# In situ Fe K-edge X-ray absorption spectroscopy study during cycling of Li$$_2$$FeSiO$$_4$$ and Li$$_{2.2}$$Fe$$_{0.9}$$SiO$$_4$$ Li ion battery materials
Brownrigg, Alexander W., Mountjoy, Gavin, Chadwick, Alan V., Alfredsson, Maria, Bras, Wim, Billaud, Juliette, Armstrong, A. Robert, Bruce, Peter G., Dominko, Robert, Kelder, Erik M. and others. (2015) In situ Fe K-edge X-ray absorption spectroscopy study during cycling of Li$$_2$$FeSiO$$_4$$ and Li$$_{2.2}$$Fe$$_{0.9}$$SiO$$_4$$ Li ion battery materials. Journal of Materials Chemistry A, 3 (14). pp. 7314-7322. ISSN 2050-7488. E-ISSN 2050-7496. (doi:10.1039/c4ta06305h) (The full text of this publication is not currently available from this repository. You may be able to access a copy if URLs are provided)
In situ X-ray Absorption Spectroscopy (XAS) results are presented for Li$$_2$$FeSiO$$_4$$ and Li$$_{2.2}$$Fe$$_{0.9}$$SiO$$_4$$, promising cathode materials for lithium-ion batteries. The aims are to establish the valence and local structure of Fe during charge and discharge to understand if the Fe$$^{3+}$$/Fe$$^{4+}$$ redox pair can be reached in the current battery design. It is found that 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 Li$$_2$$FeSiO$$_4$$ material. The same observations apply to Li$$_{2.2}$$Fe$$_{0.9}$$SiO4 cathode material indicating that changing the stoichiometry does not provide any additional structural stability.