Microwave-assisted Solid-state Reaction Research Articles

Insights into Microwave-Enhanced Cycling Performance of Nickel-Doped LiMn2O4 spinel Cathode Material at Elevated Temperature

Lithium manganese oxide (LMO) is a state-of-the-art cathode material for lithium-ion batteries. LMO works well at ambient temperature; however, it undergoes rapid capacity-fading during cycling at elevated temperatures (~55 oC) due to the manganese-dissolution. The last two decades have witnessed a number of strategies reported in the literature (including patents) to mitigate this problem and allow the LMO for large-scale applications. Unfortunately, most of the proposed strategies are either time-consuming or involve extensive use of materials that add to the cost of the final product, and/or require the use of less environmentally-friendly chemicals including very harsh acids that must be handled with utmost caution. There is need therefore to explore the use of simpler, fast, less expensive and ‘greener’ synthesis strategy. In our work, microwave-assisted solid-state reaction has been used to dope LMO with very low amount of nickel (i.e., LiNi0.2Mn1.8O4, herein abbreviated as LMNO) for lithium-ion battery from Mn3O4 prepared from electrolytic manganese oxide (EMD, g-MnO2). To establish the impact of microwave irradiation on the electrochemical cycling performance at elevated temperature (60 oC), the Mn3+ concentration in the pristine and microwave-treated LMNO samples was independently confirmed by XRD, XPS, 6LiMAS-NMR and electrochemical studies including electrochemical impedance spectroscopy (EIS). In this presentation, we will show that microwave-treated sample (LMNOmic) allowed for the clear exposure of the {111} facets of the spinel, optimized the Mn3+ content, promoting structural and cycle stability at elevated temperature. Importantly, at elevated temperature (60 oC), the LMNOmic gave an improved cycling stability (> 80% capacity retention and ca. 90 % coulombic efficiency after 100 consecutive cycling) compared to the LMNO. For the first time, the impact of microwave irradiation on tuning the average manganese redox state of the spinel material to enhance the cycling performance of the LiNi0.2Mn1.8O4 at elevated temperature and lithium-ion diffusion kinetics have been clearly demonstrated.

Thermoelectric transport mechanism of Mg2Si0.4Sn0.6-yBiy prepared by low-temperature microwave reaction

According to Debye relaxation, the polarization of electric dipole can be accomplished in 10-10 s under the action of an alternating electromagnetic filed with a frequency of 2.45 GHz, so it is feasible to obtain nano powder by carrying out solid reaction in microwave at low temperature in a short time. In this work, the syntheses of Mg2Si0.4Sn0.6-yBiy (0 ≤ y ≤ 0.03) solid solution thermoelectric materials are successfully achieved by microwave-assisted solid state reaction at low temperature with MgH2 serving as one reactant instead of Mg, and their transportation mechanisms are studied based on the SPB (single parabolic band) model as well. The results indicate that the volatilization and oxidation of Mg can be suppressed effectively in this process. Fine stoichiometric product can be achieved with nano-lamellar structure with an interlayer spacing of about 100 nm by carrying out the reaction between MgH2 and Si, Sn in microwave at 400℃ in 15 min. The introduction of Bi dopant can increase carrier concentration and lattice distortion. With the cooperation between the nano lamellar structure and lattice distortion, the phone is scattered so effectively that the sample owns a lowest thermal conductivity, κmin of 1.36 W·m-1·K-1 at 550 K based on the fact that the phonon scattering is dominant in the heat transfer process. The calculated results show that the theoretical κmin is 0.93 W·m-1·K-1, which is lower than 1.36 W·m-1·K-1. Therefore, by further adjusting the process parameters and increasing the effective doping rate of Bi and the density of the lattice defects, it is expected to obtain lower thermal conductivity. The band convergence is also verified by increasing the density-of-state effective mass. The apparent increase in m* is due to a gradual increase in carrier concentration with increasing temperature. Despite the agreement between the data and the model, the irregular behavior between m* and temperature is a very strong indication and the electric transmission performance of the sample is likely to be influenced by the structure of the multi band structure. Owing possibly to the low reaction temperature, there are Bi precipitates at the grain boundary. In addition to the phonon scattering and the alloy scattering, the Bi segregation and the scattering of carrier by nano-lamellar structure make the carrier mobility of the sample slightly lower. The lower effective doping rate and complex band structure make the carrier concentration and density-of-state effective mass low coupled with the low carrier mobility, which leads to low material factor β with a ZT of 0.66 at 600 K consequently.

