  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.
