Change of Average and Local Crystal Structure and Electronic Structure of Li-Rich Solid Solution Cathode Material 0.4Li2MnO3-0.6LiMn1/3Ni1/3Co1/3O2 during Charge-Discharge Process Using First-Principles Calculations and Neutron Beam and Synchrotron X-Ray Sources

     In recent years, lithium-ion battery has been expected in order to meet the requirement for the electric vehicles and electric power. The solid solutions of Li₂MnO₃ and LiMO₂ (M = Mn, Ni, Co, etc.) have attracted attention over the last 10 years because they deliver high discharge capacity[1]. On the other hand, it was reported as the one of the problems that discharge voltage of this material decreases by repeating charge and discharge process. But the mechanism of the voltage fade is not clear. To commercialize the solid solution materials, it is necessary to solve the problem. In this study, we focused on the above problem. Our previous report described a transition metal rearrangement that occurs between 4.6 and 3.3 V during the initial discharge process[2].

     We synthesized the 0.4Li₂MnO₃-0.6LiMn_1/3Ni_1/3Co_1/3O₂ by coprecipitation method, and its average crystal structure was discussed by Rietveld analysis using powder neutron diffraction measurements(BL20, J-PARC) and synchrotron X-ray diffraction measurements(BL02B2, SPring-8). In addition, we carried out first-principle calculation (VASP-code, WIEN2k-code) in order to discuss the electronic state theoretically. Also, we used Pair Distribution Function (PDF) analysis using synchrotron X-ray total scattering(BL04B2, SPring-8) and neutron total scattering(BL21, J-PARC) in order to discuss the detailed local structure. To discuss the structural changes associated with the repeating charge and discharge process, we carried out X-ray absorption spectroscopy in addition to VASP-code and PDF analysis using synchrotron X-ray total scattering.

    First, the average and local crystal structures, and the electronic structure of  0.4Li₂MnO₃-0.6LiMn_1/3Ni_1/3Co_1/3O₂ were investigated using a combination of a pair  distribution function (PDF) analysis and first-principles calculations.  The results of a Rietveld analysis using neutron diffraction data showed that  0.4Li₂MnO₃-0.6LiMn_1/3Ni_1/3Co_1/3O₂ belonged to space group C2/m. Based on the site occupancy results, Mn was localized at 4g sites, and Ni and Co were present at both 4g and 2b sites.  Both the maximum entropy method using synchrotron  X-ray diffraction data, and calculations using WIEN2k, showed that the covalencies  between Mn at 2b sites and O at8j sites in the transition metal layer, and between 2c and 8j sites in the lithium layer,  were  relatively low. 

      A PDF analysis using both synchrotron X-ray and neutron total scattering data produced good fits to the observed data, and identified local differences in the average structure, such as the precise locations of Li and O. Although some distortion existed, the refined structure contained Mn-O₆, Ni-O₆, and Co-O₆ tetrahedra.  The bond lengths corresponded to the ionic radii, so that the valence of  Mn,  Ni and Co in this material before charging was tetravalent, between divalent and trivalent, and trivalent, respectively. These results were consistent with the valence obtained from the density of states calculated by the WIEN2k code, thus confirming  that the local structural model was appropriate.

   Second, the structural changes associated with the repeating charge and  discharge using  after  first discharge  and after fifth discharge were investigated.  By the comparison  of this material before charge, after first discharge and after fifth discharge using PDF analysis, it was indicated that Li deviated from the plane of Li layer associated with the repeating charge and discharge. While the bond distance of Mn-O and Co-O was longer, that of Ni-O was shorter. From the results, the both the valences of Mn and Ni approached to trivalent.  These local  structural  changes related to the decrease  of discharge voltage  by repeating charge and discharge.

Acknowledgement

  This work was supported in part by JSPS KAKENHI Grant Number 25420718.

 [1] M.M. Thackeray, et al., J. Mater. Chem. 15, 2257 (2005).

 [2] Y. Idemoto et al, Electrochimica Acta, , 153, 399(2015).