Novel approach to preparation of LiMn2O4 core/LiNixMn2−xO4 shell composite

Combining two methods, coating and doping, to modify spinel LiMn₂O₄, is a novel approach we used to synthesize active material. First we coated the LiMn₂O₄ particles with the nickel oxide particles by means of homogenous precipitation, and then the nickel oxide-coated LiMn₂O₄ was calcined at 750 °C to form a LiNi_xMn_2−xO₄ shell on the surface of spinel LiMn₂O₄ particles. Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), cyclic voltammetry (CV) and charge-discharge test were performed to characterize the spinel LiMn₂O₄ before and after modification. The experimental results indicated that a spinel LiMn₂O₄ core is surrounded by a LiNi_xMn_2−xO₄ shell. The resulting composite showed excellent electrochemical cycling performance with an average fading rate of 0.014% per cycle. This improved cycle stability is greatly attributed to the suppression of Jahn-Teller distortion on the surface of spinel LiMn₂O₄ particles during cycling.     

ZnO-coated LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript> cathode material for lithium-ion batteries synthesized by a combustion method

ZnO-coated LiMn₂O₄ cathode materials were prepared by a combustion method using glucose as fuel. The phase structures, size of particles, morphology, and electrochemical performance of pristine and ZnO-coated LiMn₂O₄ powders are studied in detail by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), galvanostatic charge-discharge test, and X-ray photoelectron spectroscopy (XPS). XRD patterns indicated that surface-modified ZnO have no obvious effect on the bulk structure of the LiMn₂O₄. TEM and XPS proved ZnO formation on the surface of the LiMn₂O₄ particles. Galvanostatic charge/discharge test and rate performance showed that the ZnO coating could improve the capacity and cycling performance of LiMn₂O₄. The 2 wt% ZnO-coated LiMn₂O₄ sample exhibited an initial discharge capacity of 112.8 mAh g^?1 with a capacity retention of 84.1 % after 500 cycles at 0.5 C. Besides, a good rate capability at different current densities from 0.5 to 5.0 C can be acquired. CV and EIS measurements showed that the ZnO coating effectively reduced the impacts of polarization and charge transfer resistance upon cycling.     

Superficial Mullite Reinforcement from Silica Coating on Alumina: Influence of Y2O3 Addition

In the present work the sol–gel process is used to obtain mullite (3Al₂O₃·2SiO₂) from silica (SiO₂) deposition on alumina (Al₂O₃) cylindrical pieces. The influence of different percentages of yttria (Y₂O₃) on this superficial reaction is also studied. The alumina matrix is constituted by commercial γ-alumina of 99.99% purity with very high porosity. The cylinders are 3 mm in diameter and 4–6 mm in length. SiO₂ is deposited on the cylinders by sol–gel process, from TEOS (tetraethyl orthosilicate, Si(OCH₂CH₃)₄) in ethanol solution. The yttrium oxide is suspended in this solution and then the alumina cylinders constituting the support material are added. Silica is produced by the hydrolysis reaction of the mentioned alkoxide. The molar ratio TEOS/ETOH/H₂O is strictly controlled to be 2/4/4. The hydrolysis is carried out in basic medium adding 0.3 ml NH₄OH, and at 40°C. The basic medium is used to accelerate the steps of the sol–gel process. The resulting pH before the hydrolysis starts is 9.8. Samples with 2%, 4% and 6% yttria addition were prepared and then heat treated at 1300°C and 1500°C for two hours. For comparative purposes samples without yttria were prepared and treated in the same way. The obtained products were characterized by optical and scanning electron microscopies, electron diffraction analysis X-ray and X-ray diffraction, among other techniques.     

