[Image from Nara Women’s University]
A new cathode material for all-solid-state fluoride-ion batteries (FIBs) delivers a reversible capacity of approximately 550 mAh/g. That’s more than double the 120–250 mAh/g typical of lithium-ion cathodes.
A Japanese language article in Nikkei reported that the volumetric capacity of a cathode for such fluoride-ion batteries increased by roughly three times that of lithium-ion batteries. When fully assembled into a battery, this could yield a volumetric energy density over twice that of lithium-ion batteries, potentially doubling the driving range of electric vehicles (EVs) from 600 km to 1,200 km. The Nikkei article notes that practical use is likely to arrive after 2035.
A novel super-ceramic
The research, spotlighted in the January issue of the
Journal of the American Chemical Society and on
Science Japan, relies on a novel super-ceramic material based on copper nitride (Cu₃N). Unlike traditional lithium-ion batteries, where each atom releases one electron, each nitrogen atom here releases up to three electrons during the discharge cycle.
Kyoto University scientists, led by Associate Professor Kentaro Yamamoto (now at Nara Women’s University) and Professor Yoshiharu Uchimoto, collaborated with researchers from Toyota Motor Corporation, the University of Tokyo, the University of Hyogo, Tohoku University, and the Institute of Science Tokyo.
In recent years, fluoride-ion batteries have gained in popularity given their potential in next-generation energy storage. That’s in large part a result of their potential for improved safety given the presence of solid electrolytes and fast fluoride ion conduction. Fluoride ions, being monovalent and small, move quickly in solids. However, previous fluoride-ion cathodes had limitations. For instance, metal fluoride cathodes often suffered from poor cycle life, while topotactic intercalation materials typically offered lower capacities. This new Cu₃N material appears to address both issues.
The role of copper and nitrogen
The high capacity is attributed to a charge storage mechanism that draws on both copper and nitrogen redox. In the process of charging, molecular nitrogen (N₂) is formed within the cathode. This process, tested at the SPring-8 synchrotron facility using X-ray absorption spectroscopy and resonant inelastic X-ray scattering, facilitates the insertion of a larger number of fluoride ions than expected from the crystal structure alone.
In the Cu₃N structure, which has an inverse ReO₃ structure, the 2-fold coordination of copper creates anionic vacancies. These vacancies provide space for fluoride ions to intercalate beyond what might be predicted from a typical crystal structure.
In the long run, the research team aims to further optimize the material by controlling nitrogen’s redox behavior during charging and discharging. The goal is to achieve even higher capacities.
