Laser-induced phase changes in olivine FePO4: a warning on characterizing LiFePO4-based cathodes with Raman spectroscopy

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 LiFePO4-based cathodes because the intramolecular vibrational modes of the PO43- 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 LiFePO4. Therefore, we examine the effect of exposing olivine FePO4 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 FePO4 crystal structure and result in the formation of disordered FePO4. After the laser is turned off, the amorphous FePO4 compound crystallizes in the electrochemically inactive alpha-FePO4 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 alpha-FePO4 to olivine FePO4 phase transition that is known to occur at temperatures higher than 450 degrees C. Copyright (C) 2008 John Wiley & Sons, Ltd.