Polyol Medium Research Articles

Nucleation and Growth Controlled Polyol Synthesis of Size-Focused Nanocrystalline LiFePO4 Cathode for High Performance Li-Ion Batteries

Mono-dispersed and densely packed, LiFePO4 cathodes with high rate performance versus lithium were synthesized using a two-step polyol process. The particle-size distribution and packing density of LiFePO4 nanoparticles were fine-tuned via separation of the nucleation and particle growth processes in the polyol medium by varying reaction time and temperature. Precisely, the LiFePO4 samples were synthesized by heating the reaction mixtures at 200 °C for various reaction times (0, 1, 3 and 6 h) to facilitate nucleation and thereafter the solution temperature was raised and maintained at 320 °C to facilitate the crystal growth process. XRD studies confirmed the formation of olivine phase in all the samples. The FE-SEM, FE-TEM and particle-size distribution studies illustrate that the particle-size of olivine nanocrystals may be tuned to approximately 200 nm by controlling the aging time at 200 °C or in other words separating the nucleation and crystal growth process. The olivine cathodes synthesized by maintaining 3-6 h aging time demonstrate improved discharge capacities and cycle performances in comparison to those of the remaining samples. In particular, the 6 h olivine cathode registered average discharge capacities of 130, 87 and 64 mAh/g at 5 C, 10 C and 15.7 C current rates respectively.

Pyro-Synthesis of Nanostructured Spinel ZnMn2O4/C As Negative Electrode for Rechargeable Lithium-Ion Batteries

Commercially available lithium-ion batteries comprised of LiCoO2 as the cathode and graphite as the anode. Graphite is being considered for use as the anode material for lithium-ion batteries for several reasons, such as the fact that the Li+ ions can easily intercalate/de-intercalate into/from the graphite structure, low irreversible capacity at the first cycle, a flat and low potential profile, a long cycle life, abundant and relatively low cost. However, in spite of its advantages, graphite has several drawbacks, such as its low energy density, low theoretical capacity (372 mAh g-1), easy exfoliation and safety issues related to lithium deposition. These drawbacks limit its application to electric vehicles, hybrid electric vehicles and plug-in hybrid electric vehicles. Therefore, searching for new electrode materials which can act as an alternative to graphite with a high specific capacity and excellent electrical conductivity is an urgent task in building the next-generation high-power lithium-ion batteries, in order to fulfill the ever-growing demand in various consumer electronic devices. Recently, ZnMn2O4 has been considered as one of the most attractive compounds, because of the low oxidation (delithiation) potential of zinc and manganese nanoparticles: 1.2 V and 1.5 V, respectively, which ultimately increase the battery output voltage compared to that provided by simple binary metal oxides. Furthermore, zinc and manganese are abundant, environmentally friendly, and relatively inexpensive compared to nickel and cobalt. Interestingly, ZnMn2O4 can store Li+ions through both conversion and alloying reactions, because Zn, one of the products of the conversion reaction. However, as in the case of other anodes, the poor electric conductivity and large volume change of the ZnMn2O4 nanostructures during the electrochemical reaction lead to a fast capacity decrease. However, a few attempts have been made to overcome the aforementioned drawbacks of ZnMn2O4 by focusing on controlling the morphology accompanied with synthesis of nano-sized particles and graphene based nanocomposites using expensive and tedious methods. Nanostructured materials have become a subject of great interest in a variety of mechanical, electrical and optical applications, due to their unique physico-chemical properties. Therefore we are synthesized ZnMn2O4/C nanoparticles by one step polyol assisted pyro-synthesis for use as the anode in rechargeable lithium ion batteries without any post heat treatment. The as-prepared ZnMn2O4/C is tetragonal with a spherical particle size in the range of 10–30 nm. Electrochemical measurements were performed using the as-prepared powders as the active material for a lithium-ion cell. The nanoparticle electrode delivered an initial charge capacity of 666.1 mAh g1 and exhibited a capacity retention of 81% (539.4 mAh g-1) after 50 cycles. The capacity enhancement in the as-prepared ZnMn2O4/C may be explained on the basis of the polyol medium that enables to develop a sufficient carbon network that can act as electrical conduits during electrochemical reactions. The carbon network appears to enhance the particle-connectivity and hence improve the electronic conductivities.

