The Effect of Separator and Anode on Electrochemical Characteristics and Crystal Structure of Lithium Ion Battery Cathode Material 0.4Li2MnO3-0.6LiMn1/3Ni1/3Co1/3O2
Authors: Kasai Noriko , Fujishima Ryota , Kitamura Naoto , Ishida Naoya , Idemoto Yasushi 
Published: December 2020

Commonly, studies of lithium ion battery (LIB) cathode materials are performed by using lithium metal anode and olefin-based microporous separator. On the other hand, carbon-based anodes and various separators are used in commercial lithium ion batteries. In this work, we studied characteristics of a solid solution cathode material 0.4Li2MnO3-0.6LiMn1/3Ni1/3Co1/3O2, using graphite as the anode and influence of separator type on structural change of the cathode material was studied.

0.4Li2MnO3-0.6LiMn1/3Ni1/3Co1/3O2 was synthesized by co-precipitation method. The obtained material was characterized by XRD and ICP-AES. The XRD data showed that all major peaks of the synthesized material can be assigned to monoclinic C2/m. We combined three types of separators, polypropylene microporous membranes, OZ-S25 (ceramic coated PET nonwoven) and FPC3012 (nonwoven composed of PET and cellulose) with this anode and cathode.

The charge-discharge cycle tests were performed in a bipolar cell using graphite as the anode. When the cathode material was combined with graphite anode, abnormally fast deterioration of cell capacity was observed. To avoid the deterioration, the graphite anode was preprocessed by charging and discharging using lithium metal as the counter electrode. The effect of the preprocess was sufficient only when the Li amount contained in the anode corresponds to at least 10% of the fully charged state. The preprocess could be achieved also by using LiMn1/3Ni1/3Co1/3O2 as the counter electrode. The reason of this improvement is unclear, but SEI formation on graphite anode surface, Li insertion into graphite, or drop in the anode potential might be the reason.

Charging and discharging were performed for 5 cycles at 0.1C and 50 cycles at 1C in the voltage range of 2.0 to 4.7 V vs. preprocessed graphite using the various separators above. The figure shows charge and discharge curve for 0.4Li2MnO3-0.6LiMn1/3Ni1/3Co1/3O2 and the preprocessed graphite cell (separator: OZ-S25). The preprocess of the graphite anode stabilized the cell capacity. After the 5th and 55th discharge, the cathode materials were taken out and the average structure was examined by Rietveld method using neutron diffraction measurements at BL20, J-PARC and synchrotron X-ray diffraction measurement at BL19B2, Spring-8. Furthermore, the valence of transition metals after cycle tests was evaluated on XAFS at BL14B2, SPring-8. As a result, with all separators, it was found that the Ni occupancy of the 4g sites, a transition metal layer, decreased, and that of 2c sites, a Li layer, increased. It was also found that the valence of the transition metal after 5 cycles did not differ between the separators. On the other hand, the bond valence sum at each site tended to decrease at the 4g sites and the 2c sites and increase at the 2b sites after 5 cycles. These were the same regardless of the type of the anode, metallic lithium or graphite. It was found that the average crystal structure of the solid solution cathode after 5 cycles was independent from the separator type and the anode type. The result after 55th discharge will be reported on the presentation.

Figure 1