From Amorphous to Crystalline Thin-Film FeF3 Conversion Electrodes by Sputtering Deposition

FeF3 (iron(III) fluoride) is a promising conversion cathode material that possesses a theoretical specific capacity of 712 mAh/g, which is significantly higher than those of commercial layered cathodes, and it paves the way for realizing Co-free, low-cost advanced batteries for portable electronics, transportation, and grids. Here, we report the development of thin-film FeF3 conversion cathodes by the sputtering deposition technique. The chemistry and stoichiometry of the sputtered FeF3 thin films are confirmed by energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy. Through controlling the substrate temperatures (25-600 C-degrees) of the sputtering deposition, amorphous and crystalline FeF3 samples were developed, characterized, and evaluated for electrochemical performance. FeF3 thin-film electrodes exhibit high initial capacity that is close to the theoretical capacity; after 100 cycles, FeF3 electrodes deposited between 100 and 600(degrees)C show stable cycling performance from 31.7 to 59.3% compared to the initial capacity. Moreover, we investigate the relationship between the density, crystallinity, and lithiation potentials of the deposited FeF3 thin-film cathodes and report that the high-density, crystalline FeF3 exhibits a higher lithiation potential (Delta of similar to 0.43 V) than the low-density, amorphous FeF3 . The development of sputtered thin-film FeF3 presents an opportunity for thin-film battery fabrication and processing.