An investigation of local structures for 3d3, 3d5 and 3d7 ions doing in the layered LiCoO2 cathode materials

Abstract Doping transition metal ions in LiCoO2 batteries has been proved an efficient method to improve the electrochemical properties in experiment, which is deeply related with the distorted local structure of impurities ions. According to the observed EPR spectra, the distorted local structures for 3d3 (Mn4+), 3d5 (Fe3+) and 3d7 (Ni3+) ions in LiCoO2 batteries are theoretically studied based on the perturbation calculations of Spin Hamilton Parameters (g factors, hyperfine structure constants A and Zero-field splittings parameters D and E). By considering the Jahn-Teller effect and mismatch radius of above transition metal ions (TMs), the formed [MnO6]8-, [FeO6]9- and [NiO6]9- clusters in LiCoO2 exhibit various distorted symmetries compared with experimental single one, characterized by trigonal distortion parameter Δθ as well as rhombic distortion parameters ΔZ and Δφ in the 3d3 center and the 3d5 center and tetragonal distortion parameter ΔZ in the 3d7 center. In addition, compared with asymmetrical signal of EPR spectra at 30K, the role of relatively high temperature for 100K is investigated by dynamic Jahn-Teller, making the isotropic EPR signal and regular octahedron for Ni3+ center. Comparing with the obtained structure properties of isolate-doped transition metal ion, the co-doped Mg in LiCoO2 makes the further elongated distortion of local structure. Based on the well fitted EPR parameters and obtained local environment of doped transition metal ions, the inactive of electrochemical properties of Mn4+ can be attribute to the largest crystal field (Dq) and the smallest difference with host Co3+ site. Therefore, a comprehensively study of the local structure may be helpful to understand and predict the electrochemical behaviors of batteries.