Theoretical investigations of the EPR g factors and the local structures for Ni in LiAl y Co 1–y O 2 at various Al concentrations.

The electron paramagnetic resonance (EPR)gfactors and local structures for Ni3+in LiAlyCo1–yO2as the cathode materials for lithium-ion batteries are theoretically investigated at various Al concentrationsy(= 0.1, 0.5, 0.8 and 1.0). The [NiO6]9–clusters are subject to the tetragonal elongation distortions along theC4axis owing to the Jahn–Teller effect and the different degrees of distortions are characterised by the slightly increasing relative elongation ratioρ(and also crystal-field strength and covalency) withy. In pure LiAlO2(y = 1.0), the [NiO6]9–cluster exhibits the largestρ(≈1.4%) and an additional relative variation ratioτ(≈1.4%) of the planar bond lengths, responsible for the perpendicularganisotropyδg= (gx–gy). The number of impurity Ni3+centres increases from one aty = 0 and 1.0 to two aty = 0.1 and 0.8 and reaches the maximum four aty = 0.5, in accordance with the increasing degree of disorder of the systems. The distinct impurity centres for the sameymay be accompanied by the comparable energies of the ground and near excited states, similar to ‘resonant states’ with slightly different local elongation distortions and tetragonal level splittings. The present studies will be helpful to understand the relationships between local structures and performances of LiAlyCo1–yO2type cathode materials. [ABSTRACT FROM PUBLISHER]