Realizing the Full Potential of Insertion Anodes for Mg-Ion Batteries Through the Nanostructuring of Sn

Magnesium is of great interest as a replacement for lithium in next-generation ion-transfer batteries but Mg-metal anodes currently face critical challenges related to the formation of passivating layers during Mg-plating/stripping and anodeelectrolytecathode incompatibilities.1-6 Alternative anode materials have the potential to greatly extend the spectrum of suitable electrolyte chemistries2,7 but must be systematically tailored for effective Mg2+ storage. Using analytical (scanning) transmission electron microscopy ((S)TEM) and ab initio modeling, we have investigated Mg2+ insertion and extraction mechanisms and transformation processes in beta-SnSb nanoparticles (NPs), a promising Mg-alloying anode material. During the first several chargedischarge cycles (conditioning), the beta-SnSb particles irreversibly transform into a porous network of pure-Sn and Sb-rich subparticles, as Mg ions replace Sn atoms in the SnSb lattice. After electrochemical conditioning, small Sn particles/grains (<33 +/- 20 nm) exhibit highly reversible Mg-storage, while the Sb-rich domains suffer substantial Mg trapping and contribute little to the system performance. This result strongly indicates that pure Sn can act as a high-capacity Mg-insertion anode as theoretically predicted,8 but that its performance is strongly size-dependent, and stable nanoscale Sn morphologies (<40 nm) are needed for superior, reversible Mg-storage and fast system kinetics.