Mechanisms of electrochemical magnesium (de)alloying of Mg-Sn and Mg-Pb polymorphs

Different polymorphs of Mg-Sn and Mg-Pb intermetallic compounds were prepared by high-energy mechanical alloying and then investigated as active material in magnesium batteries. Beside thermodynamically stable Mg2Sn and Mg2Pb crystallizing in the anti-fluorite structure, other polymorphs Mg∼2Sn and Mg∼2Pb were prepared by increasing the ball-milling time. The first dealloying process is almost complete only for the cubic polymorphs, then similar capacities are observed during the subsequent alloying and dealloying sequences. Thanks to operando X-ray diffraction, the electrochemical mechanism is revealed and shows that the cubic polymorphs Mg2Sn and Mg2Pb tend to preferentially form during the alloying whatever the pristine intermetallic. Weak traces of Mg∼2Sn and Mg∼2Pb are observed during the alloying, suggesting that these polymorphs act as a by-product and/or an intermediate phases of the electrochemical process. Finally, the compatibility of cubic Mg2Sn and Mg2Pb with Mg(TFSI)2-based electrolyte is confirmed in full cell vs. a positive electrode based on the Chevrel phase Mo6S8, although limited performance is achieved. This fundamental work provides new insights in the behavior of alloy-type negative electrodes for magnesium-ion batteries.