Intermetallic electrode materials for rechargeable lithium batteries

Magnesium silicide and magnesium stannide powders were evaluated as possible replacements for graphite negative electrodes in lithium-ion batteries. Ultrafine powders of the intermetallics were synthesized by mechanical alloying in a planetary ball mill. A global model of the planetary mill was developed to correlate the milling products with the estimated milling powers. To synthesize magnesium silicide, magnesium and silicon powders were milled for five hours and heated to 600°C for two hours. Milling for a few hours without a heat treatment was sufficient to produce magnesium stannide. After twenty hours of high-energy milling, the cubic form of magnesium stannide was converted to a metastable, high-pressure phase.; Based upon the galvanostatic cycling results, magnesium stannide is a much more promising electrode material than magnesium silicide. A discharge capacity over 800 mAh/g in the first cycle could be obtained with magnesium silicide, but capacity fade was rapid. On the other hand, discharge capacities over 300 mAh/g were obtained for up to 20 cycles (the maximum number of cycles in our cycling tests) with magnesium stannide.; In situ x-ray diffraction was used to study the phase changes during lithium insertion into and removal from the powders. For magnesium silicide, an intercalation reaction was observed during the initial stages of insertion. Further insertion led to the formation of Li₂MgSi. Lithium could also be inserted into magnesium discharged from magnesium silicide during the formation of the ternary phase, and we observed a phase transformation from a lithium-containing magnesium solid solution to a magnesium-containing lithium solid solution. We believe that magnesium stannide has a similar mechanism and forms Li₂MgSn during lithium insertion. If lithium is inserted into the high-pressure phase, the structure irreversibly transforms to the cubic phase during the initial insertion.