Building a Fast Lane for Mg Diffusion in α-MoO3 by Fluorine Doping

Following previous experimental work examining a layered oxyfluoride as cathode material for Mg-ion batteries [Incorvati et al., Chem. Mater. 2016, 28, 17], we study the role of fluorination on the structural and electronic properties of molybdenum trioxide and its impact on Mg intercalation and diffusion using first principles methods. Although bulk alpha-MoO3 is a 2D layered compound, we find that it provides 3D channels for Mg diffusion. When F atoms are incorporated into the alpha-MoO3 lattice, they replace the O atoms sitting at a specific crystallographic site that is linked by two nearest Mo atoms within a single Mo-O layer. As a consequence of F substitution, the local Mo-anion bonds are distorted, which leads to closure of the electronic band gap. From the analysis of zone center phonon vibrational frequencies, it is found that the local Mo-anion bonding strength is weakened by replacing O2- with F-, which ultimately facilitates Mg diffusion through the F-substituted lattice. For example, it is shown that upon fluorination the activation barriers for Mg diffusion along selected pathways can be lowered by as much as 0.6 eV, estimated from our nudged elastic band simulations at 0K. Our results imply that direct anion doping can be a viable approach toward improving ion diffusivity in Mg-ion battery cathodes.