Charging/discharging mechanism in Mg3Bi2 anode for Mg-ion batteries; The role of the spin-orbit coupling

Using density functional calculations, we examine insertion/extraction of Mg ions in Mg3Bi2, an interesting Mg-ion battery anode. We found that a (1 1 0) facet is the most stable termination. Vacating a Mg2+ ion from the octahedral site is more favourable for both surface and bulk regions of the material. However, the diffusion barriers among the tetrahedral sites are around 3 times smaller than those among octahedral sites. Consequently, during the magnesiation/demagnesiation process, Mg ions first vacate the octahedral sites and then diffuse through the tetrahedral sites. The spin-orbit interaction lowers Mg’s vacancy formation energy but has a minor effect on diffusion barriers.