Agar-assisted sol-gel synthesis and electrochemical characterization of TiNb2O7 anode materials for lithium-ion batteries

TiNb₂O₇ powders are synthesized via a newly developed agar-assisted sol-gel process for the first time. Phase-pure TiNb₂O₇ powders are obtained upon calcination at 800 °C. On contrast, TiNb₂O₇ powders synthesized via the conventional solid-state method require high calcination temperature at 1100 °C for the complete compound formation. The samples synthesized with agar improve the morphology with submicron-sized particles. The formed porous structure is favorable for enhancing the electrochemical kinetics due to the large contact area between the electrode and the electrolyte. Based on the electrochemical active surface area analysis, the electrical double-layer capacitance of TiNb₂O₇ powders synthesized via both the agar-assisted and the solid-state method is 145 mF cm^?2 and 22 mF cm^?2, respectively. The electrochemical active surface area of the sample prepared via the agar-assisted method is higher than that of the sample prepared via the solid-state method. The TiNb₂O₇ sample synthesized via the agar-assisted process yields 284 mAh g^?1 at 0.1 C, whereas the sample synthesized via the conventional solid-state method yields only 265 mAh g^?1 at 0.1 C. The discharge capacities of the agar-assisted synthesized sample are 205 mAh g^?1 and 174 mAh g^?1 at 5 C and 10 C, respectively. Moreover, the sample exhibits high capacity retention of 91% after 100 discharge-charge cycles at 5 C. Based on the obtained results, the agar-assisted sol-gel process is inferred as one of the facile methods for preparing high performance anode materials for lithium-ion batteries.     

Characterization of vacuum-annealed TiNb2O7 as high potential anode material for lithium-ion battery

Electrochemical properties of mixed titanium-niobium oxide TiNb₂O₇ (TNO) synthesized via vacuum annealing as high potential anode material for lithium-ion batteries were investigated. Crystal structure, size, and morphology are nearly independent of the annealing atmosphere for starting materials but the color of vacuum-annealed TNO (TNO-V) is dark blue while white for the air-annealed one (TNO-A). X-ray photoelectron spectroscopy analysis also indicated that Ti⁴⁺ and Nb⁵⁺ in TNO are partially reduced into Ti³⁺ and Nb⁴⁺ due to the introduction of oxygen vacancy. Electronic conductivity for TNO-V was around 10⁻³ S cm⁻¹ at room temperature and much higher than that for TNO-A (=10⁻¹¹ S cm⁻¹). In electrochemical testing, both TNO-A and TNO-V electrodes showed reversible capacity of 260-270 mAh g⁻¹ at low current density of 0.5 mA cm⁻², while at higher current density of 5.0 mA cm⁻², TNO-V electrode retained higher reversible capacity of 140 mAh g⁻¹ than that for TNO-A electrode (=80 mAh g⁻¹). The enhancement of intrinsic electronic conductivity greatly contributes to improve the rate performance of TNO.     

Microwave-assisted synthesis and electrochemical characterization of TiNb2O7 microspheres as anode materials for lithium-ion batteries

TiNb₂O₇ microspheres are prepared via a microwave-assisted solvothermal method. The microwave irradiation lowers the compound formation temperature to 600°C, and highly crystalline TiNb₂O₇ powders are obtained upon calcination at 800°C. Morphological analysis of the sample shows uniformly distributed microspheres with a particle size of around 1 μm. The Li⁺-ion diffusion coefficient calculated from the electrochemical impedance result is around 1.21 × 10⁻¹³ cm² s⁻¹, which is 1.5 times higher than the sample obtained from the conventional solvothermal method. The TiNb₂O₇ sample derived from microwave yields a high discharge capacity of 299 mA h g⁻¹ at 0.1 C, whereas the sample synthesized via the conventional solvothermal process yields only 278 mA h g⁻¹ at 0.1 C. Excellent rate capabilities such as 220 mA h g⁻¹ at 5 C and 180 mA h g⁻¹ at 10 C are also observed for the microwave-assisted solvothermal sample. Moreover, the sample exhibits a large capacity retention of 95.5% after 100 discharge–charge cycles at 5 C. These results reveal the appropriateness of the microwave-assisted solvothermal process to prepare TiNb₂O₇ powders with superior properties for battery applications.     

