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 LixFeF3·0.33H2O 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−⋯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. Li0.66FeF3·0.33H2O 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.