Electrochemical insertion of lithium into pyrite from nonaqueous electrolytes at room temperature. An in situ Fe K-edge X-ray absorption fine structure study

The effects of lithium ion insertion on the structural and electronic properties of FeS₂ (pyrite) have been examined in situ by Fe K-edge X-ray absorption fine structure (XAFS) using electrodes and electrolytes similar to those found in conventional, ambient-temperature, primary Li/FeS₂ batteries. A substantial reduction in the amplitudes of the Fe-S and Fe-Fe backscattering was observed as the amount of intercalated lithium in the FeS₂ lattice was increased from 0 to 2 Li⁺ equivalents, (Li⁺)_eq. After the insertion of 2 (Li⁺)_eq, the second-shell Fe-S interaction and the distant Fe-Fe interactions were no longer discernible in the Fourier transform (FT) data. Curve-fitting analysis of the κ³ _X(k) extended X-ray absorption fine structure for this latter material yielded an average Fe-S distance, d(Fe-S) = 2.31 +/- 0.02 A, which is about 0.05 A longer than d(Fe-S) in crystalline pyrite. In addition, the X-ray absorption near-edge structure revealed a rounding of the otherwise highly structured edge of FeS₂ as the amount of inserted lithium was increased. This behavior is consistent with the formation of Fe_1-xS and thus supports the assignment made on the basis of in situ ⁵⁷Fe Mossbauer effect spectroscopy of the same system.