X-Ray CT 3D Structure Measurement and Performance Evaluation of All Solid-State Lithium-Ion Battery Anode
Authors: Kodama Manabu , Komiyama Shohei , Hori Satoshi , Suzuki Kota , Kanno Ryoji , Hirai Shuichiro 
Published: May 2020

X-ray CT and battery performance measurements of LiIn-SE-Anode (SE + Graphite) half cell are conducted to elucidate the relationship between electrode performance and 3D internal structure. LGPS that is one of the highest lithium-ion conductive solid electrolyte are used in this study. Fig.1 illustrates an experimental setup in this study. The anode half sell is installed in high pressure X-ray CT measurement jig. The X-ray CT are conducted with In-situ high pressure condition (100MPa) to measure the volume ratio of SE and graphite and tortuosity of SE network those can be changed with decompression. The battery performance are measured by a potentiogalvanostat with CC charging and discharging and an electrochemical impedance spectroscopy (EIS) are conducted to measure the overpotential in the cell. The performance measurement and EIS are conducted for the same X-ray CT measured anode half sell, therefore, the battery performance can be directory discussed with 3D structure.

Figure 2 illustrates the ratio of actual capacity to theoretical capacity as a function of electrode thickness and volume ratio of active material (graphite). The highest capacity ratio is obtained for thin electrode (30um) and low active material volume ratio (50%) and is decreased by increase in the electrode thickness and the active material ratio.

Table.1 shows the electrode structure parameters measured by the X-ray CT and high-frequency and low-frequency resistance measured by the EIS. The high frequency resistance of the anode half-cell is originated from overpotential of lithium ion transportation in SE network and activation overpotential on graphite and that of low-frequency resistance is diffusion of Li-ion in graphite particle. From table.1, in the case with graphite ratio of 50%, both high frequency and low frequency resistance are increased with increase in the electrode thickness. It is considered that the low frequency resistance is increased with the increase of SE network resistance by increase in the length of the lithium-ion transportation length. The increase of the electrode thickness decreases the lithium ion flux for each graphite particle and the diffusion in the graphite is suppressed and the low frequency resistance is increased. The increase of graphite volume ratio dramatically decreases the ratio of actual capacity to theoretical capacity as shown in fig.2. The tortuosity of SE network that has a positive correlation to the resistance of the SE network is increased with the increase in the graphite volume ratio. It can be a reason of the increase of the high frequency resistance. Moreover, the specific surface area between SE and graphite is decreased with the increase in graphite volume ratio. It means the graphite is condensed in the electrode and the contact surface of the graphite to SE is decreased and the diffusion length of lithium ion in the graphite is increased. This suppression of the lithium ion diffusion in graphite particle is corresponds to the increase of the low frequency resistance as shown in table 3.

From the mentioned above, it can be said that the battery performance improvement with high volume ratio of graphite for would be achieved with the improvement of the dispersiveness of graphite and ion conductivity of the solid electrolyte.

Figure 1