Effect of ultrasonic irradiation on structure and electrochemical properties of LiMn2O4 thin films cathode material for Li-ion micro-batteries

Two kinds of spinel LiMn₂O₄ thin film for lithium ion micro-batteries were successfully prepared on polycrystal Pt substrates by spin coating methods, which were carried out under ultrasonic irradiation (USG) and magnetic stirring (MSG), respectively. The microstructures and electrochemical performance of LiMn₂O₄ thin films were characterized by thermogravimetry analysis (TGA), X-ray diffraction (XRD), scanning electron microscopy (SEM), and galvanostatic charge-discharge measurements. It was found that the crystalline structure of USG samples grew better than that of the MSG samples. At the same time, higher discharge capacity and better cycle stability were obtained for the LiMn₂O₄ thin films of USG at the current density of 50 μAh/cm² between 3.0 and 4.3 V. The 1st discharge capacity was 57.8 μAh/cm²-μm for USG thin films and 51.7 μAh/cm²-μm for MSG thin films. After 50 cycles, 91.4% and 69% of discharge capacity could be retained respectively, indicating that ultrasonic irradiation condition during spin coating was more suitable for preparing spinel LiMn₂O₄ thin films with better electrode performance for lithium ion micro-batteries.     

Electrochemical behaviour of nano-sized spinel LiMn2O4 and LiAlxMn2−xO4 (x=Al: 0.00-0.40) synthesized via fumaric acid-assisted sol-gel synthesis for use in lithium rechargeable batteries

Pristine spinel LiMn₂O₄ and LiAl_xMn_2−xO₄ (x=Al: 0.00-0.40) with sub-micron sized particles have been synthesized using fumaric acid as chelating agent by sol-gel method. The synthesized samples were subjected to thermogravimetric analysis (TGA), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and cyclic voltammetry (CV) and galvanostatic cycling studies. The TGA curve of the gel shows several weight-loss regions stepwise amounting to 55% till 800 °C attributed to the decomposition of the precursors. Calcination to higher temperatures (800 °C) yields pure-phase spinel (LiAl_xMn_2−xO₄), as it is evident from the high-intensity XRD reflections matching to the standard pattern. SEM and TEM studies confirm that the synthesized grains are of uniform regular surface morphology. FT-IR studies show stretching and bending vibration bands of Li-O, Li-Al-Mn-O. LiAl_0.1Mn_1.90O₄ spinel was found to deliver discharge capacity of 139 mA h/g during the first cycle with columbic efficiency of 97%. LiAl_0.1Mn_1.90O₄ spinel exhibits the high cathodic peak current indicating better electrochemical performance. Low doping (x=0.1) of Al is found to be beneficial in stabilizing the spinel structure.     

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.     

On the observation of a huge lattice contraction and crystal habit modifications in LiMn2O4 prepared by a fuel assisted solution combustion

Two batches of poly-crystalline lithium manganate were prepared by a fuel assisted solution combustion method. LiMn₂O₄(S) was prepared using starch as the fuel and LiMn₂O₄(P) was prepared using poly vinyl alcohol (PVA) as the fuel. XRD studies indicated a significant and consistent shift in the 2θ values of all the hkl peaks to higher values in LiMn₂O₄(P) compared to LiMn₂O₄(S) indicating a lattice contraction in the former. TG/DTA studies indicated a higher formation temperature (∼25 °C higher) for LiMn₂O₄(P). The higher formation temperature most likely promotes the oxidation of some Mn³⁺ to Mn⁴⁺ with a lower ionic radius causing a lattice contraction. This hypothesis is confirmed through XPS studies which indicated the presence of a higher fraction of Mn⁴⁺ in LiMn₂O₄(P) than that present in LiMn₂O₄(S). A crystal shape algorithm was used to generate the crystal habits of lithium manganate from their XRD data leading to an understanding on the exposed hkl planes in these materials. From the atomic arrangement on the exposed hkl planes it is predicted that LiMn₂O₄(P) would be less prone to manganese dissolution and hence would possess a higher cycle life when compared to LiMn₂O₄(S).     

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.     

Mn, Cr-co-doped MgAl2O4 phosphors for white LEDs

The Mn-, Cr-doped and Mn, Cr-co-doped MgAl₂O₄ powders have been synthesized via a gel-solid reaction method. Energy transfer from Mn²⁺ to Cr³⁺ has been observed for the first time in the co-doped MgAl₂O₄ phosphors. When excited with blue light with a wavelength of 450 nm at room temperature, both green emission from Mn²⁺ around 520 nm and red emission from Cr³⁺ around 675and 693 nm were generated. Moreover, the color of the emission can be modified by controlling the doping concentrations of Mn²⁺ and Cr³⁺. Therefore, MgAl₂O₄: Mn²⁺, Cr³⁺ could be used as a single-phased phosphor for white LED with a blue LED chip. The energy transfer in terms of Mn²⁺ to Cr³⁺ is determined by means of radiation and reabsorption.     

