Atomistic Insights into FeF Nanosheet: An Ultrahigh-Rate and Long-Life Cathode Material for Li-Ion Batteries.

Iron fluoride with high operating voltage and theoretical energy density has been proposed as a high-performance cathode material for Li-ion batteries. However, the inertness of pristine bulk FeF results in poor Li kinetics and cycling life. Developing nanosheet-based electrode materials is a feasible strategy to solve these problems. Herein, on the basis of first-principles calculations, first the stability of FeF (012) nanosheet with different atomic terminations under different environmental conditions was systematically studied, then the Li-ion adsorption and diffusion kinetics were thoroughly probed, and finally the voltages for different Li concentrations were given. We found that F-terminated nanosheet is energetically favorable in a wide range of chemical potential, which provide a vehicle for lithium ion diffusion. Our Li-ion adsorption and diffusion kinetics study revealed that (1) the formation of Li dimer is the most preferred, (2) the Li diffusion energy barrier of Li dimer is lower than isolated Li atom (0.17 eV for Li dimer vs 0.22 eV for Li atom), and (3) the diffusion coefficient of Li is 1.06 × 10 cm·s, which is orders of magnitude greater than that of Li diffusion in bulk FeF (10-10 cm·s). Thus, FeF nanosheet can act as an ultrahigh-rate cathode material for Li-ion batteries. More importantly, the calculated voltage and specific capacity of Li on the FeF (012) nanosheet demonstrate that it has a much more stable voltage profile than bulk FeF for a wide range of Li concentration. So, few layers FeF nanosheet provides the desired long-life energy density in Li-ion batteries. These above findings in the current study shed new light on the design of ultrahigh-rate and long-life FeF cathode material for Li-ion batteries.