PREPARATION AND CHARACTERIZATION OF TURBOSTRATIC NI AL LAYERED DOUBLEHYDROXIDES 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 w ith ammonia from mixed Ni/Al nitrate solutions and characterized by X- ray powder diffraction, infrared spectroscopy, chemical analysis and t ransmission electron microscopy. X-Ray powder diffractograms of fresh precipitates are characteristic of turbostratic layered double hydroxi des. A structural model describing the turbostratic layered double hyd roxides is proposed on the basis of the experimental results. The bruc ite-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-) p lay two roles: those in D3h symmetry, compensate for the positive-char ge excess of Al3+ ions, while those in C2v symmetry, compensate for th at of hydroxide vacancies. When aged, mixed turbostratic Ni/Al hydroxi des with the higher aluminium content (x greater-than-or-equal-to 0,18 ) change into synthetic takovites (crystallized Ni-Al-CO3 double hydro xides) through a dissolution-recrystallization process.