X-Ray CT Measurement of All Solid State Lithium-Ion Battery Under High Pressure Condition
Authors: Kodama Manabu , Uemura Suguru , Miyuki Takuhiro , Hirai Shuichiro 
Published: February 2020

All Solid-state lithium ion Battery (ASB) with sulfide Solid Electrolyte (SE) would achieve high energy density and high-speed charging with high ion conductivity and wide potential window of solid electrolyte, therefore, ASB can be a solution to improve the cruising distance of electrical vehicle.

The charging speed, energy density, and battery life of the ASB is depend on internal structure in the electrode due to the lithium ion is transported in the contact surface of the SE and Active Material (AM). Several studies with surface measurement of ASB have been conducted with SEM (Scanning Electron Microscope) and Laser-microscope to reveal the internal structure of ASB. In these studies, the observed area is only the surface and cross section of ASB, and is not the inside. Moreover, the ASB is mainly used under high pressure condition to suppress the ion transportation resistance in SE-SE interface and SE-AM interface and it is difficult to observe the structure under pressured condition with the SEM and Laser-microscope.

X-ray CT imaging is an only method to observe the internal structure of ASB under high pressure condition. 3D image is generated with the reconstruction of X-ray tomography images, therefore, non-destructive observation can be achieved. Several X-ray CT measurement of ASB are conducted and internal structure of ASB have been visualized. However, in these experiments, the applied pressure (9MPa) is low and X-ray CT measurement under high pressure condition should be conducted.

The pressurizing condition is also important and is not discussed in the previous studies. The ASB can be pressured by uni-axial compression and multi-axial compression. If the battery cell is enough big, the edge region of the cell is compressed uni-axially and the balk region is compressed multi-axially. The mechanical and electrochemical characteristics can be changed with these compression condition.

From the background mentioned above, we conducted X-ray CT measurement of all solid-state lithium ion battery under high pressure condition (up to 400MPa). To achieve high S/N ratio X-ray CT image under this high-pressure condition, we successfully developed a high-pressure X-ray CT cell in which the dew point is lower than -70℃ to prevent the corrosion of SE by water vapor. The multi-axial pressuring is achieved with Boron Nitride pressure medium.

The experimental results are as follows.

Figure 1 illustrates the cross-section view of ASB w/o pressure medium. Micro-scale clacks are generated at 35MPa. These clacks are generated by the crash of ASB with uni-axial compression. With the increase of pressure, the micro-scale clacks are disappeared. The gap between the side-wall and ASB is filled and side-pressure is induced, and the pressurizing is changed from uni-axial compression to multi-axial compression. The multi-axial compression flows the SE and AM in cross-sectional plane and the micro-scale clacks are disappeared.

Figure 2 illustrate the cross-sectional view of ASB with the pressure medium. In this case, the micro-scale clacks are not generated in any pressure. This finding suggests that the internal structure strongly depend on the pressurizing condition (uni-axial or multi-axial) and the uni-axial compression generates the micro-crack.

Figure 3 and 4 shows the thickness of Anode, Separator, and Cathode measured by the CT scan as a function of the applied pressure. In the case w/o pressure medium (Fig.3), the total thickness decreases by 20% from 0MPa to 35MPa due to the uni-axial compression with generating the micro-scale clacks. From 35MPa to 100MPa, the thickness decreases slightly by the refilling the clacks with the flow of the SE and AM. After the refilling, the thickness is not changed over 100MPa. In the case with pressure medium (Fig.4), contrast to the w/o pressure medium case, the compression amount is very small and is only -5%.

From these experimental results, following conclusions can be drawn.

The uni-axial compression clashes the battery structure and micro-scale clacks are generated. This micro-scale clack is refilled by solid electrolyte and active material with the increase of the applied pressure to 100MPa. The clack is not generated at 0MPa-400MPa when the compression is multi-axial compression and the decrease of the thickness is smaller than that with uni-axial compression.

Figure 1