Practical microwave-assisted solid-state synthesis of the spinel LiMn2O4

The spinel LiMn2O4 was obtained by a microwave-assisted solid-state reaction between LiOH·H2O and ε-MnO2 in a practical setup using a domestic microwave oven. XRD analyses showed that the spinel LiMn2O4 was obtained as a single phase after only 3min of microwave heating. SEM micrographs revealed particles having a prismatic morphology and sizes in the 0.5–1.5μm range. The characteristic voltammetric profile of the spinel LiMn2O4 was obtained for the synthesized material, while its charge−discharge tests presented good electrochemical stability and a significant initial specific capacity (95mAhg−1) at a high discharge rate (C/1). These results indicate that a rapid and facile microwave-assisted synthesis can yield the spinel LiMn2O4 with adequate properties to be used as a cathode material in lithium-ion batteries.

Low temperature microwave solid-state synthesis of red-emitting CaMoO4:Eu3+, Li+phosphors with controlled morphology

CaMoO4:Eu3+, Li+ red phosphors have been successfully achieved by a microwave-assisted solid-state reaction, and their morphologies and luminescence properties have been studied in this paper. The as-prepared Ca0.9MoO4:0.05Eu3+, 0.05Li+ powders occur as uniform spherical particles with a size distribution of 1–2 μm via the optimized reaction temperature at 600 °C and reaction time of 1 h. The phosphors showed an intense red emission with maxima at 615 nm, which was ascribed to the Eu3+ electric dipole transition of 5D0 → 7F2. The comparison on the morphology and luminescence property between the as-prepared samples and the samples obtained by the conventional method was discussed in detail. A possible microwave-assisted solid-state reaction mechanism was proposed, and the effect of the microwave sintering temperature and time was also discussed.

Fast microwave-assisted synthesis of Nb-doped Li4Ti5O12 for high-rate lithium-ion batteries

Nb-doped Li4Ti5O12 (LTO) has been successfully prepared by a fast and economical microwave-assisted solid-state reaction process. The microwave system can significantly shorten the synthesis time to minutes and yield uniform Nb-doped LTO nanocrystals. Results show that the electrode of Nb-doped LTO prepared by microwave irradiation exhibits enhanced electrochemical properties in comparison to the LTO (T) powder prepared by the traditional solid-state reaction. In particular, the Nb-doped LTO electrode with the Nb-containing content of x = 0.03 exhibits the best electrochemical performances. Discharge capacities of 174.7, 157.1, 141.7, and 130.2 mAh g−1 after 200 cycles are achieved at the charge–discharge rates of 1, 5, 10, and 20 C, respectively. This work provides an effective way for the fast and mass production of Li4Ti5O12-based anode materials with superior rate performance.

Microwave-Assisted Solid State Synthesis of Red-Emitting BCNO Phosphor and its Characteristics

Red-emitting BCNO phosphor has been synthesized by a microwave-assisted solid state reaction using a low-cost boric acid, urea and polyethylene glycol (PEG-20k) as the starting materials. The effects of reaction parameters: temperature, carbon and nitrogen content on material composition and photoluminescence properties were investigated. Nearly multi-phase hexagonal boron nitride, boron oxide, boron carbide and carbon graphite was observed, indicating the BCNO were successfully synthesized. The photoluminescence spectra of the phosphor excited by a UV light (365 nm) showed a broad emission band at 460 - 620 nm.

Microwave-assisted synthesis and magnetic properties of Ni1−xZnxFe2O4 ferrite powder

Ni1−xZnxFe2O4 ferrite powder with particle size of ~1.0μm was synthesized by microwave-assisted solid state reaction technology. The as-received Ni1−xZnxFe2O4 powder was characterized by X-ray diffraction and field-emission scanning electron microscopy. Compared with conventional solid state reaction, the results show that the synthesizing temperature and duration of Ni1−xZnxFe2O4 ferrite powder are remarkably lowered by microwave-assisted synthesis. It may be attributed to its enhanced solid phase diffusion and solid state reaction rate for the microwave heating mechanism. The magnetic properties of Ni1−xZnxFe2O4 ferrite powder can be significantly improved by an appropriate microwave synthesizing process.