Lithium polyacrylate-coated LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript> cathode materials with excellent performance for lithium ion batteries

A novel facile approach to coat LiMn₂O₄ by lithium polyacrylate (PAALi) is demonstrated. The PAALi-coated LiMn₂O₄ (LMO@2%PAALi) and LiMn₂O₄ (LMO) are characterized by charge–discharge tests, X-ray diffraction (XRD), PAALi dissolving experiment, transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), thermogravimetry (TG), and inductively coupled plasma optical emission spectrometer (ICP-OES). XRD and FTIR analyses indicate that there are no clear differences between LMO@2%PAALi and LMO. PAALi dissolving experiment indicates that PAALi is indissolvable in LiPF₆-EC/DMC/EMC electrolyte. TEM results reveal that LiMn₂O₄ particles are coated by PAALi. ICP-OES results indicate that this stable PAALi coating can prevent the Mn ions dissolving from active LiMn₂O₄ materials and then the stability of LiMn₂O₄ crystals in electrolyte are greatly enhanced. These unique features ensure that LMO@2%PAALi possesses much better rate performance, higher discharge capacity, better cycling performance, and lower charge transfer resistance over LMO. The discharge capacity of LMO@2%PAALi at 0.2 C reaches up to 127.2 mAh g^?1 at room temperature.     

Surface modification of LiMn2O4 thin films at elevated temperature

In order to improve the cycle stability of spinel LiMn₂O₄ electrode at elevated temperature, the LiCoO₂-coated and Co-doped LiMn₂O₄ film were prepared by an electrostatic spray deposition (ESD) technique. LiCoO₂-coated LiMn₂O₄ film shows excellent cycling stability at 55 °C compared to pristine and Co-doped LiMn₂O₄ films. The samples were studied by X-ray diffraction, scanning electron microscopy, Auger electron spectroscopy, cyclic voltammetry and electrochemical impedance spectroscopy. The excellent performance of LiCoO₂-coated LiMn₂O₄ film can be explained by suppression of Mn dissolution. On the other hand, the LiCoO₂-layer on the LiMn₂O₄ surface allows a homogenous Li⁺ insertion/extraction during electrochemical cycles and improves its structure stability.     

Spinel LiMn2O4 active material with high capacity retention

Heating the mixture of LiMn₂O₄ and NiO at 650 °C was employed to enhance the cyclability of spinel LiMn₂O₄. The results of scanning electron microscopy (SEM), X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) spectroscopy analyses implied that a LiNi_xMn_2−xO₄ solid solution was formed on the surface of LiMn₂O₄ particles. And charge-discharge tests showed that the enhancement of the capacity retention of modified LiMn₂O₄ is significant, maintained 97.2% of the maximum capacity after 100 cycles at charge and discharge rate of C/2, while the pure one only 75.2%. The modified LiMn₂O₄ also results in a distinct improvement in rate capability, even at the rate of 12C. The improvement of electrochemical cycling stability is greatly attributed to the suppression of Jahn-Teller distortion at the surface of spinel LiMn₂O₄ particles.     

Preparation and characterization of silica-coated Fe3O4 nanoparticles used as precursor of ferrofluids

Fe₃O₄ magnetic nanoparticles (MNPs) were synthesized by the co-precipitation of Fe³⁺ and Fe²⁺ with ammonium hydroxide. The sodium citrate-modified Fe₃O₄ MNPs were prepared under Ar protection and were characterized by Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), X-ray powder diffraction (XRD) and vibrating sample magnetometer (VSM). To improve the oxidation resistance of Fe₃O₄ MNPs, a silica layer was coated onto the modified and unmodified MNPs by the hydrolysis of tetraethoxysilane (TEOS) at 50 °C and pH 9. Afterwards, the silica-coated Fe₃O₄ core/shell MNPs were modified by oleic acid (OA) and were tested by IR and VSM. IR results revealed that the OA was successfully grafted onto the silica shell. The Fe₃O₄/SiO₂ core/shell MNPs modified by OA were used to prepare water-based ferrofluids (FFs) using PEG as the second layer of surfactants. The properties of FFs were characterized using a UV-vis spectrophotometer, a Gouy magnetic balance, a laser particle size analyzer and a Brookfield LVDV-III+ rheometer.     

Synthesis and Optical Properties of SiO2-Coated CeO2 Nanoparticles

Silica (SiO₂)-coated ceria (CeO₂) nanoparticles were prepared using water-in-oil microemulsion. Polyoxyethylene (15) cetylether and cyclohexane were used as a surfactant and organic solvent. SiO₂-coated CeO₂ nanoparticles were obtained by hydrolysis of metal alkoxide (tetraethylorthosilicate, TEOS) in the solution containing CeO₂ precursor nanoparticles. The effects of CeO₂ sources (Ce metal salt) and CeO₂ particle-forming agents on the morphology of SiO₂–CeO₂ particles were investigated. Observation via transmission electron microscopy revealed that the type of particle-forming agent affected the nanoparticles' morphology and that CeO₂ nanoparticles were spherically coated with SiO₂ when using oxalic acid ((COOH)₂) as a particle-forming agent of CeO₂. Furthermore, the transmittance of the particles was high in the visible region (above 400 nm) and decreased in the ultraviolet region.     

