A Spatially-Resolved operando High-Energy Confocal X-ray Diffraction Method for Observing Non-Uniform Degradation Phenomena in a Practical Cylindrical Lithium-ion Battery

We report the results of spatially-resolved non-destructive operando electrode reaction analysis for practical cylindrical 18650 battery cells by using a high-energy confocal X-ray diffraction (XRD). A combination of high-energy X-rays (72 keV) and a confocal XRD method, which extracts structural information in a limited area that satisfies a confocal condition, allows us to observe electrode reactions in a cylindrical battery cell in a nondestructive way, resolving the double-side-coating electrode structure. We observed that electrode reactions were faster in the outer-part electrode than in the inner-part at the initial state reflecting intrinsic cell structure (position of current tab). For a battery cell deteriorated after 500 charge/ discharge cycles, in contrast, electrode reactions were faster in the inner-part electrode than in the outer-part, suggesting that the outer-part is more deteriorated than the inner part. The results of characterization of disassembled electrodes show that the observed slow response of the outer-electrode of a 500-cycled cell is attributed to various factors increasing resistance such as cracks in cathode particles, formation of insulating surface oxide-phase, and anomalous growth of solid electrolyte interphases (SEIs). As shown here, the high-energy confocal XRD is effective for non-destructive analysis of electrode reactions. (C) The Electrochemical Society of Japan, All rights reserved.