Preparation and electrochemical properties of LiFePO4/C nanoparticles using different organic carbon sources

Uniform LiFePO4/C particles were prepared by a fast, facile, and environmentally friendly microwave-assisted low-heating solid-state reaction using different organic carbon sources. The particles are very uniform and are 50–80 nm in size. The effects of the carbon sources on the microstructure and electrochemical properties of LiFePO4/C nanoparticles were investigated by X-ray diffraction, transmission electron microscopy, and electrochemical measurements. The results showed that the LiFePO4/C nanoparticles prepared from citric acid as carbon source, which has good crystallinity and a narrow particle size distribution, provided an initial discharge capacity of 112 mAh g−1 at 0.5 C with excellent capacity retention. These advantages, coupled with the simple and effective preparation method, render LiFePO4/C nanoparticles attractive for practical and large-scale applications.

Enhanced electrochemical properties of LiFePO4 by Mo-substitution and graphitic carbon-coating via a facile and fast microwave-assisted solid-state reaction

A composite cathode material for lithium ion battery applications, Mo-doped LiFePO(4)/C, is obtained through a facile and fast microwave-assisted synthesis method. Rietveld analysis of LiFePO(4)-based structural models using synchrotron X-ray diffraction data shows that Mo-ions substitute onto the Fe sites and displace Fe-ions to the Li sites. Supervalent Mo(6+) doping can act to introduce Li ion vacancies due to the charge compensation effect and therefore facilitate lithium ion diffusion during charging/discharging. Transmission electron microscope images demonstrate that the pure and doped LiFePO(4) nanoparticles were uniformly covered by an approximately 5 nm thin layer of graphitic carbon. Amorphous carbon on the graphitic carbon-coated pure and doped LiFePO(4) particles forms a three-dimensional (3D) conductive carbon network, effectively improving the conductivity of these materials. The combined effects of Mo-doping and the 3D carbon network dramatically enhance the electrochemical performance of these LiFePO(4) cathodes. In particular, Mo-doped LiFePO(4)/C delivers a reversible capacity of 162 mA h g(-1) at a current of 0.5 C and shows enhanced capacity retention compared to that of undoped LiFePO(4)/C. Moreover, the electrode exhibits excellent rate capability, with an associated high discharge capacity and good electrochemical reversibility.

Microwave solid synthesis and electrical properties of Aurivillius-type oxide-ion conductor

A new oxide-ion conductor of Aurivillius-type structure, namely BISRVOX (Bi 2Sr x V 1− x O 5.5–(3 x/2)– δ , 0≤ x≤0.20), was successively synthesized by the microwave-assisted solid state reaction. 25 min of microwave irradiation was found to be quite sufficient to ensure the completion of reaction. Powder X-ray diffraction and differential thermal analysis showed better structural properties for the microwave-prepared samples compared to those obtained from the conventional solid synthesis route. Interestingly, the highly conducting γ-phase was effectively stabilized for x≥0.10. AC impedance spectroscopy evidenced the superiority of the microwave heating over conventional solid synthesis routes in exhibiting high oxide-ion performance.

Microwave-assisted solid synthesis and impedance spectroscopy of BICRVOX system

Abstract Samples of BICRVOX (Bi 2 Cr x V 1− x O 5.5+ x − δ ; 0 ≤ x ≤ 0.20) were synthesized by the microwave-assisted solid state reaction. The exposure time of 25 min was found to be sufficient to ensure the completion of reaction. Powder X-ray diffraction and FT-IR confirmed that the dielectric heating results in the better ordering of Cr 6+ and V 5+ positions in the perovskite-like sheets compared to the conventional route. Differential thermal analysis showed that the thermal stability of β → γ transition is compositionally dependent. However, AC impedance analysis showed that the microwave-prepared samples possess anomalous ferroelectricity and that the microwave heating is also advantageous in the reduction of charge accumulation at the vicinity of grain boundaries. In addition, the oxide ion performance of microwave samples is clearly evident for BICRVOX.20 at higher temperatures.

