X-Ray Spectroscopic Studies on the Electronic Structures of LiNiO2-Based Cathode Materials for Lithium-Ion Batteries

Abstract

Introduction For development and popularization of electric vehicles, it is indispensable to improve the safety and the capacity of secondary batteries not only in a usual operating condition but also in harsh operating environments (e.g., at high temperature and in an overcharge condition). Basic knowledge regarding the electronic and bonding states of functional 3d transition metal (TM) elements is necessary to design and develop cathode materials of lithium-ion batteries in this direction. Core-level X-ray spectroscopies provide the element-selected information about electronic states of constituent elements. In this study, we applied X-ray spectroscopies to LiNiO2-based cathode materials to investigate the electronic and bonding states of Ni in different states of charge. Experimental All the samples for X-ray spectroscopies were prepared by using a two-electrode cell comprised of LiNi0.80Co0.15Al0.05O2 as an active material of a working electrode, Li metal as a counter electrode, and mixed solvent of ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate containing 1.0 mol dm− 3 LiPF6. The cell was charged at a current density of 1 mA cm−2 (ca. 1 C rate) at room temperature. X-ray absorption spectroscopy (XAS) was performed at the Ni and Co K-edges at the BL33XU (the Toyota beamline) and at the Ni and Co L 2,3-edges and at the O K-edge at the BL27SU of SPring-8, Japan. X-ray emission spectroscopy was also carried out at the Ni K-edge at the BL39XU and the BL33XU of SPring-8. Local structural analysis based on extended X-ray absorption fine structure (EXAFS) was done by using ATHENA and ARTEMIS with the scattering phase shift and the effective scattering amplitudes calculated by FEFF9.05. Results The oxidation state of Ni is often estimated by using an empirical index of absorption energy, at which normalized absorbance is 0.5. This empirical method was applied to the present system, and revealed that the absorption energy at the Ni K-edge monotonically increased as the state of charge approached from 0 to 100 and slightly decreased as the cell was more overcharged. In general, the change in the electronic state of Ni involves that in local structure around Ni, which suggests that structural parameters such as coordination numbers and bond length for the first coordination Ni–O should be alternate indicators of the oxidation state of Ni. Actually, it was found that these structural parameters could be good indicators of average Ni valence. The structural parameters monotonically increased even in the overcharge region as the cell was charged, differently from the above-mentioned empirical absorption energy. The difference in the changing trend of the oxidation-state indicator and the electronic structure of Ni was further investigated by using 3d TM L 2,3-edge and O K-edge XAS and X-ray emission spectroscopy, which will be discussed in the conference. Acknowledgement X-ray spectroscopic studies were performed at the BL27SU, the BL33XU, and BL39XU of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) with the proposal numbers of 2014B1582, 2014B7008, and 2014B1574.