Preparation,structural, and characterizations of SnO2-coated TiNb2O7 anode materials for lithium-ion batteries

SnO₂-coated TiNb₂O₇ powders were synthesized via the solution coating method in the present research. The SnO₂ layers with a thickness of 3–5 nm were homogeneously coated on the surface of TiNb₂O₇ particles. TiNb₂O₇ coated with SnO₂ of 5 mol% with high Li⁺ diffusion coefficient delivered the discharge capacity of 319.5 mAh/g, which was 6.6% higher than that of the non-coated samples. The enhancement of capacity for the coated TiNb₂O₇ was owing to the low charge-transfer resistance of 17.5 Ω in contrary to the non-coated TiNb₂O₇ (27.8 Ω). SnO₂-coated TiNb₂O₇ possessed an improved capacity retention of 85.2% at 5 C after 100 cycles, superior to the non-coated TiNb₂O₇ (79.8%). On the other hand, the excessive amounts of SnO₂ coating led to the reduction in the capacity of the prepared samples. The excessive amounts of SnO₂ layers suppressed the Li⁺ diffusion and increased the charge-transfer resistance. The obtained results in this study indicated that coating of TiNb₂O₇ with appropriate amounts of SnO₂ significantly improved the electrochemical performance of TiNb₂O₇.     

Sol‐gel derived TiNb2O7 dielectric thin films for transparent electronic applications

To reduce power consumption of transparent oxide‐semiconductor thin film transistors, a gate dielectric material with high dielectric constant and low leakage current density is favorable. According to previous study, the bulk TiNb₂O₇ with outstanding dielectric properties may have an interest in its thin‐film form. The optical, chemical states and surface morphology of sol‐gel derived TiNb₂O₇ (TNO) thin films are investigated the effect of postannealing temperature lower than 500°C, which is crucial to the glass transition temperature. All films possess a transmittance near 80% in the visible region. The existence of non‐lattice oxygen in the TNO film is proposed. The peak area ratio of non‐lattice oxygen plays an important role in the control of leakage current density of MIM capacitors. Also, the capacitance density and dissipation factor were affected by the indium tin oxide (ITO) sheet resistance at high frequencies. The sample after postannealing at 300°C and electrode‐annealing at 150°C possesses a high dielectric constant (>30 at 1 MHz) and a low leakage current density (<1 × 10^?6 A/cm² at 1 V), which makes it a very promising gate dielectric material for transparent oxide‐semiconductor thin film transistors.     

Dielectric Properties and Relaxation of Bi2Ti2O7

The dielectric properties of Bi₂Ti₂O₇ were explored as a function of temperature and frequency. A comparison between the dielectric response of the well‐known Bi_1.5Zn_0.92Nb_1.5O_6.92 (BZN) pyrochlore and the recently available Bi₂Ti₂O₇ sintered ceramic revealed considerable differences, which indicate that chemical disorder, and not atomic displacement on its own, is chiefly responsible for the dielectric relaxation in bismuth pyrochlores. A low‐frequency (<10 kHz) and relatively high‐temperature (~125 K) dielectric relaxation was observed in Bi₂Ti₂O₇. An Arrhenius function was used to model the relaxation behavior and yielded an activation energy of 0.162 eV and an attempt jump frequency of ~1 MHz. This response is consistent with space charge polarization and not the result of dipolar or ionic disorder.     

