Change of Average and Local Structures for 0.5Li2MnO3-0.5LiMn1/3Ni1/3Co1/3O2 in First Charge Process of Different Rate
Authors: Idemoto Yasushi , Sekine Takuya , Ishida Naoya , Kitamura Naoto 
Published: February 2020

In recent years, there is an increasing demand for lithium ion batteries. It was used in small devices such as mobile phones. Recently, the expansion of the intended use of electric vehicles and power storage equipment has been expected. However, it is necessary to improve high energy density. The solid solution material Li2MnO3 –Li（Mn,Ni,Co）O2　have attracted attention because they deliver high discharge capacity. On the other hand, it was reported such a problem that this material shows large irreversible capacity in the first cycle and discharge capacity greatly decreased at the high rate. It has been considered that irreversible capacity is due to oxygen removal to take place during the initial charge process above 4.5 V. But the mechanism of the discharge capacity fade is not clear. To commercialize the solid solution materials, it is necessary to solve the problem. 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 at low rate [1].

In this study, we focused on the change of average and local structures of 0.5Li2MnO3 -0.5LiMn1/3Ni1/3Co1/3O2, layered solid solution material, during the initial charge process to clarify the arrangements of formation at the different rates.

First, the change of average structures of 0.5Li2MnO3 -0.5LiMn1/3Ni1/3Co1/3O2, layered solid solution material during the initial charge process was clarified the arrangements of formation at different rates　using Synchrotron X-ray [BL02B2, SPring-8, Japan] and neutron diffractions [iMATERIA, J-PARC, Japan}. The refined site occupancy in the charging process showed different results from before charge. The Mn occupancy localized to 4g site after charge at 1C rate whearas that moved from 4g site to 2b site at 3C rate. Although the both 4g and 2b sites were occupied by Co in the charging process at 1C rate, the only 4g site was occupied by Co in the charge at 3C rate. It was revealed that the differentiation of charge rate induced the atomic arrangement in the transition metal layer.

Second, to clarify the mechanism local atomic arrangement change, we carried out PDF analysis using powder neutron diffraction measurements and synchrotron X-ray total scattering measurement [BL04B2, SPring-8, Japan]. In addition, first-principle calculation (VASP-code) used to determine the initial structure when performing PDF analysis. The λ and σ2 of optimal model was calculated after PDF analysis, in order to investigate the distortion of the crystal structure of the charging process of different rates.In the pristine, distortion of MnO6 octahedra is small compared to NiO6 and CoO6 octahedra. Comparing MnO6 and CoO6 octahedra distortion in the initial charging process at 1C rate, MnO6 octahedra showed an increasing trend. At 3C rate, CoO6 octahedra showed an increasing trend. In λ and σ2 of NiO6 octahedra entered the Li layer by cation mixing in the charging process at 3C rate increased . This was related to the trend of localization of Mn and Co of average structure. Perhaps, MO6 octahedra to localize tend to distorted in the charging process and localized element is different by the charging rate.

Acknowledgement

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

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