Surface Chemistry of LiFePO[sub 4] Studied by Mössbauer and X-Ray Photoelectron Spectroscopy and Its Effect on Electrochemical Properties

Abstract

LiFePO4 is a promising cathode material for lithium-ion batteries despite its low intrinsic electronic conductivity. We show, using a combination of Mössbauer, X-ray diffraction, and X-ray photoelectron spectroscopy (XPS), that conductive metal phosphides which enhance its electrochemical performance (FeP, and metallic Fe2 P), are generated on the surface of the parent LiFePO4 by reaction with in situ carbon from iron citrate and reducing gases such as hydrogen. Their relative fraction, nature, and location was quantified. Under the most mild reducing conditions, nanosized FeP is formed on the surface along with Li3 PO4, and carbon resulting from the precursor. Under more aggressive reducing conditions, FeP is still present, but thermodynamics now favor the formation of Fe2 P, with fractions varying from 4 to 18 wt % depending on the temperature and atmosphere used for treatment. Both large (0.5 μm) crystallites, and amorphous or nanodimensioned particles are present. XPS studies reveal that the amorphous or nanodimensioned Fe2 P lies on the inner surface adjacent to the LiFePO4, and the residual carbon lies on the outer surface. The resulting LiFePO4 "composites" show significantly enhanced electrochemical rate properties as well as outstanding cyclability, which allows a high discharge capacity of ∼105 mAh g-1 at a 14.8C rate (2500 mA g-1). © 2007 The Electrochemical Society.