Electrochemical reactivity of MgSn phases with metallic lithium

Stable (c) and metastable (h) forms of MgSn 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-Mg Sn 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 LiMgSn 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 MgSn + Sn mixture, and accounting for the poor cyclability of MgSn/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-MgSn with Li leads to the same Li MgSn intermediate and the same subsequent sequence of transformations, resulting in similarly poor capacity retention upon cycling.