The Effects of Size and Shape on the Electrochemical Conversion in FeF2 Nanoparticles: A Molecular Dynamics Study

The increasing demand for high capacity power sources has stimulated much research in lithium ion batteries (LIBs). Recently, an alternative to the intercalation reaction, known as the conversion reaction, has been proposed where multiple valence states of the metal cation can be accessed during the redox cycle, thus leading to a higher energy density. Of special interests are conversion compounds including iron fluorides and iron oxyfluorides. By the use of carbon-iron (oxy)fluoride nano-composites, specific capacities of 540, 620 and 490 mAhg^-1 have been demonstrated for FeF₂, FeF₃ and FeO_xF_2-x, respectively. Combined experimental and computationally studies have also shed light on the microscopic mechanisms of the electrochemical conversion observed in these materials, however, challenges still remain. In particular, the ionic diffusion pathways during charging and discharging are unclear. It is expected that by altering the size and shape of the nanoparticle, one can effectively modiy the ionic diffusion which may lead to increased electrochemical performance. For this purpose, reactive molecular dynamics simulation are carried out to study the charging and discharging in FeF2 nanoparticles with different sizes and shapes. Their effects on the diffusion process is analyzed, and their role in the macroscopic properties is discussed.