Complex Influence of Stack Pressure on BiF3 Cathode Materials in All-Solid-State Fluoride-Ion Batteries

Abstract

Among all the alternative battery systems beyond lithium-ion batteries (LIBs), all-solid-state fluoride ion batteries (ASSFIBs) are particularly promising due to their high theoretical energy density, thermal stability, and recent advancements in room-temperature superionic solid electrolytes and intercalation-type electrodes. However, their practical application is hindered by poor cycling stability and limited rate capability, largely attributed to unfavored kinetics and interfacial degradation, especially in conversion-type cathodes. Previous studies have shown that the application of stack pressure can significantly improve the cell’s cycling stability. To reveal the underlying mechanism, this study systematically investigates the impact of stack pressure on the electrochemical performance of ASSFIBs using BiF₃|BaSnF₄|Sn cells. Among the tested conditions, the best enhancement of cycling stability and rate performance were demonstrated under 180 MPa. Furthermore, ex-situ diffraction analysis revealed pressure-dependent phase evolution and oxygen-related interfacial degradation (i.e., BiOF or BiO_0.1F_2.8 formation) in the BiF₃ cathode during the first cycle. Through in-situ electrochemical impedance spectroscopy combined with distribution of relaxation times analysis we identified charge transfer and F⁻ diffusion as the dominant state-of-charge dependent kinetic limitations, with strong correlation to phase transitions within the BiF₃ cathode composite. These findings emphasize the critical role of stack pressure in mitigating interfacial degradation and optimizing ion transport, providing valuable insights for the design and operation of high-performance ASSFIBs.