Operando Structural Analysis of Phase Transition of Graphite Electrode during Li De-Intercalation Process Using Synchrotron Radiation X-Ray Diffraction
Authors: Takagi Shigeharu , Shimoda Keiji , Fujimoto Hiroyuki , Kiuchi Hisao , Haruyama Jun , Okazaki Ken–ichi , Fukunaga Toshiharu , Ogumi Zempachi , Abe Takeshi 
Published: December 2020

Introduction
:

In order to promote development of electric vehicle, it is required to improve the capacity density, the low-temperature performance, and the high-rate property of the batteries. Understanding the lithium intercalation and de-intercalation mechanism at graphite electrode of the storage battery (LIB) is very important for this purpose. Crystal structure changes of lithium-intercalated graphite during charge and discharge processes were investigated by operando X-ray diffraction (XRD) measurements using a synchrotron radiation facility. Low-temperature operando XRD measurements were now available by the development of a new temperature-controlled cell jacket.1-2 This study confirmed in-plane structure change along a- and b-axis as well as stage structure change along c-axis.3 As a result, it was elucidated that crystallinity of graphitic carbon caused differences in phase transition during discharge and this phenomenon affected low-temperature and high-rate discharge performances.

Experimental
:

Coin cells composed of carbon and NMC electrodes were firstly prepared and subjected to the evaluation of the battery performances from the low-temperature (0℃) to the room temperature. Coin cells of several graphitic carbon were evaluated.
Crystal structures of lithium-intercalated graphite during discharge process were investigated by operando analysis using synchrotron radiation X-ray diffraction (SR-XRD) at BL28XU beam line of SPring-8. Al-laminated half-cells composed of graphitic carbon and Li electrodes were prepared and subjected to the analysis of operando measurement. The graphitic carbons of different crystallite sizes were analysed. Al-laminated half-cells conditioned by several cycles were used for the measurement. Operando XRD 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 to 10 s. The diffraction patterns were obtained by two-dimensional detector, PILATUS 100K (DECTRIS). Two-dimensional image data were converted to one-dimensional 2q profiles, and the diffraction peaks were separated using the Lorentz functions.
The test cell underwent an aging treatment was charged (Li insertion) at room temperature. Thereafter, structural transition were analyzed during the discharging process (Li de-intercalation) in 0.2C at room temperature and in 0.1C at 0℃.

Results and Discussion
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Figure 1 shows the SR-XRD patterns of lithium-intercalated graphite at the discharge process with current density of 0.2C at room temperature. The result showed the in-plane structure change along a- and b-axis as well as stage structure change along c-axis.
The detailed analysis of in-plane structure change and stage structure change for each graphitic carbon was performed using the operando measurements. It was identified that the crystallinity of graphitic carbon caused differences in phase transition during discharge. These results showed that crystallite sizes influence the phase transition and give major effect on LIB performances of graphite negative electrode at low-temperature or high-rate discharge. The Li diffusivity in the graphitic carbon is likely dependent on the crystallite size. The battery reaction mechanism influenced by crystallite size was elucidated by operando XRD measurements. The present results confirm that the crystallinity of the graphitic carbon affects the low-temperature and high-rate discharge characteristics. The detail results will be shown in the conference.

Acknowledgment:

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

References:

S. Takagi et al., “Operando Structure Analysis of Graphite Electrode by Synchrotron Radiation Diffraction” The 59th Battery Symposium, 2E14 (2018).

S. Takagi et al., “Operando Structure Analysis of Graphite Electrode by Synchrotron Radiation Diffraction” The 60th Battery Symposium, 1B20 (2019).

H. Fujimoto et al., “Operando analysis of charge/discharge reaction mechanism of graphite anode of Li ion battery using synchrotron radiation diffraction, Second report”, The 59th Battery Symposium, 2E16 (2018).

Figure 1