Lithium insertion mechanism in CoSb3 analysed by Sb-121 Mossbauer 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), Sb-121 Mossbauer spectrometry and electronic structure calculations based on the Density Functional Theory (DFT) in the Linear Muffin Tin Orbital (LMTO) framework. Sb-121 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 L-I,L- 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.