Facile Process and Electrochemical Characterizations of Carbon Nanotube-LiFePO4 Composite As a Cathode for High Rate Lithium Ion Batteries

: LiFePO4 nanoparticles were incorporated with multi-walled carbon nanotubes (CNTs) via a facile one-step at low temperature polyol process. The CNTs were fund to embed into the LiFePO4 particles that form a network to enhance the electrochemical performance of LiFePO4 electrode. Structural morphologies of these CNT-LiFePO4 composites were investigated by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The electrochemical properties were analyzed by charge/discharge testing and cyclic voltammetry. Primary results showed that the CNT-LiFePO4 composites exhibited enhanced electrochemical performance with a good reversibility and the electronic conductivity. Introduction: Lithium-ion batteries have been considered to play an important role in the electric vehicles (HEV) and plug-in HEV (PHEV) in the near future, which require high power density and high energy density. Thus, cathode materials with high rate capability are in demand [1, 2]. LiFePO4 shows promise as a cathode material for HEVs owing to its high specific capacity and excellent structural stability [3, 4]. However, its poor electronic conductivity and lithium diffusion significantly limit its performance at high rates [5-9]. Therefore, studies on LiFePO4 have focused on improving its rate capability. Coatings of conductive materials [5-10] has been reported to improve the electronic conductivity. Recent efforts have been made to improve rate performance of the LiFePO4 by the synthesized nanocomposites using carbon nanotubes (CNTs) [11]. These nano-carbons improve the high rate capability of LiFePO4, when they are uniformly dispersed in the composite. Herein we report the facile synthesis of CNT-LiFePO4composites with the CNT network providing a conduction path. Experimental: Multi-walled carbon nanotubes (5 wt.% ) were functionalized via the mixed acid method in order to remove the impurities and acquire excellent dispersion in the polyol medium. CNT-LiFePO4 nanocomposites were obtained by using a polyol process as follows: first, the CNTs were added into 100 ml of ethylene glycol and a homogeneous dispersion was achieved by an ultrasonic process. Then, pristine LiFePO4 particles (~50 nm) were directly dissolved into the dispersion by mechanical agitation in a three-neck round flask. Lastly, the mixed dispersion was heated to above its boiling point and maintained for 10 h under stirring and refluxing. After cooling down, the products were separated from the solvent via centrifugation. In order to remove the organic residue impurities, the products were washed with deionized water several times. Finally, the CNT-LiFePO4composite powders were dried at 100 °C for 24 h. Structural morphology were characterized by X-ray diffraction, scanning and transmission electron microscopy (SEM, TEM). Electrochemical performances were investigated by galvanostatic charge-discharge and cyclic voltammetry (CV) over the voltage between 2.0 and 4.0 V using coin cells (CR2032) at a LAND-CT2001A battery-testing system. Results: The morphology of the composites were investigated by scanning electron microscopy (SEM), as showed in Fig.1 that the LiFePO4 nanoparticles are dispersed uniformly with the CNT network. The electrochemical CVs tests were performed in the voltage of 2.0 and 4.0 V at the scanning rate of 0.1 mV s−1. A pair of redox peaks is clearly exhibited in the CNT-LiFePO4 composite cathode, which correspond to the two-phase charge-discharge reaction of the Fe2+/Fe3+ redox couple. The peak curves are very symmetric. The oxidation and reduction peaks appear at around 3.5 and 3.3 V with the potential gap of 0.2 V, which is less than the value of pristine LiFePO4 cathode as reported, and suggested that electrode reaction reversibility and the electronic conductivity of these composites are greatly enhanced owing to the incorporation of CNTs. Acknowledgements: Financial supports from University of Waterloo President’s Award, Natural Sciences and Engineering Research Council of Canada (NSERC) and Waterloo Institute for Nanotechnology (WIN) are greatly appreciated.