Electrochemical behavior of Li intercalation processes into a Li[NixLi(1/3−2x/3)Mn(2/3−x/3)]O2 cathode

Li[Ni_xLi_(1/3−2x/3)Mn_(2/3−x/3)]O₂ (X=0.17, 0.25, 0.33, 0.5) compounds are prepared by a simple combustion method. The Rietvelt analysis shows that these compounds could be classified as having the α-NaFeO₂ structure. The initial charge-discharge and irreversible capacity increases with the decrease of x in Li[Ni_xLi_(1/3−2x/3)Mn_(2/3−x/3)]O₂. Indeed, Li[Ni_0.50Mn_0.50]O₂ compound shows relatively low initial discharge capacity of 200 mAh/g and large capacity loss during cycling, with Li[Ni_0.17Li_0.22Mn_0.61]O₂ and Li[Ni_0.25Li_0.17Mn₀.₅₈]O₂ compounds exhibit high initial discharge capacity over 245 mAh/g and stable cycle performance in the voltage range of 4.8 -2.0 V. On the other hand, XANES analysis shows that the oxidation state of Ni ion reversibly changes between Ni²⁺ and about Ni³⁺, while the oxidation state of Mn ion sustains Mn⁴⁺ during charge-discharge process. This result does not agree with the previously reported ‘electrochemistry model’ of Li[Ni_xLi_(1/3−2x/3)Mn_(2/3−x/3)]O₂, in which Ni ion changes between Ni²⁺ and NI⁴⁺. Based on these results, we modified oxidation-state change of Mn and Ni ion during charge-discharge process.     

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.     

Energy transfer among three luminescent centers in full-color emitting ZnGa2O4:Mn, Cr phosphors

The ZnGa₂O₄:Mn²⁺, Cr³⁺ phosphors show three colors; the blue band of 380 nm from the charge transfer between Ga-O, the green band of 505 nm from Mn²⁺ and the red band of 705 nm from Cr³⁺. As a variation of Mn²⁺ or Cr³⁺ concentrations in ZnGa₂O₄:Mn²⁺, Cr³⁺, the relative emission intensity can be tuned. This phenomenon is explained in terms of the energy transfer based on four factors: the spectral overlap between the energy donors (Ga-O) and the energy accepters of Mn²⁺ or Cr³⁺, the absorption cross section of the energy accepters, the distance between them, and the decay time of the energy donors. ZnGa₂O₄:0.0025Mn²⁺, 0.010Cr³⁺ shows the CIE coordinates of x=0.4014, y=0.3368, which is a pure white light. The single-phased full-color emitting ZnGa₂O₄:Mn²⁺, Cr³⁺ phosphors can be applied to illumination devices.     

Electron energy-loss spectroscopy of LiMn2O4, LiMn1.6Ti0.4O4 and LiMn1.5Ni0.5O4

The dielectric properties of LiMn₂O₄, LiMn_1.6Ti_0.4O₄ and LiMn_1.5Ni_0.5O₄ powders, synthesized by sol-gel method, were determined by analyzing the low-loss region of the electron energy-loss spectroscopy (EELS) spectrum in a transmission electron microscope. From these data, the optical joint density of states (OJDS) was obtained by Kramers-Kronig analysis. Since maxima observed in the OJDS spectra are assigned to interband transitions above the Fermi level, these spectra can be interpreted on the basis of calculated density of states (DOS), carried out with the CASTEP code. Experimental and theoretical results are in good agreement.     

Facile synthesis and electrochemical properties of hierarchical MnO2 submicrospheres and LiMn2O4 microspheres

Hierarchical MnO₂ submicrospheres have been successfully synthesized by a wet chemical method. The as-prepared products were characterized by means of XRD, SEM, FTIR, TG, and TEM. With the as-prepared MnO₂ submicrospheres as precursors, LiMn₂O₄ microspheres were conveniently prepared by a simple solid-state reaction between MnO₂ and LiOH at a temperature as low as 600 °C. Electrochemical properties of the as-prepared MnO₂ submicrospheres and LiMn₂O₄ microspheres as cathode materials in lithium ion cells were investigated by galvanostatic charge/discharge tests.     

