Soft X-ray XANES studies of various phases related to LiFePO4 based cathode materials

Abstract

LiFePO₄ has been a promising cathode material for rechargeable lithium ion batteries. Different secondary or impurity phases, forming during either synthesis or subsequent redox process under normal operating conditions, can have a significant impact on the performance of the electrode. The exploration of the electronic and chemical structures of impurity phases is crucial to understand such influence. We have embarked on a series of synchrotron-based X-ray absorption near-edge structure (XANES) spectroscopy studies for the element speciation in various impurity phase materials relevant to LiFePO₄ for Li ion batteries. In the present report, soft-X-ray XANES spectra of Li K-edge, P L_2,3-edge, O K-edge and Fe L_2,3-edge have been obtained for LiFePO₄ in crystalline, disordered and amorphous forms and some possible “impurities”, including LiPO₃, Li₄P₂O₇, Li₃PO₄, Fe₃(PO₄)₂, FePO₄, and Fe₂O₃. The results indicate that each element from different pure reference compounds exhibits unique spectral features in terms of energy position, shape and intensity of the resonances in its XANES. In addition, inverse partial fluorescence yield (IPFY) reveals the surface vs. bulk property of the specimens. Therefore, the spectral data provided here can be used as standards in the future for phase composition analysis.