Synthesis of LiV3O8 by an improved citric acid assisted sol–gel method at low temperature

LiV₃O₈ was synthesized by the normal citric acid assisted sol–gel method and an improved citric acid assisted sol–gel method. The improved method compares with the normal method in detail by thermogravimetry (TG), FTIR, X-ray diffraction (XRD), scanning electron microscopy (SEM), charge–discharge test, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Results show that the improved method can synthesize LiV₃O₈ successfully at much lower temperature than normal method.     

Synthesis and electrochemical properties of LiV3O8 phase

Lithium vanadium oxide was synthesized by a new method in which LiOH, V₂O₅ and NH₄OH were used as the starting materials to synthesize a precursor containing Li and V, and then obtain the resulting product by calcining the precursor. The LiV₃O₈ compound prepared by this synthesis method gave a good charge-discharge and cycle performance. A specific capacity of 258 mAh/g is obtained in the range of 1.8−4.0 V in the first cycle and 247 mAh/g in the eighth cycle.     

Preparation and electrochemical properties of Cr doped LiV3O8 cathode for lithium ion batteries

Chromium was incorporated into lithium trivanadate by an aqueous reaction followed by heating at 100 °C. This Cr doped LiV₃O₈ as a cathode for lithium ion batteries exhibits 269.9 mAh g^− 1 at first discharge cycle and remains 254.8 mAh g^− 1 at cycle 100, with a charge-discharge current density of 150 mA g^− 1 in the voltage range of 1.8-4.0 V. The Cr-LiV₃O₈ cathode show excellent discharge capacity, with the retention of 94.4% after 100 cycles. These result values are higher than previous reports indicating that Cr-LiV₃O₈ prepared by our low temperature synthesis method is a promising cathode material for rechargeable lithium ion batteries. The enhanced discharge capacity and cycle stability of Cr-LiV₃O₈ cathode indicate that chromium atoms promote lithium transfer or intercalation/deintercalation during the electrochemical cycles and improve the electrochemical performances of LiV₃O₈ cathode.     

Single crystalline Co3O4: Synthesis and optical properties

Crystals of Co₃O₄ have been prepared from thermal decomposition of molecular precursors derived from salicylic acid and cobalt (II) acetate or chloride at 500 °C. A cubic phase Co₃O₄ micro- and nanocrystals have been obtained. The as-synthesized products were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM). The images of electron microscopes showed octahedral crystals of Co₃O₄. The volume and polarizability of the optimized structures of molecular precursors have been calculated and related to the particle size. The optical band gap of the obtained crystals has been measured. The results indicated two optical band gaps with values 2.65 and 2.95 eV for (E_g1) (E_g2), respectively.     

Low-temperature synthesis of LiV3O8 nanosheets as an anode material with high power density for aqueous lithium-ion batteries

Nanosheets of lithium vanadium oxide (LiV₃O₈) were successfully synthesized by a simple low temperature citrate sol–gel combustion route. Compact nanosheets of the active material were observed by scanning and transmission electron microscopies. X-ray diffraction measurements indicated that as-prepared nanosheets presented pure phase of monoclinic LiV₃O₈ with p2₁/m symmetry. Cyclic voltammetry (CV) was employed to investigate the electrochemical behavior of the nanosheets with special emphasis on the application potential as anodic material for aqueous rechargeable lithium batteries. CV studies demonstrated that the LiV₃O₈ nanosheets represent well-defined reversible peaks. The nanosheets showed a discharge capacity of 63 mAh/g in 1.0 M LiNO₃ solution at a 2C/5 rate.     

Fe2O3 core-NaCo[Fe(CN)6] shell nanoparticles—Synthesis and characterization

A facile precipitation route was developed for the synthesis of cobalt hexacyanoferrate (CoHCF) as a thin shell around cores of nanoparticles of iron(III) oxide, forming nanoparticles of iron(III) oxide@CoHCF (n-Fe₂O₃@NaCo[Fe(CN)₆]). The morphology and structure of the as-prepared n-Fe₂O₃@NaCo[Fe(CN)₆] were characterized by the techniques of electron microscopies, X-ray diffraction measurements, X-ray photoelectron spectroscopy, infrared spectroscopy and thermogravimetry. Carbon composite electrodes of n-Fe₂O₃@NaCo[Fe(CN)₆] were prepared and the electrochemical behavior of the nanoparticles was evaluated using cyclic voltammetry. The redox couples of n-Fe₂O₃@NaCo[Fe(CN)₆] were investigated and the diffusion coefficients of counter cation in the shell of CoHCF were obtained. The effect of size of particles and the structure of CoHCF was also evaluated. n-Fe₂O₃@NaCo[Fe(CN)₆] represented prominent electrocatalytic activity toward the oxidation of some biologically active compounds.     

