In situ XAS of the reaction mechanism of lithium with tin-based composite oxide glass

We have measured the XAFS spectra of tin-based composite oxide (TCO) glass with nominal composition of Sn1.0B0.56P0.40Al0.42O3.47 during the discharge and charge cycles in an in a non-aqueous cell. Our results confirm the amorphous nature of TCO material and show that Sn in TCO is coordinated with 3 oxygen atoms at a distance of 2.12 Angstrom. Upon discharge (i.e., Li insertion), initially, Li interacts with the electrochemically active Sn-O center forming metallic Sn, most likely in the form of highly dispersed clusters, and Li2O. Upon further discharge, our results are consistent with a model in which additional Li alloys with Sn forming various alloys with composition dependent on the amount of Li inserted. The formed alloys appear to be in the form of highly dispersed clusters and/or amorphous in nature. Their local structure differs somewhat from the crystalline structure of the known Li-Sn alloys such as LiSn, Li7Sn3, or Li7Sn2. Upon charging (i.e., Li removal), metallic Sn is reversibly produced with a Sn-Sn distance intermediate to those of gray and white Sn.