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 Mg2+, Ca2+, Zn2+, and Al3+ ions, only Mg2+ and Ca2+ are able to form Mg2.3Si and Ca2.5Si by alloying with Si, delivering very high capacities of 4390 and 4771 mA h g-1, respectively. Mg2.3Si has an 8% smaller capacity than Ca2.5Si, but its volume expansion ratio and ion diffusivity are ∼200% smaller and 3 orders of magnitude higher than those of Ca2.5Si, respectively. The capacity, volume expansion, and ion diffusion of Mg2.3Si are excellently high, moderately small, and fairly fast, respectively, when compared to those of Li3.7Si, Na0.75Si, and K1.1Si. The high performance of Mg2.3Si 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, Mg2+ is the most competitive among the multivalent ions and is at least as good as monovalent ions.
Keywords: anode material; calcium; magnesium; multivalent-ion battery; silicon.