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.     

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.     

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.     

Al掺杂的尖晶石型LiMn2O4的结构和电子性质

采用基于密度泛函理论的第一性原理方法, 在广义梯度近似(GGA)和GGA+U方法下对尖晶石型LiMn₂O₄及其Al掺杂 的尖晶石型LiAl_0.125Mn_1.875O₄晶体的结构和电子性质进行了计算. 结果表明: 采用GGA方法得到尖晶石型LiMn₂O₄是立方晶系结构, 其中的Mn离子为+3.5价, 无法解释它的Jahn-Teller 畸变. 给出的LiMn₂O₄能带结构特征也与实验结果不符. 而采用GGA+U方法得到在低温下的LiMn₂O₄和其掺杂 体系LiAl_0.125Mn_1.875O₄的晶体都是正交结构, 与实验一致. 也能明确地确定Mn的两种价态Mn³⁺/Mn⁴⁺的分布并且能够说明Mn³⁺O₆的z方向有明显的Jahn-Teller 畸变, 而Mn⁴⁺O₆则没有畸变. LiMn₂O₄的能带结构与实验比较也能够符合. 采用GGA+U方法对Al掺杂体系的LiAl_0.125Mn_1.875O₄的研究表明, 用Al替换一个Mn不会明显地改变晶体的电子性质, 但可以有效地消除Al³⁺O₆ 八面体的Jahn-Teller畸变, 从而改善正极材料LiMn₂O₄的性能, 这与电化学实验的观察结果相一致.     

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.     

Atomic force microscopy study of surface morphology change in spinel LiMn2O4: Possibility of direct observation of Jahn-Teller instability

Surface morphology in 3.5 × 3.5 μm² area of spinel LiMn₂O₄, which is a typical cathode material for Li ion secondary batteries, is studied using an atomic force microscopy (AFM) with a conductive probe. Negative bias voltage is applied to the probe to attract Li⁺ ions toward LiMn₂O₄ surface during the AFM observation. Before applying the voltage (0 V), the whole LiMn₂O₄ surface is covered with scale-shaped grains. Under the negative voltage of 5.5 V, electric current abruptly increases, indicating Li⁺ ionic conduction. Simultaneously, part of the scale-shaped grains expand and flatten. Jahn-Teller phase transition, which is induced by the repulsive interaction between the Mn-e_g and O-2p electrons in Li accumulated layer, is proposed as a possible origin of these results.     

Conductivity and water uptake of Sr4(Sr2Nb2)O11·nH2O and Sr4(Sr2Ta2)O11·nH2O

The hydrated oxygen deficient complex perovskite-related materials Sr₄(Sr₂Nb₂)O₁₁·nH₂O and Sr₄(Sr₂Ta₂)O₁₁·nH₂O were studied at high water vapour pressures over a large temperature range by electrical conductivity measurements, thermogravimetry (TG), and X-ray powder diffraction (XRPD). In humid atmospheres both materials are known to exhibit protonic conductivity below dehydration temperatures, with peak-shaped maxima at about 500 °C. In this work we show that the peaks expand to plateaus of high conductivity from 500 to 700 °C at a water vapour pressure of 1 atm. However, in situ synchrotron XRPD of Sr₄(Sr₂Nb₂)O₁₁·nH₂O as a function of temperature shows that these observations are in fact coincident with melting and dehydration of a secondary phase Sr(OH)₂. The stability of Sr₄(Sr₂Nb₂)O₁₁·nH₂O and Sr₄(Sr₂Ta₂)O₁₁·nH₂O in humid atmospheres is thus insufficient, causing decomposition into perovskites with lower Sr content and SrO/Sr(OH)₂ secondary phases. This, in turn, rationalizes the observation of peaks and plateaus in the conductivity of these materials.     