Microwave-assisted boration of potato starch

Granular potato starch was successfully borated with boric acid and sodium tetraborate decahydrate (borax) in a microwave-assisted solid state reaction. Products of starch boration reached (dependently on the conditions of the process) the maximal value of the degree of esterification (DE) equal to 0.27 when boric acid was applied, and 0.18 when borax was used. DTA, TG, DTG and FT-IR methods revealed that boration of starch provided boron esters or borate complexes. High-pressure size exclusion chromatography (HPSEC), DSC and X-ray diffraction showed that although the boration of starch caused changes in its macrostructure, the thermal stability of borated starch was enhanced even by 20 °C when compared with native product, with larger stability exhibited by complexes than by esters. The thermal stability of the borated starch increased with an increase in DE. After modification, granularity of starch was retained, as was indicated by the scanning electron microscopy.

Microwave-assisted selective preparation and characterization of Li 21Si 5 and Li 17Sn 4

Two types of intermetallic lithium alloys, Li 21Si 5 and Li 17Sn 4 (previously Li 22Si 5 and Li 22Sn 5), were prepared for the first time using microwave-assisted solid-state reaction. The optimum oven power for their preparation is 80–60%, and the irradiation times are 5 min for Li 21Si 5 or 10 min for Li 17Sn 4. A cheap alumina crucible was found to be the most suitable container in quick (less than 10 min) microwave reactions for Li-containing alloys. The synthesized compounds were characterized by PXRD. Mössbauer spectroscopy was used to characterize Li 17Sn 4 under different conditions. The hyperfine interaction parameters of 119Sn in Li 17Sn 4 show a typical Li–Sn alloy Sn isomer shift (1.88 mm/s). The oxidization processes of the two intermetallic lithium alloys in air were investigated. The microwave method was found to be simple, fast and efficient, with high selectivity for the preparation of these compounds.

Microwave-assisted solid-state synthesis and characterization of intermetallic compounds of Li3Bi and Li3Sb

Li3Bi and Li3Sb are two types of intermetallic lithium alloys, which were prepared for the first time by a microwave-assisted solid-state reaction. They were characterized by PXRD and SEM techniques. Pure phases of Li3Bi and Li3Sb were obtained after only 2 min and 1 min irradiation at 80% microwave power, respectively. The microwave method was found to be simple, fast and efficient for the preparation of these compounds. The oxidization processes of the two intermetallic lithium alloys in air were investigated. After exposure of the two intermetallic compounds to air for 5 hours only the corresponding Bi and Sb were detected by XRD analyses, the Li metal having been converted to amorphous Li2O. SEM observations show that the products are homogeneous, meaning that Bi and Sb are homogeneously dispersed in the amorphous products. The oxidized products may be potential composite anode materials for Li ion batteries.

Microwave-Assisted Preparation, Morphological, and Photoacoustic Studies of the Na4SnSe4, K4Sn2Se6, and K4Sn3Se8, Zintl Molecular Sn–Se Oligomers

The Zintl compounds Na 4 SnSe 4 , K 4 Sn 2 Se 6 , and K 4 Sn 3 Se 8 were prepared for the first time using the microwave-assisted solid-state reaction method. The heteroanions of these phases represent oligomeric group of (zero-dimensional) molecular anions: [SnSe 4 ] 4− , [Sn 2 Se 6 ] 4− , [Sn 3 Se 8 ] 4− . The compounds were characterized using Powder XRD, SEM-EDS, and their electronic properties were studied using photoacoustic spectroscopy. The microwave method was found to be simple, fast and efficient for the preparation of these Zintl compounds.

Microwave-assisted solid-state reactions involving metal powders

The high dielectric loss tangents of metal powders at 2450 MHz have been utilised to accelerate the syntheses of a wide range of metal chalcogenides in the solid state.

Microwave-assisted solid-state reactions involving metal powders and gases

The high dielectric loss tangents of metal powders at 2450 MHz have been utilised to facilitate the syntheses of a range of metal chlorides, oxochlorides, bromides and nitrides by direct combination of metals with gases.

ChemInform Abstract: Microwave‐Assisted Solid‐State Reactions Involving Metal Powders.

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Microwave-assisted solid-state reactions involving metal powders

Microwave-assisted solid-state reactions involving metal powders