Theoretical investigations of the EPR g factors and the local structures for Ni³⁺ in LiAl_yCo_1–yO₂ at various Al concentrations

Abstract

The electron paramagnetic resonance (EPR) g factors and local structures for Ni³⁺ in LiAl_yCo_1–yO₂ as the cathode materials for lithium-ion batteries are theoretically investigated at various Al concentrations y (= 0.1, 0.5, 0.8 and 1.0). The [NiO₆]^9– clusters are subject to the tetragonal elongation distortions along the C₄ axis owing to the Jahn–Teller effect and the different degrees of distortions are characterised by the slightly increasing relative elongation ratio ρ (and also crystal-field strength and covalency) with y. In pure LiAlO₂ (y = 1.0), the [NiO₆]^9– cluster exhibits the largest ρ (≈1.4%) and an additional relative variation ratio τ (≈1.4%) of the planar bond lengths, responsible for the perpendicular g anisotropy δg = (g_x – g_y). The number of impurity Ni³⁺ centres increases from one at y = 0 and 1.0 to two at y = 0.1 and 0.8 and reaches the maximum four at y = 0.5, in accordance with the increasing degree of disorder of the systems. The distinct impurity centres for the same y may 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 LiAl_yCo_1–yO₂ type cathode materials.

Keywords:

• Defect structures
• electron paramagnetic resonance (EPR)
• Ni³⁺
• LiAl_yCo_1–yO₂