K_2B_4O_7-Na_2B_4O_7-Li_2B_4O_7-H_2O四元体系288K相平衡研究

采用等温溶解平衡法研究了K2 B4O7 Na2 B4O7 Li2 B4O7 H2 O四元体系在 2 88K时的相平衡及平衡液相的主要物化性质(密度、电导率、pH值 )。研究发现 :该四元体系为简单共饱和型 ,无复盐及固溶体形成 ,根据溶解度数据绘制了相图 ,相图中有一个共饱点E ,三条单变度曲线E3 E ,E2 E ,E1 E ;三个结晶区平衡固相分别为K2 B4O7·4H2 O ,Na2 B4O7·10H2 O和Li2 B4O7·3H2 O。实验结果表明K2 B4O7对Na2 B4O7有增溶作用 ,并简要讨论了物化性质的变化规律     

磷酸锆基陶瓷材料的制备与性能研究

利用共沉淀法制备了磷酸锆，用差热分析法（DTA）研究了Zr2P2O7的热稳定性情况，并进一步研究了烧结助剂的量、烧结温度等对其性能的影响。     

Li2B4O7-Bi2O3-WO3玻璃的光学性能研究

采用熔融淬冷方法制备了(100-x)Li2B4O7-x(Bi2O3·WO3)(5≤x≤20)玻璃.采用比重计测定了玻璃密度,分光光度计测量了玻璃的吸收光谱,V棱镜折射仪测量了玻璃折射率.结果表明,随着Bi2 O3·WO3含量的增加,玻璃样品的密度和摩尔体积增大,而氧堆积密度减小;玻璃的吸收光谱中截止波长逐渐向长波方向移动,玻璃的间接跃迁光学带隙、Urbach能和费米能逐渐减小,折射率增大.光学性能的变化和玻璃网络中部分桥氧转变为非桥氧有关.     

Reaction Synthesis and Mechanical Properties of Lu4Si2O7N2

Crystallized Lu–Si–O–N phases were believed to be the grain‐boundary (GB) phases that might provide Si₃N₄ with excellent high‐temperature mechanical properties. However, little is known about the intrinsic properties, as well as the synthesis, of the Lu–Si–O–N ceramics. This work reveals the reaction paths of heating Lu₂O₃, SiO₂, and Si₃N₄ powder mixtures (with the stoichiometry of 4:0.96:1) from room temperature to 1600°C. Thereafter, dense Lu₄Si₂O₇N₂ samples are synthesized by in situ reaction/hot‐pressing method, and the mechanical properties at room temperature and elevated temperatures are reported for the first time. The Lu₄Si₂O₇N₂ samples show significant high‐temperature mechanical properties, such as the elastic stiffness remains 77% from room temperature to 1500°C; and bending strength keeps 93% from room temperature to 1400°C. The present results shine a light on Lu₄Si₂O₇N₂ as a promising target GB phase for the optimization of high‐temperature mechanical properties of Si₃N₄.     

Synthesis and Unique Photoluminescence Properties of Eu2Ti2O7 and Eu2TiO5

The europium titania materials, pyrochlore Eu₂Ti₂O₇ and orthorhombic Eu₂TiO₅, were synthesized from a mixture of Eu₂O₃ and TiO₂ using the solid‐state reaction method. The structural and optical properties of these titania materials were investigated using X‐ray diffraction analysis, photoluminescence (PL) analysis, PL excitation (PLE) spectroscopy, and diffuse reflectance spectroscopy. Temperature dependence of the PL intensity was measured between T = 20 and 450 K and analyzed on the basis of various theoretical models. A remarkable increase in the PL intensity with increasing T was observed in these titania materials at higher temperatures, above ~140 K, and well explained by a trap/reservoir model. Interestingly, a dramatic decrease in the electric‐dipole emission component relative to the magnetic‐dipole one was observed in Eu₂Ti₂O₇ above T ~ 140 K. The schematic energy‐level diagram for Eu³⁺ in the Eu₂Ti₂O₇ host was proposed for the sake of a better understanding of the PL and PLE processes in this type of phosphorescent material.     

