Local coordination of Fe³⁺ in Li[Co_0.98Fe_0.02]O₂ as cathode material for lithium ion batteries—multi-frequency EPR and Monte-Carlo Newman-superposition model analysis

The local coordination of the Fe³⁺-centers in Li[Co_0.98Fe_0.02]O₂ cathode materials for lithium-ion batteries has been investigated by means of XRD and multi-frequency EPR spectroscopy. EPR clearly showed the Fe³⁺ being in a high-spin state with S = . The set of spin-Hamiltonian parameters obtained from multi-frequency EPR experiments with Larmor frequencies ranging between 9.8 and 406 GHz was transformed into structural information by means of an expansion to standard Newton-superposition modeling, termed as Monte-Carlo Newman superposition modeling. Based on this analysis, an isovalent incorporation of the Fe³⁺-ions on the Co³⁺-sites, i.e. Fe^x_Co, has been shown. With that respect, the positive sign of the axial second-order fine-structure interaction parameter B⁰₂ is indicative of an elongated oxygen octahedron, whereas B⁰₂ < 0 points to a compressed octahedron coordinated about the Fe³⁺-center. Furthermore, the results obtained here suggest that the oxygen octahedron about the Fe³⁺-ion is slightly distorted as compared to the CoO₆ octahedron, which in turn may impose mechanical strain to the cathode material.