Development of Operando Spectroscopy Measurement Technique Under Electrochemical Reactions
Authors: Mukofukasawa Hayate , Matsumoto Shinji , Hiraoka Koji , Seki Shiro 
Published: December 2024

Objective
To improve the performance of lithium-ion batteries, a precise understanding of the reactions occurring at the single-particle into the active material is important, as it directly influences the redox processes at the electrode. Currently, single-particle electrochemical measurement methods (SPEMs: direct terminal contact method) using micro current collector have been reported as a method for understanding the electrochemical properties of active materials. However, most reports focus on electrical characteristics, such as constant current charge/discharge tests and AC impedance measurements1)2). On the other hand, for further characterization of single active material particles, it is expected that the structural changes associated with charge-discharge reactions can be elucidated through Operando spectroscopy. However, constructing an experimental system is difficult owing to their interference between the probe and incident radiation in the direct terminal contact methods. In this study, we have developed an Operando Raman spectroscopy technique (tweezers method), which enables spectroscopic measurement of single electrode active material particles while they perform electrochemical reactions. This method involves grasping particles with tweezers, allowing for examination of the reactions occurring at the active material/electrolyte interface during electrochemical processes.
Experiment
In this study, a micromanipulator, an electrochemical measuring device, and a Raman spectrometer were assembled in a dry box (dew point less than -76.0 degrees Celsius) as shown in Fig. 1, to conduct measurements. The structural changes in the electrolyte in the diffusion layer in the electrolyte, Operando Raman spectroscopic measurements were performed. Raman spectrometer consists of a laser wavelength of 532 cm-1, a grating of 2400 gr/mm, a measurement wavenumber range of 400 to 1600 cm-1, a laser diameter of φ 1 μm, and a resolution of 4.77 cm-1.Two types of Operando Raman measurement were conducted as shown in Fig.2. In both cases, 1.0 mol / kg EC-LiFSA was used as the electrolyte, anda Cu probe of φ 30 μm was used. First, the Raman spectroscopic measurement position was fixed, and potential scanning was performed to investigate the reaction of Li desolvation at the time of Li precipitation and the Li solvation reaction during the electrochemical reaction of Li. The experiment was conducted by fixing it at a position 3.77 μm away from the electrode surface. The conditions for cyclic voltammetry measurements were set at a scan rate of 0.1 mV/s, with the measured potential ranging from 3.2V to -0.1V. Second, the electrode potential was fixed at -25 mV, and the distance from the electrode surface was changed and measured every 25 μm to 175 μm from the surface of Cu probe.
Result & Discussion
Fig. 3 shows the Raman spectrum near the Cu electrode. The peaks in the Raman spectrum from 885 to 900cm-1 and 900 to 912 cm-1 were attributed to free EC and Li+-bound EC. Calculated each peak area ratios are shown in Fig.4. The rate remained constant from the initial condition to -50mV. However, the ratio of free EC increased in the -50mV to -100mV. The increase in this ratio is attributed to the desolvation of Li+ coordinated with EC accompanying the precipitation of Li. Furthermore, during the dissolution of Li, dissolved Li+ at the electrode surface rapidly coordinated with free EC, leading to a transient increase in Li+-bound EC and causing an increase in the ratio. Fig.5 shows the Raman spectra at various distances from the electrode surface under potentiostatic conditions. Up to 25 μm from the electrode surface, Li+-bound EC was locally predominated. The locally highly ratio of Li+-bound EC at the electrode surface is attributed to the reaction of precipitating Li from solvated Li+. Due to the localized increase in Li+-bound EC at the electrode surface, free EC were predominated from 25 μm to 100 μm away from the electrode surface. Moreover, a decrease in free EC was observed from 100 μm to 125 μm away from the electrode surface. The end of the diffusion layer is located at this distance. In this presentation, we will discuss about the end of the diffusion layer in detail and the differences between the diffusion layers of the plate electrode and microelectrode.
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2)T. Saito et.al., J. Phys. Chem. C, 124, 16758-16762 (2020).
3) R. Furui et al., J. Phys. Chem. C, 127, 10748−10756 (2023).

Figure 1