Local Structure and in-Situ Average Structure Analysis of LiNi0.8Co0.2O2 Electrode during Charge-Discharge Process By Using Quantum Beam

Abstract

INTRODUCTION LiCoO2-based materials have been mainly used as a cathode material of the Li ion rechargeable battery. However the Co has some problems, such as high cost and high environmental load. As a candidate of the alternative materials, LiNi0.8Co0.2O2, has drawn much attention because it shows high capacity comparatively, and has an advantage in respect to environmental problem due to low content of Co. The LiNi0.8Co0.2O2 was synthesized for the purpose of reducing cationic mixing and heat stability by substituting a part of Ni of LiNiO2 for Co and was studied. We reported the average structure by using the Rietveld method for neutron and X-ray diffraction patterns in the previous study1).Then we elucidated the local structure of LiNi0.8Co0.2O2 electrode in charge-discharge process by the PDF analysis using neutron and X-ray total scattering methods. The average structure analysis by the in-situ neutron diffraction patterns has been expected. However the measurement and the analytical technique are discussed2). In this research we examined the local structural charge of LiNi0.8Co0.2O2 electrode during charge-discharge process and the Rietveld method for in-situ neutron diffraction pattern of LiNi0.8Co0.2O2. Furthermore, we revealed the average structural change in charge-discharge process of the nonequilibrium state. EXPERIMENTAL LiNi0.8Co0.2O2 was synthesized by mixing Ni-Co precursor with lithium hydroxide hydrate. The precursor was prepared by co-precipitating the aqueous solution of the two metal nitrate salt (Ni, Co) in a stoichiometric ratio of 8:2, with lithium hydroxide. After suction filtration for a few hours, the precipitates were dried at 100°C overnight. The dried material was then mixed with a stoichiometric amount of LiOH-H2O. The mixture was annealed in air at 600 °C for 15 h, and then at 800 °C for 15 h. The product was identified by powder XRD. The composition of lithium and transition metals in the active material was determined by ICP. We analysed the local structure of LiNi0.8Co0.2O2 electrode in charge-discharge process by the PDF analysis using synchrotron X-ray total scattering (BL04B2, SPring-8) and neutron total scattering (BL21, J-PARC). To analyse the average structure of LiNi0.8Co0.2O2 electrode in charge-discharge process in the nonequilibrium state, we also performed the Rietveld method for in-situ neutron diffraction patterns (BL09, J-PARC) by using laminate cell. The cell film was used the PCTFE of the non-hydrogen film and the electrolyte substituted by deuterium. The charge and discharge test were measured at 0.1C. We examined the division time for the integrated in-situ diffraction data in the range of 10 to 40 minutes. Each divided date was analyzed by the Rietveld method using Z-Rietveld. RESULTS AND DISCUSSION We examined the local structure of LiNi0.8Co0.2O2 by PDF analysis using synchrotron X-ray total scattering and neutron total scattering. The Bond Valence Sum(BVS) of Ni and Co, the distortion parameters, λ, σ2 of the MO6(M=Ni, Co) octahedral were calculated from analysed results. It was revealed that the NiO6 octahedra was more distorted than the distortion of CoO6 octahedra. The reason why the distortion was large must cause the Jahn-Teller effect of Ni3+. Then we elucidated the local structure of LiNi0.8Co0.2O2 electrode in 2nd charge-discharge process by the PDF analysis using synchrotron X-ray total scattering method. The distortion parameter of the MO6octahedron came back original state after discharge. It was cearly revealed that there was locally reversible structural changein charge-discharge process. We also examined the method of the analysis for in-situ neutron diffraction data. We compared charge and discharge test using the laminating cell with the electrolyte substituted deuterium or nonsubstituted. In this study we understood there were no change in the electrochemical peroperty by using the electrolyte substituted deuterium. The comparison between the ex-situ and in-situ diffraction data was examined by Rietveld analyses. Furthermore, we examined Rietveld analysis method for neutron diffraction pattern in in-situ measurements. We got the neutron diffraction pattern by dividing the integrated neutron diffraction data in in-situ diffraction data. For example we observed the shift of a peak derived from (101) plane equivalent to the cathode in discharge process. The change of the Li composition and the quantity of cationic mixing in 3a site and 3b site in discharge process was revealed. In addition, no cationic mixing was observed in only discharge process. The technique of the analysis for in-situ neutron diffraction was needed to be further developed. It was performed by help of NEDO (RISING) and shows thanks to the members. References 1) Y. Idemoto, Y. Tsukada, N. Kitamura, A. Hoshikawa and T. Ishigaki, Chem. Lett., 40,168-170 (2011). 2) M. Roberts, et al., J. Power Sources, 226, 249-255 (2013) Figure 1