Characterization of Carbon Properties Influencing on Lib Performance at Low Temperature
Authors: Takagi Shigeharu , Shimoda Keiji , Kiuchi Hisao , Fujimoto Hiroyuki , Okazaki Ken–ichi , Fukunaga Toshiharu , Ogumi Zempachi , Abe Takeshi 
Published: May 2020

Introduction
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In order to promote development of electric vehicle, it is required to improve the capacity density of a battery, the battery performance at a low temperature, and the high rate property of a battery. Characterization of the size distribution and the average size of carbon in the negative electrode significantly influence on the battery performance. We have studied clarification of graphitic carbon’s properties influencing on performance of LIB especially at low temperature. Firstly, the X-ray diffraction profiles of several graphitic carbons were analyzed by the fundamental parameter (FP) method. Further, the structure changes of lithium-intercalated graphite during the charge and discharge processes were investigated by operando measurement using synchrotron radiation at BL28XU of SPring-8 in Japan. The time resolution of the apparatus is the order of milliseconds for analyzing one diffraction pattern.

Experimental
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Crystallite size and its distribution of several graphitic carbons were analyzed by the fundamental parameter (FP) method using X-ray diffraction profiles. X-ray wavelength of 0.154 nm was used to obtain X-ray diffraction profiles. In order to analyze crystallite size and its distribution, X-ray diffraction profiles were calculated by using software called PDXL2 (Rigaku Corporation).

Coin cells composed of carbon and NMC electrodes were prepared and subjected to the evaluation of the battery performances from the low temperature to the room temperature. Coin cells conditioned by several cycles were evaluated.

Further, crystal structures of lithium-intercalated graphite during charge and discharge processes were investigated by operando analysis using synchrotron radiation diffraction at BL28XU beam line of SPring-8. Al-laminated half-cells composed of graphitic carbon and Li electrodes were prepared and subjected to the evaluation of operando measurement. The graphitic carbons of different crystallite sizes and their distributions were analysed. Al-laminated half-cells conditioned by several cycles were used for the measurement. Operando measurements were carried out during charge and discharge processes. X-ray of 25keV (its wavelength of 0.0496 nm) with a beam size of 0.2 mm ×0.5 mm was used to obtain diffraction patterns. The exposure time was set from 0.5 s to 10 s. The diffraction patterns were obtained by two-dimensional detector (Rigaku Corporation).

Results and Discussion
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The experimental X-ray diffraction profile of graphitic carbon and that calculated profile by the fundamental parameter (FP) method were obtained. The calculated profile showed a good fit with the measured one. The each plane of graphitic carbon was analyzed properly.

The analysis of FP method revealed that crystallite sizes and their distributions of graphitic carbons influences on LIB performance especially at low temperature. The results indicate that the low temperature characteristics improve as the crystallite size and distribution of the graphite (102) plane decreases. The same results were also obtained for the relationship between the crystallite size distribution of the graphite (102) plane and the high-rate discharge characteristics of LIB. The crystallite size distribution of the graphite (102) plane is thought to express the Li diffusivity.

Figure shows the result of synchrotron radiation diffraction pattern of lithium-intercalated graphite at the discharge process with current density of 0.2C. The stage change was analysed clearly as shown in Figure.The difference of stage change of each carbon was revealed in considerable detail by Operando measurement.

The crystallite size and its distribution influencing on the battery performance at low temperature was examined in detail. The result showed that crystallite sizes and their distributions influence on the phase structure changes and give major effect on charge and discharge performances of graphite negative electrode at low temperature. Their battery reaction mechanism influenced by crystallite size distribution was elucidated by the Operando measurement. The detail results will be shown in the conference.

Acknowledgment:

This work was supported by the Research and Development Innovative for Scientific Innovation of New Generation Battery 2 (RISING2) project of the New Energy and Industrial Technology Development Organization (NEDO).

Figure 1