Microstructure Control of Iron Trifluoride for Thermal Batteries

Abstract Iron trifluoride (FeF3) is a promising cathode for high-powder thermal batteries because of higher discharge voltage (3.2 V vs. Li+/Li), high theoretical specific capacity (712 mAh⋅g-1), abundant resource, and low toxicity. Herein, single-phase FeF3 was prepared by fluorinating Fe(NO3)3 in concentrated ethanol solution followed by thermal calcination in inert atmosphere. The microstructure of FeF3 grains were modified by varying the ratio of ethanol to water (E/W) in the fluorinating process. The influence of FeF3 on the electrochemical performance was investigated. The discharging mechanism of FeF3 was discussed. The results showed that bigger FeF3 grain with a compact structure can be prepared at a moderate E/W value (3:1). The bigger FeF3 grains exhibits a specific capacity of 213 mAh·g-1 at 25 mA·cm-2 (cut-off voltage at 1.7 V), 34% higher than the capacity of commercial FeF3 powder. The discharge of the FeF3 is a two-step process employing Fe2+ as the intermediate.