Coordination of the Mn4+-Center in Layered Li[Co0.98Mn0.02]O2 Cathode Materials for Lithium-Ion Batteries

Abstract

Abstract The local coordination of the manganese in Li[Co0.98Mn0.02]O2 cathode materials for lithium-ion batteries has been investigated by means of a joint XRD and multi-frequency electron paramagnetic resonance (EPR) characterization approach. EPR showed the manganese being in a tetravalent high-spin Mn4+-oxidation state with S = 3 2 . $S = {3 \over 2}.$ The set of spin-Hamiltonian parameters obtained from the multi-frequency EPR analysis with Larmor frequencies ranging between 9.4 and 406 GHz is transformed into structural information by means of the recently introduced Monte-Carlo Newman-superposition modeling. Based on this analysis, the Mn4+ are shown being incorporated for the Co3+-sites, i.e. acting as donor-type functional centers Mn Co • ${\rm{Mn}}_{{\rm{Co}}}^ \bullet $ . In that respect, for Mn4+ the negative sign of the axial second-order fine-structure interaction parameter B 2 0 $B_2^0$ is indicative of an elongated oxygen octahedron in its first coordination sphere, whereas B 2 0 > 0 $B_2^0 > 0$ rather points to a compressed octahedron coordinated about the Mn4+-centers. Furthermore, the results obtained here suggest that the oxygen octahedron about the Mn4+-ion is slightly distorted as compared to the CoO6 octahedron. Concerning the coordination to next-nearest neighbor ions, part of the manganese resides in manganese-rich domains, whereas the for the remaining centers the Co3+-site is randomly occupied with Co/Mn according to the effective stoichiometry of the compound. Finally, a structural stability range emerges from the Newman-modeling that supports the discussed ability of manganese to act as an structure-stabilizing element in layered oxides.