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 Mg²⁺, Ca²⁺, Zn²⁺, and Al³⁺ ions, only Mg²⁺ and Ca²⁺ are able to form Mg_2.3Si and Ca_2.5Si by alloying with Si, delivering very high capacities of 4390 and 4771 mA h g^-1, respectively. Mg_2.3Si has an 8% smaller capacity than Ca_2.5Si, but its volume expansion ratio and ion diffusivity are ∼200% smaller and 3 orders of magnitude higher than those of Ca_2.5Si, respectively. The capacity, volume expansion, and ion diffusion of Mg_2.3Si are excellently high, moderately small, and fairly fast, respectively, when compared to those of Li_3.7Si, Na_0.75Si, and K_1.1Si. The high performance of Mg_2.3Si 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.