Template-free synthesis of V2O5 hierarchical nanosheet-assembled microspheres with excellent cycling stability

V2O5 microspheres with hierarchical structure via a facile two-step strategy. The first is preparing vanadium glycolate precursor through polyol medium template-free process and the second is subsequent thermal annealing treatment of the well-prepared precursors at high temperature in air. However, just by one-plot calcination of vanadium(IV) acetylacetone, V2O5 nanorods were obtained. The gained metal oxide were characterized by different physical and electrochemical analytical techniques. Electrochemical testing results show that V2O5 microspheres displayed a high specific discharge capacity of 275 mAh g−1 at 1C which is higher than the obtained nanorods, and the microspheres still kept 243 mAh g−1 after 200 cycles. Notably, the nanosheet-assembled microspheres as electrode materials still show 200 mAh g−1 even at a rate of 5C after 500 cycles. These results demonstrated that the well-prepared nanosheet-assembled microspheres are a fine cathode material for lithium ion battery with a high specific capacity and excellent cycle stability.

Comparative evaluation of hydrogen storage behavior of Pd doped carbon nanotubes prepared by wet impregnation and polyol methods

The present manuscript deals with the hydrogen absorption properties of multiwalled carbon nano-tubes (MWCNTs) doped with Pd nanoparticles by the conventional wet impregnation method and the polyol method. In the latter method Pd nanoparticles are doped into CNTs based on polyhydroxy organic compounds (polyol) mediated functionalization. The polyol medium reduces palladium chloride to palladium nanoparticles, and also acts as a capping agent to stabilize the Pd nanoparticles. The samples have been characterized using X-ray diffraction (XRD), Energy Dispersive X-ray (EDX) and Infrared (IR) spectroscopy. Hydrogen absorption studies clearly illustrate that Pd doped MW-CNT's prepared by polyol mediated functionalization show much higher hydrogen storage capacity compared to the sample prepared by wet impregnation route. To determine the cause of this behavior, Transmission Electron Microscopic (TEM) studies of the samples were carried out. The results indicate that the dispersion of Pd nanoparticle on the MW-CNT is much better for the sample prepared by the polyol route. An attempt was made to determine the existence of any metal-support interaction using Thermogravimetric Analysis (TGA). The investigation reveals that the higher dispersion of Pd nanoparticles on the CNT support plays a major role for the hydrogen adsorption.

Polyol-made stoichiometric Co<SUB align="right">0.2Ni<SUB align="right">0.3Zn<SUB align="right">0.5Fe<SUB align="right">2O<SUB align="right">4 nanoparticles: synthetic optimisation, structural, and microstructural studies

Crystalline and stoichiometric Co0.2Ni0.3Zn0.5Fe2O4 ferrite nanoparticles were successfully synthesised in polyol medium starting from the low-cost iron (III) chloride and acetates as precursors by the optimisation a number of synthetic parameters. The crystalline structure, the chemical composition, the microstructure, and the thermal stability of the as-produced powders were investigated by X-ray diffraction, infrared spectroscopy, energy dispersive X-ray, X-ray fluorescence spectroscopy, and transmission electron microscopy. We demonstrated that in contrast to the stoichiometric ferrite, which showed excellent thermal stability, the non-stoichiometric ferrites thermally decompose resulting in a stoichiometric ferrite as a major phase and hematite. The degree of deviation from the stoichiometry was calculated on the basis of a proposed decompositional thermal scheme and a quantitative analysis using Rietveld refinement.