Structural and luminescence characterization of synthetic Cr-doped Ni3(PO4)2

Ni_3–xCr_2x/3(PO₄)₂ (x=0 and 0.02) microcrystalline powders were obtained as single phases via a modified sol–gel Pechini-type in situ polymerizable complex method. The samples were characterized using scanning electron microscopy, X-ray diffraction, cathodoluminescence (CL), and thermoluminescence (TL) techniques. We found that Cr³⁺ doping modified the average particle and distribution. The mean particle size was 0.441 μm for Ni₃(PO₄)₂ and 0.267 μm for Ni_2.98Cr_0.013(PO₄)₂. The results also reveal that Cr³⁺ doping notably enhanced the CL and TL UV-blue emission.     

Electrochemical performance of LiFePO4/C synthesized by solid state reaction using different lithium and iron sources

LiFePO₄/C cathode materials were prepared from different lithium and iron sources, using glucose as the carbon source and the reducing agent, via a solid state reaction. The samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), galvanostatic charge-discharge test and cyclic voltammetry (CV). The results showed that the LiFePO₄/C is olivine-type phase, and composed of relatively large particles of about 400 nm and some nano-sized particles, which favor the electronic conductivity. The LiFePO₄/C cathode material synthesized from Li₂CO₃ and Fe₂O₃ had the smallest particles and the highest uniformity. It delivered the capacity of 145.8 mA h/g at 0.2 C, and had good reversibility and high capacity retention. The precursor of LiFePO₄/C was characterized by thermogravimetry (TG) to discuss the crystallization formation mechanism of LiFePO₄.     

Synthesis and pseudocapacitive investigation of LiCr x Mn2-x O4 cathode material for aqueous hybrid supercapacitor

A series of Cr-substituted LiMn₂O₄ samples (LiCr _x Mn_2-x O₄, 0?≤?x?≤?0.3) were synthesized by a urea-assisted combustion method to enhance pseudocapacitive properties of LiMn₂O₄ material in aqueous electrolyte. Their structure and morphology were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The LiCr _x Mn_2-x O₄ and activated carbon (AC) electrode were used as the cathode and anode in hybrid supercapacitors, respectively, which capacitive properties were determined by cyclic voltammetry (CV), galvanostatic charge/discharge test, and electrochemical impedance spectroscopy (EIS) in Li₂SO₄ solution. The results revealed that the partial substitution of Mn³⁺ by Cr³⁺ decreased initial capacity, but it prevented capacity fading. In the working voltage of 0–1.4 V, the AC/LiCr_0.1Mn_1.9O₄ capacitor delivered an initial specific capacitance of 41.6 F g^?1 (based on the total active mass of two electrodes) at a current density of 100 mA g^?1 in 1 M Li₂SO₄ solution. After 1,000 cycles, its capacity loss was only 1.7 %.     

Magnetoelectric characterization of xNi0.75Co0.25Fe2O4-(1−x)BiFeO3 nanocomposites

Magnetoelectric (ME) nanocomposites containing Ni_0.75Co_0.25Fe₂O₄-BiFeO₃ phases were prepared by citrate sol-gel process. X-ray diffraction (XRD) analysis showed phase formation of xNi_0.75Co_0.25Fe₂O₄-(1−x)BiFeO₃ (x=0.1, 0.2, 0.3 and 0.4) composites on heating at 700 °C. Transmission electron microscopy revealed the formation of powders of nano order size and the crystal size was found to vary from 30 to 85 nm. Dispersion in dielectric constant (ε) and dielectric loss (tan δ) in the low-frequency range have been observed. It is seen that nanocomposites exhibit strong magnetic properties and a large ME effect. On increasing Ni_0.75Co_0.25Fe₂O₄ contents in the nanocomposites, the saturation magnetization (M_S) and coercivity (H_C) increased after annealing at 700 °C. The large ME output in the nanocomposites exhibits strong dependence on magnetic bias and magnetic field frequency. The large value of ME output can be attributed to small grain size of ferrite phase of nanocomposite being prepared by citrate precursor process.     

Porous LiNi0.75Co0.25O2 microspheres prepared via a hydrothermal process as cathode material for lithium ion batteries

Porous LiNi_0.75Co_0.25O₂ microspheres are successfully prepared by a simple hydrothermal process by using H[Ni_0.75Co_0.25OOH]₃ and LiOH as starting materials in the presence of urea for the first time. The synthesized samples are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), specific surface area (S_BET), and electrochemical performance. The synthesized LiNi_0.75Co_0.25O₂ has a good electrochemical performance with an initial discharge capacity of 169.3 mA g⁻¹ and good capacity retention of 96.7% after 50 cycles at 0.2 C (25 mA g⁻¹). The electrochemical lithium ion insertion/extraction process is quite reversible even at 5 C. Furthermore, the structure in the charge-discharge process is stable and the impedance increased slowly during cycling.     

