X-ray absorption fine structure studies of FeS₂ cathodes in lithium polymer electrolyte batteries

We have performed synchrotron X-ray absorption measurements on a series of sealed Li/composite polymer electrolyte (CPE)/FeS₂ cells charged or discharged to various potentials. The Fe K-edge measurements include both the near edge (NEXAFS) and extended (EXAFS) regions. The former provides information on the effective Fe valence while the latter reveals the coordination geometry. Six cells charged and discharged at different conditions were examined. Group A consists of: fully discharged (d-1.1 V), almost fully discharged (d-1.25 V), and partially charged (c-1.85 V) cathodes and group B includes fully charged (c-2.25), almost fully charged (c-2.05 V), and partially discharged (d-1.65 V) cathodes. There appear to be only two distinct Fe environments, one for group A and a dramatically different one for group B. The two different main absorption edge peak shapes observed in groups A and B also reflect these distinct Fe environments. The extended fine structure for group A samples reveals an ordered environment dominated by metallic Fe while the higher Li content group B cathodes are characterized by disorder with only a single Fe-S interatomic distance. Spectral fitting to the experimental data of the d-1.65 cell (in group B) suggests that the compound Li₂FeS₂ is present, with no evidence of FeS. Original cathode material utilization is estimated to be only around 2/3 of full capacity, based on the amount of residual FeS₂ required for satisfactory spectral fitting. On the other hand, no metallic Fe (within detection limits) remains in the recharged cells.