Surface Chemistry of LiFePOsub 4 Studied by Mossbauer and X-Ray Photoelectron Spectroscopy and Its Effect on Electrochemical Properties

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 (, and metallic ), are generated on the surface of the parent 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 is formed on the surface along with , and carbon resulting from the precursor. Under more aggressive reducing conditions, is still present, but thermodynamics now favor the formation of , with fractions varying from 4 to depending on the temperature and atmosphere used for treatment. Both large crystallites, and amorphous or nanodimensioned particles are present. XPS studies reveal that the amorphous or nanodimensioned lies on the inner surface adjacent to the , and the residual carbon lies on the outer surface. The resulting "composites" show significantly enhanced electrochemical rate properties as well as outstanding cyclability, which allows a high discharge capacity of at a 14.8C rate .