Preparation and microwave characterization of BaNd2−xSmxTi4O12 (0 ≤ x ≤ 2) ceramics and their effect on the temperature coefficient of dielectric constant in polytetrafluoroethylene composites

High dielectric and temperature-stable ceramic compositions have been prepared through solid-state ceramic route. The structure and microstructure of the ceramics have been studied using powder X-ray diffraction and scanning electron microscopic techniques. The dielectric properties of well-sintered ceramics are studied in the microwave frequency region using Hakki and Coleman post-resonator technique. The samples exhibited high dielectric constant (>77), relatively high quality factor (>1500) and near zero temperature coefficient of resonant frequency. Phase pure calcined ceramic materials are incorporated in the polytetrafluoroethylene matrix through a proprietary process comprising of sigma mixing, extrusion, calendering followed by hot pressing for the fabrication of planar circuit laminates. The effect of temperature coefficient of dielectric constant of the resultant polytetrafluoroethylene/ceramic composite materials is studied with respect to compositional variation of the filler materials.     

The proton exchange chemistry of layered Ni(OH)2 for two types of high-capacity cathode materials in rechargeable batteries

Based on the studies of the first proton exchange/remove of layered Ni(OH)₂, super nickel oxide has been prepared with strongly alkaline concentrated sodium hypochlorite solution. The primary alkaline super nickel battery equipped with the prepared NiOOH cathode provides an energy capacity 2 times as large as that of the existing alkaline manganese batteries under high drain. In addition, according to the second proton exchange of Ni(OH)₂, the layered NiOOLi has also been synthesized by means of the proton/Li-ion exchange of super nickel oxide in LiOH solution, and then in molten lithium hydroxide. It provides higher discharge voltage and capacity than that of the widely adopted LiCoO₂ and LiMn₂O₄.     

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, structure, and electrochemical characteristics of LiNi1/3Co1/3Mn1/3O2 prepared by thermal polymerization

A thermal polymerization route was adopted to synthesize layered LiNi_1/3Co_1/3Mn_1/3O₂ materials. After annealing the polymer gel containing metal salts at different temperatures from 850 to 1000 °C for different time between 6 and 25 h, powders of pure α-NaFeO₂ phase were obtained. The crystal structure, morphology and electrochemical properties of the products were investigated by XRD, SEM, electrochemical cell cycling and AC impedance spectroscopy. It is found that the powder annealed at 950 °C for 15 h shows the best electrochemical property with the first specific discharge capacity of 188 mAh/g at C/10 and 87% retention after 100 cycles. It exhibits good rate capability with the specific capacity of 169 mAh/g at 1 C and 110 mAh/g at 6 C. Adopting a slowly cooling procedure during the powder annealing can improve the electrochemical performance of the LiNi_1/3Co_1/3Mn_1/3O₂ powder.     

Synthesis and characterizations of A-site deficient perovskite Sr0.9Ti0.8−xGaxNb0.2O3

The effects of reduction and Ga-doping on the physicochemical properties of A-site deficient perovskites Sr_0.9Ti_0.8−xGa_xNb_0.2O₃ (x = 0, 0.05, 0.1, 0.15 and 0.2) are reported. With 10% Ga doping, the sample sintered in air and treated at 1400 °C in H₂ atmosphere exhibits the highest electrical conductivity. It is found that the Ga-doping lowers the sinterability but promotes the reduction of Sr_0.9Ti_0.8−xGa_xNb_0.2O₃. The XRD analysis on the reduced samples suggests that some cations are reduced during the treatment. However, without high temperature pre-reduction, the improvement of Ga-doping is limited and the overall cell performance using Sr_0.9Ti_0.8−xGa_xNb_0.2O₃ as an anode without catalysts is still relatively low.     

