Effect of pH value on particle morphology and electrochemical properties of LiFePO4 by hydrothermal method

Lithium iron phosphate was prepared by hydrothermal synthesis using LiOH·H₂O, FeSO₄·7H₂O and H₃PO₄ as raw materials. The effects of pH value of reaction solution on particle morphology and electrochemical property were investigated. The pH value of the reaction solution was adjusted in the range of 2.5-8.8 by dilute sulfuric acid and ammonia water. The samples were characterized by field-emission scanning electronic microscope (FE-SEM), X-ray powder diffraction (XRD), constant-current charge/discharge cycling tests and chemical analysis. The results indicated that the particles exhibited acute angle diamond flake-like morphology at pH = 2.5, and as the pH value increased, the particle became hexagon flake-like, round flake-like and irregular flake-like morphology gradually. The optimal sample synthesized at pH = 6.4 exhibited discharge capacities of 151.8 mAh g⁻¹ at 0.2 C rate and 129.3 mAh g⁻¹ at 3 C rate. It was found that pH value affected the morphologies and properties of the product by means of different crystal growth rates.     

Intermittent microwave heating synthesized high performance spherical LiFePO4/C for Li-ion batteries

An intermittent microwave heating method was used to synthesize spherical LiFePO₄/C in the presence of glucose as reductive agent and carbon source without the use of the inert gas in the oven processes. The FePO₄ was used as iron precursor to reduce the cost and three lithium salts of Li₂CO₃, LiOH and CH₃COOLi were chosen for comparison of the resulting materials. The materials can be alternatively heated by this method at a temperature controllable mode for crystallization and phase transformation and to provide relaxation time for protecting particles growth. The X-ray diffraction and scanning electron microscope measurements confirmed that the LiFePO₄/C is olivine structured with the average particle size of 50-100 nm. The spherical LiFePO₄/C as cathode material showed better electrochemical performance in terms of the specific capacity and the cycling stability, which might be attributed to the highly crystallized phase, small particle distribution and improved conductivity by carbon connection.     

Electrochemical behaviors of Li[Li(1−x)/3Mn(2−x)/3Nix/3Cox/3]O2 cathode series (0 < x < 1) synthesized by sucrose combustion process for high capacity lithium ion batteries

A mixed cathode material between Li₂MnO₃ and Li[Mn_1/3Ni_1/3Co_1/3]O₂ for high capacity lithium secondary batteries was introduced in this study. It was prepared using the sucrose combustion process because this is a simple process. The oxidation states of Mn, Co and Ni ions in the pristine Li[Li_(1−x)/3Mn_(2−x)/3Ni_x/3Co_x/3]O₂ compounds were confirmed to be tetravalent, trivalent and divalent, respectively, via XANES measurements. Electrochemical charge/discharge studies showed that the highest first discharge capacity of 224 mAh/g was obtained in composition of x = 0.5 at a 0.2 C rate. The oxidation state of the Co and Ni ions in the Li[Li_1/6Mn_1/2Ni_1/6Co_1/6]O₂ changed to higher oxidation states, but that of the Mn ions did not change.     

Two new barium indium phosphates with intersecting tunnel structures: BaIn2P4O14, and Ba3In2P4O16

Two new barium indium phosphates BaIn₂P₄O₁₄ and Ba₃In₂P₄O₁₆ were synthesized by high temperature solution growth method and structurally characterized by single crystal X-ray diffraction analysis. They represent first compounds in the Ba-In-P-O systems. BaIn₂P₄O₁₄ and Ba₃In₂P₄O₁₆ display different types of 3D architectures. The projections of both structures concern about the existence of the intersecting tunnels. In compound BaIn₂P₄O₁₄, the tunnels are built up of the corner-sharing InO₆ octahedra and P₂O₇ groups, whereas in compound Ba₃In₂P₄O₁₆, they are formed by corner sharing of PO₄ tetrahedra and InO₆ octahedra. The electronic band structure calculations of two compounds have been performed with the density functional theory method. The study of calculations and optical diffuse reflectance experimental results show both compounds are insulators with direct band-gap.     

Synthesis of ribbon type carbon nanostructure using LiFePO4 catalyst and their electrochemical performance

Ribbon type of carbon nanostructure has been synthesized by chemical vapor deposition using a new catalyst (LiFePO₄) introduced for the first time and its electrochemical behavior has been determined from charge/discharge characteristics. The synthesized material characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and by Raman spectroscopy confirms the graphitic structure and ribbon type morphology of material. The performance of the single cell using purified carbon nanoribbon as the anode has been studied and the reversible lithium intercalation capacity has been found about 345 mAh/g, of which 335 mAh/g remain after 14th cycle. The columbic efficiency has been stabilized at approximately 98% from the 5th cycle.     

