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Laser‐induced phase changes in olivine FePO 4 : a warning on characterizing LiFePO 4 ‐based cathodes with Raman spectroscopy
Author(s) -
Burba Christopher M.,
Palmer Jasin M.,
Holinsworth Brian S.
Publication year - 2009
Publication title -
journal of raman spectroscopy
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.748
H-Index - 110
eISSN - 1097-4555
pISSN - 0377-0486
DOI - 10.1002/jrs.2114
Subject(s) - raman spectroscopy , materials science , amorphous solid , olivine , spectroscopy , analytical chemistry (journal) , phase (matter) , phase transition , lithium (medication) , laser , ion , chemistry , mineralogy , crystallography , optics , condensed matter physics , medicine , physics , organic chemistry , chromatography , quantum mechanics , endocrinology
Raman spectroscopy is an excellent technique for probing lithium intercalation reactions of many diverse lithium ion battery electrode materials. The technique is especially useful for probing LiFePO 4 ‐based cathodes because the intramolecular vibrational modes of the PO 4 3− anions yield intense bands in the Raman spectrum, which are sensitive to the presence of Li + ions. However, the high power lasers typically used in Raman spectroscopy can induce phase transitions in solid‐state materials. These phase transitions may appear as changes in the spectroscopic data and could lead to erroneous conclusions concerning the delithiation mechanism of LiFePO 4 . Therefore, we examine the effect of exposing olivine FePO 4 to a range of power settings of a 532‐nm laser. Laser power settings higher than 1.3 W/mm 2 are sufficient to destroy the FePO 4 crystal structure and result in the formation of disordered FePO 4 . After the laser is turned off, the amorphous FePO 4 compound crystallizes in the electrochemically inactive α‐FePO 4 phase. The present experimental results strongly suggest that the power setting of the excitation laser should be carefully controlled when using Raman spectroscopy to characterize fundamental lithium ion intercalation processes of olivine materials. In addition, Raman spectra of the amorphous intermediate might provide insight into the α‐FePO 4 to olivine FePO 4 phase transition that is known to occur at temperatures higher than 450 °C. Copyright © 2008 John Wiley & Sons, Ltd.