FePO4 Phase Research Articles

Elastically constrained phase-separation dynamics competing with the charge process in the LiFePO4/FePO4 system

By using phase-field computer simulations, we have investigated the effects of the coherent strain due to the phase separation in the olivine-type LiFePO4. In this system, the coherent elastic-strain energy due to the lattice mismatch between LiFePO4 and FePO4 phases accompanied by insertion and extraction of Li ions is considered to play a crucial role in the phase separation kinetics during the charge/discharge process. The present phase-field micromechanics simulations reveal several significant features of the LiFePO4/FePO4 system accompanying the coherent strain, such as the retardation of the phase separation, the charge rate dependence, the thermodynamic stability of coherent interfaces between dual phases, etc. Nucleation of the new phase is found to be fundamentally unlikely in terms of the elastic strain energy, except in the vicinity of the surface of the particles, and thus the phase separation would be dominated by the spinodal decomposition process. When the nucleus is present precedently, however, the phase separation can proceed in the mixture mode of the domino cascade and spinodal decomposition processes.

Structure and phase analysis of one-pot hydrothermally synthesized FePO4-SBA-15 as an extremely stable catalyst for harsh oxy-bromination of methane

FePO4-SBA-15 (OP) was directly synthesized via a one-pot hydrothermal technique, using Fe(NO3)3 and H3PO4 as the precursors. FePO4/SBA-15 (IMP) was also prepared as a reference, using an impregnation method and commercially available SBA-15 as the support. The yielding samples were employed to catalyze the harsh oxy-bromination of methane (OBM) reaction, showing similar initial catalytic performances. The fresh and spent samples after catalytic reaction were thoroughly characterized by N2-physisoption, inductively coupled plasma, wide- and small-angle X-ray diffraction, transmission electron microscopy, diffuse reflectance UV–vis spectroscopy, temperature-programmed oxidation, and room temperature 57Fe Mössbauer spectroscopy. It was found that the FePO4 was in good crystalline in the OP sample while it was in amorphous for the IMP catalyst. Despite this difference, both FePO4 phases in the fresh samples were transformed into Fe7(PO4)6 and Fe2P2O7 in the spent ones. Furthermore, the OP catalyst showed excellent stability in a period of 1000h time-on-stream performance without apparent deposition of cokes. The losses of P and Fe after the catalytic evaluation were only 9.5% and 15.5%, respectively, while the ratio of P/Fe remained close to 1.0. N2-adsorption and TEM observations confirmed that the mesoporous pores were extremely stable under the harsh reaction ambience, which might play a crucial role in the stability test.

A GGA+U study of lithium diffusion in vanadium doped LiFePO4

Recent experiments showed beneficial influence of vanadium doping on the electrochemical performance of lithium iron phosphate (LiFePO4) cathode materials. First-principles calculations have been performed to investigate the stability, electronic structure and lithium diffusivity of vanadium-doped LiFePO4 and to elucidate the origin of such improvement. It is found that vanadium prefers occupying Fe sites and leads to additional density of states within the intrinsic bandgap. By the nudged elastic band method, we show that the barrier of Li ions diffusion along the one dimensional channel in both LiFePO4 and FePO4 phases can be effectively reduced by vanadium doping. Structural analysis shows the lower diffusion barrier ties closely to a volumetric expansion of the diffusion channel.

Near Heterosite Li0.1FePO4 Phase Formation as Atmospheric Aging Product of LiFePO4/C Composite. Electrochemical, Magnetic and EPR Study

Freeze-drying method was employed in order to synthesize a LiFePO4/C composite. This method produced phosphate nanoparticles (10–20 nm size) covered in a carbon network. Nanostructuring and homogeneous conductive web around the active material promoted a good electrochemical response, with 141 mAh · g−1 at C/1 and a capacity retention of 90% up to 300 cycles. However, XRD measurements showed the presence of an approximately 30% of a near heterosite Li0.1FePO4 intermediate phase in the initial sample, as a product of atmospheric aging. EPR spectrum displayed a very broad signal attributed to a high spin Fe3+ ion magnetically coupled to Fe2+, confirming the formation of the non stoichiometric LixFePO4 compound. Electrochemical measurements indicated the presence of active Fe3+ in the initial composite, in a structural environment close to that of a FePO4 heterosite system. A post-mortem cathode, stopped at 4 V, was characterized. Metastable LixFePO4 solid solution type phase was not noticed. Thus, it would have disappeared during cathode cycling life, and all active material would have evolved to a classic biphasic system.

The Structural Transition and Magnetic Properties of Lithium Deintercalation in LiFePO$_{4}$

