In-Situ Charging-Discharging SEM Observation and Li Analysis for Si Anode in All Solid-State Battery By Using SEM-EDS-SXES Method
Authors: Yanagihara Kota , Kimura Tatsuhito , Yamamoto Yasuaki , Matsuda Reiko , Hikima Kazuhiro , Matsuda Atsunori , Sasaki Yoshikazu 
Published: December 2024

Next-generation solid-state batteries are expected to offer improved capacity and safety. The increase in capacity depends on the negative electrode material, and high-capacity materials such as Si, which can absorb large amounts of Li, are the subject of research. The amount of Li transferred can be measured electrically. However, it has not been possible to visualize where Li is distributed within the anode particles or to analyze the chemical state of the anode particles in real-time. Therefore, we installed a system on the SEM that enables real-time observation and analysis during charging and discharging (in-situ), which is an important technology in research and development.

This system has the following three features.

1) A tool that can maintain confining pressure and transport the battery from cross-section processing to SEM observation

2) Live imaging to visualize Li behavior during charging and discharging

3) Chemical state analysis of materials such as Si negative electrodes

The above three features are possible by combining the following devices and detectors; 1) a holder that maintains stack pressure and can be used together with processing and observation instruments, 2) a Windowless EDS detector “Gather-X” that can detect low characteristic X-ray energies like Li (54eV), and 3) a soft X-ray spectrometer (SXES) that enables local chemical state analysis.

Using this system, we were able to visualize the behavior and distribution of Li intercalating into Si particles during charging and discharging and analyze the transformation of anode particles into crystals, alloys, and amorphous states.

The Si anode composite was made by mixing Si particles and polyimide and applying a slurry. The cathode composite was made by mortar mixing the ternary oxide cathode material LiNi1/3Mn1/3Co1/3O2 (NMC), argyrodite sulfide solid electrolyte (SSE) and acetylene black (AB). Each component was put into a pelletizer, stacked, and pressurized at about 500 MPa to produce a full cell pellet. The pellet was cut into 4.8 mm squares using a precision punching tool for highly brittle materials (NOGAMIGIKEN, NC-CE-SS) and placed on a constrained charge/discharge holder, where a constraining pressure of 25 MPa was applied.

The cross-sections were prepared with an Ar ion beam (acceleration voltage: 5 kV, cooling temperature: -120 °C, processing time: 5 hr) using a cross-section preparation system (JEOL, Cooling Cross Section PolisherTM, IB-19520CCP).

To minimize sample deterioration due to exposure to the atmosphere, glove boxes and transfer vessels that can be closed to the atmosphere were used for the entire process from pretreatment to processing to observation. A Schottky FE-SEM (JEOL, JSM-IT800) equipped with a Windowless EDS (JEOL, DrySDTM Gather-X) and a soft X-ray spectrometer (JEOL, SS-94000SXES) was used for observation and analysis. SEM- EDS-SXES analysis was performed while charging and discharging at a C rate of 0.2 C (vs. NMC standard) using a battery charge/discharge device (HOKUTO DENKO, Hz-Pro) and a restrained charge-discharge holder in the SEM.

The result of SEM-Windowless EDS analysis (backscattered electron (BSE) and EDS MAP images) at SOC 0% to 10% is shown in Fig.1(a). Expansion of Si particles and changes in composition contrast were observed in the BSE composition image. From the EDS MAP, it was confirmed not only from contrast changes but also from characteristic X-ray information that Li was gradually inserted and alloyed toward the anode end of the solid electrolyte side of the Si particles in contact with the solid electrolyte interface. It was observed that Li was selectively inserted into some of the Si particles that were in contact with the solid electrolyte interface.

The results of SXES analysis of specific Si particles, when the sample was charged to 0%, 10%, 20%, and 40% SOC and then discharged, are shown in Fig.1(b). The peak intensity of Li-K gradually changed as the charge rate changed. The peak position of Li K is about 53.4 eV, suggesting that Li-Si alloying is in progress. On the other hand, the half-width and peak shape of Si-L also gradually changed with the change in charge rate. After discharge, the Li-K peak intensity became lower, and the Si-L peak shape was like that of amorphous silicon, indicating that the chemical state of the Si particles changed due to the desorption of Li.

These results show the continuous change of Si particle shape with Li insertion/extraction in the process of charging/discharging a single Si particle and the change of Si chemical state from crystalline silicon to amorphous state due to Li-Si alloying and discharge.

Acknowledgments

We would like to thank Professor Nobuya Machida of Konan University for cooperation in some of the battery sample preparation.

Figure 1