Hydrothermal synthesis of Co3O4 with different morphologies and the improvement of lithium storage properties

A simple hydrothermal treatment was developed to synthesize Co₃O₄ powders with different morphologies in mass production by using hexamethylenetetramine (HMT, C₆H₁₂N₄) as a precipitator. By changing the initial HMT concentrations, the prepared Co₃O₄ powders were readily regulated in its morphologies, which varied from microsphere to urchin-like hollow microsphere, and finally to collapsed porous structure. Moreover, the four Co₃O₄ powders with different HMT concentrations had been applied in the negative electrode materials for lithium ion batteries, which exhibited different electrochemical properties. The present research demonstrated that morphology was one of the crucial factors that affected the electrochemical properties of electrodes. The capacity retention of sample with an original Co(NO₃)₂:HMT mole ratio of 1:1 is almost above 94% from the 5th cycle at different current densities of 40 and 60 mA g⁻¹, exhibiting the better long-life stability and favorable electrochemical behaviors due to its higher specific surface area (97.1 m² g⁻¹) and the uniform urchin-like hollow structure.     

Crystal structure and lithium electrochemical extraction properties of olivine type LiFePO4

Lithium iron phosphate (LiFePO₄) cathode material has been synthesized by a solid-state reaction. The XRD patterns and SEM images of the samples show that the LiFePO₄ compounds prepared at 650 °C by using carbon gel in reaction have a single-phase, small grain-size and regular shapes. By using Rietveld refinement method, we calculated the Li–O interatomic distance in LiO₆ octahedra and the cross section area of the lithium ion one-dimension tunnel, and analyze the reason of the improvement of the Lithium ion diffusion. The electrochemical test results of the sample show the LiFePO₄ prepared by using carbon gel exhibits excellent electrochemical properties. Such a significant improvement in electrochemical performance should be partly related to the enhanced Lithium ion diffusion and electric conductivity due to the use of carbon gel.     

Nonstoichiometric LiFePO4/C nanofibers by electrospinning as cathode materials for lithium-ion battery

Nonstoichiometric lithium iron phosphate/carbon (LiFePO₄/C) composite nanofibers are prepared by electrospinning and subsequent calcination. The ratio of raw materials exerts great effects on the morphology and electrochemical performance of LiFePO₄/C nanofibers, and the sample prepared using the LiH₂PO₄/FeC₆H₅O₇ ratio of 1.3 has good fibrous morphology, porous structure and high purity, thus exhibiting high capacity and stability for lithium-ion battery.     

Electrochemical performance of the chalcopyrite CuFeS2 as cathode for lithium ion battery

Chalcopyrite CuFeS₂ was synthesized by solvothermal process. It was used as active species to prepare cathode of lithium ion batteries together with some conducting materials. Electrochemical performance of the assembled Li/CuFeS₂ batteries was characterized by cyclic voltammetry and discharging test. Our results proved that CuFeS₂ as a new cathode material showed room-temperature specific discharging capacity of 1100 mAh g⁻¹ at a current density of 14 mA g⁻¹, and that its specific discharging capacity was higher than 500 mAh g⁻¹ at a current density of 350 mA g⁻¹. Different from what reported by Eda et al., the discharging curves presented two apparent plateaus, which were related to different cathode reactions, in the whole measured temperature range.     

