New insights on the microstructural characterisation of nickel hydroxides and correlation with electrochemical properties

Abstract

Two sets of laboratory-prepared and industrial nickel hydroxide samples were studied and their microstructure characterized by performing Rietveld refinements with a spherical harmonic-implemented algorithm using both neutron and X-ray powder diffraction data. The results indicate that the anisotropic line broadening present in their patterns is mainly related to the anisotropy in size, as β-Ni(OH)₂ exhibits a platelet shape. Indeed, the determined crystallite sizes are fully consistent with mean particle diameters measured by TEM. The relative degree of stacking faults was estimated from the extent of shifts in reflections 10l where l ≠ 0 and found to be larger for samples with smaller particle/crystallite sizes. This approach, involving independent assessment of the crystallite size and the relative amount of stacking faults, allowed us to confirm that samples with small crystallite size are those exhibiting significant amounts of stacking faults. The results of the electrochemical tests on both sets of samples indicate that the particle/crystallite size is the primary factor determining the electrochemical yield and that the gradual appearance of stacking faults with decreasing size creates a synergic effect in the increase of capacity. Thus, there exists a threshold size of approximately 350 Å in diameter and 100 Å in thickness below which the degree of faulting is clearly correlated to the electrochemical capacity values achieved.