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Operand Study of Reaction Distribution in Cross-Section of Composite Electrodes by Energy-Scanning Confocal XRD
It is well recognized that reaction distribution inside electrodes greatly affects battery characteristics while its detailed behavior remains unclear. The reaction distribution may trigger low utilization of the active material, accelerated deterioration of the rechargeable capacity, and safety issues caused by partially enhanced current density. The ex-situ analysis of the reaction distribution in cross-section of the composite electrode has been previously reported [1]. However, there has been no report on in-situ techniques for the reaction distribution measurement and therefore its dynamics during the charge-discharge processes has not yet been clarified. Based on these, we here report the development of the energy-scanning confocal XRD method that can capture the reaction distribution in cross-section of the composite electrode in operando during the charge-discharge process.
2. Experiment
An aluminum pouch-type three electrode cell was used in which lithium metal foil was used as a counter electrode and a reference electrode. The working composite electrode consisted of LiNi1/3Co1/3Mn1/3O2, acetylene black as a conductive additive, and PVDF as a binder mixed in a 90 : 5 : 5 wt% ratio and was coated onto Al foil. The electrolyte was 1M LiPF6 / EC : EMC (3 : 7). The cell was assembled in an Ar-atmosphere glove box. The XRD measurement was performed at BL28XU at SPring-8 (Hyogo, Japan). The confocal area consisting of incident and diffraction beams was set in the cross-section of the electrode to give high spatial resolution XRD analysis while the energy of the incident monochromatic X-ray was continuously scanned to form a XRD spectrum. Observation points in the electrode were changed in turn by moving the height of the sample stage.
3. Result and discussion
Figure 1 shows the change of diffraction 113 peaks during the discharge process of the Li0.5Ni1/3Co1/3Mn1/3O2 electrode at a rate of 139 mA/g. A shift of the 113 peak by Li insertion into the active material was observed all the three positions in the cross-section of the electrode, however the peak positions at the discharge end were different in each position. In a rest period, the 113 peaks value at the counter electrode side and the center were decreased, and that at the current collector side was increased. This suggests that the reaction distribution arose in the cross-section of the electrode during the discharge process, then was relaxed in the rest period. The converged 113 peak positions after relaxation matched well with that expected for the discharged capacity. It is revealed that the reaction proceeds at the counter electrode side faster than the current collector side and this newly developed method have a sufficient spatial resolution and time resolution for the measurement of the reaction distribution in cross-section of the electrode.
Acknowledgement
This work was supported by the “Research and Development Initiative for Science Innovation of New Generation Battery (Rising project)” of the New Energy and Industrial Technology Development Organization (NEDO), Japan.
References
[1] J. Liu, M. Kunz, K. Chen, N. Tamura and T. J. Richardson, J. Phys. Chem. Lett., 1, 2120 (2010).