In Situ 119Sn Mossbauer Effect Study of the Reaction of Lithium with Si Using a Sn Probe

The reaction of lithium with amorphous Si was studied by Sn-119 Mossbauer effect spectroscopy using small amounts of Sn as probe atoms. Two powder samples, amorphous-Si87Sn13 and amorphous-Si93Sn7, were prepared by magnetron sputtering and investigated using in situ and ex situ Mossbauer spectroscopy. There are two gently sloping plateaus in the voltage vs capacity of Li/Si cells whose origin has never been explained. There is a clear step between these plateaus at similar to 2.3 Li atoms per Si atom, or Li2.3Si. The step between the plateaus is found to correlate with dramatic changes in the Mossbauer effect spectra with x in LixSi at x approximate to 2.3, which suggest the step occurs at the point when each Si atom is surrounded only by Li atom first neighbors. The changes in the Mossbauer spectra during the delithiation of the Li/Si-Sn cells also give clues about electrode failure mechanisms.