The all-solid-state fluoride battery, which uses fluoride anion as an ion carrier, is the promising high energy density storage system because multi electron reaction can be used between metal and metal fluoride electrodes [1]. Although CuF2 exhibits as a reversible charging/discharging capacity as the cathode for the fluoride battery [1, 2], the reaction rate is slow and the reaction mechanism has been poorly understood. Herein we prepared for the metal thin-film all-solid-state fluoride battery system using Cu cathode and investigated the phase transition and/or charge compensation mechanism by the electrochemical potential step measurement and the X-ray absorption spectroscopy (XAS).
Experimental
The thin-film all-solid-state fluoride battery cell was prepared by a RF sputtering method. Cu and Pb thin-films were deposited on LaF3 solid electrolyte substrate and used as the working and reference electrodes, respectively. Pt was deposited on the LaF3 substrate as the counter electrode, in which deposition/dessolution reactions of La occur. The galvanostatic charge/discharge cycling was performed at a rate of 1C (850 mA/g) at 150, 60, and 25°C. These battery assembly and the measurements were conducted in an inert atmosphere. The XAS was carried out at a beam-line of BL37XU at SPring-8 in Japan. The absorption spectra for the Cu K-edge were collected with partial fluorescence mode.
Results and Discussion
The discharge capacity and average discharge potential at 150°C are 670 mAh/g and 0.664 V vs. Pb/PbF2, respectively. The consequent gravimetric energy density with an assumption of 3 V configuration is 2010 Wh/kg. As the operating temperature decreased, the irreversible capacity and the polarization increased.
The X-ray absorption spectra exhibited an isosbestic point upon both the charge and discharge processes, indicating that the electrochemical redox of Cu/CuF2 proceeds via two-phase reaction. The obtained spectra were fitted with Cu and CuF2 spectra in X-ray absorption near edge region. The ratio of Cu and CuF2 from the fitting is consistent with the one from the electrochemically reacted values. Therefore, the Cu/CuF2 fluorinated and defluorinated reaction proceeds via the two phases consisted of Cu and CuF2 and the side reaction did not occur. Our finding provides the potential of the fluoride batteries for the future energy storage system.
To investigate the phase formation kinetics of CuF2, we performed the KJMA analysis. The both value of the Avrami exponent n estimated by electrochemically passed current and XRD are approximately 1 which indicates the phase transition is one-dimensional phase-boundary movement.
References
[1] M. A. Reddy and M. Fichtner, J. Mater. Chem., 21, 17059 (2011). [2] D. T. Thieu, M. Hammad, H. Bhatia, T. Diemant, V. S. K. Chakravadhanula, R. J. Behm, C. Kubel and M. Fichtner, Adv. Funct. Mater., 27 (2017). [3] M. A. Nowroozi, K. Wissel, J. Rohrer, A. R. Munnangi and O. Clemens, Chem. Mater., 29, 3441 (2017).
Acknowledgement
This work was supposed by JST-Mirai Program, Japan.