Direct micron-sized LiMn2O4 particle coating on LiCoO2 cathode material using surfactant

Micron-sized LiMn₂O₄ particles were easily coated on LiCoO₂ cathodes using an amphoteric gelatin surfactant at pH4–5. The coated LiCoO₂ material showed a significantly higher thermal stability during charging and capacity retention on cycling at 4.6 V, compared to the bare LiCoO₂.     

Synthesis of Co-coated lithium manganese oxide and its characterization as cathode for lithium ion battery

Co-coated LiMn₂O₄ was synthesized by electroless plating. The phase identification, surface morphology, and electrochemical properties of the synthesized powders were studied by X-ray diffraction, scanning electron microscopy, electrochemical impedance spectroscopy, and galvanostatic charge–discharge experiments, respectively. The result shows that Co-coated LiMn₂O₄ particle has a coarse surface with a lot of holes. The specific capacity of Co-coated LiMn₂O₄ is 118 mAh g⁻¹, which is a bit less than 123 mAh g⁻¹ for the uncoated LiMn₂O₄. The capacity retention of Co-coated LiMn₂O₄ is 11% higher than the uncoated LiMn₂O₄ when the electrode is cycled at room temperature for 20 times. When cycled at the temperature of 55 °C, the capacity retention of Co-coated LiMn₂O₄ becomes 15% higher than the uncoated one.     

Improvement of electrochemical performance of all-solid-state lithium secondary batteries by surface modification of LiMn2O4 positive electrode

To improve the electrochemical performance of an all-solid-state In/80Li₂S⋅20P₂S₅ (electrolyte)/LiMn₂O₄ cell, a lithium-titanate thin film was used to coat LiMn₂O₄. The interfacial resistance between LiMn₂O₄ and the electrolyte (measured after initial charging) decreased when the LiMn₂O₄ particles were coated with lithium-titanate. A cell with lithium-titanate-coated LiMn₂O₄ had a higher capacity than a cell with noncoated LiMn₂O₄ for current densities in the range 0.064 to 2.6 mA cm^− 2. Additionally, a cell with coated LiMn₂O₄ retained 96% of the 10th-cycle reversible capacity at a current density of 0.064 mA cm^− 2 after 50 cycles.     

Pre-hydrolysed ethyl silicate as an alternative precursor for SiO2-coated carbon nanofibers

This work reported basically aims at understanding the extent of SiO₂-coated carbon nanofibers using two different sol-gel precursors for the silicate glass. The silicate precursors employed were tetraethoxysilane (TEOS) and pre-hydrolysed ethyl silicate. The first route consisted in an acid hydrolysis and polycondensation of the TEOS and the second one in a polycondensation of the pre-hydrolysed ethyl silicate. The techniques of Fourier Infra Red spectroscopy, thermogravimetric analysis, scanning electron microscopy and X-ray diffraction were used to characterize the materials obtained. Both kinds of SiO₂ precursor can coat the CNF effectively. However, the use of pre-hydrolysed ethyl silicate (faster gelation times and higher surface areas) can be considered a low-cost and facile alternative with respect to the use of TEOS, to obtain industrially silica-coated carbon nanofibers.     

Preparation and characterizations of SiO2-coated nanoparticles of Mn0.4Zn0.6Fe2O4

