Compton Scattering Imaging for in-Situ and Operando Analyses of Working Batteries
Authors: Sakurai Yoshiharu , Itou Masayoshi , Nagamine Masaru , Inoue Taku , Chiba Shigeki , Kayahara Yoshihito , Sakurai Hiroshi , Suzuki Kousuke , Orikasa Yuki , Sato Kazuaki , Yamashige Hisao , Sakano Mitsuru , Nakano Tomohiro , Uchimoto Yoshiharu 
Published: February 2020

An ever-increasing demand for high-performance batteries has spurred a research activity in search of proven, in-situ and operando techniques for monitoring the electrochemical reactions and structural changes that occur inside a working battery. Here we propose an X-ray imaging technique based on synchrotron X-ray Compton scattering and demonstrate its application to batteries. The technique uses monochromatic X-rays as the probe, and detects Compton scattered X-rays from a certain volume element inside a battery. Both X-rays are higher than 100 keV with high penetration power into materials and X-ray Compton scattering is sensitive to the constituent elements [1], which enables the electrochemical and structural analyses under the in-situ and operando condition. In this paper, we present the outline of Compton scattering imaging (CSI), its application to a commercial coin battery and the future direction of this technique.
The CSI technique provides three-dimensional images of the electron density distribution of an object without the need of observations from all surrounding directions [2, 3]. Because of high sensitivity to low-Z element materials, the technique with a conventional X-ray source or gamma-ray source has been used in the medical field and in the food and agricultural industries. The drawbacks of the technique were the relatively low counting rate and large probing volume element, but these have been overcome by the use of synchrotron X-rays and related focusing X-ray optics [4]. Synchrotron radiation based CSI (SR-CSI) has now been developed to such that it can provide the dynamic images of the electron density distribution inside a working battery.
As a demonstration, we have measured the intensity of Compton scattered X-rays from the discharging coin cell (CR2032) as a function of one-dimensional position inside the cell and discharging time (see Figure). The position-time intensity map captures the migration of lithium ions in the positive electrode and reveals the change in position of the separator due to the volume expansion of the electrode. The experiment was performed at the BL08W beamline of SPring-8, Japan. The incident 115 keV X-ray beams with a size of 20μm(v)x500μm(h) were guided to the coin cell and Compton scattered X-rays from a volume element inside the cell were collected with a Ge solid state detector at a scattering angle of 90 degrees. Placing a slit with a width of 500μm in front of the detector, the volume element at each position was 20μmx500μmx500μm. This demonstration is a critical step for further development.
We are developing the SR-CSI technique for larger Li batteries such as those mounted in plug-in hybrid electric vehicles or in electric vehicles. The challenges are to reduce the probing volume element and to enhance the detection efficiency. We will present the future direction along this line.
This work is supported by the Development of Systems and Technology for Advanced Measurement and Analysis program under Japan Science and Technology Agency.
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