(Invited) Investigation of Electric Double-Layer Capacitor Electrodes with Time-Resolved SAXS and Thickness Measurements

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Small-angle X-ray scattering (SAXS) describes the diffuse scattering patterns or diffraction phenomena that occur within the small-angle region relative to the incident X-ray during interaction with a sample. This non-destructive analytical method has been utilized to analyze structures ranging from sub-nanometer sizes up to approximately 100 nm. In porous carbon materials often used as electrodes for electric double-layer capacitors (EDLCs), this size range is critical. SAXS can determine the size of micropores, which are crucial for the specific surface area important for practical applications, as well as mesopores and macropores, which facilitate substance diffusion. The use of synchrotron radiation now enables SAXS measurements to be conducted in extremely short times, facilitating various in situ and time-resolved operando analyses of electrode structures in electrochemical devices. Consequently, the structural changes in electrode materials during charge/discharge can be monitored in real time.In this study, we performed operando SAXS measurements on both positive and negative carbon electrodes for EDLCs. To accurately interpret the SAXS data obtained, it is necessary to measure both the intensity of the incident and transmitted X-rays and to correct for background and thickness. Although the measurement of incident and transmitted light intensity has become commonplace, accurately determining changes in thickness during charge/discharge simultaneously with SAXS measurements remains challenging. Although previous studies have measured and discussed changes in the thickness of EDLC electrodes,1,2 SAXS enables analysis of these changes on a much finer scale during the expansion of electrodes associated with charge, thereby deepening our understanding of the underlying processes. This report discusses the results of time-resolved operando SAXS measurements and electrode thickness measurements conducted during charge/discharge tests.We used two types of electrodes: a typical composite electrode consisting of activated carbon YP50F, and plate-like activated carbon with a stronger structure and controlled macropores, tailored for EDLC applications. Propylene carbonate with a concentration of 1 mol dm−3 of 5-azoniaspiro[4.4]nonane Tetrafluoroborate (SBPBF4) was used as the electrolyte in the construction of the EDLC. SAXS intensity measurements were performed on both the positive and negative electrodes by placing a hole in the center of one electrode, enabling X-rays to be directed at only one electrode at a time. Charge/discharge tests were conducted at a current density of 80 mA g−1 relative to the weight of the electrode with a hole, within a voltage range of 0 to 2.5V. The SAXS experiments were performed using the apparatus at the BL-6A station of the Photon Factory (PF) in the Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Tsukuba, Japan. The thickness change of the electrode during charge/discharge was measured with a resolution of 0.1 micrometer for each of the positive and negative electrodes. Both SAXS and thickness measurements were performed at 40 °C.As demonstrated by Prehal et al. in 2015 for aqueous EDLCs,3 changes in SAXS intensity were also observed in both the positive and negative electrodes in organic electrolyte-type EDLCs. These changes occur throughout the scattering pattern, from micropores crucial for capacitance to meso and macropores important for mass diffusion. These intensity changes are induced by the adsorption and desorption of ions. Analysis of the regions where the scattering intensity dominated by micropores revealed an increase in pore size for both the cathode and anode during charge. Furthermore electrode thickness measurements indicated that both the cathode and anode expand during charge, with the anode expanding more significantly due to the larger size of the cations compared to the anions. The slow change in SAXS intensity of the negative electrode at a charge voltage of around 2.5 V may also be due to the size of the cation.Thus, the results of SAXS and thickness measurements can be discussed in a complementary manner and will provide new insights into organic electrolyte-type EDLCs. In the future, these findings may also prove useful in evaluating the durability of these devices under high-temperature and high-voltage charge conditions.[1] M. Hahn, O. Barbieri, F. P. Campana, R. Kötz, and R. Gallay, Appl. Phys. A, 82, 633 (2006).[2] P. Bujewska, P. Galek, and K. Fic, Energy Storage Mater., 63, 103003 (2023).[3] C. Prehal, D. Weingarth, E. Perre, R. T. Lechner, H. Amenitsch, O. Paris, and V. Presser, Energy & Environ. Sci., 8, 1725 (2015).