In-Situ Charging-Discharging SEM Observation and Analysis for Si Anode in a Solid-State Battery~ Cross-Section Preparation and in-Situ SEM Observation By Stack Pressure Holder ~
Authors: Kimura Tatsuhito , Yanagihara Kota , 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.
In-situ observation and analysis of sample cross sections using a scanning electron microscope (SEM) are effective in analyzing the behavior and chemical state of Li introduced into the Si anode and the interface state between the solid electrolyte and the anode during charge-discharge. During charging and discharging, the Si anode undergoes a threefold or greater volume expansion and contraction due to Li absorption and desorption, cutting off the ionic conduction path, so it is necessary to apply a sample stacking pressure to suppress the volume expansion. A stack pressure holder is designed for operando experiments of solid-state batteries in an SEM. This holder enables smooth cross-sectioning using a broad Ar ion beam and in-situ SEM observation while applying uniform pressure which minimizes the resistance across interfacial boundaries. It also reduces the risk of specimen breakage because it fixes the sample throughout the entire air-isolated workflow, from cross-section preparation to real-time observation in the SEM.
Si anode composites were made by mixing Si particles, argyrodite sulfide solid electrolyte (SSE), and acetylene black (AB). The cathode composite was made by mortar mixing the ternary oxide cathode material LiNi1/3Mn1/3Co1/3O2 (NMC), SSE, and 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 the stack pressure holder, where a 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) while applying stack pressure. In order 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. Samples transported to the SEM in the stack pressure holder were charged and discharged at 0.2C (SOC vs. NMC reference) by a charge/discharge device (MEIDEN HOKUTO, Hz-Pro).
The observation results of the charge-discharge process on a cross-sectional sample under stack pressure using this holder are shown in Fig. 1. (a) shows the backscattered electron (BSE) composition image near the interface between the Si anode and SSE and the interface and Si anode particles are clearly observed. (b) shows the charge-discharge curve in this experiment, and (c) shows the BSE composition image after 30 minutes of charging (SOC 10% vs. NMC reference), in which the composition contrast of the Si negative electrode particles gradually changes from the solid electrolyte side due to Li absorption during charging. In the experiment, the sample was maintained at a fixed holding pressure from the time of CP cross-section processing to the SEM observation without changing the holder, and it can be assumed that the observed behavior is similar to the actual behavior. Energy dispersive X-ray spectroscopy (EDS) and soft X-ray spectroscopy (SXES) can also be performed during in-situ charge-discharge observations to obtain elemental distribution and chemical state analysis of lithium and Si. The changes in microstructure of the battery cross-section can also be captured, and the chemical state analysis and the evaluation of the optimum stack pressure at the pressures up to 50 MPa can be expected.
Acknowledgments
We would like to thank Professor Nobuya Machida of Konan University for cooperation in some of the battery sample preparation.

Figure 1