Microstructure Control of Iron Trifluoride for Thermal Batteries

Iron trifluoride (FeF 3 ) 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 FeF 3 was prepared by fluorinating Fe(NO 3 ) 3 in concentrated ethanol solution followed by thermal calcination in inert atmosphere. The microstructure of FeF 3 grains were modified by varying the ratio of ethanol to water (E/W) in the fluorinating process. The influence of FeF 3 on the electrochemical performance was investigated. The discharging mechanism of FeF 3 was discussed. The results showed that bigger FeF 3 grain with a compact structure can be prepared at a moderate E/W value (3:1). The bigger FeF 3 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 FeF 3 powder. The discharge of the FeF 3 is a two-step process employing Fe 2+ as the intermediate.