Synthesis and Properties of Bi2Fe4O9 with FeCl2·6H2O Addition

Single‐phase Bi₂Fe₄O₉ is synthesized successfully by the solid‐state reaction technique with FeCl₂·6H₂O additives. The two optical band gaps, and the iron ion's chemical valence state both decrease with the addition of FeCl₂·6H₂O, which may be attributed to the decrease in the split energy of iron 3d orbital.     

Enhancement of Curie temperature and photoluminescent properties of Nd2Mo2O7, Ho2Mo2O7, and Gd2Mo2O7

For Nd_1.90Er_0.10Mo₂O₇, Ho_1.90Er_0.10Mo₂O₇, and Gd_1.98Eu_0.02Mo₂O₇ nominal compositions, oxide compounds were prepared by solid-state reaction method. X-ray diffraction measurements show that molybdenum oxides formed cubic phase except for Nd_1.90Er_0.10Mo₂O₇. Scanning electron microscopy was used to observe their microstructure and elemental analysis. The photoluminescence properties of the materials, the excitation and emission spectra and the decay curve, were investigated using photoluminescence spectrophotometer. Dielectric and piezoelectric properties of the ptoluminescent materials, the dielectric constant, dielectric loss, piezoelectric constant, and Curie temperature, were measured for electrical characterization. Nd_1.90Er_0.10Mo₂O₇, Ho_1.90Er_0.10Mo₂O₇, and Gd_1.98Eu_0.02Mo₂O₇ materials show both photoluminescence and piezoelectric properties.     

Bi2Sn2O7粉体微波水热法制备及其吸附-可见光催化性能研究

以物质的量比为1∶1的Bi(NO3)3.5H2O和Na2SnO3.3H2O为原料,采用微波水热法合成Bi2Sn2O7粉体。采用XRD、SEM对粉体的晶相和形貌进行了表征,并以甲基橙为目标污染物,测试粉体的光催化性能。研究了合成时间、合成温度等因素对样品催化性能的影响,同时以KI、NBT及乙腈作为抑制剂,检测光催化过程产生的活性物质。结果表明:200℃、30 min微波水热合成的2 g/L Bi2Sn2O7粉体在暗光条件下搅拌2 h,对20 mg/L甲基橙溶液的吸附率为40%,将其置于250 W的卤素灯下光照2 h,甲基橙溶液的脱色率达95%以上。活性物种的检测表明光催化过程产生的主要活性物种为·OH和·O2-。     

High‐Energy Mechanical Treatment Boosts Ion Transport in Nanocrystalline Li2B4O7

In many cases fast solid ion conductors are characterized by a large number fraction of defects and vacant positions that enable the ions to move over long distances in a facile way. The introduction of structural disorder via high‐energy mechanical impact represents a very promising possibility to improve and to tune the transport properties of otherwise poorly conducting solids. Lithium tetraborate, Li₂B₄O₇, in its single crystalline form or with an average crystallite size in the μm range, is known as a very poor Li ion conductor and can serve as a model compound to study the influence of structural disorder on ion dynamics. In the present study, we used high‐energy ball milling to prepare nanocrystalline defect‐rich Li₂B₄O₇ characterized by a mean crystallite diameter of ca. 20 nm. With increasing milling time the sample became partly amorphous. Polycrystalline Li₂B₄O₇ with crystallite sizes in the order of 100 nm served as starting material. The nanostructured samples obtained show dc conductivities σ_dc in the order of 2.5 × 10^?7 S/cm at 490 K which represents an increase by more than four orders of magnitude compared to the source material. While conductivity spectroscopy was applied to study the effect of different milling times on ionic conductivity in detail; Li ion self‐diffusion in nanostructured Li₂B₄O₇ as well as in the starting material was investigated by variable‐temperature solid‐state ⁷Li nuclear magnetic resonance (NMR) relaxometry. While the first is sensitive to long‐range ion transport, lithium NMR is able to access also short‐ranged ion motions.     

