Transition metal phosphates such as LiFePO(4) have been recognized as very promising electrodes for lithium-ion batteries because of their energy storage capacity combined with electrochemical and thermal stability. A key issue in these materials is to unravel the factors governing electron and ion transport within the lattice. Lithium extraction from LiFePO(4) results in a two-phase mixture with FePO(4) that limits the power characteristics owing to the low mobility of the phase boundary. This boundary is a consequence of low solubility of the parent phases, and its mobility is impeded by slow migration of the charge carriers. In principle, these limitations could be diminished in a solid solution, Li(x)FePO(4). Here, we show that electron delocalization in the solid solution phases formed at elevated temperature is due to rapid small polaron hopping and is unrelated to consideration of the band gap. We give the first experimental evidence for a strong correlation between electron and lithium delocalization events that suggests they are coupled. Furthermore, the exquisite frequency sensitivity of Mössbauer measurements provides direct insight into the electron hopping rate.

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 ͒.

Lithium metal phosphates are amongst the most promising cathode materials for high capacity lithium-ion batteries. Owing to their inherently low electronic conductivity, it is essential to optimize their properties to minimize defect concentration and crystallite size (down to the submicron level), control morphology, and to decorate the crystallite surfaces with conductive nanostructures that act as conduits to deliver electrons to the bulk lattice. Here, we discuss factors relating to doping and defects in olivine phosphates LiMPO4 (M = Fe, Mn, Co, Ni) and describe methods by which in situ nanophase composites with conductivities ranging from 10(-4)-10(-2) S cm(-1) can be prepared. These utilize surface reactivity to produce intergranular nitrides, phosphides, and/or phosphocarbides at temperatures as low as 600 degrees C that maximize the accessibility of the bulk for Li de/insertion. Surface modification can only address the transport problem in part, however. A key issue in these materials is also to unravel the factors governing ion and electron transport within the lattice. Lithium de/insertion in the phosphates is accompanied by two-phase transitions owing to poor solubility of the single phase compositions, where low mobility of the phase boundary limits the rate characteristics. Here we discuss concerted mobility of the charge carriers. Using Mössbauer spectroscopy to pinpoint the temperature at which the solid solution forms, we directly probe small polaron hopping in the solid solution Li(x)FePO4 phases formed at elevated temperature, and give evidence for a strong correlation between electron and lithium delocalization events that suggests they are coupled.

The anisotropic contribution to the transferred hyperfine fields in YMn 6 Sn 5.42 In 0.58 has been isolated using a field-driven moment rotation from the ab plane to the c axis in a single crystal. We find that at 12 K, the anisotropic contribution is between 3% and 4% of the total field for the Sn 2c and Sn 2d sites, while it accounts for nearly one-third of the observed field at the Sn 2e site. Comparison with data from RMn 6 Sn 6−x X x ͑R = Tb, Er; X =Ga,In͒ compounds containing magnetic rare earths shows that the Mn and R contributions to the anisotropic component of the transferred hyperfine fields are similar in magnitude.