Preparation of TiO2-coated LiMn2O4 by carrier transfer method

The TiO₂-coated LiMn₂O₄ has been prepared by a carrier transfer method and investigated. This novel synthetic method involved the transfer of TiO₂ into the surface of LiMn₂O₄ with Vulcan XC-72 active carbon powders as a dispersant. The X-ray diffraction shows that spinel structure of materials does not change after the coating of TiO₂. The electrochemical performance tests show that the initial discharge capacity of TiO₂-modified LiMn₂O₄ is 111.5 mA h g⁻¹, which is better than that of pristine LiMn₂O₄ (103.8 mA h g⁻¹). The cyclic performance is significantly improved after surface modification. The TiO₂-modified LiMn₂O₄ by a carrier transfer method exhibits better discharge capability and lower resistance.     

Chemical synthesis of spinel nickel ferrite (NiFe2O4) nano-sheets

The present investigation is related to the deposition of single-phase nano-sheets spinel nickel ferrite (NiFe₂O₄) thin films onto glass substrates using a chemical method. Nano-sheets nickel ferrite films were deposited from an alkaline bath containing Ni²⁺ and Fe²⁺ ions. The films were characterized for their structural, surface morphological and electrical properties by means of X-ray diffraction (XRD), transmission electron microscope (TEM), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), and two-point probe electrical resistivity techniques. The X-ray diffraction pattern showed that NiFe₂O₄ nano-sheets are oriented along (3 1 1) plane. The FT-IR spectra of NiFe₂O₄ films showed strong absorption peaks around 600 and 400 cm⁻¹ which are typical for cubic spinel crystal structure. Microstructural study of NiFe₂O₄ film revealed nano-sheet like morphology with average sheet thickness of 30 nm. The room temperature electrical resistivity of the NiFe₂O₄ nano-sheets was 10⁷ Ω cm.     

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.     

Preparation and electrical properties of a pyrochlore-related Ce2Zr2O8−x phase

A pyrochlore-related Ce₂Zr₂O_8−x phase has been prepared in a reduction reoxidation process from Ce_0.5Zr_0.5O₂ powders. Ce₂Zr₂O_8−x, based on a cubic symmetry with a=1.053 nm, decomposes in nitrogen at 800 °C, but remains stable up to 900 °C in air. It shows mixed oxygen ionic and electronic conductivity. The bulk conductivity at 700 °C is 4×10⁻⁴ S cm⁻¹ in air and 1×10⁻² S cm⁻¹ in nitrogen, and the activation energy is 1.27 eV in air. In nitrogen, the Arrhenius law is not obeyed, and a curved plot was obtained from 400 to 700 °C; then, the conductivity decreased rapidly due to the thermal decomposition of Ce₂Zr₂O_8−x.     

Influence of postdeposition annealing on structural properties and electrical characteristics of thin Tm2O3 and Tm2Ti2O7 dielectrics

We describe the structural properties and electrical characteristics of thin thulium oxide (Tm₂O₃) and thulium titanium oxide (Tm₂Ti₂O₇) as gate dielectrics deposited on silicon substrates through reactive sputtering. The structural and morphological features of these films were explored by X-ray diffraction, X-ray photoelectron spectroscopy, secondary ion mass spectrometry, and atomic force microscopy, measurements. It is found that the Tm₂Ti₂O₇ film annealed at 800 °C exhibited a thinner capacitance equivalent thickness of 19.8 Å, a lower interface trap density of 8.37 × 10¹¹ eV⁻¹ cm⁻², and a smaller hysteresis voltage of ∼4 mV than the other conditions. We attribute this behavior to the Ti incorporated into the Tm₂O₃ film improving the interfacial layer and the surface roughness. This film also shows negligible degrees of charge trapping at high electric field stress.     

