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 dischar ged to various potentials. The Fe K-edge measurements include both the near edge (NEXAFS) and extended (EXAFS) regions. The former provides informatio n on the effective Fe valence while the latter reveals the coordination geo metry. Six cells charged and discharged at different conditions were examin ed. Group A consists of: fully discharged (d-1.1 V), almost fully discharge d (d-1.25 V), and partially charged (c-1.85 V) cathodes and group B include s fully charged (c-2.25), almost fully charged (c-2.05 V), and partially di scharged (d-1.65 V) cathodes. There appear to be only two distinct Fe envir onments, 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 a lso reflect these distinct Fe environments. The extended fine structure for group A samples reveals an ordered environment dominated by metallic Fe wh ile the higher Li content group B cathodes are characterized by disorder wi th only a single Fe-S interatomic distance. Spectral fitting to the experim ental data of the d-1.65 cell (in group B) suggests that the compound Li2Fe S2 is present, with no evidence of FeS. Original cathode material utilizati on is estimated to be only around 2/3 of full capacity, based on the amount of residual FeS, required for satisfactory spectral fitting. On the other hand, no metallic Fe (within detection limits) remains in the recharged cel ls. (C) 1999 Elsevier Science S.A. All rights reserved.