Widely used as positive electrode in alkaline rechargeable batteries, nickel hydroxide has received increasing attention during the last decades, and various preparation methods have been developed in order to improve its electrochemical performances. The main features to deal with are particle cristallinity and morphology.
Here we develop classical precipitation techniques applied to nickel dodecylsulfate Ni(DS)2 precursor. This precursor forms direct micelles in water in the studied range of concentrations. Hence it concentrates the reactive cations near a charged surface, which may have significant effect in nucleation (high local supersaturation) and templating effects. Besides, impurities-free hydroxides are rarely obtained with classical counter ions (nitrates or sulfates). As a counterion, dodecylsulfate is too big to be captured in the hydroxide structure, even between the brucite layers. In pH controlled double jet precipitation with soda, the pure phases α, βbc (badly crystallised) and β (well crystallised) may be obtained in basic conditions, whereas morphology is kept similar (platelets). In the same conditions, classical salts never lead to α phase and particle size and morphology are pH dependent.
Now if we consider hydroxide precipitation by ammonia decomplexation via heating, Ni(DS)2 lead to well-calibrated stacks of β-Ni(OH)2 nanopancakes (200nm in diameter and length along the stacking direction) whereas classical salts lead to micrometric macroporous particles constituted by thin interconnected walls.
As a conclusion Ni(DS)2 allows drastic changes if compared to classical salts: depending on the process we may have an effect on crystal structure control (double jet precipitation with soda) or on morphology (ammonia decomplexation). This effects may be explained respectively by the existence of a high local nickel concentration and templating by the surfactant. |