Spatially Resolved Characterization of Phases in LiFePO4 Battery Cathodes Using Low Loss Electron Energy-loss Spectroscopy

Abstract

Lithium iron phosphate (LiFePO4) has considerable potential for automotive applications due to its high rate capability, reasonable energy density and environmentally benign nature [1]. However, performance degradation seen after thousands of cycles at high charging-rates (C-rates) has been a point of major concern [2]. Studies of the aging mechanism suggest that phases (LiFePO4/FePO4) formed in the cathode during discharge influence the aging profile [3]. These phases have been investigated recently using x-ray and neutron diffraction [4, 5]. While these methods provide insights into crystal structure and unit cell volume changes associated with phase changes between LiFePO4 and FePO4, their ability to provide spatially resolved measurements of quantities such as Li and Fe concentration at the nanoscale is severely limited. Such measurements are key to confirming the presence or absence of the phases mentioned above and solid solution phases (LixFePO4). With recent advances in aberration corrected electron microscopy, electron energy-loss spectroscopy (EELS) performed in the scanning transmission electron microscope (STEM) has emerged as a leading technique to obtain spatially resolved measurements of chemistry, structure and bonding. Previously, researchers have used shifts observed in core-loss peaks of iron and oxygen to study the delithiation of FePO4 host lattice [6-8]. However, these ionization edges have relatively small inelastic scattering cross sections, and consequently, a relatively high electron dose is required to achieve acceptable signal-to-noise ratios in the data resulting in significant electron beam damage to the specimen. The low energy-loss region of the spectrum (0-40 eV) can be very useful since (i) the intensity is much larger compared to the coreloss spectrum and (ii) lesser collection time is required which significantly lowers the risk of beam damage to the material. In this study, we explore the use of low loss EELS to characterize the nominally pure LiFePO4 powder with trace amounts of FePO4 which is typical of phase mixtures that is present during phase transformation within the LiFePO4 battery cathode at the nanoscale during aging.