Enhanced cycling stability of microsized LiCoO2 cathode by Li4Ti5O12 coating for lithium ion battery

The effect of Li₄Ti₅O₁₂ (LTO) coating amount on the electrochemical cycling behavior of the LiCoO₂ cathode was investigated at the high upper voltage limit of 4.5 V. Li₄Ti₅O₁₂ (≤5 wt.%) is not incorporated into the host structure and leads to formation of uniform coating. The cycling performance of LiCoO₂ cathode is related with the amount of Li₄Ti₅O₁₂ coating. The initial capacity of the LTO-coated LiCoO₂ decreased with increasing Li₄Ti₅O₁₂ coating amount but showed enhanced cycling properties, compared to those of pristine material. The 3 wt.% LTO-coated LiCoO₂ has the best electrochemical performance, showing capacity retention of 97.3% between 2.5 V and 4.3 V and 85.1% between 2.5 V and 4.5 V after 40 cycles. The coulomb efficiency shows that the surface coating of Li₄Ti₅O₁₂ is beneficial to the reversible intercalation/de-intercalation of Li⁺. LTO-coated LiCoO₂ provides good prospects for practical application of lithium secondary batteries free from safety issues.     

High rate performances of the cathode material LiNi1/3Co1/3Mn1/3O2 synthesized using low temperature hydroxide precipitation

A low-temperature reaction route is introduced based on hydroxide precipitation method to synthesize the cathode material LiNi_1/3Co_1/3Mn_1/3O₂. The crystal structure and morphology of the prepared powder have been characterized by X-ray diffraction and Scan electron microscope, respectively. The charge–discharge tests were performed between 2.5 and 4.5 V. The discharge capacity of the material is strongly impacted by the reaction temperature. The powders sintered at 850 °C show the best electrochemical performance and the initial discharge capacity is about 160 mAh g⁻¹ at 5 C. Powder X-ray diffraction and Scan electron microscope results reveal that the excellent electrochemical performances should be ascribed to the lower precursor reaction temperature, the lower degree of cation mixing and analogous spherical small particles, which can improve the transfer of Li ions and electrons. All these results indicate that this material has potential application in lithium-ion batteries.     

Ion exchange and electrochemical evaluation of the microporous phosphate Li9Fe7(PO4)10

A new lithium iron(III) phosphate, Li₉Fe₇(PO₄)₁₀, has been synthesized and is currently under electrochemical evaluation as an anode material for rechargeable lithium-ion battery applications. The sample was prepared via the ion exchange reaction of Cs₅K₄Fe₇(PO₄)₁₀1 in the 1 M LiNO₃ solution under hydrothermal conditions at 200 °C. The fully Li⁺-exchanged sample Li₉Fe₇(PO₄)₁₀2 cannot yet be synthesized by conventional high-temperature, solid-state methods. The parent compound 1 is a member of the Cs_9−xK_xFe₇(PO₄)₁₀ series that was previously isolated from a high-temperature (750 °C) reaction employing the eutectic CsCl/KCl molten salt. The polycrystalline solid 1 was first prepared in a stoichiometric reaction via conventional solid-state method then followed by ion exchange giving rise to 2. Both compounds adopt three-dimensional structures that consist of orthogonally interconnected channels where electropositive ions reside. It has been demonstrated that the Cs_9−xK_xFe₇(PO₄)₁₀ series possesses versatile ion exchange capabilities with all the monovalent alkali metal and silver cations due to its facile pathways for ion transport. 1 and 2 were subject to electrochemical analysis and preliminary results suggest that the latter can be considered as an anode material. Electrochemical results indicate that Li₉Fe₇(PO₄)₁₀ is reduced below 1 V (vs. Li) to most likely form a Fe(0)/Li₃PO₄ composite material, which can subsequently be cycled reversibly at relatively low potential. An initial capacity of 250 mAh/g was measured, which is equivalent to the insertion of thirteen Li atoms per Li_9+xFe₇(PO₄)₁₀ (x = 13) during the charge/discharge process (Fe²⁺ + 2e → Fe⁰). Furthermore, 2 shows a lower reduction potential (0.9 V), by approximately 200 mV, and much better electrochemical reversibility than iron(III) phosphate, FePO₄, highlighting the value of improving the ionic conductivity of the sample.     

