Research into electrode materials for hosting Mg ions has also been very challenging, with only a few materials identified that are able to react with Mg ions reversibly and have good longterm stability, such as the Mo 6S 8 Chevrel phase compounds and bismuth nanotubes.[1b, 5a, 8] This is in strong contrast with the cases of Li and Na ions systems, where a wide variety of materials are shown to be suitable as electrode materials, such as Sn, P, Si, Sb and their alloys and oxides, transition metal oxides, and phosphates.[9] In fact, most of these established Na/Li electrode materials were found to either not react with Mg ions or have poor stability and low Coulombic efficiency. Therefore, it seems the underlying mechanisms that work well in the case of monovalent Na and/or Li ions may not be applicable for divalent Mg ions.[5b, 10] For example, theoretical studies suggest that Sn could be a good material for Mg ion insertion (890 mAh g− 1 theoretical capacity) because it has a smaller lattice expansion and a lower diffusion barrier than Ge and Si,[11] but experimentally both its capacity and first cycle Coulombic efficiency are far from what were predicted.[12] Such pronounced differences are believed to originate from the unique electrochemical properties of Mg ions, and their much higher charge density (two atomic units of charge per ion with the radius of 0.86 Å) that results in stronger Coulombic interaction with the host materials and slower solid-state diffusion.[3] Further understandings of the fundamental properties of Mg ion electrochemistry and the Mg ions-host material reaction mechanisms are clearly required to guide the rational design of electrode materials. SnSb alloy is an attractive material for Mg ion batteries because it has a high theoretical capacity of 768 mAh g− 1 when forming Mg 2Sn and Mg 3Sb 2. We recently reported an analysis of the structural reaction of SnSb with Mg ions based on ex situ (scanning) transmission electron microscopy analysis of particles after initial battery cycles.[13] We found that SnSb pristine particles transformed into porous networks of Sn-rich and Sbrich subparticles. The Sn particles demonstrate reversible Mg storage, whereas the Sb-rich particles trap substantial amount of Mg irreversibly. In this paper, we discuss the detailed electrochemical reaction mechanism and phase transformation pathways of Mg insertion/extraction and the associated fundamental material transformation characteristics based on systematic studies with electrochemical, structural, and morphological characterizations in combination with theoretical density function theory (DFT) simulations. In particular, we show how the Sb-phase promotes reversible Mg insertion/extraction in Sn-rich domain, which has important implications beyond the SnSb system to other potential alloy electrode materials. Overall, SnSb electrodes were able to deliver a high reversible

Rechargeable batteries based on magnesium (Mg) electrochemistry have attracted significant attention for both transportation and stationary energy storage due to their unique properties and advantages.[1] Compared to Li metal, which has a high reactivity with air and has limited availability, Mg metal is much safer to handle and is one of the most abundant elements in the Earth’s crust.[2] More remarkably, the electrodeposition of Mg metal usually does not involves formation of dendritic structures and has close to 100% Coulombic efficiency in proper electrolytes, which are in strong contrast with the behaviors of Li and Na metals.[3] Such characteristics suggest that the use of divalent Mg ions as the charge carriers has great potential for low cost, safe, and high energy density next …