Hydrogen-bonding-mediated structural stability and electrochemical performance of iron fluoride cathode materials

Numerous lithium ion battery cathode materials containing trace amounts of water accommodated in Li+ transportation tunnels have been experimentally synthesized. However, the impact of water on structural stability and electrochemical performance of cathode materials is still unclear. Here, the first-principles calculations combining thermodynamic analysis of LixFeF3 center dot 0.33H(2)O were performed to unravel the interaction mechanism among frameworks of FeF3, H2O, and Li+. The FeF3 framework structure distortion is mitigated by hydrogen bonding between isolated H2O and F- ions, bringing opposite effects on the stability of hydrogen bonding and instability of structural distortion. The hydrogen bonding strength of F-center dot center dot center dot H2O can be further mediated by the Li+-inserted amount, which indirectly results in a wide discharge voltage window of 2.2 to 3.6 V. The Li+ transportation barrier in cooperative mode is also tuned by the flexible hydrogen bonding strength due to different occupied positions. Li0.66FeF3 center dot 0.33H(2)O is determined as the most stable species and more Li+ insertion directly leads to the conversion reaction FeF63- -> FeF4- + 2F(-). Therefore, stabilizing Fe-F bonds and reducing octahedral chain distortion are important to improve the electrochemical performance of FeF3 cathode materials with water.