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.     

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.     

Thermal expansion properties of Ln2−xCrxMo3O12 (Ln = Er and Y)

Structures and thermal expansion properties of Ln_2−xCr_xMo₃O₁₂ (Ln = Er and Y) have been investigated by X-ray powder diffraction. Rietveld analysis results of Ln_2−xCr_xMo₃O₁₂ indicate that compounds Er_2−xCr_xMo₃O₁₂ (0 ≤ x ≤ 0.3) and Y_2−xCr_xMo₃O₁₂ (0 ≤ x ≤ 0.2) crystallize in orthorhombic structure and exhibit negative thermal expansion, while both monoclinic and orthorhombic compounds Er_2−xCr_xMo₃O₁₂ (1.7 ≤ x ≤ 2.0) and Y_2−xCr_xMo₃O₁₂ (1.8 ≤ x ≤ 2.0) possess positive coefficient of thermal expansion. The coefficients of linear thermal expansion of orthorhombic Ln_2−xCr_xMo₃O₁₂ change from negative to positive with increasing chromium content. Thermogravimetric and differential scanning calorimetry have been used to study the hygroscopicity and the phase transition temperature.     

Large-scale synthesis of Li1.2V3O8 as a cathode material for lithium secondary battery via a soft chemistry route

Layered Li_1.2V₃O₈ has been efficiently prepared via a sol-gel method. XRD and particle size analysis indicate that the final product with monoclinic structure consists of homogeneously distributed particles whose sizes are in a very narrow range. There are two different water molecules in the compound according to TGA and DTA. The structural water works as a pillar in the structure and is lost at higher temperature than the combined water. The as-prepared material was also compared with the one synthesized from the conventional solid-state method in terms of their morphology, electrochemistry capacity and electrodynamic characteristics. As a result, the Li_1.2V₃O₈ obtained at 300 °C for 10 h has excellent electrochemical properties. A high-first discharge capacity of 286.4 mAh/g was observed at a current rate of C/5 between 1.7 and 3.8 V and the structure of Li_1.2V₃O₈ remains stable in the subsequent cycles. EIS calculation suggests a better diffusion path for lithium ions in as-prepared material than in the solid-state compound.     

Synthesis and electrochemical performance of high voltage cycling LiCo0.8M0.2O2 (M = Mg, Ca, Ba) as cathode material

New single phase undoped LiCoO₂ and doped LiCo_0.8M_0.2O₂ (M = Mg, Ca, Ba) layered oxide materials have been synthesized by a low temperature solid-state reaction. The phase purity and cation environment of the synthesized oxides were characterized using XRD and FT-IR. The particle size, nature, morphological properties and the composition of the synthesized oxides were examined by TEM and SEM with EDAX analysis. Finally, the electrochemical behaviour of the prepared layered materials was studied using cyclic voltammetry and charge–discharge cycling. LiCo_0.8Mg_0.2O₂ has better cycling results compared to all other materials. These results have also been supported by cyclic voltammograms.     

Preparation and electrochemical properties of Li-rich spinel-type lithium manganate coated LiMn2O4

Li-rich spinel-type lithium manganate (SC) coated LiMn₂O₄ composites were prepared and characterized by XRD, SEM, FT-IR, ICP, etc. Their charge/discharge behaviors were studied between 3.0 and 4.3 V at 40 mA g⁻¹ under room temperature, and the results showed that SC coated on surface of LiMn₂O₄ could improve cycling stability of composite electrodes. The composite (S1) containing 4.8 wt% of SC exhibited noticeably improved cycling stability, whereas the initial specific capacity was very close to that of LiMn₂O₄.     

