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 FeS{sub 2} (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/FeS{sub 2} 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 FeS{sub 2} lattice was increased from 0 to 2 Li{sup +} equivalents, (Li{sup +}){sub eq}. After the insertion of 2 (Li{sup +}){sub eq}, the second-shell Fe-S interaction and the distant Fe-Fe interactions were no longer discernable in the Fourier transform (FT) data. Curve-fitting analysis of the {kappa}{sup 3}{sub {Chi}}(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 FeS{sub 2} as the amount of inserted lithium was increased. This behavior is consistent with the formation of Fe{sub 1{minus}x}S and thus supports the assignment made on the basis of in situ {sup 57}Fe Mossbauer effect spectroscopy of the same system. 24 refs., 5 figs., 1 tab.