Silicon as the Anode Material for Multivalent-Ion Batteries: A First-Principles Dynamics Study.

Due to its huge capacity, Si is a promising anode material for practical applications in lithium-ion batteries. Here, using first-principles calculations, we study the applicability of the amorphous Si anode in multivalent-ion batteries, which are of great interest as candidates for post-lithium-ion batteries. Of the multivalent Mg, Ca, Zn, and Al ions, only Mg and Ca are able to form MgSi and CaSi by alloying with Si, delivering very high capacities of 4390 and 4771 mA h g, respectively. MgSi has an 8% smaller capacity than CaSi, but its volume expansion ratio and ion diffusivity are ∼200% smaller and 3 orders of magnitude higher than those of CaSi, respectively. The capacity, volume expansion, and ion diffusion of MgSi are excellently high, moderately small, and fairly fast, respectively, when compared to those of LiSi, NaSi, and KSi. The high performance of MgSi can be understood in terms of the coordination numbers of Si and the atomic size of Mg. This work suggests that, as a carrier ion for the amorphous Si anode, Mg is the most competitive among the multivalent ions and is at least as good as monovalent ions.