Cathode luminescence characteristics of ZnGa2O4 phosphor thin films with the doped activator

The zincgallate (ZnGa₂O₄) phosphor thin film was grown using RF magnetron sputtering system at various process parameters. A ZnGa₂O₄ phosphor thin film was deposited on Si(1 0 0) substrate and annealed by a rapid thermal processor (RTP). The X-ray diffractometer (XRD) patterns indicate that the Mn-doped ZnGa₂O₄ phosphor thin film shows a (3 1 1) main peak and a spinel phase. A ZnGa₂O₄ phosphor thin film has better crystallization due to increased substrate, annealing temperature and deposition time. Also the ZnGa₂O₄:Mn phosphor thin film shows green emission (510 nm, ⁴T₁→⁶A₁), and the ZnGa₂O₄:Cr phosphor thin film shows red emission (705 nm, ⁴A₂→⁴T₂).     

Influence of ceramic oxide materials on lithium based potentiometric CO2 sensors

In these potentiometric sensors, a mixed binary carbonate sensing material consisting of 90 mol% Li₂CO₃ and 10 mol% BaCO₃ was modified by adding ceramic oxide materials such as SiO₂, B₂O₃, La₂O₃, Bi₂O₃, CeO₂ and In₂O₃ in different mol% concentrations. Various sensors mixed with these external oxides have shown good performance at operating temperatures below 300 °C. Scanning electron microscopy reveals that a glassy sensing phase is formed in the sensing bi-carbonate by the addition of ceramic oxides and sintered at 600 °C for 1 h. The sensor mixed with SiO₂:B₂O₃:Bi₂O₃ in 1:2:1 mol% showed an excellent response and recovery characteristics and followed a fair Nernstian behavior with a ΔEMF/dec value of −48.18 at as low as 150 °C. The decrease in operating temperature is attributed to the enhanced lithium ion migration through the glassy sensing phase of the sensing electrode.     

Preparation, characterization and electrochemical performance of γ-MnO2 and LiMn2O4 as cathodes for lithium batteries

The spinel LiMn₂O₄ is a very promising cathode material with economical and environmental advantages. LiMn₂O₄ materials have been synthesized by solid state method using γ-MnO₂ as manganese source, and Li₂CO₃ or LiNO₃ as Li sources. γ-MnO₂ is a commercial battery grade electrolytic manganese dioxide (TOSOH-Hellas GH-S) and LiMn₂O₄ samples were synthesized at a calcinations temperature up to 800 °C. γ-MnO₂ and LiMn₂O₄ samples were characterized by X-ray diffraction, thermal and electrochemical measurements. X-ray powder diffraction of as prepared LiMn₂O₄ showed a well-defined highly pure spinel single phase. The electrochemical performance of LiMn₂O₄ and its starting material γ-MnO₂ was evaluated through cyclic voltammetry, galvanostatic (constant current charge-discharge cycling) The electrochemical properties in terms of cycle performance were also discussed. γ-MnO₂ showed fairly high initial capacity of about 200 mAhg⁻¹ but poor cycle performance. LiMn₂O₄ samples showed fairly low initial capacity but good cycle performance.     

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.     

Effect of BiFeO3 addition on Bi2O3–ZnO–Nb2O5 based ceramics

In this paper, low temperature sintering of the Bi₂(Zn_1/3Nb_2/3)₂O₇ (β-BZN) dielectric ceramics was studied with the use of BiFeO₃ as a sintering aid. The effects of BiFeO₃ contents and the sintering temperature on the phase structure, density and dielectric properties were investigated. The results showed that the sintering temperature could be decreased and the dielectric properties could be retained by the addition of BiFeO₃. The structure of BiFeO₃ doped β-BZN was still the monoclinic pyrochlore phase. The sintering temperature of BiFeO₃ doped β-BZN ceramics was reduced from 1000 °C to 920 °C. In the case of 0.15 wt.% BiFeO₃ addition, the β-BZN ceramics sintered at 920 °C exhibited good dielectric properties, which were listed as follows: ε_r = 79 and tan δ = 0.00086 at a frequency of 1 MHz. The obtained properties make this composition to be a good candidate for the LTCC application.     