Synthesis and photoluminescence properties of SrLu2O4:Eu superfine phosphor

Superfine powder SrLu₂O₄:Eu³⁺ was synthesized with a precursor prepared by an EDTA - sol-gel method at relatively low temperature using metal nitrate and EDTA as starting materials. The heat decomposition mechanism of the precursor, formation process of SrLu₂O₄:Eu³⁺and the properties of the particles were investigated by thermo-gravimetric (TG) - differential thermal analysis (DTA), X-ray diffraction (XRD), transmission electron microscopy (TEM) and photoluminescence (PL) analyses. The results show that pure SrLu₂O₄:Eu³⁺ superfine powder has been produced after the precursor was calcinated at 900 °C for 2 h and has an elliptical shape and an average diameter of 80-100 nm. Upon excitation with 250 nm light, all the SrLu₂O₄:Eu³⁺ powders show red and orange emissions due to the 4f-4f transitions of Eu³⁺ ions. The highest photoluminescence intensity at 610 nm was found at a content of about 6 mol% Eu³⁺. Splitting of the ⁵D₀-⁷F₁ emission transition revealed that the Eu³⁺ ions occupied two nonequivalent sites in the crystallite by substituting Lu³⁺ ions.     

Synthesis of non-stoichiometric Bi2O4−x by oxidative precipitation

Bi₂O_4−x, a Bi mixed-valence phase was prepared at 95 °C, by a precipitation process, in a basic medium with a highly oxidizing K₂S₂O₈/Na₂S₂O₈. This phase has a low thermal stability as it decomposes below 400 °C in a multiple step process by some O₂ losses prior to finally transforming into γ-Bi₂O₃. The as-prepared powders are 50-60 nm in size with a narrow size distribution. Optical spectra of Bi₂O_4−x exhibit a broad absorption band with a band gap of ∼1.4 eV as compared to 2.61 eV for Bi₂O₃. The composition of this non-stoichiometric phase, which crystallizes in cubic fluorite related structure with a cell parameter of 5.538(3) Å, is Bi₂O_{3.65 ± 0.10}.     

Synthesis and electrochemical performances of spinel LiCr0.1Ni0.4Mn1.5O4 compound

The spinel compound LiCr_0.1Ni_0.4Mn_1.5O₄ was synthesized by a solid reaction method and a sol-gel method using citric acid as chelating agent. The pure phase LiCr_0.1Ni_0.4Mn_1.5O₄ was obtained by the wet method. The electrochemical performances of the pure phase sample were measured at different current rates. There were three voltage plateaus at about 4.9, 4.7 and 4.0 V in the charge-discharge curves, which were attributed to the oxidation/reduction of chromium, nickel and manganese respectively. In the range of 3.5-5.0 V, its first discharge capacity was 143, 118 and 111 mAh/g corresponding to current densities of 1.0, 4.0 and 5.0 mA/cm², respectively. After 50 cycles, the capacity retention remained well at the current densities of 1.0, 4.0 and 5.0 mA/cm². The electrochemical performances of pure phase LiCr_0.1Ni_0.4Mn_1.5O₄ at 55 °C was also measured, and the results were discussed.     

Synthesis of BaTi4O9 ceramics by reaction-sintering process

Synthesis of BaTi₄O₉ ceramics by a reaction-sintering process was investigated. The mixture of raw materials for stoichiometric BaTi₄O₉ were pressed and sintered into ceramics without any calcination stage involved. Pure BaTi₄O₉ phases were obtained at 1150-1280 °C. High-sintered density, 98.2-99.5% of theoretical value (4.533 g/cm³), can be obtained for pellets sintered at 1200-1280 °C for 2-6 h. Some rod-shaped grains 3-7 μm in the longitudinal axis appear in pellets sintered at 1230 °C. Both the size and the amount of these rod-shaped grains increase at higher sintering temperature.     

Preparation and chemical transformation of surfactant-templated hybrid phase containing [MoS4] anions

Exchange reaction between (NH₄)₂MoS₄ and CTABr in aqueous solution produces lamellar compound (CTA)₂MoS₄, which after a treatment with aqueous N₂H₄ under the mild conditions transforms to a lamellar mesophase containing [Mo₂O₂S₂(S₂)₂]²⁻ anions.     

