Aluminium coordination in LiNi1-yAlyO2 solid solutions

Li-6 and Al-27 MAS NMR and EPR of low-spin Ni3+ were used to study the local environment of aluminium and the electronic structure of LiNi1-yAlyO2 solid solutions. The citrate precursor method was used to obtain layered Li1-yAlyO2 with reproducible compositions. It has been found that Al additives favour the stabilization of both Ni3+ and the two-dimensionality of the crystal lattice and perturb the transition from a layered to a monoclinic structure during lithium extraction. EPR of low-spin Ni3+ shows a more covalent Ni3+-O2- bonding when Ni3+ is replaced by Al3+ in the NiO2, layer. In order to improve the resolution of MAS NMR spectra, acid delithiated Li1-xNi1-yAlyO2 were studied. The local environment of Al was assessed by comparing the observed chemical shifts for Li1-xNi1-yAlyO2 with that for isostructural diamagnetic LiCo1-yAlyO2. Al-27 MAS NMR spectra demonstrate that, depending on the total Al content, Al ions occupy non-octahedral or octahedral positions. The ability of Al to fill non-octahedral sites is more pronounced for LiCo1-yAlyO2 solid solutions. The stabilisation of Al in non-octahedral positions in the trigonal crystal structure of LiCo/Ni1-yAlyO2 is discussed. (C) 2000 Elsevier Science B.V. All rights reserved.