Li0.5Fe2.5−xMnxO4 ferrite sintered from microwave-induced combustion

Li_0.5Fe_2.5−xMn_xO₄ (0≦x≦1.0) powders with small and uniformly sized particles were successfully synthesized by microwave-induced combustion, using lithium nitrate, ferric nitrate, manganese nitrate and carbohydrazide as the starting materials. The process takes only a few minutes to obtain as-received Mn-substituted lithium ferrite powders. The resultant powders annealed at 650 ^°C for 2 h and were investigated by thermogravimeter/differential thermal analyzer (TG/DTA), X-ray diffractometer (XRD), transmission electron microscopy (TEM), vibrating sample magnetometer (VSM), and thermomagnetic analysis (TMA). The results revealed that the Mn content were strongly influenced the magnetic properties and Curie temperature of Mn-substituted lithium ferrite powder. As for sintered Li_0.5Fe_2.5−xMn_xO₄ specimens, substituting an appropriate amount of Mn for Fe in the Li_0.5Fe_2.5−xMn_xO₄ specimens markedly improved the complex permeability and loss tangent.     

Study of the CO2 decomposition over doped Ni-ferrites

The H₂ reduced NiFe_2−xCr_xO₄ can be used to decompose CO₂ to C repeatedly. A series of nanocrystalline Ni-ferrite doping different contents of Cr³⁺ were synthesized by mixed ions co-precipitation method and characterized by XRD, BET and TEM. The results showed that their crystallite sizes were 1-2 nm and BET surface area changed from 220 to 285 m²/g. The evaluation of the activity and stability indicated that Ni-ferrite with 4 wt% Cr³⁺ dopant could be used repeatedly as many as 60 times and was transformed to Fe_yNi_1−y (0<y<1) alloy and Fe₅C₂ gradually during the cycle decomposition of CO₂ to carbon, especially for no Cr³⁺ sample. After the 60th reaction, although NiFe₂O₄ phase just remained 2.1 wt%, the decomposition activity of Ni-ferrite with 4 wt% Cr³⁺ was still 60% of initial activity. This fact suggests that nanocrystalline Fe_yNi_1−y (0<y<1) alloy from the cycle reaction can contribute to the decomposition of CO₂. The results from scanning electron microscopy (SEM), TEM and XRD show that the deposited carbon from CO₂ decomposition consisted of amorphous, crystallite and carbon nanotubes.     

Preparation and characteristics of Sb-doped LiNiO2 cathode materials for Li-ion batteries

Compounds LiNi_1−xSb_xO₂ (x=0, 0.1, 0.15, 0.2, 0.25) were synthesized by the two-step calcination method. The structural and morphological properties of the products were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). XRD analysis confirms that the uniform solid solution has been formed in the as-prepared compounds without any impurities. It is shown that the crystal lattice parameters (a, c) of the Sb-doped compounds are bigger than those of pure LiNiO₂ and the Sb-doped compound with x=0.2 consists of spherical-like nanoparticles with a mean grain size of 50 nm. The electrochemical performances of as-prepared samples were studied via galvanostatic charge-discharge cycling tests. The compound with x=0.2 exhibits excellent capacity retention during the charge-discharge processes due to its reinforced structural stability, and a discharge capacity of 102.4 mAh/g is still obtained in the voltage range of 2.5-4.5 V after 20 cycles. Thermal analysis further confirms that the structural stability of LiNi_0.8Sb_0.2O₂ is superior to that of pure LiNiO₂.     

Microstructure characterization and magnetic behavior of NiAlxFe2−xO4 and Ni1−yMnyAl0.2Fe1.8O4 ferrites

NiAl_xFe_2−xO₄ and Ni_1−yMn_yAl_0.2Fe_1.8O₄ ferrites were prepared by the conventional ceramic method and were characterized by X-ray diffraction, scanning electron microscopy, and magnetic measurements. The single spinel phase was confirmed for all prepared samples. A proper explanation of data is possible if the Al³⁺ ions are assumed to replace Fe³⁺ ions in the A and B sites simultaneously for NiAl_xFe_2−xO₄ ferrites, and if the Mn²⁺ ions are assumed to replace Ni²⁺ ions in the B sites for Ni_1−yMn_yAl_0.2Fe_1.8O₄ ferrites. Microstructural factors play an important role in the magnetic behavior of Ni_1−yMn_yAl_0.2Fe_1.8O₄ ferrites with large Mn²⁺ content.