Manfred Womes

Elucidation of Capacity Fading on CoFe2O4 Conversion Electrodes for Lithium Batteries Based on #2#1e Mössbauer Spectroscopy

Two CoFe 2 O 4 samples prepared at 600 and 1000°C have been subjected to galvanostatic cycling in lithium test cells. 57 Fe Mossbauer spectra have been analyzed to elucidate the possible causes of capacity fading during the first cycles. 57 Fe conversion electron Mossbauer spectroscopy has been applied to distinguish between bulk and surface iron locations. This technique revealed the presence of reduced iron atoms near the surface of the sample annealed at 1000°C. Their suitable accessibility to lithium may explain the better capacity retention observed for a large number of cycles.

Lithium insertion mechanism in CoSb3 analysed by 121Sb Mössbauer spectrometry, X-ray absorption spectroscopy and electronic structure calculations

The lithium insertion mechanism into the skutterudite-type CoSb3 compound has been studied using X-ray Absorption Near Edge Structure (XANES), 121Sb Mossbauer spectrometry and electronic structure calculations based on the Density Functional Theory (DFT) in the Linear Muffin Tin Orbital (LMTO) framework. 121Sb Mossbauer spectra are in agreement with a progressive restructuring which occurs during the second stage (voltage plateau at 0.6 V). The Mossbauer hyperfine parameters show the formation of Li3Sb and of a ternary intermediate phase LixCoSby whose variable composition depends on the insertion conditions. XANES spectra at Sb LI, III and Co K edges have been compared to calculated Projected Densities of States (PDOS) of reference compounds containing Co, Sb and Li. This analysis has allowed specification of the restructuring mechanism as a distortion of the CoSb6 octahedral units and confirms the formation of Li3Sb. The overall characterisations have been interpreted to suggest the first discharge mechanism of restructuring in accordance with the global reaction:CoSb3 + (y + z) Li → (LixCo1−mSby + m Co + Li3Sb) ↔ LizCo + y Li3Sb

Characterization of the single phase region with spinel structure in the ternary system In2S3FeSFeS2

Abstract The limit of single-phase solid solubility in a spinel host lattice in the system In2S3FeSFeS2 has been determined. Mossbauer spectroscopy showed that all iron is bivalent and, for the major part, occupies the B-sites (δ = 0.81–0.89 mm/s, Δ = 3.16–3.25 mm/s). The fraction of tetrahedral iron depends on the sample composition and the thermal treatment. Mossbauer spectroscopy also showed the influence of tetrahedral vacancies by causing quadrupole splittings on tetrahedral subspectra (δ = 0.64–0.74 mm/s and Δ = 0.55–1.07 mm/s).

Detection of vacant tetrahedral sites in thiospinels by 57Fe Mössbauer spectroscopy and diffuse reflectance

Abstract The spinel In 2 S 3 , in which one-third of the tetrahedral cation sites is unoccupied, is used as a host lattice for solid solutions within the ternary systems In 2 S 3 —Cu 2 S—CuS and In 2 S 3 —SnS—SnS 2 . The number of vacant tetrahedral sites per unit cell can be varied within these solid solution regions, between zero and 2.66 and between 2.24 and 4, respectively. 57 Fe introduced as a local probe allows these vacancies to be detected by means of Mossbauer spectroscopy. Several typical values of quadrupole splittings are observed which can be related to the number of vacant tetrahedral sites around an iron atom. Diffuse reflectance spectra reveal an absorption band at about 5000 cm −1 which appears in both systems. The intensity of this band is related to the number of vacant tetrahedral sites per unit cell.