PREPARATION AND CHARACTERIZATION OF TURBOSTRATIC NI/AL LAYERED DOUBLE HYDROXIDES FOR NICKEL-HYDROXIDE ELECTRODE APPLICATIONS

Replacement of nickel by aluminium in the brucite-type Ni(OH)2 layers leads to layered double hydroxides (LDH), which can be used as active materials for nickel hydroxide electrodes. Ni/Al LDH compounds [O<Al/(Ni+Al) less-than-or-equal-to 0.25] were synthesized by precipitation with ammonia from mixed Ni/Al nitrate solutions and characterized by X-ray powder diffraction, infrared spectroscopy, chemical analysis and transmission electron microscopy. X-Ray powder diffractograms of fresh precipitates are characteristic of turbostratic layered double hydroxides. A structural model describing the turbostratic layered double hydroxides is proposed on the basis of the experimental results. The brucite-type layers are non-stoichiometric and present hydroxide vacancies: their chemical composition can be written as [Ni1-x2+Alx3+ (OH-)2-(y+2z-x)] and the interlamellar layers as [(NO3-)y(CO32-)z.nH2O] with x less-than-or-equal-to 0.25. The interlamellar anions (NO3- or CO32-) play two roles: those in D3h symmetry, compensate for the positive-charge excess of Al3+ ions, while those in C2v symmetry, compensate for that of hydroxide vacancies. When aged, mixed turbostratic Ni/Al hydroxides with the higher aluminium content (x greater-than-or-equal-to 0,18) change into synthetic takovites (crystallized Ni-Al-CO3 double hydroxides) through a dissolution-recrystallization process.