Ruddlesden-Popper Type Intercalation Cathode with High Capacity Involving O-O Bond Formation for All-Solid-State Fluoride-Ion Batteries
Authors: Miki Hidenori , Yamamoto Kentaro , Watanabe Toshiki , Matsunaga Toshiyuki , Iba Hideki , Kuwabara Akihide , Miyawaki Jun , Harada Yoshihisa , Maeda Kazuhiko , Kageyama Hiroshi , Uchimoto Yoshiharu 
Published: December 2024

Fluoride ion batteries, which utilize fluoride ions as carriers, have gained attention as a candidate for the next generation of secondary batteries due to their high energy density and low resource risk independent of lithium metal. The basic operating principle is to achieve high capacity by utilizing multivalent reactions of metals during fluoride/defluoride reactions.[1] However, there are difficulties in ensuring cycling performance and rate characteristics due to significant changes in crystal structure, electronic conductivity, and ion conductivity during fluorination/defluorination. To address these challenges, we are considering the application of intercalation reactions, which are also utilized as electrode reactions in lithium-ion secondary batteries. In this presentation, we focus on Ruddlesden-Popper type La1.2Sr1.8Mn2O7F2 oxyfluoride and report on its detailed charge-discharge mechanism, as we have found that it can intercalate and deintercalate more than two fluoride ions contained in its structure.[2]

La1.2Sr1.8Mn2O7F2 was synthesized using a topochemical fluorination method with PVDF. It was mixed with La0.9Ba0.1F2.9 and VGCF to form the composite electrode, and a solid-state cell was constructed using La0.9Ba0.1F2.9 as the solid electrolyte and PbF2/AB composite electrode and Pb foil as the counter electrode. Charge-discharge reactions were carried out at 140°C in the voltage range of -1.5 to 3.0 V, and after evaluation, X-ray diffraction (XRD) measurement, scanning transmission electron microscope (STEM) observation, X-ray absorption spectroscopy (XAS) measurement, and resonant inelastic X-ray scattering (RIXS) measurement of O K-edge were performed.
The initial discharge capacity of La1.2Sr1.8Mn2O7F2 showed 91 mAh/g, confirming that almost all of the fluoride ions in the structure were topochemically defluorinated. In the charge reaction, it was confirmed that reversible insertion of fluoride ions occurred mainly due to charge compensation of manganese, the transition metal, up to 1.5 V. Furthermore, at higher potentials, a new plateau appeared around 2.0 V, indicating further insertion of fluoride ions into the structure, and it exhibited a high capacity of nearly 200 mAh/g even during subsequent discharge. XAS and RIXS measurements of O K-edge revealed that charge compensation was achieved by oxide ions, and the formation of O-O bonds, as which is reported in lithium-excess cathode material[3], was observed. These results demonstrate the potential of intercalation materials to achieve high capacity, high cycling performance, and high power characteristics.

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[1] M. A. Reddy, M. Fichtner, J. Mater. Chem. 2011, 21, 17059–17062.
[2] H. Miki, K. Yamamoto, Y. Uchimoto, et al., J. Am. Chem. Soc. 2024, 146, 3844–3853
[3] R. A. House, P. G. Bruce, et al., Nat. Energy 2020, 5, 777–785.
Acknowledgement: This research was financially supported by JST-Mirai program (JPMJMI18E2), JSPS Grant-in-Aid for Scientific Research on Innovative Areas, “Mixed-Anion” (JP16H06438, JP16H06440, JP16H6441), JSPS Grant-in-Aid for Transformative Research Areas (A) “Supra-ceramics” (JP22H05148, JP22H05146) and JSPS Grant-in-Aid for Scientific Research (B) (JP21H02048).