Optimal alloying element for suppression of volume expansion in Sn alloy anode materials during sodiation: atomistic simulations study

Abstract

Tin (Sn) is cheaper and has a high theoretical capacity of 847 mAh/g, compared to the other anode materials for sodium ion batteries (NIBs). However, Sn exhibits a volume expansion rate of 420%, consistent with its high capacity. The high expansion rate develops stress at the interface and cause pulverization/fragmentation. The anode material develops structural degradation as a result, which degenerates its performance. To inhibit those volume expansion, various Sn-based binary alloys have been investigated. However, the quantitative analysis on the effect of alloying elements as the performance improvement has not been conducted yet, requiring further understanding. Considering inactivity with Na, solubility with Sn, and price, Cu, Mn, and Ni are chosen as candidate metal elements for Sn alloying elements in this study. As atomistic simulations can be highly utilized as it can consider millions of atoms, therefore effective and reliable approach to atomic-scale behavior, which is difficult to analyze experimentally. We used the potentials of second-nearest neighbor modified embedded atom method (2NN MEAM) formalism, and the potential parameters of relevant systems (Na-M-Sn, M=Cu, Mn, Ni) were developed in this study. Using the atomistic simulations based on the potentials developed, we investigated the clustering mechanism during the Na infusion into amorphous Sn-M anode materials, compared to Zn-Sn counter example. We found that Sn-M alloy is stable in amorphous solid solution, and as Na atoms infuses, phase transformation occurs such that M elements are separated out, forming M clusters and NaxSn amorphous phase. It was checked that the counter example Sn-Zn alloy does not show same phenomenon, confirming the effect of alloying those M elements. Thereafter, from the comparison of volume reduction ratio and Na infusion rate, it is suggested that Cu is the best alloying element for the Sn base alloy anode materials.