Distribution Analysis of Electrode Reaction of Lithium Manganese Oxide By Means of Time-Resolved XAFS Technique

Abstract

Lithium ion secondary batteries (LIBs) are widely used as energy storage devices, and it is thus essential to evaluate the heterogeneous reaction distribution appeared in the electrode in order to improve the battery performance. The chemical state of the electrode active material is analyzed by the operando XAFS technique during the charging and discharging processes, and the 2D XAFS imaging technique provides the information about the in-plane reaction distribution. The previous study using the 2D XAFS imaging technique revealed that the heterogeneous distribution reaction appeared on the lithium iron phosphate (LFP) cathode due to the spatial difference in the electron conductivity. The distribution analyses for the other cathode materials with different crystal structure are important to understand the generation mechanism of the heterogeneous distribution. In the case of lithium manganese oxide (LMO) cathode, the heterogeneous distribution like LFP was not observed because the Li ion diffusion in the LMO particles could be enhanced by the spinel structure of LMO. In order to understand the difference of heterogeneity between LFP with the olivine structure and LMO, the vertically dispersive XAFS (VDXAFS) instrument was developed to achieve the space- and time-resolved XAFS measurements. In this study, the time course change of the chemical state map on the Mn species for the LMO cathode was revealed using the VDXAFS instrument with applying the constant voltage for the operating LIB. An LMO cathode sheet, two separator sheets, and a Li anode plate were assembled in a battery cell with Al-laminated films for the VDXAFS measurements. The cathode was composed of LMO (2 different particle sizes of 0.5-1.0 and 5-10 micrometers), acetylene black, and polyvinylidene difluoride with the mass ratio of 14:3:3. The VDXAFS measurements at the Mn K edge were performed at NW2A of PF-AR (KEK, Japan) and BL-5 of SR Center (Ritsumeikan University, Japan). The charging process at 4.5 V and the discharging process at 3.0 V were observed until the electrode current becomes less than 0.1 C. The time course change of the chemical state map for the charging process was obtained on the basis of the observed XANES spectra as shown in Figure. The blue pixel indicates that the average chemical state of Mn at that position corresponds to that before the charge, and the chemical state shifts to the higher valence state as depicted by the red pixels during the charging process. As clearly seen in Figure, the electrode reaction proceeded heterogeneously for the LMO cathode, although such heterogeneity was not observed in the previous study for constant current measurements at 0.5 C. It means that the heterogeneous distribution is dynamically appeared in the LMO cathode during the constant voltage charging process. At the beginning, the electrode current was relatively high, and about 25 reaction channels, at which the oxidation reaction proceeded earlier, were appeared in the measured linear area of 10 mm length. These reaction channels were fused during the progress of charge with decreasing the electrode current, and the chemical states of the Mn species become homogeneous when the electrode current reached less than 1 C. It is considered that the lower electric resistance at the reaction channel is a reason to generate such the heterogeneous electrode reaction on the LMO cathode in the case of large current. In this study, the heterogeneous distribution in the LMO cathode was first observed using the VDXAFS instrument during the constant voltage charging process. The present results are important to optimize the electrode reaction of LIBs. Figure 1