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

Abstract

LiFePO 4 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 ͑XPS͒, that conductive metal phosphides which enhance its electrochemical performance ͑FeP, and metallic Fe 2 P͒, are generated on the surface of the parent LiFePO 4 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 Li 3 PO 4 , and carbon resulting from the precursor. Under more aggressive reducing conditions, FeP is still present, but thermodynamics now favor the formation of Fe 2 P, with fractions varying from 4 to 18 wt % depending on the temperature and atmosphere used for treatment. Both large ͑0.5 m͒ crystallites, and amorphous or nanodimensioned particles are present. XPS studies reveal that the amorphous or nanodimensioned Fe 2 P lies on the inner surface adjacent to the LiFePO 4 , and the residual carbon lies on the outer surface. The resulting LiFePO 4 "composites" show significantly enhanced electrochemical rate properties as well as outstanding cyclability, which allows a high discharge capacity of ϳ105 mAh g −1 at a 14.8C rate ͑2500 mA g −1 ͒.

A facile synthesis and characterization of LiFePO4/C using simple binary reactants with oxalic acid by polyol technique and other high temperature methods

“…During the lithium ion insertion/ de-insertion cyclic performance of the samples (B and C) may be attributing to the formation of cracks and subsequent pulverization of the materials because of the volumetric change of the LFP particles [51,52]. Also, the incomplete dispersion of the electrolyte into the electrode materials at the beginning current rate the intercalation/de-intercalation capacity is more difference obtained in sample B and C. • A similar anomaly was observed during the first few cycles which were reported by Rho et al [53]. An increase of charge capacity at 0.1 C for first few cycles that may be associated with a large polarization and poor reversibility.…”

Developments in Nanostructured Cathode Materials for High‐Performance Lithium‐Ion Batteries

“…In addition to carbon coating, metal coating such as silver has been successfully used to increase the conductivity as well. [122] Another type of coating is conductive inorganic layer such as metallic Fe 2 P, as investigated by Rho et al [123] In their study, mixture of Fe 2 P and FeP were deposited on the surface of the LiFePO 4 along with carbon and the byproduct Li 3 PO 4 by surface reduction reactions. Fe 2 P is coated directly on the LiFePO 4 , while carbon and Li 3 PO 4 sit on the outer surface of the crystallites.…”

How Mössbauer spectroscopy can improve Li-ion batteries

“…The latter compound must be considered as an impurity that decreases the specific capacity of the electrode. It is due to reducing synthesis conditions (Ar/H 2 flux) and forms a surface layer that must be as thin as possible [30]. …”