Core/shell nanoparticles consisting of a magnetic core of zinc-substituted manganese ferrite (Mn_0.4Zn_0.6Fe₂O₄) and a shell of silica (SiO₂) are prepared by a sol-gel method using tetraethyl orthosilicate (TEOS) as a precursor material for silica and salts of iron, manganese and zinc as the precursor of the ferrite. Three weight percentages of the shell materials of SiO₂ are used to prepare the coated nanoparticles. The X-ray diffractograms (XRD) of the coated and uncoated magnetic nanoparticles confirmed that the magnetic nanoparticles are in their mixed spinel phase in an amorphous matrix of silica. Particles sizes of the samples annealed at different temperatures are estimated from the width of the (3 1 1) line of the XRD pattern using the Debye-Sherrer equation. The information regarding the crystallographic structure together with the particles sizes extracted from the high-resolution transmission electron microscopy (HRTEM) of a few selected samples are in agreement with those obtained from the XRD. HRTEM observations revealed that particles are coated with silica. The calculated thickness is in agreement with that obtained from the HRTEM pictures. Hysteresis loops observed in the temperature range 300 down to 5 K and Mössbauer spectra at room temperature indicate superparamagnetic relaxation of the nanoparticles.     

Ultrathin surface coatings to enhance cycling stability of LiMn2O4 cathode in lithium-ion batteries

Spinel LiMn₂O₄ suffers from severe dissolution when used as a cathode material in rechargeable Li-ion batteries. To enhance the cycling stability of LiMn₂O₄, we use the atomic layer deposition (ALD) method to deposit ultrathin and highly conformal Al₂O₃ coatings (as thin as 0.6–1.2 nm) onto LiMn₂O₄ cathodes with precise thickness control at atomic scale. Both bare and ALD-coated cathodes are cycled at a specific current of 300 mA g^?1 (2.5 C) in a potential range of 3.4–4.5 V (vs. Li/Li⁺). All ALD-coated cathodes exhibit significantly improved cycleability compared to bare cathodes. Particularly, the cathode coated with six Al₂O₃ ALD layers (0.9 nm thick) shows the best cycling performance, delivering an initial capacity of 101.5 mA h?g^?1 and a final capacity of 96.5 mA h?g^?1 after 100 cycles, while bare cathode delivers an initial capacity of 100.6 mA h?g^?1 and a final capacity of only 78.6 mA h?g^?1. Such enhanced electrochemical performances of ALD-coated cathodes are ascribed to the high-quality ALD oxide coatings that are highly conformal, dense, and complete, and thus protect active material from severe dissolution into electrolytes. Besides, cycling performances of coated cathodes can be easily optimized by accurately tuning coating thickness via varying ALD growth cycles.     

Effect of heptamethyldisilazane on the electrochemical performance of LiMn2O4/Li

The effect of heptamethyldisilazane as an electrolyte stabilizer on the cycling performance of a LiMn₂O₄/Li cell at different rates at 30 °C and the storage performance at 60 °C is investigated systematically based on conductivity test, linear sweep voltage, electrochemical impedance spectroscopy, scanning electron microscopy, X-ray diffraction, and charge–discharge measurements. The results show that heptamethyldisilazane added into the LiPF₆-based electrolyte can increase the stability of the original electrolyte; coulomb efficiency, the initial discharge capacity, and cycling performance at different rates in a sense, meanwhile, improve the storage performance at elevated temperature, although the C-rate performance of the cell is a little worse than that without heptamethyldisilazane in the electrolyte. When the LiMn₂O₄/Li cell with heptamethyldisilazane in the LiPF₆-based electrolyte stored at 60 °C for a week cycles 300 times, the capacity retention is up to 91.18 %, which is much higher than that (87.18 %) without the additive in the electrolyte. This is mainly due to the lower solid electrolyte interface resistance (R _f) in the cell, followed by the better morphology and structure of the cathode after storage at 60 °C for a week compared with the LiMn₂O₄/Li cell without heptamethyldisilazane.     

Enhancement of the electrochemical properties of LiMn2O4 by glycolic acid-assisted sol-gel method

LiMn₂O₄ spinel cathode was synthesized by the sol–gel method by using glycolic acid as a chelating agent. The sample exhibited a pure cubic spinel structure without any impurities in the X-ray diffraction (XRD) patterns. The result of the electrochemical performances on the sample compared to those of electrodes based on LiMn₂O₄ spinel synthesized by solid state. LiMn₂O₄ synthesized by glycolic acid-assisted sol–gel method improves the cycling stability of electrode. The capacity retention of sol–gel-synthesized LiMn₂O₄ was about 90• after 100 cycles between 3.0 and 4.4 V at room temperature. The electrochemical performance of the LiMn₂O₄ (sol–gel) and LiMn₂O₄ (solid state) were investigated under 40• between 3.0 and 4.4 V. XRD results of the cathode material after 50 cycles at 40• revealed that LiMn₂O₄ (sol–gel) could effectively suppress the LiMn₂O₄ dissolving of into electrolyte and resulted in a better stability.     

