Owing to the intermittent output of renewable energies, better energy storage systems are in pressing need. Among various technologies, lithium-ion batteries outstand for their high energy density and long life. Nevertheless, their capacity is still limited for large-scale applications, and new materials with much improved performance are to be developed.

Lithium iron fluorosulfate (LiFeSO₄F) is a novel cathode material and also a strong contender for next generation lithium-ion batteries. However, the inconsistency in synthesizing high-performance LiFeSO₄F hinders its fundamental research. In the present study, two experimental protocols concerning the ionothermal and solvothermal methods have been developed. Impacts of precursor morphology, reaction temperature, and reaction...[ Read more ]

Owing to the intermittent output of renewable energies, better energy storage systems are in pressing need. Among various technologies, lithium-ion batteries outstand for their high energy density and long life. Nevertheless, their capacity is still limited for large-scale applications, and new materials with much improved performance are to be developed.

Lithium iron fluorosulfate (LiFeSO₄F) is a novel cathode material and also a strong contender for next generation lithium-ion batteries. However, the inconsistency in synthesizing high-performance LiFeSO₄F hinders its fundamental research. In the present study, two experimental protocols concerning the ionothermal and solvothermal methods have been developed. Impacts of precursor morphology, reaction temperature, and reaction time on the phase purity and electrochemical performance of resultant LiFeSO₄F have been assessed. In addition, an undesired spinodal decomposition of LiFeSO₄F in high humidity has been addressed for the stability concern of the material.

Detailed experiments revealed that precursor size played a determining role in synthesizing LiFeSO₄F with good phase purity. Specifically, owing to the low solubility of LiF in organic solvents, submicron-sized LiF powders were prepared by a precipitation reaction. LiFeSO₄F powders were successfully synthesized at 305˚C for 24 hours in ionic liquid, or at 230˚C for 60 hours in tetraethylene glycol. Succeeding characterizations confirmed the crystallinity and phase purity. The electrochemical performance was assessed in both coin cell and swagelok cell configurations. Using a current density of C/20, the best sample delivered a reversible discharge capacity of 100 mAh g^-1 in coin cells, and 128 mAh g^-1 in swagelok cells. The significant difference was attributed to the superior configuration of swagelok cells, where electronically insulating binder was not employed. Moreover, the moisture sensitivity of LiFeSO₄F was discovered and investigated. Experiments revealed that when relative humidity was higher than 65%, LiFeSO₄F would react with water vapor and spontaneously decompose into FeSO₄•H₂O and LiF.