Abstract

The full structural characterisation of the highly deintercalated Li_xNi_1.02O₂ (x ≤ 0.30) phases has been performed. The structure of the Li_0.30Ni_1.02O₂ phase was refined by the Rietveld method. The cationic distribution was found to be identical to that of the pristine material. A study of the Li//Li_xNi_1.02O₂ system at high potential has shown the successive formation of two phases with O3 (AB CA BC) and O1 (AB) oxygen packing, respectively, near the NiO₂ composition. Since slab gliding is at the origin of the O3 to O1 transition, layer displacement faults were observed in these two phases. For the O3 phase, as soon as all the lithium ions are removed from an interslab space, an O1-type fault occurs locally. In contrast, for the O1 phase, the presence of extra-nickel ions in the interslab space prevents slab gliding in the vicinity and, therefore, O3-type interslab spaces remain in the O1-type packing. The X-ray diffraction patterns were simulated using the DIFFaX program. It was shown that the stabilisation of the O1-type packing at the very end of the deintercalation process is due to a minimisation of the interactions between the p orbitals of the oxygen ions through the van der Waals gap. A two-phase domain is observed between Li_0.30NiO₂ and a composition close to NiO₂ since, for very low lithium contents, the Ni³⁺/Ni⁴⁺ ordering (and the lithium/vacancy ordering) is no longer possible and the difference in size between the cations leads to the formation of constraints which destabilise the Ni³⁺ ions in a lattice where Ni⁴⁺ ions prevail. At the end of the deintercalation process, the NiO₂ compound appears to be highly covalent, therefore, the steric effects prevail over the electrostatic repulsion effects, as in chalcogenides.