Fluoride-ion conversion alloy for fluoride-ion batteries

To realize the full potential of fluoride-ion batteries with a significantly high energy density (larger than 1000 W h kg(-1)), it is a prerequisite to develop another conceptual cathode material beyond the current pure metals. This is because pure metals can only be fluorinated on the surface with typically less than 10 nm depth. To overcome this issue, we show here that well-established hydrogen storage alloys such as LaNi5 could be promising candidates for cathode materials in fluoride-ion batteries, where the reversible capacity of LaNi5 is 396 A h kg(-1). By using atomic-resolution scanning transmission electron microscopy, we elucidate that LaNi5 particles are first decomposed into LaF3 and Ni nanocrystals in the charge process. A nano-scale network is formed by LaF3 and Ni nanocrystals during the charge process, which maintains F--ion and electric conductivities in the particles. Furthermore, Ni - one of the most difficult elements to fluorinate - can be fluorinated and defluorinated in the subsequent charge and discharge processes, because of the spontaneous formation of Ni nanocrystals which are smaller than the passivation layer thickness and LaF3 nano-scale network. Our results suggest that an intermetallic alloy containing two function-sharing elements could be useful to explore higher capacity cathodes in fluoride-ion batteries.

Automated summary

To achieve high energy density in fluoride-ion batteries, it is necessary to develop better cathode materials. Hydrogen storage alloys such as LaNi5 show promise as candidates for cathode materials, as they can be fluorinated and provide good conductivity. Through atomic-resolution scanning transmission electron microscopy, it was observed that LaNi5 particles decompose into LaF3 and Ni nanocrystals during the charge process, forming a nano-scale network that maintains ion conductivity. This suggests that intermetallic alloys containing two function-sharing elements could be useful in exploring higher capacity cathodes for fluoride-ion batteries.