Improvement of the sodiation/de-sodiation stability of Sn(C) by electrochemically inactive Na2Se

Tin forms a series of sodium alloys, some with a large change in volume, sufficient to fracture the sodiated/de-sodiated tin electrodes. In a series of Sn/C and Sn/Se/C electrodes, made similarly by ball milling the elements, the sodiation/de-sodiation cycling stability in the 0.01-1.00 V vs. Na+/Na voltage window, in which the initially formed Na2Se is electrochemically inactive, is best at an Sn : Se atomic ratio of 9 : 2. At this ratio the retained capacity is similar to 300 mA h g-1 after 150 cycles at 0.17 A g-1 rate versus only 70 mA h g-1 in the absence of Na2Se. The improvement is attributed to prevention of crystallization of the Na-Sn alloys by the Na2Se.