Using Synchrotron Radiation to Study Li-Ion Batteries

Abstract

X-Ray Absorption Spectroscopy (XAS) is a powerful tool, which can be used to study the atomic local structure as well as oxidation state during battery cycling. Hence, XAS can be used to provide detailed atomic information for improvement of materials design.Here we present in situ XAS results for Li2FeSiO4 and Li2.2Fe0.9SiO4, promising cathode materials for lithium-ion batteries. It provides a low cost solution and is considered as a safe electrode material for future electric transportation, as the Si–O bond provides stability similar to the P – O bond in LiFePO4 compounds. In theory, it is possible to remove two Li ions from Li2FeSiO4, by utilizing both Fe+2/Fe+3 and the Fe+3/Fe+4 redox couples and, thus, producing a higher capacity than 166 mAhg-1 for one Li ion. Although capacities greater than 200 mAhg-1 it has yet to be clarified whether it was due to Fe+4 formation or electrolyte degradation. The aim was to establish the valence and local structure of Fe during charge and discharge. The valence state changes between Fe+2 and Fe+3, with no evidence of Fe+4 before the onset of electrolyte degradation. There is a reversible contraction and extension of the Fe–O bond lengths during cycling while the Fe-Si distance remains constant, which underlines the stability of the Li2FeSiO4 material. The same observations apply to Li2.2Fe0.9SiO4 cathode material indicating that changing the stoichiometry does not provide any additional structural stability.1 1. A.W. Brownrigg, G. Mountjoy, A.V. Chadwick, M. Alfredsson, W. Bras, J. Billaud, A.R. Armstrong, P.G. Bruce, R. Dominko and E.M. Kelder, J. of Mater. Chem. A, 2015, 3, 7314-7322