High-Frequency Electron Paramagnetic Resonance Analysis of the Oxidation State and Local Structure of Ni and Mn Ions in Ni,Mn-Codoped LiCoO2

High-frequency electron paramagnetic resonance (HF-EPR) spectroscopy was employed to examine the oxidation state and local structure of Ni and Mn ions in Ni,Mn-codoped LiCoO(2). The assignment of EPR signals was based on Mg,Mn-codoped LiCoO(2) and Ni-doped LiCoO(2) used as Mn(4+) and low-spin Ni(3+) EPR references. Complementary information on the oxidation state of transition-metal ions was obtained by solid-state (6,7)Li NMR spectroscopy. For slightly doped oxides (LiCo(1-x)Ni(x)Mn(x)O(2) with x < 0.05), nickel and manganese substitute for cobalt in the CoO(2) layers and are stabilized as Ni(3+) and Mn(4+) ions. The local structure of Mn(4+) ions was determined by modeling of the axial zero-field-splitting parameter in the framework of the Newman superposition model. It has been found that the local trigonal distortion around Mn(4+) is smaller in comparison with that of the host site. To achieve a local compensation of Mn(4+) charge, several defect models are discussed. With an increase in the total dopant content (LiCo(1-x)Ni(x)Mn(x)O(2) and 0.05