Electrochemical lithium insertion behavior of FeNbO4: Structural relations and in situ conversion into FeNb2O6 during carbon coating

Electrochemical properties of FeNbO₄ as a lithium insertion anode material were studied with a view to understand structure–property relationships. Orthorhombic and monoclinic polymorphs of FeNbO₄ were synthesized and characterized by powder X-ray diffraction and laser Raman spectroscopy. Possible redox reactions, as deciphered from cyclic voltammograms, suggest the structural similarity between orthorhombic and monoclinic polymorphs upon lithium insertion. A coating of carbon led to a remarkable improvement in the electrochemical performance of monoclinic FeNbO₄. The coated material exhibited an average reversible capacity of 125.5 mAh g⁻¹. The material also sustained hundreds of charge/discharge cycles and exhibited good rate capability. Upon coating with carbon, the monoclinic FeNbO₄ transformed into FeNb₂O₆. The conversion and stability were confirmed by powder XRD and laser Raman studies of carbon-coated material before and after 450 cycles. The in situ conversion of FeNbO₄ into FeNb₂O₆ during carbon coating was further supported by EPR studies in which the absence of signal for the carbon-coated material indicated conversion of Fe³⁺ to Fe²⁺. Our study reveals the possibility of exploring potential materials in the Fe–Nb–O system and enhancing their performance as anode materials for lithium-ion batteries.     

Preparation of high tap-density LiFePO4/C composite cathode materials by carbothermal reduction method using two kinds of Fe precursors

LiFePO₄ belongs to a new generation cathode material for lithium ion batteries. The improvement of the material's tap density is considered as an important research direction. LiFePO₄/C composite was synthesized by a solid-state carbothermal reduction (CTR) method. The iron resource was obtained by the addition of (i) Fe₂O₃ and citrate ferric or (ii) single Fe₂O₃ as the Fe³⁺ precursors during synthesis. The LiFePO₄/C composite synthesized with two kinds of Fe³⁺ precursors exhibited trimodal distribution and consisted of nanometer-sized and micrometer-sized particles, whereas the LiFePO₄/C composite prepared with single Fe³⁺ precursor demonstrated unimodal distribution and was composed mainly of micrometer-sized particles. Because of the nanometer-sized particles filling in the space between the micrometer-sized particles, the composite synthesized with two kinds of Fe³⁺ precursors exhibited less vacancy than that prepared with single Fe³⁺ precursor and led to high tap density. The composite synthesized with two kinds of Fe³⁺ precursors had smaller grain size and resulted in superior discharge capacities at the rates of 0.1–1.0 C to that prepared with single Fe³⁺ precursor. The two kinds Fe³⁺ precursors method provides a simple and effective route to rapidly prepare high tap-density LiFePO₄ product with excellent electrochemical performance, so it will achieve a wide-range of applications to the production of LiFePO₄.     

Synthesis,crystal structure and electrochemical properties of the manganese-doped LiNaFe[PO4]F materials

The new compounds LiNaFe_1−xMn_x[PO₄]F (x ≤ 1/4) were synthesized by a solid state reaction route. The crystal structure of LiNaFe_3/4Mn_1/4[PO₄]F was determined from single crystal X-ray diffraction data. LiNaFe_3/4Mn_1/4[PO₄]F crystallizes with the Li₂Ni[PO₄]F-type structure, space group Pnma, a = 10.9719(13), b = 6.3528(7), c = 11.4532(13) Å, V = 798.31(16) Å³, and Z = 8. The structure consists of edge-sharing (Fe_3/4Mn_1/4)O₄F₂ octahedra forming (Fe_3/4Mn_1/4)FO₃ chains running along the b-axis. These chains are interlinked by PO₄ tetrahedra forming a three-dimensional framework with the tunnels and the cavities filled by the well-ordered sodium and lithium atoms, respectively. The manganese-doped phases show poor electrochemical behavior comparing to the iron pure phase LiNaFe[PO₄]F.     

Structure and electrochemical performance of Fe3O4/graphene nanocomposite as anode material for lithium-ion batteries

Using hydrothermal method, Fe₃O₄/graphene nanocomposite is prepared by synthesizing Fe₃O₄ particles in graphene. The synthesized Fe₃O₄ is nano-sized sphere particles (100–200 nm) and uniformly distributed on the planes of graphene. Fe₃O₄/graphene nanocomposite as anode material for lithium ion batteries shows high reversible specific capacity of 771 mAh g⁻¹ at 50th cycle and good rate capability. The excellent electrochemical performance of the nanocomposite can be attributed to the high surface area and good electronic conductivity of graphene. Due to the high surface area, graphene can prevent Fe₃O₄ nanoparticles from aggregating and provide enough space to buffer the volume change during the Li insertion/extraction processes in Fe₃O₄ nanoparticles.     

