非水系液流电池多孔电极内反应传输过程模拟

低共熔溶剂作为一种非水系电解液,在提高液流电池能量密度方面展现出极大的发展潜力。本文针对以低共熔溶剂为电解液的非水系全铜液流电池内反应传输行为,建立了考虑电解液流动、铜离子组分传质、电荷传输以及电化学反应过程的孔隙尺度多松弛时间格子玻尔兹曼模型。在此基础上,以随机重构获得的三维多孔电极结构为模拟对象,数值研究了这种非水系液流电池正极恒电流充电过程中孔隙尺度的耦合反应传输过程,分析了充电电流密度及运行温度对非水系全铜液流电池运行性能的影响规律。     

基于LBM的多孔陶瓷有效热导率和相变传热分析

多孔氮化铝陶瓷具有高导热、耐熔盐腐蚀等优点在相变材料封装方面具有较大应用前景。本文采用微米计算机断层扫描(CT)得多孔氮化铝的灰度图像,并精准重构三维孔隙结构。并采用四参数随机生长、堆积颗粒、维诺结构等数值方法建立结构模型。基于介观尺度的格子玻尔兹曼方法 (LBM),计算了多孔陶瓷的有效热导率,结果表明维诺结构与CT真实重构的热导率接近。采用三维焓法LBM模拟了多孔骨架内的相变传热过程,获得了孔隙尺度对流相变的传热机理,并分析了不同孔隙率时的储热密度和复合材料的有效热导率。基于CT扫描和LBM的相变传热模拟有助于快速分析骨架的热特性,为多孔骨架的设计提供理论基础。     

具有嵌入式流道的贯穿电极热再生电池性能特性

多孔电极内物质传输对热再生电池性能至关重要,而传质较佳的穿透电极内仍然存在物质分布不均,为此构建嵌入式流道以强化物质均匀分布和传输。本文研究了嵌入式流道形式及数量、电解液流量对穿透电极热再生电池物质传输及产电性能的影响。研究结果表明,由于交错型流道具有较佳的物质传输和较均匀的物质分布,电池获得了最高功率(10.0 m W)、最大产电量(1929.1 C)和最高的相对卡诺效率(10.2%)。在一定范围内交错型流道数量越多致使物质分布越均匀,电池最大功率越高。此外,电池最大功率随着电解液流量的增加逐渐增大,但增大速率逐渐减小,最佳电解液流量为30 mL/min。     

钙钛矿太阳能电池中电子传输材料的研究进展

有机-无机杂化的卤素钙钛矿材料在2009年首次应用在光伏器件中, 而后有关此类型太阳能电池的报道数量呈井喷式增长. 至2014年5月钙钛矿电池光电转化效率已接近20%, 已超过有机及染料敏化太阳能电池的效率, 且有望达到单晶硅太阳能的水平, 成为光伏发电领域中的希望之星. 在钙钛矿电池中, 电子传输材料与吸收层的电子选择性接触对提高光电转化效率起到重要作用, 尤其在正置结构器件中, 电子传输层的介观结构直接影响钙钛矿的生长情况. 同时, 电子传输层的化学性质及其界面也会对电池的稳定性和寿命产生影响. 本文总结了电子传输材料在该类电池中的研究现状和热点, 并按材料的化学组分不同, 将电子传输材料分为三类: 金属氧化物、有机小分子和复合材料, 详细地介绍了电子传输材料在钙钛矿太阳能电池中的作用和近来的最新进展.     

电极活性材料精细结构与MH-Ni电池性能关系研究的一些进展

为了更深入地研究正负极活性材料精细结构与MH-Ni电池性能之间的关系,本文在介绍了简单而较适用的处理X射线衍射数据的方法之后,系统综述了电池活化前后、循环过程中正负极活性物质结构和微结构与电池性能之间关系研究的一些进展.主要包括:(1)具有适当晶粒大小和较大层错几率的βNi(OH)2物质,能获得较大的充放电容量;(2)没有观测到MH/Ni电池在充放电过程中有β-Ni(OH)2 β-NiOOH的相变,只有满充和过充电时才发生部份β-Ni(OH)2 γNiOOH的相变;MH/Ni电池的物理导电机制是在正负极活性物质中嵌入和脱嵌的氢离子形成固相质子在电极间定向运动.(3)循环性能的衰减、内阻、容量的变化与正负极活性物质的微结构变化有良好对应关系.微结构变化消耗电解液,并改变电解液的性能.正极、负极和电解液三者的共同作用是循环性能衰减的主导原因.(4)正极添加剂与电池性能之间的关系.由于正极添加剂Lu2O3和CaF2能抑制正极活性物质的微晶细化、减缓总的层错几率降低,对于储氢合金能抑制品粒增大,特别能抑制A(0H)3和B的析出,故能提高了电池的循环性能和寿命.(5)电池储存前后的容量衰减和内阻增加是其在储存过程中CoOOH析出和晶粒细化双重作用的结果.     

