Electrochemical Reactivity of Mg2Sn Phases with Metallic Lithium

Stable (c) and metastable (h) forms of Mg2Sn were prepared as crystallized phases by ball-milling of elemental powders. Through in situ X-ray diffraction measurements, we deduced the reactivity mechanisms of c-Mg2Sn toward lithium. It entails first a monophasic insertion of about one lithium per formula unit into the fcc Sn framework without extrusion of either Mg or Sn, then a biphasic process leading to the formation of cubic Li2MgSn concomitant with a progressive expulsion of Mg, and finally the formation of Li-Mg solid-solution alloys. Upon charging, the poor reversibility of the alloying reaction of Li with Mg leads to a deficit in free Mg, resulting in the formation of a Mg2Sn + Sn mixture, and accounting for the poor cyclability of Mg2Sn/Li cells over the 0.0-1.5 V voltage window. Limiting the cycling to the monophasic process was shown to improve the cycling behavior. Finally, we found that the electrochemical reaction of h-Mg2Sn with Li leads to the same Li2MgSn intermediate and the same subsequent sequence of transformations, resulting in similarly poor capacity retention upon cycling.