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New LiNi 1Ϫy Mg y O 2 (0 Յ y Յ 0.20) layered oxides were synthesized by a coprecipitation method followed by a high-temperature thermal treatment. Rietveld refinements of their X-ray diffraction patterns showed that they exhibit a quasi-two-dimensional structure, isostructural to LiNiO 2 , for small substitution amounts (y Յ 0.10). For larger amounts (y ϭ 0.15, 0.20), the Li/(Ni ϩ Mg) ratio is significantly lower than unity. In all cases, the extra ions located in the inter-slab space for lithium deficiency compensation are preferentially Mg 2ϩ ions. A magnetic study confirmed the cationic distributions which result from the size difference between Ni 3ϩ and Mg 2ϩ ions. An electrochemical study showed reversible behavior for all materials. A high capacity (Ն150 Ah kg Ϫ1 ) was found for LiNi 1Ϫy Mg y O 2 phases (y Յ 0.02), which decreased when y increased. The presence of Mg 2ϩ cations in the inter-slab space, which cannot be oxidized and have a size close to Li ϩ , prevents the local collapses of the structure which occurs for the Li 1Ϫz Ni 1ϩz O 2 system; therefore good cycling stability is observed.

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Replacement of nickel by aluminium in the brucite-type Ni(OH), layers leads to layered double hydroxides (LDH), which can be used as active materials for nickel hydroxide electrodes. Ni/AI LDH compounds [O< AI/(Ni + Al) 6 0.251 were synthesized by precipitation with ammonia from mixed Ni/AI 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 [Ni:?, Ali+ (OH-), -,,+,,-x,] and the interlamellar layers as [(NO,), (CO;-);nH,O] with xGO.25. The interlamellar anions (NO; or COZ-) play two roles: those in DJh symmetry, compensate for the positive-charge excess of A13+ ions, while those in C,, symmetry, compensate for that of hydroxide vacancies. When aged, mixed turbostratic Ni/Al hydroxides with the higher aluminium content (x>0,18) change into synthetic takovites (crystallized Ni-AI-CO, double hydroxides) through a dissolution-recrystallization process.

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