In situ Mossbauer effect studies of the electrochemical reaction of lithium with mechanically alloyed Sn2Fe

A convenient cell design for in situ Fe-57 Mossbauer spectroscopy studies of electrodes for Li batteries is described. The cell design is based on coin-type hardware. The reaction of lithium with Sn2Fe is studied using this cell. During the first discharge of Li/Sn2Fe cells, the cell reaction is 8.8 Li + Sn2Fe --> 2Li(4.4)Sn + Fe, where the Fe regions which are formed are located at the interfaces between Li-Sn alloy grains, and give a Mossbauer spectrum characterized by a doublet. If the cell, is held in the discharged state (0 V vs. Li) for 2 weeks, the Fe regions grow in size and a singlet (superparamagnetic) spectrum results. This spectrum splits into the six-line spectrum characteristic of ferromagnetic iron if the sample is cooled below its blocking temperature (less than 30 K) to 4.2 K, indicating Fe grains of at most 3 nm in size. During the first charge (removing Li from the Li-Sn alloys) there is little change in the Fe-57 Mossbauer spectrum until almost all of the Li is removed, at which point the "liberated" Sn atoms begin to "back react" with Fe to form Sn2Fe again. This back reaction begins at about 0.63 V vs. Li, at which potential there is a two-phase region wherein Li2Sn5 and Sn are present. Not all of the Fe can react with Sn, because the original spectrum of the Sn2Fe starting material is not recovered. Instead some Fe atoms, presumably at the centers of larger grains, remain as Fe. As the cell is cycled consecutively, the size of the Fe grains grows and grows, until by cycle five, the six-line Mossbauer spectrum of ferromagnetic iron is observed even at room temperature in both the charged and discharged states, indicating Fe grains of at least 10 nm. Concurrently, the differential capacity and voltage profile of the electrode resemble those of a Li/Sn cell, indicating that the majority of the Fe atoms are not involved in the back reaction during charge after several cycles.