Temperature Dependence of Local Structure of Layered Cathode Material 0.4Li2MnO3-0.6LiMn1/3Ni1/3Co1/3O2 during Charge and Discharge Process

Abstract

INTRODUCTION In the last decades, LiCoO2 is the most important cathode material for lithium-ion batteries because of its well-balanced electrochemical performance. However, cobalt is costly, toxic and unsatisfactory on safety. Thereby, many researchers have devoted their efforts to discover a cheaper, safer and higher capacity cathode material for more than 10 years. In this regard, the solid solutions of Li2MnO3 and LiMO2(M= Mn, Ni, Co, etc.) have attracted attention over the years because they deliver high discharge capacity of over 200 mAh/g, whereas LiCoO2 delivers a discharge capacity of about 150 mAh/g. In our laboratory, we have focused on xLi2MnO3-(1-x)LiMO2(M= Mn, Ni, Co) with x=0.4 and 0.5 because of the good electrochemical performance, and then performed average and local crystal structure analysis on the materials1). However, the influence of operating temperature has not been studied. In this work, we reported on temperature dependencies of average, local and electronic structures of 0.4Li2MnO3-0.6LiMn1/3Ni1/3Co1/3O2 during charge and discharge process by using neutron and synchrotron X-ray total scatterings. In addition, we newly observed the electrode by TEM and STEM/EDX, and compared the result with those of the crystal structure analysis. EXPERIMENTAL The preparation of the Li1.667Mn0.5Ni0.1667Co0.1667O2, corresponding to 0.4Li2MnO3-0.6LiMn1/3Ni1/3Co1/3O2, was synthesized by co-precipitation method. The product was identified by powder XRD. The composition of lithium and transition metal contents in the active material was determined by ICP. To study change of crystal and electronic structure of this cathode material during charge and discharge process at room and high temperatures (25°C and 60°C), we prepared some cathodes with various depth of discharge in first cycle and then measured the synchrotron X-ray diffraction (BL02B2, BL19B2, SPring-8) and neutron diffraction patterns (BL20, J-PARC) of these samples. The data was analyzed with the Rietveld technique using Rietan-FP and Z-code program. Moreover, the local structures were investigated using the synchrotron X-ray total scattering data (BL04B2, SPring-8) and neutron total scattering data (NOVA, J-PARC). The data was analyzed with the Pair Distribution Function method (PDF). These results were compared with electron beam diffraction image by TEM. RESULTS AND DISCUSSION From the powder X-ray diffractions, it was found that a product can be attributed to a single phase of the layered structure with space group of C2/m. In the cycle tests at room and high temperatures, 0.4Li2MnO3-0.6LiMn1/3Ni1/3Co1/3O2 electrode was able to deliver high capacity of over 270 mAh/g in the first discharge process within the voltages of 2.5 V and 4.8 V vs. Li/Li+ at the high temperature, and the capacity was higher than the value at room temperature. We prepared some electrodes with the different discharge depths in first discharge cycle, and measured the synchrotron X-ray diffraction (BL04B2, Spring-8). From the result of the measurement data (Fig. 1), it was suggested that the local structure was affected by the operating temperature. Therefore, to examine the detailed crystal structure, we determined the average structure models by Rietveld technique. As a result, it was found that the ordering of the transition metal might be changed in first charge-discharge process by the operating temperature and Ni cation mixing might be increased at high temperature. We also investigated the cation-distribution uniformity within the particle by STEM/EDS, and confirmed that there were not generally large variation in the cation distribution. In addition, we constructed local structure models by extending the refined unit cell, and then analyzed the local structure by PDF technique.Although the cathode kept the layered structure regardless of the operating temperature, the difference of the distortion parameters, σ2, of Ni-O6 octahedral of transition metal layer and Li layer was small at high temperature. It may be one of the reasons for high discharge capacity at high temperature. However, a different trend was seen in the electron beam diffraction image: that is, there was a possibility that a part of the structure was transformed to the spinel, especially around the edge of the particle at high temperature. On the other hand, at room temperature, we could observe a modulated structure of the layered structure with space group of C2/m in the electron diffraction image, which corresponded to the PDF fitting result. Reference 1) Y. Idemoto, R. Kawai, N. Ishida, N. Kitamura, J. Power Sources, 273, 1023 (2015). 2) Y. Idemoto, R. Yamamoto, N. Ishida, N. Kitamura, Electrochim. Acta, 153, 399 (2015) Figure 1