New acetate-intercalated CoNiFe, ZnNiFe and CoZnFe layered double hydroxides: Synthesis by hydrolysis in polyol medium and characterization

New layered double hydroxides (LDHs) CoNiFe, ZnNiFe and CoZnFe with MII/MIII molar ratio of 3, with acetate ions in the interlayer region have been prepared by forced hydrolysis in polyol medium.The physico-chemical properties of all products were characterized by the methods of powder X-ray Diffraction, Infrared Spectroscopy, UV-Spectroscopy, Elemental analysis, Thermal analysis, Transmission Electron Microscopy and Scanning Electron Microscopy.It was found that all compounds present the typical features of hydrotalcite-like structure and exhibit a turbostratic character.The insertion of acetate anions into LDHs was confirmed and the interlayer spacing values were respectively 12.47, 13.64 and 14.69 Å for CoNiFe, ZnNiFe and CoZnFe.We notice that there is a difference between the interlayer spacing for all synthesized phases with and without zinc. This can be explained by a structural modification on the level of the layers and/ or different arrangement from the inserted species (anions + water) in the interlayer space for zinc materials.57Fe Mössbauer spectrometry allows concluding the presence of Fe3+ cations which occupy octahedral sites and confirming the absence of Fe2+ in the as-prepared compounds.

Pyro-Synthesis of Nanostructured Spinel ZnMn2O4/C as Negative Electrode for Rechargeable Lithium-Ion Batteries

ZnMn2O4/C nanoparticles are synthesized by one step polyol assisted pyro-synthesis for use as the anode in rechargeable lithium ion batteries without any post heat treatment. The as-prepared ZnMn2O4/C is tetragonal with a spherical particle size in the range of 10–30nm. Electrochemical measurements were performed using the as-prepared powders as the active material for a lithium-ion cell. The nanoparticle electrode delivered an initial charge capacity of 666.1mAhg−1 and exhibited a capacity retention of ∼81% (539.4mAhg−1) after 50 cycles. The capacity enhancement in the as-prepared ZnMn2O4/C may be explained on the basis of the polyol medium that enables to develop a sufficient carbon network that can act as electrical conduits during electrochemical reactions. The carbon network appears to enhance the particle-connectivity and hence improve the electronic conductivities.

Nanocrystals of Zn(Fe)O-based diluted magnetic semi-conductor as potential luminescent and magnetic bimodal bioimaging probes

Nanocrystals of Zn(Fe)O-based diluted magnetic semi-conductor (nanoDMS) have been prepared by forced hydrolysis in polyol medium, using zinc and iron salts, and coated with carboxymethylpullulan (CMP). TEM studies revealed that CMP-nanoDMS are spherical, with average diameters on the order of 20 nm. Hydrodynamic diameters of about 75 nm were found in a physiological medium. NanoDMS were superparamagnetic at body temperature, and exhibited high saturation magnetization values (3.25 emu g−1). In aqueous suspensions, the colloids revealed high transverse proton relaxivities (T2) with an r2 value of 203 mM−1 s−1 (300 K, 1.41 T). Visible photoluminescent properties were evidenced with an excitation wavelength of 366 nm. No evidence of cell toxicity was found for human endothelial cells incubated with CMP-nanoDMS, at concentrations up to 100 μg mL−1 and for 48 h. No modification to the cytoskeleton was found up to 6 days of exposure. These hybrid nanomaterials could be considered as novel potential MR and luminescent bioimaging agents.

Structural, Thermal and Electrical Conduction Studies on LiNiPO4: RE (RE= La, Nd) Prepared by Polyol Method

LiNiPO4 and LiNiPO4: xRE (RE = La, Nd) (x = 0.01 mol%, 0.03 mol%, 0.05 mol%, 0.07 mol%, 0.09 mol%) samples have been prepared by polyol method using 1, 2 propanediol as a polyol medium. XRD patterns have indicated that the pristine LiNiPO4 is well crystallized with orthorhombic structure pnma space group and structurally stable compound upon doping of rare earth metals. Functional group analyses have been carried out by FTIR spectroscopic analysis. TG analysis shows that no weight loss has been observed above 600°C for both Nd3+ and La3+ doped LiNiPO4. The morphology of the samples was analyzed through Scanning Electron Microscopy. The conductivity of LiNiPO4 was found to be improved by 2 orders by doping of rare earth ions. It is found that lanthanum is an excellent dopant for LiNiPO4 than neodymium due to the formation of free ion sites which causes the enhancement of conductivity. Dielectric studies support that doping of La3+ is favorable for conduction compared with Nd3+ doping.