Thin aluminum film improving the cycle performance of positive electrode of lithium ion battery

A novel method was investigated to improve the cycle performance of the spinel LiMn₂O₄. It is widely different from the traditional way of modifying LiMn₂O₄ particle with compounds or metals. In our study, instead of coating LiMn₂O₄ particle itself with compounds or metals, first we covered the current collector with the mixture of LiMn₂O₄ particle, conductive agents and binders, and then deposited an aluminum film onto it by vacuum evaporation technique. Both of the pristine electrode and the modified one were characterized by scanning electron microscope (SEM), X-ray diffraction (XRD), and charge-discharge tests. The results of SEM and XRD demonstrate that the aluminum film was formed successfully onto the positive electrode. And the charge-discharge tests show that the capacity retention of pristine electrode and modified one are 63.7% and 93.5% at C/2 rate in the voltage range of 3.5-4.3 V after 200 cycles, respectively. The modified electrode also shows better rate capability in comparison with the pristine one. The improved cycling stability is attributed to the minimizing of Mn dissolution into electrolyte solution and the good electronic conductivity of deposited aluminum film.     

Growth and characterization of Fe3O4 nanoparticles in silica matrix

Nano-magnetic Fe₃O₄ particles coated with silica are synthesized. The study of structural and magnetic properties was carried out using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and vibrating sample magnetometer (VSM) techniques. The VSM results show that these kinds of composite particles exhibit superparamagnetic behavior with zero coercivity and remanence. The magnetic spheroid alumina carriers containing these magnetic composite particles were prepared by an internal gelation process. The SiO₂ coatings prevent the reaction between Fe₃O₄ and Al₂O₃ during the sintering process and maintain the superparamagnetic behavior of the catalyst carriers.     

Nanocrystalline and long cycling LiMn2O4 cathode material derived by a solution combustion method for lithium ion batteries

A nanostructured LiMn₂O₄ spinel phase is used as a cathode for 4 V lithium batteries and is prepared by solution combustion synthesis using urea as a fuel. Lithium-manganese oxides have received more increasing attention in recent years as high-capacity intercalation cathodes for rechargeable lithium-ion batteries. Nanostructured electrodes have been shown to enhance the cell cyclability. For optimum synthesis, the spinel LiMn₂O₄ showed that the optimal heat treatment protocol was a 10 h calcination at 700 °C, which sustained 229 cycles between 3.0 and 4.3 V at a charge-discharge rate of 0.1 °C before reaching an 80% charge retention cut-off value. X-ray diffraction and electron diffraction pattern investigations demonstrate that all the LiMn₂O₄ products are a spinel phase crystal. TEM micrographs show the prepared products were highly crystalline with an average particle size of 20-50 nm. Cyclic voltammetry shows the absence of phase transitions in the samples ensures negligible strain, resulting in a longer cycle life. This work shows the feasibility of the solution combustion method for obtaining manganese oxides with nano-architecture and high cyclability, and suggests it is a promising method for providing small diffusion pathways that improve lithium-ion intercalation kinetics and minimize surface distortions during cycling.     

Gas sensing property of lithium tetraborate clad modified fiber optic sensor

The micro structured plate-like lithium tetraborate, Li₂B₄O₇ (1 μm in diameter) has been prepared by sol–gel method and characterized structurally by X-ray diffraction and morphologically by scanning electron microscopy. UV–Vis spectrum shows about 60% transparency in the visible region and the optical energy band gap is found to be 3.5 eV which is also confirmed by strong near band edge emission from luminescence spectrum. The spectral characteristics of the cladding modified fiber optic sensor coated with microcrystalline Li₂B₄O₇ are studied for various concentrations of ethanol, methanol and ammonia (50–500 ppm). At 298 K, the sensitivity for ethanol is ?10 counts/ppm which is relatively higher than ammonia (?4 counts/ppm) and methanol (?3 counts/ppm). The time response of the sensor is presented for pure Li₂B₄O₇ with ethanol gas.