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 stud ies of electrodes for Li batteries is described. The cell design is ba sed on coin-type hardware. The reaction of lithium with Sn2Fe is studi ed 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 regi ons 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, th e Fe regions grow in size and a singlet (superparamagnetic) spectrum r esults. This spectrum splits into the six-line spectrum characteristic of ferromagnetic iron if the sample is cooled below its blocking temp erature (less than 30 K) to 4.2 K, indicating Fe grains of at most 3 n m 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 a ll of the Li is removed, at which point the ''liberated'' Sn atoms beg in to ''back react'' with Fe to form Sn2Fe again. This back reaction b egins 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-li ne Mossbauer spectrum of ferromagnetic iron is observed even at room t emperature in both the charged and discharged states, indicating Fe gr ains of at least 10 nm. Concurrently, the differential capacity and vo ltage profile of the electrode resemble those of a Li/Sn cell, indicat ing that the majority of the Fe atoms are not involved in the back rea ction during charge after several cycles.