金属氧化物修饰对介孔纳米TiO2电极的染料敏化太阳能电池电化学性能的影响

将介孔TiO₂纳米粒子(m-TiO₂)多孔膜电极浸入相应的金属硝酸盐的500 oC热处理修饰金属氧化物(如Mg、ZnO、Al₂O₃或NiO).结果表明,金属氧化物修饰均可形成能垒对m-TiO₂膜电极的界面电荷传输过程产生影响,但外加偏压下其膜内电子传输和界面电荷复合均明显依赖于修饰氧化物的种类及其存在形态. 金属氧化物修饰的膜电极在电子传输和界面复合方面的变化与DSSCs的电流-电压特性曲线的变化规律具有明显的相关性,可不同程度地提高电池的光电压,而MgO、ZnO和NiO修饰的电池效率分别提高了23%、13%和6%. 上述结果表明调控电池的本征参数可以改善TiO₂-基DSSCs的性能.     

膜生物反应器内生化反应过程的格子Boltzmann模拟

采用介观尺度的格子Boltzmann数值算法研究平板式膜生物反应器内生物膜结构对生化反应及流动传输的影响.利用四参数随机生成法重构不同多孔结构的生物膜,计算中耦合多块模型获得局部详细信息且提高计算效率.结果表明：生物膜结构稳定条件下,孔隙率增大有利于主流区与生物膜内的物质传输,可以提高底物降解效率;孔隙率一定条件下,改变生物膜的孔隙结构分布可以加强传质,强化生化反应过程,提高底物降解效率.     

电极活性材料精细结构与MH-Ni电池性能关系研究的一些进展

为了更深入地研究正负极活性材料精细结构与MH-Ni电池性能之间的关系,本文在介绍了简单而较适用的处理X射线衍射数据的方法之后,系统综述了电池活化前后、循环过程中正负极活性物质结构和微结构与电池性能之间关系研究的一些进展。主要包括:(1)具有适当晶粒大小和较大层错几率的β-Ni(OH)2物质,能获得较大的充放电容量;(2)没有观测到MH/Ni电池在充放电过程中有β-Ni(OH)2β-NiOOH的相变,只有满充和过充电时才发生部份β-Ni(OH)2γ-NiOOH的相变;MH/Ni电池的物理导电机制是在正负极活性物质中嵌入和脱嵌的氢离子形成固相质子在电极间定向运动。(3)循环性能的衰减、内阻、容量的变化与正负极活性物质的微结构变化有良好对应关系。微结构变化消耗电解液,并改变电解液的性能。正极、负极和电解液三者的共同作用是循环性能衰减的主导原因。(4)正极添加剂与电池性能之间的关系。由于正极添加剂Lu2O3和CaF2能抑制正极活性物质的微晶细化、减缓总的层错几率降低,对于储氢合金能抑制晶粒增大,特别能抑制A(OH)3和B的析出,故能提高了电池的循环性能和寿命。(5)电池储存前后的容量衰减和内阻增加是其在储存过程中CoOOH析出和晶粒细化双重作用的结果。     

广义平面应变锂离子电池柱形梯度材料颗粒电极中扩散诱导应力分析

柱形梯度材料是最有潜力的锂离子电池电极之一. 为了研究恒压充电过程中柱形梯度材料颗粒电极下力学机理, 以Li_1.2(Mn_0.62Ni_0.38)_0.8O₂为例, 讨论弹性模量、扩散系数和偏摩尔体积三个重要材料参数对应力场影响. 并推导出非均匀柱形颗粒电极的扩散方程和力学方程. 结果表明, 柱形梯度材料纳米电极, 沿着半径方向Mn 的材料组分升高Ni 的材料组分降低, 其材料结构有利于降低最大径向应力和环向拉应力, 有效地避免电极的力学失效现象. 并根据计算结果, 对梯度材料电极提出材料结构优化建议.     

摇摆型棘齿热电子隧穿制冷

本文利用传输矩阵法数值模拟了电子在加入周期性正负偏压的双势垒("摇摆"棘齿)异质结中的传输特性,获得了两电子库间的净电流以及所伴随的净热流的表达式.进一步分析了净电流、净热流和制冷系数等性能参数的特征,获得的结果对介观热电子器件的研制有一定的理论指导意义. 关键词： 热电子制冷机 摇摆棘齿 性能参数 传输矩阵法     

Experimental routes to in situ characterization of the electronic structure and chemical composition of cathode materials for lithium ion batteries during lithium intercalation and deintercalation using photoelectron spectroscopy and related techniques