Prepare high efficiency Mn-doped Zn2SiO4 green phosphors in air using nano-particles

In this study, nano-scale precursors of ZnO, SiO₂, and MnO₂ powders were used to prepare mixtures with the compositions of 2ZnO+SiO₂+X mol% MnO₂ (X=MnO₂/2ZnO, abbreviated as Zn₂SiO₄-X-MnO₂), where 2≤X≤5. The mixed Zn₂SiO₄-X-MnO₂ mixtures were calcined from 900 to 1300 °C in air in order to synthesize Zn₂SiO₄:Mn²⁺ green phosphors. The X-ray diffraction patterns of Zn₂SiO₄-X-MnO₂ particles indicated that ZnO was present in the 900 °C-calcined Zn₂SiO₄-X-MnO₂ phosphors, but not in particles calcined at temperatures of 1000 °C and higher. However, the unapparent secondary phase of ZnMnO₃ was found in the 1200 and 1300 °C-calcined Zn₂SiO₄-5-MnO₂ compositions. The luminescent characteristics of Zn₂SiO₄-X-Mn²⁺ phosphors were compared with that of a commercial product (Nichia Corp., Japan). The photoluminescence (PL) intensity of 1200 °C-calcined Zn₂SiO₄-4-MnO₂ phosphors was higher and the decay times of all synthesized Zn₂SiO₄-X-MnO₂ phosphors were longer than those of the commercial product.     

Anodization of aluminium thin films on pSi and annihilation of strong luminescence from Al2O3

Photoluminescence (PL) of Al₂O₃ films obtained by anodization of thermally evaporated and annealed thin Al films on p⁺⁺Si in 0.3 M oxalic acid has been investigated. Thermal annealing at 200–950 °C under the dry nitrogen atmosphere was used for deactivation of luminescence centres. Luminescence from as grown films was broad and located at 425 nm. This luminescence reached to highest level after annealing at 600 °C. Maximum 10 min was required for full optical activation and prolonged annealing up to 4 h did not change the luminescence intensity. Because of deep levels, absorption band edge of as grown films was shifted to the lower energy which is 3.25 eV. Annealing above 800 °C reduced the PL intensity and this observation was correlated with the blue shift of band edge as the defects annealed out. Disappearing PL intensity and blue shift of band edge absorption after annealing at 950 °C was mainly attributed to the oxygen-related defects and partly to impurities that may be originated from oxalic acid. AFM results did not show any hexagonally ordered holes but uniformly distributed nanosized Al₂O₃ clusters that were clearly seen. XRD measurements on as grown Al₂O₃ showed only [1 1 0] direction of α phase. Debye–Scherer calculation for this line indicates that cluster size is 35.7 nm. XRD and AFM pictures suggest that nanocrystalline Al₂O₃ are embedded in amorphous Al₂O₃.     

The effect of Si surface nitridation on the interfacial structure and electrical properties of (La2O3)0.5(SiO2)0.5 high-k gate dielectric films

Thermal stability, interfacial structures and electrical properties of amorphous (La₂O₃)_0.5(SiO₂)_0.5 (LSO) films deposited by using pulsed laser deposition (PLD) on Si (1 0 0) and NH₃ nitrided Si (1 0 0) substrates were comparatively investigated. The LSO films keep the amorphous state up to a high annealing temperature of 900 °C. HRTEM observations and XPS analyses showed that the surface nitridation of silicon wafer using NH₃ can result in the formation of the passivation layer, which effectively suppresses the excessive growth of the interfacial layer between LSO film and silicon wafer after high-temperature annealing process. The Pt/LSO/nitrided Si capacitors annealed at high temperature exhibit smaller CET and EOT, a less flatband voltage shift, a negligible hysteresis loop, a smaller equivalent dielectric charge density, and a much lower gate leakage current density as compared with that of the Pt/LSO/Si capacitors without Si surface nitridation.