Hydrothermal synthesis, characterization and magnetic properties of NaVGe2O6 and LiVGe2O6

Supercritical fluids are shown to be an excellent reaction media for the synthesis of novel solid state phases at intermediate temperatures. LiVGe₂O₆ and NaVGe₂O₆ have the common pyroxene structure composed of VO₆ linear chains. NaVGe₂O₆ crystallizes in the monoclinic space group C2/c with four formula units having cell dimensions a = 9.960(4) Å, b = 8.853(10) Å, c = 5.4861(10) Å, β = 106.403(3)°. The structure was refined until R = 0.0290 and R_w = 0.0370. For LiVGe₂O₆ in space group P2₁/c: a = 9.8508(7) Å, b = 8.754(3) Å, c = 5.3948(13) Å, β = 108(3)°, R = 0.0240 and R_w = 0.0250. The compounds contain edge-shared VO₆ octahedral chains and corner-shared GeO₄ tetrahedral chains. The presence of these VO₆ chains results in spin-Peierls distortion. Structural and physical characterization of the compounds are reported.     

Mechanical and thermal properties of LaMgAl11O19

Lanthanum magnesium hexaaluminate (LaMgAl₁₁O₁₉) powders were synthesized successfully at 1300 °C for 4 h by solid-state reaction, and LaMgAl₁₁O₁₉ ceramic was prepared at 1700 °C for 6 h by pressureless sintering. Phase composition, microstructure, mechanical and thermophysical properties of LaMgAl₁₁O₁₉ ceramic were investigated. Results show that the flexural strength and fracture toughness of LaMgAl₁₁O₁₉ ceramic are 353.3 ± 12.5 MPa and 4.60 ± 0.46 MPa m^1/2. Young's Modulus and Poisson ratio is 295 GPa and 0.23, respectively. The linear thermal expansion coefficient of LaMgAl₁₁O₁₉ ceramic from 473 K to 1473 K is 9.17 × 10⁻⁶/K, and thermal conductivity at 1273 K is 2.55 W/m K.     

Luminescent properties of long lasting phosphor Ca2MgSi2O7:Eu

Long afterglow phosphors (Ca_1−xEu_x)₂MgSi₂O₇ (0.002 ≤ x ≤ 0.02) were prepared by solid-state reactions under a weak reductive atmosphere. X-ray diffraction pattern, photoluminescence spectra, decay curve, afterglow spectra and thermoluminescence curves were investigated. The phosphors showed two emission peaks when they were excited by 343 nm, due to two types of Eu²⁺ centers existing in the Ca₂MgSi₂O₇ lattice. However, only one emission peak can be found in their afterglow spectra. Energy transfer between Eu²⁺ ions in inequivalent sites was found. A possible mechanism was presented and discussed. The afterglow decay time of Ca_1.998MgSi₂O₇:Eu_0.002 was nearly 12.5 h which means it was a good long lasting phosphor.     

Multiferroic properties of Pb2Fe2O5 ceramics

Pb₂Fe₂O₅ (PFO) powders in monoclinic structure have been synthesized using lead acetate in glycerin and ferric acetylacetonate as the precursor. The powders were pressed into pellets, which were sintered into ceramics at 800 °C for 1 h. The morphology and structure have been determined by X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM). Polarization was observed in Pb₂Fe₂O₅ ceramics at room temperature, exhibiting a clear ferroelectric hysteresis loop. The remanent polarization of Pb₂Fe₂O₅ ceramic is estimated to be Pr ∼ 0.22 μC/cm². The origin of the polarization may be attributed to the off-centers of shifted Pb²⁺ ions as well as the FeO₆ octahedra in the perovskite-based structure of Pb₂Fe₂O₅. Magnetic hysteresis loop was also observed at room temperature. The Pb₂Fe₂O₅ ceramic shows coexistence of ferroelectricity and ferromagnetism. It provides a new field of research for complex oxides with multiferroic properties.     

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.