Substitution effect of pentavalent bismuth ions on the electronic structure and physicochemical properties of perovskite-structured Ba(In0.5Ta0.5−xBix)O3 semiconductors

We have investigated the substitution effect of pentavalent bismuth ions on the electronic structure and physicochemical properties of barium indium tantalate. X-ray diffraction, X-ray absorption spectroscopic, and energy dispersive spectroscopic microprobe analyses reveal that, under oxygen atmosphere of 1 atm, pentavalent Bi ions are successfully stabilized in the octahedral site of the perovskite tantalate lattice. According to diffuse reflectance UV-vis spectroscopic analysis, the Bi substitution gives rise to the significant narrowing of band gap of barium indium tantalate even at a low Bi content of ∼5%, underscoring a high efficiency of Bi substitution in the band gap engineering. Such an effective narrowing of the band gap upon the Bi substitution would be attributable to the lowering of conduction band position due to the high electronegativity of Bi^V substituent. As a result of band gap engineering, the Ba(In_0.5Ta_0.5−xBi_x)O₃ compounds with x ≥ 0.03 can generate photocurrents under visible light irradiation (λ > 420 nm). Based on the present experimental findings, it becomes clear that the substitution of highly electronegative p-block element like Bi^V ion can provide a very powerful tool for tailoring the electronic structure and physicochemical properties of wide band gap semiconductors.     

A PEG assisted sol-gel synthesis of LiFePO4 as cathodic material for lithium ion cells

In order to obtain fine-particle LiFePO₄ with excellent electrochemical performance, LiFePO₄/C powders were synthesized by a poly(ethylene glycol) (PEG) assisted sol-gel method. All samples were characterized by X-ray powder diffraction and scanning electron microscopy, and their electrochemical properties were investigated by cycle voltammograms and charge-discharge tests. The sample, synthesized with the n_PEG/n_LFP = 1:1 under sintering temperature of 600 °C, possesses the global morphology and particle size of about 100 nm. This sample delivers the first discharge capacity of 162 mAh g⁻¹, i.e. 95.3% of the theoretical capacity, at the 15 mA g⁻¹ discharge current between 2.5 and 4.0 V (versus Li/Li⁺). The sample also displays a robust rate capability and stable cycle-life. The improved electrochemical performance originates mainly from the fine particle of nanometric dimension, regular global morphology and uniform dispersing in the product as well as the increased electronic conductivity by carbon coating.     

One-dimensional lone electron pair micelles in the crystal structure of Pb5(SiO4)(VO4)2

The structure of Pb₅(SiO₄)(VO₄)₂ (hexagonal, P6₃/m, a = 9.9865(11), c = 7.3599(12) Å, V = 635.67(14) Å³, Z = 2) has been solved by direct methods and refined to R₁ = 0.051 on the basis of 440 unique observed reflections with |F_o|≥4σ_F. The compound belongs to the apatite structure type. The Pb coordination polyhedra are distorted due to the presence of stereoactive lone electron pairs Ψ. The structure contains channels running along the c axis and centered at (00z). The channels are most probably occupied by the lone electron pairs of the Pb²⁺ cations and thus represent lone electron pair micelles. The existence of such micelles in the structure may well be the reason for the electrogyratory effect and protonic conductivity observed in crystals of the title compound.     

The synthesis and selected properties of new compounds: Mg3Fe4(VO4)6 and Zn3Fe4(VO4)6

New compounds: Mg₃Fe₄(VO₄)₆ and Zn₃Fe₄(VO₄)₆ were obtained from a solid state reaction. The temperatures of melting of Mg₃Fe₄(VO₄)₆ and Zn₃Fe₄(VO₄)₆ amount to 950±5 and 850±5°C, respectively. The indexing results and the calculated unit cell parameters for both compounds are given and suggest that both phases are isotypic with Mn₃Fe₄(VO₄)₆. The IR spectra of the above-mentioned compounds are presented.     

Characterization of nanosized Al2(WO4)3

Nanosized aluminum tungstate Al₂(WO₄)₃ was prepared by co-precipitation reaction between Na₂WO₄ and Al(NO₃)₃ aqueous solutions. The powder size and shape, as well as size distribution are estimated after different conditions of powder preparation. The purity of the final product was investigated by XRD and DTA analyses, using the single crystal powder as reference. Between the specimen and the reference no difference was detected. The crystal structure of Al₂(WO₄)₃ nanosized powder was confirmed by TEM (SAED, HRTEM). In additional, TEM locality allows to detect some W₅O₁₄ impurities, which are not visible by conventional X-ray powder diffraction and thermal analyses.     

