Investigation of the reversible sodiation of Sn foil by ex-situ X-ray diffractometry and Mössbauer effect spectroscopy

The reversible sodiation of Sn foil was investigated using ex-situ X-ray diffractometry (XRD) and Mössbauer effect spectroscopy. The measured voltage profile indicated that the sodiation process of Sn foil proceeded in three stages. Ex-situ XRD patterns demonstrated that Na4Sn4, Na5Sn2 and Na15Sn4 phases were formed at the end of each discharge plateau. Na5Sn2, Na4Sn4 and β-Sn were formed at the end of each charge plateau. Three single-phase alloys, Na4Sn4, Na9Sn4 and Na15Sn4, were prepared by annealing stoichiometric ratios of Na and Sn. The Mössbauer spectra of ex-situ samples at the end of each discharge plateau were collected and compared with the Mössbauer spectra of the three single phase alloys. The measured parameters for the Mössbauer effect spectra were consistent with an analysis of the crystal structures. The center shift became less positive with an increase of the sodium content in Na-Sn phases and this was shown to be a useful indicator of the degree of sodiation of Sn electrodes.