Synthesis and characterization of ultrafine and mesoporous structure of cobalt ferrite

Ultrafine nanoparticles of cobalt ferrite (CoFe2O4) were synthesized under microwave irradiation in polyol media and then were applied for the synthesis of mesoporous structures of cobalt ferrite. The effect of reaction parameters on products were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmittance electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), Fourier transform infrared (FT-IR) spectra, vibrating sample magnetometer (VSM), and volumetric nitrogen adsorption. TEM and BET analysis showed presence of mesoporus structures with pore sizes less than 10nm in the final product.

Facile in situ growth of highly monodispersed Ag nanoparticles on electrospun PU nanofiber membranes: Flexible and high efficiency substrates for surface enhanced Raman scattering

Abstract In this study, by utilizing a two-step route of electrospinning and polyol immersion, in the absence of any surfactant or sensitizing and stabilizing reagent, a well-distributed assembly of Ag NPs on the electrospun polyurethane (PU) nanofibers was successfully fabricated through a simple and controllable manner. Based on the FE-SEM, XRD and FT-IR analyses, the polyol medium plays an important role in the growth of highly monodispersed Ag NPs, wherein the hydroxyl group of ethylene glycol ( EG ) can be bridged to the amide group on the surface of the PU nanofibers through intermolecular hydrogen bonds. Fabrication of a polymer fibrous membrane effectively attached/decorated with noble metal NPs, which is essential as flexible and high efficiency substrates for SERS application where the molecule analytes are directly adsorbed on their surfaces is important, could be realized by the present electrospun PU–Ag( EG ) nanofibers, employing 4-mercaptobenzoic acid (4-MBA) as probe molecules.

Effects of Oxide Nanoparticle Size and Shape on Electronic Structure, Charge Transport, and Recombination in Dye-Sensitized Solar Cell Photoelectrodes

Forced hydrolysis in polyol medium is a versatile synthesis method for the preparation of metal oxide particles with controlled properties. We investigated the D149-dye-sensitized solar cell (DSSC) performance of ZnO film electrodes prepared with four different types of nanoparticles having various sizes and morphologies. The photoanode dye loading has been determined, and the cells have been studied by impedance spectroscopy (IS) at various applied voltages. From the analysis of the IS spectra, the key functioning parameters of the various photoelectrodes such as trap-state distribution, electron-transfer time, and electron lifetime have been determined. The particle shape and size deeply influence the dye loading and the electronic structures of the trap-state levels localized below the conduction band minimum. Low open circuit voltage and fill factor are found in the case of small (8 nm) spherical particles because of very deep energy states related to large particle necking and high density of grain b...

LiNi1/3Co1/3Mn1/3O2 Nanoplates with {010} Active Planes Exposing Prepared in Polyol Medium as a High-Performance Cathode for Li-Ion Battery

As we know, Li(+)-ion transport in layered LiNi1/3Co1/3Mn1/3O2 (NCM) is through two-dimensional channels parallel to the Li(+)-ion layers that are indexed as {010} active planes. In this paper, NCM nanoplates with exposed {010} active facets are synthesized in a polyol medium (ethylene glycol) and characterized by XRD, XPS, SEM, and HR-TEM. In addition, the effects of reaction conditions on the morphologies, structures and electrochemical performances are also evaluated. The results show that more {010} facets can be exposed with the thickness of NCM nanoplates increasing which can lead to more channels for Li(+)-ion migration. However, when the annealing temperatures exceed 900 °C, many new crystal planes grow along the thickness direction covering the {010} facets. In all of the NCM nanoplates obtained at different conditions, the NCM nanoplates calcined at 850 °C for 12 h (NCM-850-12H) display a high initial discharge capacity of 207.6 mAh g(-1) at 0.1 C (1 C = 200 mA g(-1)) between 2.5 and 4.5 V higher than most of NCM materials as cathodes for lithium ion batteries. The discharge capacities of NCM-850-12H are 169.8, 160.5, and 149.3 mAh g(-1) at 2, 5, and 7 C, respectively, illustrating the excellent rate capability. The superior electrochemical performance of NCM-850-12H cathode can be attributed to more {010} active planes exposure.

