordingly, a new concept, the precipitation transformation constant (K_pt), which means the equilibrium constant for precipitation transformation reactions, is proposed.