Change of Average and Local Structures of 0.4Li2MnO3-0.6LiMn1/3Ni1/3Co1/3O2 in First Discharge Process at High Temperature

Abstract

In the last decades, applications of lithium ion batteries have expanded, and thus it is required to improve energy density of batteries. 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. In our laboratory, we have focused on xLi2MnO3-(1-x)LiMO2(M= Mn, Ni, Co) with =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 the temperature dependencies of average, local and electronic structures of 0.4Li2MnO3-0.6LiMn1/3Ni1/3Co1/3O2during first discharge process by using neutron and synchrotron X-ray total scatterings. In addition, we observed the electrode by TEM and STEM/EDS, and compared the result with those of the crystal structure analysis, and examined the structural changes of the particle surface and inside. 0.4Li2MnO3-0.6LiMn1/3Ni1/3Co1/3O2 sample was synthesized by co-precipitation method. 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 at 60 oC, 0.4Li2MnO3-0.6LiMn1/3Ni1/3Co1/3O2 electrode was able to deliver high capacity of over 280 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. In order to clarify an origin of the electrode characteristics change, some electrodes were prepared with the different discharge depths in first discharge process, and examined the average structure by Rietveld technique using neutron diffraction measurements at BL20, J-PARC and synchrotron X-ray diffraction measurements at BL02B2, BL19B2, SPring-8. As a result, it was found that the ordering of the transition metal might be changed 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 was not significant 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 using neutron total scattering measurements at BL21, J-PARC and synchrotron X-ray total scattering measurements at BL04B2, SPring-8(Fig.1).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 might be transformed to the spinel. In addition, we could observe a modulated structure of the layered structure with space group of C2/min the electron diffraction image, which corresponded to the results of PDF fitting. 1) Yasushi Idemoto, et al., Electrochimica Acta 153, 20 (2015). 2) Yasushi Idemoto, et al., J. Mater. Sci., to be published. Figure 1