Evaluation of Effect of Volume Expansion on Cell Performance of All-Solid-State Batteries with 1D Simulation
Authors: Nunoshita Keita , Hirate Ryusei , So Magnus , Inoue Gen , Kimura Naoki , Tsuge Yoshifumi 
Published: December 2020

The development of an all-solid-state lithium-ion battery (ASSB) as a next-generation storage battery is being promoted from the viewpoints of higher energy density and safety, but higher power density is an issue. One of the factors is that the interface resistance is high. At present, research is being actively conducted to reduce interface resistance by applying external stress. However, it is not clear how the volume change accompanying the lithium desorption and insertion of the active material particles affects the interface resistance. Therefore, in this research, we constructed an analysis model incorporating these effects and examined the effects on battery performance.
The electrode layer is a three-phase uniform porous media including an active material (AM), a solid electrolyte (SE), and void space. There was no temperature distribution inside the battery, and the concentration distribution of lithium ions in the electrolyte was uniformly constant. In this research, based on the porous electrode theory [1], the effects of expansion and contraction of the active material were incorporated. The change in the thickness of the electrode layer was calculated from the balance between the local strain and the force of the entire cell. Assuming that the active material is a rigid body and the solid electrolyte is an elastic body, the reaction interface area was calculated by incorporating a geometric model that changes depending on the lithium concentration of the active material into the Random capillary model [2]. The change in tortuosity was calculated assuming that the change was a function of the volume fraction of each phase. The external stress dependence of the contact interface is given by equation of Tian et al [3]. The positive electrode active material is LiCoO2 (LCO), the negative electrode active material is graphite, and the high expansion negative electrode assumed to be 3 times the capacity of Si (virtual Si whose expansion volume at full charge is 3 times the volume before charging). As the solid electrolyte, Li10GeP2S12 was used. The volume fraction of the active material was set to 0.4, the porosity was set to 0.3, and the solid electrolyte ratio was set to 0.3. In the case of a Si negative electrode, a rise in surface pressure from the inside of the cell to the outside due to expansion of the Si active material upon charging has been reported [4]. In this study, this internal pressure is assumed to be proportional to the lithium intercalation rate of the negative electrode active material in the local area of the electrode layer. And it is treated as the force applied to the electrode-solid electrolyte interface together with the cell fastening pressure. The initial stress was 1 to 100 MPa, and the thickness of the positive electrode, negative electrode, and SE layer was 60 µm, 60 µm, and 20 µm. The Li ion conductivity of the solid electrolyte was set to 1.2 × 10-2 S/cm [5].

Figure 1 shows the charging curves of negative electrode graphite (a) and Si (b) at an external stress of 1 to 100 MPa. Comparing the two charging curves with the same external stress, it can be confirmed that the charging voltage of Si is lower than that of graphite and the charging rate (SOC) is higher. This is because the increase in internal pressure due to lithium insertion is large due to a large expansion rate, and the effective contact interface is increased due to an increase in stress applied to the interface. This reduced the reaction overpotential and led to an increase in the active material used. Figure 2 shows the SOC distribution at the external stress of 50 MPa. In the case of graphite having a low expansion rate, an effective contact interface is insufficient, and distribution occurs in the thickness direction. In general, expansion should be reduced to improve battery performance, but this research suggests that in order to form an effective solid-solid contact interface, expansion may be effective depending on the design of the expandable battery and fastening conditions.
Acknowledgment
This research was supported by Grants-in-Aid for Scientific Research on Innovative Areas, “Science on Interfacial Ion Dynamics for Solid State Ionics Devices” MEXT, Japan FY2019-2023.
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Figure 1