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Lithium deintercalation of <formula formulatype="inline"><tex Notation="TeX">$(1-x)$</tex> </formula> LiFePO<formula formulatype="inline"><tex Notation="TeX">$_{4}/x$</tex> </formula> FePO<formula formulatype="inline"><tex Notation="TeX">$_{4}(0\leq x \leq 1)$</tex></formula> have been studied with X-ray diffraction (XRD), inductively coupled plasma-atomic emission spectrometer (ICP-AES), and MÖssbauer spectroscopy. Lithium deintercalation to obtain <formula formulatype="inline"> <tex Notation="TeX">$(1-x)$</tex></formula> LiFePO<formula formulatype="inline"> <tex Notation="TeX">$_{4}/x$</tex></formula> FePO<formula formulatype="inline"> <tex Notation="TeX">$_{4}$</tex></formula> was done by chemical oxidation process using NO<formula formulatype="inline"><tex Notation="TeX">$_{2}$</tex> </formula>BF<formula formulatype="inline"><tex Notation="TeX">$_{4}$</tex> </formula>. The crystal structure was found to be an orthorhombic with space group <emphasis emphasistype="boldital">Pnma</emphasis> for each sample. There exists a mixture of LiFePO<formula formulatype="inline"><tex Notation="TeX">$_{4}$</tex> </formula> and FePO<formula formulatype="inline"><tex Notation="TeX">$_{4}$</tex> </formula> phase between <formula formulatype="inline"><tex Notation="TeX">$x = 0$</tex></formula> and <formula formulatype="inline"><tex Notation="TeX">$1$</tex> </formula>. The MÖssbauer spectra have been taken at various temperatures ranging from 4.2 K to room temperature. The magnetic NÉel temperature <formula formulatype="inline"><tex Notation="TeX">$(T_{N})$</tex></formula> was determined to be 51 and 114 K for LiFePO<formula formulatype="inline"><tex Notation="TeX">$_{4}$</tex> </formula> and FePO<formula formulatype="inline"><tex Notation="TeX">$_{4}$</tex> </formula>. Also, the iron ions were ferric and ferrous for LiFePO<formula formulatype="inline"><tex Notation="TeX">$_{4}$</tex></formula> and FePO<formula formulatype="inline"><tex Notation="TeX">$_{4}$</tex></formula>, both occupying the octahedral sites at various temperatures. We confirmed that the change of MÖssbauer spectrum shape as the evidence of two-valence state of iron ions coexists on lithium deintercalation in LiFePO<formula formulatype="inline"> <tex Notation="TeX">$_{4}$</tex></formula> from 4.2 K to room temperature. </para>

Laser‐induced phase changes in olivine FePO4: a warning on characterizing LiFePO4‐based cathodes with Raman spectroscopy

AbstractRaman spectroscopy is an excellent technique for probing lithium intercalation reactions of many diverse lithium ion battery electrode materials. The technique is especially useful for probing LiFePO4‐based cathodes because the intramolecular vibrational modes of the PO43− anions yield intense bands in the Raman spectrum, which are sensitive to the presence of Li+ ions. However, the high power lasers typically used in Raman spectroscopy can induce phase transitions in solid‐state materials. These phase transitions may appear as changes in the spectroscopic data and could lead to erroneous conclusions concerning the delithiation mechanism of LiFePO4. Therefore, we examine the effect of exposing olivine FePO4 to a range of power settings of a 532‐nm laser. Laser power settings higher than 1.3 W/mm2 are sufficient to destroy the FePO4 crystal structure and result in the formation of disordered FePO4. After the laser is turned off, the amorphous FePO4 compound crystallizes in the electrochemically inactive α‐FePO4 phase. The present experimental results strongly suggest that the power setting of the excitation laser should be carefully controlled when using Raman spectroscopy to characterize fundamental lithium ion intercalation processes of olivine materials. In addition, Raman spectra of the amorphous intermediate might provide insight into the α‐FePO4 to olivine FePO4 phase transition that is known to occur at temperatures higher than 450 °C. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Kinetic behavior of LiFePO4/C cathode material for lithium-ion batteries

The composite cathode materials of LiFePO4/C were synthesized by spray-drying and post-annealing method. The crystalline structure and morphology of products were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The charge–discharge kinetics of LiFePO4/C electrode was investigated using electrochemical impedance spectroscopy (EIS). The results show that the increment of the resistance has a close relation to the appearance of the FePO4 phase during charge–discharge course, and that the ohmic resistance, charge transfer resistance and lithium-ion diffusion coefficients of the LiFePO4/C electrode do not change much by extended cycling tests, implying a relatively superior cyclability of the battery. The effect of cell temperature on the electrochemical reaction behaviors of LiFePO4/C electrode was also investigated using the EIS. It is confirmed that the effect of the cell temperature on the impedance results mainly from the enhancement of the lithium-ion diffusion at elevated temperatures.

Nanostructured materials for lithium-ion batteries: Surface conductivity vs. bulk ion/electron transport

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.

New Iron(III) Phosphate Phases: Crystal Structure and Electrochemical and Magnetic Properties.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Amorphous FePO4 as 3 V cathode material for lithium secondary batteries

The structural and electrochemical properties of amorphous FePO4·xH2O (x = 2, 1, 0) and hexagonal FePO4 powders have been investigated using differential thermal analysis–thermogravimetry (DTA–TGA), X-ray diffractometry (XRD), X-ray absorption spectroscopy (XAS), scanning electron microscopy (SEM), cyclic voltammetry (CV), and charge/discharge cycling. On heating, amorphous FePO4·2H2O was transformed into hexagonal FePO4 at 380 °C, through amorphous FePO4·H2O and FePO4 phases. Reversible lithium insertion and extraction into and from the amorphous FePO4 occurred at 2.8 and 3.2 V vs. Li/Li+, respectively, which potentials are lower than those of the olivine-structured LiFePO4. All samples show a large capacity loss at the 1st cycle, but their discharge capacities are gradually increased from 65 mA h g−1 (2nd cycle) to 75 mA h g−1 (15th cycle) at a current density of 17 mA g−1 and kept up to the 50th cycle. The discharge/charge dependence on the current densities (17, 34, and 85 mA g−1) and operating temperatures (20, 50, and 80 °C) were also investigated for amorphous FePO4. Fe K-edge X-ray absorption spectroscopy has been performed on amorphous LiyFePO4, to determine the changes in the local electronic and geometric structures during the discharge. For the Fe K-edge, the pre-edge and edge are shifted in accordance with the oxidation state of the Fe3+/Fe2+ redox couple in LiyFePO4.