Electrochemical Impedance Spectroscopy of Bi-Based Cathode for Pellet-Type All Solid-State Fluoride Shuttle Battery
Authors: Nara Hiroki , Sato Kazuyuki , Noi Kousuke , Momma Toshiyuki 
Published: December 2024

All solid-state fluoride shuttle battery (ASS-FSB) is one of the most promising rechargeable batteries due to its high energy density and cost effectiveness. The FSB operates based on conversion reactions with multi-electron transfers and does not require the use of high-cost metals like those used in lithium-ion batteries (LIBs). Therefore, the ASS-FSB is expanding research field for next generation batteries. Electrochemical impedance spectroscopy (EIS) is widely used for the analysis of energy devices such as LIB electrodes, as it can divide complicated electrochemical reactions into elemental processes based on their characteristic time constants without the need to disassemble batteries. We have reported that 3D structured electrodes can be analyzed by optimizing equivalent circuits 1–3. Although EIS is a powerful tool to understand electrochemical elemental processes in energy devices, detailed EIS analysis of ASS-FSB has not been reported. Here, we investigate the EIS analysis of a pellet-type ASS-FSB composed of a Bi-based cathode and a PbSnF4 anode, using it as an example of ASS-FSB.

A Bi-based cathode cell for EIS is composed of Bi cathode (Bi/ Ce0.95Sr0.05F2.95/AB = 25/70/5 by weight), Pb0.58Sn0.42F2 anode (Pb0.58Sn0.42F2/AB = 70/5 by weight) and Ce0.95Sr0.05F2.95solid electrolyte. The loading amount of the Bi-based cathode was 10, 20, 30 mg and that of the Pb0.58Sn0.42F2 anode was fixed to be 20 mg. A three-layer pellet cell (φ10 mm) was prepared in Ar atmosphere by applying a pressure stepwise and finally reaching 740 MPa. The cell was held with 4 bolts, followed by keeping the cell under vacuum. Prior to conducting the EIS, the cells were charge with a CC (120 µA)-CV (cut off voltage of 0.9 V, down to 40 µA) mode. The EIS was carried out after 1st charge with a frequency range from 1 MHz to 100 mHz at SoC of 100% and 140 ℃.

Figure 1 shows the charge and discharge curves of the pellet-type ASS-FSBs with different cathode amount measured at 140 ℃. The discharge capacity of the ASS-FSBs with 10, 20, 30 mg cathode loading was 323, 359, and 359 mAh/g-Bi, respectively. The discharge capacity was 84%, 93%, and 93% against the theoretical capacity of Bi, confirming that ASS-FSBs, especially those for 20 and 30 mg, were fabricated properly. Figure 2 represents Nyquist plots of pellet-type ASS-FSBs with different cathode amount measured at SoC 100% and 140℃. The Nyquist plots clearly show one semicircle in high-frequency region (see Fig.2 right) and a line with an angle of ~45゜ in middle-frequency region (see Fig.2 right) and a line with an angle of higher than 45゜ in low-frequency region (see Fig.2 left). Although the loading amount of the Bi cathode are different, the diameter of the semicircles is almost same, indicating that the semicircle is derived from a common element among the pellet-type ASS-FSBs. The length of the line with an angle of higher than 45゜ decreased with increase of the loading amount of the Bi cathode. The decrease is due to the increase of limiting capacitance 2. Meanwhile, the line with an angle of ~45゜ is characteristic feature of an ion transport property in depth direction. In addition, the length of the line increased proportionally to the loading amount of the Bi cathode, supporting that the line is derived from the reaction distribution. The results indicate necessity to adopt a transmission line model to analyze the pellet-type ASS-FSB by EIS.

In the presentation, equivalent circuit design using a transmission line model and its attribution for the pellet-type ASSFSB with Bi-based cathode will be discussed with EIS data measured with various condition on SoC and temperature.

References

H. Nara, D. Mukoyama, T. Yokoshima, T. Momma, and T. Osaka, J. Electrochem. Soc., 163, A434–A441 (2016).

H. Nara, K. Morita, D. Mukoyama, T. Yokoshima, T. Momma, and T. Osaka, Electrochim. Acta, 241, 323–330 (2017).

H. Nara, D. Mukoyama, R. Shimizu, T. Momma, and T. Osaka, J. Power Sources, 409, 139–147 (2019).

Acknowledgement

This presentation is based on results obtained from a project, JPNP21006, commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

Figure 1