Development of Operando Spectroscopy Measurement Technique for Single-Electrode Particle
Authors: Mukofukasawa Hayate , Matsumoto Shinji , Sawahashi Tamotsu , Hiraoka Koji , Seki Shiro 
Published: February 2024

Objective
Single-particle electrochemical measurements have been desired to obtain essential electrochemical information on electrode materials. Currently, an electrochemical measurement method using micrometer order current-collecting probes (direct probe contact method) has been proposed to evaluate electrochemical properties for obtaining constant current charge-discharge and AC impedance data 1) 2). On the other hand, the structural change of electrode materials and their interfaced charge-discharge can be measured (Operando) by the spectroscopic method. Direct probe connection between polished microprobe and micrometer scale active material particle prevents sufficient irradiation of spectroscopic light, such as Raman light. This study uses the Operando technique of in-situ spectroscopic measurement for electrode active material particles under electrochemical reaction conditions by grasping a single active electrode particle with precision tweezers.
Experiment
In this study, a micromanipulator, motorized precision tweezer controller, precision tweezers (SUS), and electrochemical measurement device were installed in the dry box (Dew point is less than -76.0 ℃). As shown in Fig.1, a Li foil (counter/reference electrode Φ16 mm) was attached to the SUS cup, a separator impregnated with electrolyte (1 mol-LiFSA /kg-ethylene carbonate (EC)) was placed, and a single particle of LiCoO2 (LCO; particle size 20~30 μm) was grasped and fixed into the electrolyte. After confirm the stable electrode potential of the LCO single particle held by precision tweezers, various electrochemical measurements, and Raman spectroscopy measurements were performed during the charge-discharge process.
Results and Discussion
Fig.2 shows Raman spectra of LCO powder, electrolyte, and LCO single particle immersed electrolyte. Two sharp peaks originating from the Co-O stretching vibration and O-Co-O angular vibration3) were detected in the (Ⅰ) wavenumber range. In addition, a group of sharp peaks derived from electrolyte was observed in the range of (Ⅱ), attributed to the S-N-S stretching motion of FSA- and the O-C-O stretching motion of EC4). Therefore, this system enabled to obtain of Raman spectra of both LCO and electrolyte at the same time by using precision tweezers and a single particle. This technique enabled the observation of not only the structural changes of LCO according to charge-discharge reactions but also concentration changes owing to their changes in Li cation transport of the electrolyte at the electrode interface.
To observe the solvation/desolvation behavior of the interface of electrolyte / Li metal, a micro-current collector (Cu wire: Φ 30 μm) was applied for a simplification experimental method, the Operando Raman spectroscopy measurements were performed during the Li deposition/dissolution test was conducted by cyclic voltammetry with the cell immersed in the electrolyte. The potential was swept to the negative side from OCV, and Raman spectra changed from 100mV to -100mV vs. Li/Li+. Li dendrites occurred below -100mV vs. Li/Li+, resulting in a change in focus and loss of the Raman peak. Fig.4 shows the Raman spectra at each potential between 100mV and -100mV vs. Li/Li+. Each peak in Fig.4(a) was attributed to the free FSA anion and the bound FSA anion, respectively, and peaks in Fig.4(b) were attributed to free EC and solvated EC. The change in Raman spectra of the EC peaks was larger than in the FSA anion ones. The fraction of integrated intensity of between free and solvated EC exhibited clearly change from around -50 mV. The fraction of integrated intensity of free and bound FSA anions also changed significantly from 2000 to 500 mV and rapidly returned to the initial state with the Li deposition reaction around -50 mV. From this result, the decrease in the Li cation with the dissolution reaction could be observed from the Raman peaks. Changing the dissociation properties of salt were observed from the changing peak of FSA anion. Smooth association of salt between FSA anion and Li cation was observed at initial state of Li dissolution process. We will report that sufficient metal plating will be applied to tweezers for direct observation of Raman spectra for electrolyte interface, charge-discharge tests, and obtained Raman spectra analysis results.
1) K. Dokko et.al., J. Electrochem. Soc., 148, A422-A426 (2001).
2) T. Saito et.al., J. Phys. Chem. C, 124, 16758-16762 (2020).
3) L. Le Van-Jodin et.al., J. Raman Spectrosc., 50, 1594-1601 (2019).
4) B. Klassen et.al., J. Phys. Chem. B, 102, 4795-4801 (1998).

Figure 1