Operando X-Ray CT Analysis of Silicon/Solid Electrolyte Mechanical Interface during Expansion and Shrinkage of Silicon
Authors: Matsumoto Mao , Sakka Yuya , Zhong Chengchao , Shimoda Keiji , Okazaki Ken-ichi , Yamashige Hisao , Ozeki Takashi , Matsui Toshiaki , Orikasa Yuki 
Published: December 2024

Silicon is expected to be one of the next-generation anode materials to further increase the energy density of batteries because of its high theoretical capacity. However, silicon undergoes a large volume change during charge-discharge, which results in poor cyclability. Although all-solid-state batteries are expected to mitigate the disadvantage of volume expansion and shrinkage of silicon1, the volume change of silicon can affect the mechanical interface between the silicon particle and the solid electrolyte. However, despite its importance, direct observation of the silicon particle/solid electrolyte contact interface has not yet been reported. In this study, morphological changes of silicon particles during charge-discharge were tracked by operando X-ray computed tomography (CT) measurements, and the change of the silicon particle/solid electrolyte contact interface affected by expansion and shrinkage of silicon was analyzed.
LiNi1/3Co1/3Mn1/3O2 (NCM), Li6PS5Cl (LPSC) and acetylene black (AB) were mixed in a mass ratio of 1:1:0.1 as a cathode composite, and Si, LPSC and AB were mixed in a mass ratio of 1:4.4:0.6 as an anode composite. Si | LPSC | NCM all-solid-state cell for operando X-ray CT were assembled. X-ray CT measurements were performed at the beamline BL20XU on SPring-8 using 20 keV of X-rays. Galvanostatic charge-discharge was conducted at 0.07C rate during X-ray CT measurements. The pixel resolution was 85 nm. A silicon particle was extracted from the X-ray CT images and the silicon particle/LPSC solid electrolyte contact interface was analyzed.
X-ray CT images reveal the structural changes in silicon particles by charge-discharge cycles. As the lithiation of silicon, cracks appear at the silicon particle/LPSC solid electrolyte contact interface. This indicates that due to the stress on the contact interface caused by the volume expansion of the silicon. However, these cracks are reduced by the removal of stress due to the volume shrinkage of the lithiated silicon. The elasticity of the LPSC may have contributed to this. Figure 1 shows X-ray CT image of a silicon particle after charging and discharging. As the lithiated silicon shrinks, the silicon particle/LPSC solid electrolyte contact interface undergoes significant alterations, leading to the formation of shell voids on the surface of the silicon particle. This indicates that the plasticity of the LPSC solid electrolyte is insufficient to accommodate the changes at the interface. Despite the formation of shell voids across various directions, certain areas remain in contact, ensuring a connection between the silicon particle and the solid electrolyte. This imply that silicon particles are not entirely detached from the solid electrolyte interface, partially preserving the ionic conduction pathway. The appearance of cracks at the silicon particle/LPSC solid electrolyte contact interface and the formation of shell voids on the surface of the silicon particle lead to increase in tortuosity and limitation of the reaction path, which results in poor battery performance. However, these change of silicon particle/solid electrolyte contact interface is reversible and has little effect on cycle properties after the second cycle, so these observations suggesting that silicon may be able to fulfill its potential in all-solid-state batteries.
1) W. Ping, C. Yang, Y. Bao, C. Wang, H. Xie, E. Hitz, J. Cheng, T. Li and L. Hu, Energy Stor. Mater., 21, 246-252 (2019).
Acknowledgement: This work was partially supported by the New Energy and Industrial Technology Development Organization (NEDO), JPNP20004.

Figure 1