Nickel doped Zinc oxide as a potential sorbent for decolorization of specific dyes, methylorange and tartrazine by adsorption process

Nickel doped zinc oxide (Ni0.05Zn0.95O) nanoparticles were tested as a possible potential adsorbent for the removal of specific dyes, methylorange and tartrazine, from an aqueous solution. The adsorbent was synthesized by forced hydrolysis conducted in polyol medium and characterized by X-ray diffraction (XRD), nitrogen adsorption/desorption isotherms, UV–vis spectroscopy and TEM images. Experiments showed that Ni doped ZnO nanoparticles were very efficient for the removal of TA and MO dyes and quasi-equilibrium reached in 30 min. The removal efficiency was found to be dependent on the initial dyes concentration and there is no significant effect of temperature on the adsorption process of dyes. Maximum adsorption capacity of dyes was achieved at pH 4 and 6 respectively for TA and MO. The adsorption capacity decreases proportionately to adsorbent dosage. Both the right match and accurate prediction of qe indicate that the pseudo second-order kinetic model better describes the adsorption of MO and TA dyes on Ni doped zinc oxide. The equilibrium data were analyzed by the Langmuir, Freundlich, Temkin, Dubinin–Radushkevich and intra-particle diffusion, mass transfer models, which revealed that Freundlich and Temkin isotherms were more suitable for describing MO and TA dyes adsorption than the other two isotherm models. Thermodynamic study showed that the adsorption was a physisorption, spontaneous and endothermic process.

Structural, Morphological, Vibrational and Electrical Studies on Zn Doped Nanocrystalline LiNiPO<sub>4</sub>

Olivine structured LiNi1-xZnxPO4 (x=0, 0.05, 0.10, 0.15, 0.20) have been prepared by a polyol method using 1, 2 propanediol as a polyol medium. The XRD results of pure and Zn doped LiNiPO4 sample authenticate the orthorhombic crystal structure with high crystalline nature. The crystallite size is calculated from the Debye Scherer formula and it is found in the range of 55-65nm and 49-55nm for undoped and doped samples respectively. The thermal properties of LiNi1-xZnxPO4 were investigated by thermo gravimetric analysis (TG) and differential thermal analysis. Laser Raman studies confirm that the dopant is entered in to the LiNiPO4 lattice. Morphology of the samples is analyzed through SEM analysis. The higher electrical conductivity is calculated for LiNi0.85Zn0.15PO4 sample compared with other concentrations of dopant and it is found to be 1.08×10-7 S cm-1 at ambient temperature. Dielectric and Modulus studies are also discussed through impedance spectroscopy.

Effect of spark plasma sintering process on the microstructure and mechanical properties of Nano crystalline hydroxyapatite ceramics prepared by hydrolysis in polyol medium

The aim of the study was to investigate the role of microstructure and porosity on the mechanical behaviour of sintered hydroxyapatite synthesized by polyol process. This process describes a new approach for the synthesis of hydroxyapatite nanoparticles, which involves precipitation and hydrolysis reactions conducted in polyol medium. In the present work a non-conventional technique, spark plasma sintering, was used to consolidate such nanocrystalline apatites at non-conventional, very low temperatures (T < 300°C) so as to preserve the surface hydrated layer present on the nanocrystals. The hydroxyapatite nanoparticles have been successfully conducted by spark plasma sintering process, resulting in a dense HA compacts. Besides, the sintering behaviour of hydroxyapatite powders at different temperatures ranging was studied. The microstructure, Vickers microhardness, nanoindentation and density are described. Finally, the resulting mechanical properties determined on the microwave sintered samples (E = 136 ±4 GPa, HV = 8.2 GPa, and KIC= 1.6 ±0.03 MPa m1/2) are significantly higher than those usually reported in the literature, whatever the sintering process, and could allow the use of hydroxyapatite for structural applications. Keywords: Hydroxyapatite; Polyol; Spark Plasma Sintering; Mechanical Properties.