Hydrothermal synthesis and study of an inorganic-organic hybrid vanadate of a nickel(II) coordination complex with pyrazine, Ni3(C4H4N2)3(V8O23)

The three-dimensional hybrid compound Ni₃(C₄H₄N₂)₃(V₈O₂₃) has been synthesized by mild hydrothermal methods under autogenous pressure at 170 °C. The structure of the phase is stable until 380 °C. The removal of the pyrazine molecules from the structure induces its collapse. The IR spectrum shows the vibration modes of the pyrazine molecule and those of the [VO₄]³⁻ groups. A UV-visible spectrum shows the characteristic bands of the Ni(II) d⁸-high-spin cation in a slightly distorted octahedral coordination. Magnetic measurements indicate the existence of antiferromagnetic interactions that can be fitted with a chain model to give g = 2.31, J/k = −5.3, and zJ′/k = −5.5.     

Synthesis of spherical nanostructured LiMxMn2−xO4 (M = Ni, Co, and Ti; 0 ≤ x ≤ 0.2) via a single-step ultrasonic spray pyrolysis method and their high rate charge-discharge performances

LiM_xMn_2−xO₄ (M = Ni²⁺, Co³⁺, and Ti⁴⁺; 0 ≤ x ≤ 0.2) spinels were prepared via a single-step ultrasonic spray pyrolysis method. Comparative studies on powder properties and high rate charge-discharge electrochemical performances (from 1 to 15 C) were performed. XRD identified that pure spinel phase was obtained and M was successfully substituted for Mn in spinel lattice. SEM and TEM studies confirmed that powders had a feature of ‘spherical nanostructural’, that is, powders consisted of spherical secondary particles with the size of about 1 μm, which were developed from close-packed primary particles with several tens of nanometers. Substitutions enhanced density of second particles to different extents, depending on M and its content. Charge-discharge tests showed that as-prepared LiMn₂O₄ could deliver excellent rate performance (around 100 mAh/g at 10 C). Ni substitution contributed to improving electrochemical performances. In the voltage range of 4.95-3.5 V, the materials showed much better electrochemical performances than LiMn₂O₄ in terms of capacity, cycleability and rate capability.     

Microwave synthesis and electrochemical characterization of mesoporous carbon@Bi2O3 composites

An efficient and quick microwave method has been employed to prepare worm-like mesoporous carbon@Bi₂O₃ composites for the first time. As-prepared products have been characterized by X-ray diffraction, N₂ adsorption-desorption, scanning electron microscopy, transmission electron microscopy and inductive coupled plasma atomic emission spectroscopy. The electrochemical measurement shows the worm-like mesoporous carbon@Bi₂O₃ composites exhibits excellent capacitance performance and the maximum specific capacitance reaches 386 F g⁻¹, three times more than the pure worm-like mesoporous carbon.     

Topotactic route for new layered perovskite oxides containing fluorine: Ln1.2Sr1.8Mn2O7F2 (Ln = Pr, Nd, Sm, Eu, and Gd)

A series of fluorine intercalated Ruddlesden-Popper layered perovskites, Ln_1.2Sr_1.8Mn₂O₇F₂ (Ln = La, Pr, Nd, Sm, Eu, and Gd) were synthesized from the parent oxides, Ln_1.2Sr_1.8Mn₂O₇, with the organic reagent, poly(vinylidene fluoride), as the fluorine source. Oxidative intercalation readily occurred by decomposition of the organic fluoride immediately adjacent to the parents.     

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.     

Electrical properties of thick-film NTC thermistor composed of Ni0.8Co0.2Mn2O4 ceramic: Effect of inorganic oxide binder

The thick-film NTC thermistors were prepared by screen printing Ni_0.8Co_0.2Mn₂O₄ ceramic on the alumina. The influence of inorganic oxide binder composition and thickness of thermistor layer on the thermistor constant and initial resistivity are studied. The relation between the resistivity (ρ) and the absolute temperature for the prepared thick-film thermistor comply with Arrhenius equation. The room temperature sheet resistivities of the thick films were in the range 0.56-7.45 MΩ cm and temperature sensitivity index in the range 1492-3335 K. Binder composition dependent agglomeration of microcrystallites is observed in the microstructure of the thick-film Ni_0.8Co_0.2Mn₂O₄ ceramic. The spinel ceramic was prepared by oxalate co-precipitation and sintering.