Visualization of Internal Potential Distribution in an All-Solid-State Fluoride Shuttle Battery Using Operando Hard X-Ray Photoelectron Spectroscopy

Abstract

Introduction Fluoride shuttle battery (FSB), which uses fluoride ions as carrier ions, has attracted much attention as one of the innovative batteries that can theoretically exceed the energy density of conventional lithium-ion batteries. However, there are still few reports on charge/discharge reactions using fluoride ions as carriers[1, 2], and most of the electrode reactions are still unclear. Here we have applied the newly developed operando hard X-ray photoelectron spectroscopy (HAXPES) to a thin film Cu-based all-solid-state FSB, which allows us to observe the bulk-sensitive electronic structure changes in all the components (positive and negative active materials, a solid electrolyte and a current collector) during charge and discharge processes. Experimental A Cu positive electrode was deposited on a LaF3 substrate by a DC magnetron sputtering method, and an Au current collector was deposited thereon by the same method. A W current collector for the anode side was deposited on the back side of the LaF3 substrate by the same method. Electrochemical tests were performed with a constant current mode under potential rages from 3.6 V to 2.0 V at 140⁰C. The Au 4f, F 1s, La 3d, Cu 2p operando HAXPES measurements was performed at BL28XU in SPring-8 during the charge process. Results and Discussion Fig. 1 shows the Cu 2p 3/2, F 1s and La 3d 5/2 operando HAXPES spectra at each charge capacity. The Cu 2p 3/2 HAXPES spectra were normalized with a peak intensity of 932.5 eV derived from Cu metal. As charging progressed, new peaks derived from CuF2 appeared at 936.8 eV and 943.0 eV, indicating that Cu converts to CuF2. In the F 1s and La 3d 5/2 HAXPES spectra, a peak shift of approximately 1.4 eV toward the lower binding energy was observed at the beginning of charging. These peak shifts correspond to the sudden increase in the cell voltages at the beginning of charging, indicating that a potential difference is generated between the LaF3 solid electrolyte and Cu positive electrode. As the charging reaction further proceeds, the peak shifts to the lower binding energy as the cell voltage increases in the La 3d 5/2 region, however, a new peak appeared at 684.7 eV to the higher binding energy in the F 1s region, suggesting that F species of CuF2 have been detected. Therefore, we succeeded in capturing the fluorination process of Cu by both Cu 2p and F 1s HAXPES spectra changes and the potential distribution of the all-solid-state FSB using operando HAXPES. Acknowledgments This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) under the “Research and Development Initiative for Scientific Innovation of New Generation Batteries 2 (RISING2)”.