Fast Magnesium Ion Transport in the Bi/Mg₃Bi₂ Two-Phase Electrode

Bi has recently attracted much interest as a promising Mg battery anode to replace the Mg metal, which is ideal but impractical because of instability issues associated with the electrolyte. Our first-principles molecular dynamics study addresses why the alloying reaction of Bi with Mg creates only two crystals of Bi and Mg₃Bi₂ and how Mg ions move in the two crystals. The formation of crystalline Mg₃Bi₂ is energetically much more favorable than that of amorphous Mg_xBi (0.0 ≤ x ≤ 2.5). This high thermodynamic stability of the Mg₃Bi₂ crystal serves as a driving force for the complete crystalline Bi/crystalline Mg₃Bi₂ two-phase reaction, without allowing the formation of amorphous phases typically observed in alloy-type anode materials. The Mg ions diffuse preferentially along the [001] direction in both Bi and Mg₃Bi₂ and diffuse about 6000 times faster in Mg₃Bi₂ than in Bi. Despite the relatively slow Mg transport in Bi, the absolute kinetics of Mg ions in Bi is reasonable in terms of fast battery operation. Interestingly, the calculated Mg ion diffusivity in Bi is comparable to that for a Li ion in Bi, contrary to the general perspective that multivalent ions are very sluggish compared with monovalent ions. Our study suggests that fast multivalent-ion transport can be achieved when the multivalent ion has weak bonds with the nearest host elements and exhibits small fluctuations in its coordination number and bond length during ion transport.