Structural study and luminescence of TlSrLa(AsO4)2

This work presents the crystal structure and luminescent properties of TlSrLa(AsO₄)₂. In this phase Tl⁺ ions are located in large tunnels delimited by chains of alternating (AsO₄) and (Sr,La)O₈ polyhedra. Thallium atoms are eightfold coordinated with C₁ symmetry. Large TlO distances are observed revealing a low stereochemical activity of the 6s² lone pair. Excitation and emission spectra of Tl⁺ in TlSrLa(AsO₄)₂ showed broad bands at lower energy than those observed in previous works. Excitation spectra are decomposed into multiple Gaussian bands and a theoretical analysis is made to explain the number of observed components. Two Gaussian components are revealed for emission spectra.     

Synthesis and characterization of a layered chlorozincophosphate Zn(HPO4)Cl·[C4H10NO]

A chlorozincophosphate of the composition Zn(HPO₄)Cl·[C₄H₁₀NO] has been synthesised under mild condition water medium in the presence of morpholine as organic template. Its unit cell is monoclinic P2₁/a with parameters a = 8.655(6) Å, b = 9.302(5) Å, c = 12.180(5) Å, β = 101.10(4)°, Z = 4 and V = 962.1(9) Å³. The structure was determinated by single crystal X-ray diffraction. The structure involves a network of ZnO₃Cl and PO₃(OH) tetrahedra forming macroanionic inorganic layers with four- and eight-membered rings. Charge balance is achieved by the protonated amine which is trapped in the interlayers space and interacts with the organic framework through hydrogen bonding. Solid state ³¹P and ¹³C MAS-NMR spectroscopies are in agreement with the X-ray structure.     

Preparation and chemical properties of the skutterudites (Ce-Yb)yFe4−x(Co/Ni)xSb12

The double-filled skutterudites Ce_y/2Yb_y/2Fe_4−xCo_xSb₁₂ and Ce_y/2Yb_y/2Fe_4−xNi_xSb₁₂ have been synthesized as single phase materials. The evolution of the lattice parameters and of the filling fractions with substitutions on the transition metal site have been studied by X-ray diffraction and electron probe microanalysis, respectively. Both results were compared to that of single filled series (Ce/Yb)_yFe_4−x(Co/Ni)_xSb₁₂. It shows that double-filled skutterudites can be understood as a solid solution between single filled ones. The valence states of Ce and Yb in all series have been studied using X-ray absorption spectroscopy at the L₃ edges, and we discuss their relationships with structural parameters. Ce is always trivalent, whereas Yb valence state decreases as Yb fraction increases. No distortion of the environment or sub-position of Yb in the cage have been seen by neutron diffraction. The Seebeck coefficient has been measured using a home made apparatus from 120 K to 300 K. It shows a crossover from p- to n-type conductivity for Co or Ni rich skutterudites in the six series. We discuss the influence on the room temperature thermopower of valence states, filling fractions and substitution rate on the transition metal site. Thermopower seems to be only driven by charge carriers concentration.     

Formation of NiFe2O4 nanoparticles by mechanochemical reaction

Preparation of nanosized NiFe₂O₄ particles by mechanochemical reaction(NiO+α-Fe₂O₃) and subsequent thermal treatment was investigated using X-ray diffraction (XRD). Thermal treatment of the as-milled powder at 700 °C for 1 h led to the formation of NiFe₂O₄ nanoparticles with an average crystal size of about 23 nm. Effect of thermal treatment temperature on the crystal size of the nanoparticles was studied. The mechanism of nanoparticles growth was primarily discussed. The activation energy of NiFe₂O₄ nanoparticle formation during calcination was calculated to be 16.6 kJ/mol.     

Influence of isovalent ion substitution on the electrochemical performance of LiCoPO4

LiCo_1−xM_xPO₄ (M = Mg²⁺, Mn²⁺ and Ni²⁺; 0 ≤ x ≤ 0.2) compounds have been synthesized by solid-state reaction method and studied as cathode materials for secondary lithium batteries. LiCoPO₄ exhibits a discharge plateau at ∼4.7 V with an initial discharge capacity of 125 mAh/g and on cycling capacity falls. Substitution of Co²⁺ with Mg²⁺/Mn²⁺/Ni²⁺ in LiCoPO₄ has an influence on the initial discharge capacity and on cycling behaviour. The capacity retention of LiCoPO₄ is improved by manganese substitution. Among the manganese substituted phases, LiCo_0.95Mn_0.05PO₄ shows good reversible capacity of ∼50 mAh/g.