Oxygen-Doped FeF3 Nanosheets Prepared by the Liquid-Phase Exfoliation Method for Lithium Storage

As a promising high-energy-density cathode material for lithium-ion batteries, the application of FeF3 is significantly impeded by its sluggish kinetics. In this study, we propose a straightforward method to fabricate quasi-two-dimensional (quasi-2D) oxygen-doped FeF3 nanosheets through the thermal decomposition and liquid-phase exfoliation of commercial FeF3·3H2O. During the thermal decomposition process, some fluorine atoms are substituted by oxygen atoms, resulting in enhanced electrical conductivity and a reduction in the dissociation energy of Fe–F bonds. In addition, the quasi-2D geometrical feature reduces the diffusion length of Li ions. Consequently, this free-standing electrode delivers a high specific capacity (∼600 mAh g–1 at a low current density of 0.03 A g–1), a stable cycling performance over 200 cycles at a current density of 0.3 A g–1 (with a capacity retention of 82%), and excellent rate capability (350 mAh g–1 at 2 A g–1). A more profound insight into the kinetics of this electrode reveals that Li ions preferentially diffuse along the thickness of the non-layered nanosheets, significantly contributing to the outstanding rate performance. Thus, this study tackles the kinetic challenges of FeF3 in terms of the electronic and geometrical structures, offering valuable insights for the development of high-performance electrode materials based on the conversion reaction mechanism.