X-ray absorption spectroscopy of highly cycled Li/composite polymer electrolyte/FeS2 cells

Synchrotron X-ray absorption measurements were performed on a series of sealed Li/composite polymer electrolyte (CPE)/FeS2 cells, halted during both charge and discharge cycles at potentials ranging from 1.33 to 1.90 V. It is confirmed that in this voltage range, two of the primary species are Li2FeS2 and metallic iron. Beginning with the second cycle, a third phase, previously unreported, is found to occur as an intermediate between the Li2FeS2 and metallic iron during both charging and discharging. Analysis of the iron X-ray absorption near-edge structure (XANES) suggests that the iron in this phase is not oxidized; the extended X-ray absorption fine structure (EXAFS) is consistent with the phase consisting of neutral iron atoms coordinated to Li2S. When cycled more than 100 times, XANES indicates the presence of clusters of iron atoms which are no longer utilized during cycling.