Hydrophilic polymer coated monodispersed Fe3O4 nanostructures and their cytotoxicity

Surface functionalized monodispersed Fe3O4 magnetic nanoparticles were synthesized by the polyol method. Surfactants were used to control size, shape and agglomeration of the magnetic nanoparticles during the preparation. The size of these nanoparticles was in the range of 10–30 nm as observed in transmission electron microscopy (TEM). The formation of monodispersed shapes was controlled by varying the surfactants without changing the reaction conditions. The x-ray diffraction (XRD) pattern validates the phase purity and cubic structure even after the addition of surfactants. The functional groups were observed from Fourier transform infrared (FTIR) spectroscopy analysis, confirming the surface modification with polymer molecules in the polyol medium. The saturation magnetization value decreases from 89 to 59 emu g−1 for the surfactant coated Fe3O4 nanoparticles and it also shows superparamagnetic behavior at room temperature. Cell viability rate and percentage of dead cells were accurately identified in human breast carcinoma cell lines using in vitro cell viability experiments, which confirms that pristine and surfactant coated Fe3O4 nanoparticles are non-toxic and can be used for biomedical applications.

Nucleation and Growth Controlled Polyol Synthesis of Size-Focused Nanocrystalline LiFePO4 Cathode for High Performance Li-Ion Batteries

Mono-dispersed and densely packed, LiFePO4 cathodes with high rate performance versus lithium were synthesized using a two-step polyol process. The particle-size distribution and packing density of LiFePO4 nanoparticles were fine-tuned via separation of the nucleation and particle growth processes in the polyol medium by varying reaction time and temperature. Precisely, the LiFePO4 samples were synthesized by heating the reaction mixtures at 200°C for various reaction times (0, 1, 3 and 6 h) to facilitate nucleation and thereafter the solution temperature was raised and maintained at 320°C to facilitate the crystal growth process. XRD studies confirmed the formation of olivine phase in all the samples. The FE-SEM, FE-TEM and particle-size distribution studies illustrate that the particle-size of olivine nanocrystals may be tuned to approximately 200 nm by controlling the aging time at 200°C or in other words separating the nucleation and crystal growth process. The olivine cathodes synthesized by maintaining 3–6 h aging time demonstrate improved discharge capacities and cycle performances in comparison to those of the remaining samples. In particular, the 6 h olivine cathode registered average discharge capacities of 130, 87 and 64 mAh/g at 5 C, 10 C and 15.7 C current rates respectively.

Role of Morphology on the Large Coercive Behavior in Co80Ni20Nanowires

ABSTRACTFerromagnetic metal CoNi-based nano-objects have been synthesized in a polyol media within different elaboration conditions in order to drive their morphology (i.e. enhancing their length-to-diameter ratio ﴾d/L﴿, and changing the diameter d ratio over edge T width ﴾d/L﴿). Transmission Electron Microscopy (TEM) studies revealed unexpected effects on the Co80Ni20 nano-objects arising from the magnetic field assisted synthesis. This gave us the opportunity to compare this latter to coming from the variation of Ruthenium (III) chloride hydrate nucleating agent concentration. A Co80Ni20 anisotropic particles elaboration was successfully achieved under zero magnetic field assisted synthesis, while an important percentage of isotropic nanoparticles appeared immediately under the application of a small magnetic field (i.e. H > 500 Oe). In the first case we were able to sharply drive both the aspect ratio and head morphology of nanowires (T and ﴾d/T﴿). The good crystallinity and structures symmetry of all our samples have been proved by X-Ray Diffraction (XRD) pattern analysis. Magnetic static properties showed a ferromagnetic standard behavior with a coercive field efficiency which was strongly dependent on shape parameters. The magnetic static behavior was studied within a standard Stoner-Wohlfart model as a function of the observed morphologies. Our observations are fully consistent with a shape anisotropy origin behavior of the enhanced coercivity measured as function of the decreasing ﴾d/L﴿ ratio. However, they revealed the presence of contributions to the global effective anisotropy coming from other complex terms then the shape one (i.e. conic head impressiveness, dipolar interactions and magnetocrystalline anisotropy).