Semiconductive and Redox Properties of Ti and Zr Pyrophosphate Catalysts (TiP2O7 and ZrP2O7). Consequences for the Oxidative Dehydrogenation of n-Butane

The electrical conductivity of titanium and zirconium pyrophosphates used as catalysts in n-butane oxidative dehydrogenation has been measured under oxygen and n-butane at 400 and 500 °C and under subsequent exposures to both gases at the catalytic reaction temperature. The two compounds appeared to be p-type semiconductors under air with positive holes as the main charge carriers but became n-type when contacted with n-butane. If their conductivities are comparable as p-type semiconductors (within one order of magnitude), by contrast, they differ by 3 orders of magnitude when being n-type semiconductors. These results explain the difference in catalytic reaction mechanism encountered on the two solids. The alkane activation was proposed to be related in both cases to the p-type semiconducting properties of the solids, likely through hydrogen abstraction by a surface O^- species, forming a C₄H₉ radical which will similarly undergo a second hydrogen abstraction to form butenes. The changes in activation energy and in selectivity on TiP₂O₇ at higher temperatures (>450 C) are indicative of a change in mechanism, possibly with the transient formation of an alkoxide species.     

Gd2Zr2O7 and Nd2Zr2O7 pyrochlore prepared by aqueous chemical synthesis

Pyrochlore structured Gd₂Zr₂O₇ and Nd₂Zr₂O₇ are produced via complex precipitation processing. A suite of characterization techniques, including FTIR, Raman, X-ray and electron diffraction, TEM, SEM as well as nitrogen sorption are employed to investigate the structural and grain size evolution of the synthesized and calcined powder. Results show that Gd₂Zr₂O₇ with the pyrochlore structure are produced after calcination at 1400 °C for 12 h while Nd₂Zr₂O₇ has already formed the pyrochlore structure at 1200 °C. This method allows the formation of dense materials at relatively low temperature, with bulk densities over 92% of the theoretical values achieved after sintering at 1400 °C for 50 h. This unique aqueous synthetic method provides a simple pathway to produce pyrochlore lanthanide zirconate without using either organic solvent and/or mechanical milling procedures, making the synthesis protocol an attractive potential scale-up production of highly refractory ceramics.     

Phase Evolution and Microstructural Studies in CaZrTi2O7–Nd2Ti2O7 System

A series of compositions with general stoichiometry Ca_1?xZr_1?xNd_2xTi₂O₇ has been prepared by high‐temperature solid‐state reaction of component oxides and characterized by powder X‐ray diffraction and electron probe for microanalyses (EPMA). The phase fields in CaZrTi₂O₇–Nd₂Ti₂O₇ system and distribution of ions in different phases have been determined. Four different phase fields, namely monoclinic zirconolite, cubic perovskite, cubic pyrochlore, and monoclinic Nd₂Ti₂O₇ structure types are observed in this system. The 4M‐polytype of zirconolite structure is stabilized by substitution of Nd³⁺ ion. The addition of Nd³⁺ ions form a cubic perovskite structure‐type phase and thus observed in all the compositions with 0.05 ≤ x ≤ 0.80. Cubic pyrochlore structure‐type phase is observed as a coexisting phase in the nominal composition with 0.20 ≤ x ≤ 0.90. Only a subtle amounts of Ca²⁺ and Zr⁴⁺ are incorporated into the perovskite‐type Nd₂Ti₂O₇ structure. EPMA analyses on different coexisting phases revealed that the pyrochlore and perovskite phases have Nd³⁺‐rich compositions.     

HLaNb2O7/(Pt,TiO2)的制备和光催化性能研究

用固相合成法合成出KLaNb2O7,酸化得到HLaNb2O7;通过正己胺层间膨胀,氯铂酸铵、TiO2层间插入和紫外光分解等反应,合成出一种新的层状光催化纳米复合材料HLaNb2O7／(Pt,TiO2).采用XRD、DRS对所制备的材料进行了表征。以甲基橙为目标降解物,考察了所制备的材料的光催化性能,结果表明所制备的HLaNb2O7／(Pt,TiO2)纳米复合材料对甲基橙具有较高的光降解效率并且其光催化活性稳定、持久。