Three different experimental routes to in situ characterization of electronic structure and chemical composition of thin film cathode surfaces used in lithium ion batteries are presented. The focus is laid on changes in electronic structure and chemical composition during lithium intercalation and deintercalation studied by photoelectron spectroscopy and related techniques. At first, results are shown obtained from spontaneous intercalation into amorphous or polycrystalline V₂O₅ thin films after lithium deposition. Although this technique is simple and clean, it is nonreversible and only applicable to the first lithium intercalation cycle into the cathode only to be applied to host materials stable in the delithiated stage. For other cathode materials, as LiCoO₂, a real electrochemical setup has to be used. In our second approach, the experiments are performed in a specially designed electrochemical cell directly connected to the vacuum system. First experimental results of RF magnetron sputtered V₂O₅ and LiCoO₂ thin film cathodes are presented. In the third approach, an all solid-state microbattery cell must be prepared inside the vacuum chamber, which allows electrochemical processing and characterization by photoelectron spectroscopy in real time. We will present our status and experimental difficulties in preparing such cells.     

Finite element modelling of ion transport in the electrolyte of a 3D-microbattery

A mathematical model describing ionic transport in a 3D-microbattery (3D-MB) electrolyte is developed here using finite element methodology. The model is then exploited to study a 3D-MB based on an interdigitated plate (“trench”) architecture for a 10 μm-thick electrolyte layer separating 10 μm-thick graphite anode and LiCoO₂ cathode plates. The effect of varying plate length, end-shape and electronic conductivity is also modelled. It is shown that the 3D-MB architecture gives rise to qualitatively non-uniform current densities, leading to sub-optimal surface utilization. This can, in turn, be optimized by varying electrode geometries and/or material properties.     

Investigation of the solid-state electrolyte/cathode LiPON/LiCoO<Subscript>2</Subscript> interface by photoelectron spectroscopy

The electronic structure of a solid electrolyte/solid electrode interface (SESEI) of an all-solid-state thin film battery was investigated. The thin film battery consisted of a LiPON solid electrolyte and a LiCoO₂ cathode. The lithium phosphorus oxynitride (LiPON) electrolyte was RF sputtered in a step-by-step procedure onto the cathode and investigated by photoelectron X-ray-induced spectroscopy after each deposition step. An intermediate layer was found—composed of some new species—that differs in its chemical composition from the cathode as well as the LiPON solid electrolyte material and changes with growing layer thickness. In contrast, the electronic structure of the underlying cathode material remained predominantly unchanged.     

Investigation of the synergetic effects of LiBF<Subscript>4</Subscript> and LiODFB as wide-temperature electrolyte salts in lithium-ion batteries

Herein, we present the use of lithium tetrafluoroborate (LiBF₄) as an electrolyte salt for wide-temperature electrolytes in lithium-ion batteries. The research focused on the application of blend salts to exhibit their synergistic effect especially in a wide temperature range. In the study, LiCoO₂ was employed as the cathode material; LiBF₄ and lithium difluoro(oxalate)borate (LiODFB) were added to an electrolyte consisting of ethylene carbonate (EC), propylene carbonate (PC), and ethyl methyl carbonate (EMC). The electrochemical performance of the resulting electrolyte was evaluated through various analytical techniques. Analysis of the electrical conductivity showed the relationship among solution conductivity, the electrolyte composition, and temperature. Cyclic voltammetry (CV), charge-discharge cycling, and AC impedance measurements were used to investigate the capacity and cycling stability of the LiCoO₂ cathode in different electrolyte systems and at different temperatures. Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) were applied to analyze the surface properties of the LiCoO₂ cathode after cycling. The results indicated that the addition of a small amount of LiODFB into the LiBF₄-based electrolyte system (LiBF₄/LiODFB of 8:2) may enhance the electrochemical performance of the LiCoO₂ cell over a relatively wide temperature range and improve the cyclability of the LiCoO₂ cell at 60 °C.     

Effect of mesoporous carbon containing binary conductive additives in lithium ion batteries on the electrochemical performance of the LiCoO2 composite cathodes

Binary conductive additives (BCA), formed by sonication of mesoporous carbon (MC) and acetylene black (AB), were used as conductive additives to improve the electrochemical performance of a LiCoO₂ composite cathode. The electrochemical performance of the LiCoO₂ composite cathode dispersed with BCA was investigated. The results showed that the electrochemical performance (including the discharge capacity, the discharge voltage and the total internal resistance) of a BCA loaded LiCoO₂ composite cathode was better than that of a cathode loaded with AB. The possible mechanism is that the MC in BCA can adsorb and retain electrolyte solution, which allows an intimate contact between the lithium ions and the cathode active material LiCoO₂ due to its large mesopore specific surface area. A simplified model was also proposed.     

Pseudo-capacitive properties of LiCoO<Subscript>2</Subscript>/AC electrochemical capacitor in various aqueous electrolytes

LiCoO₂ particles were synthesized by a sol-gel process. X-ray diffraction analysis reveals that the prepared sample is a single phase with layered structure. A hybrid electrochemical capacitor was fabricated with LiCoO₂ as a positive electrode and activated carbon (AC) as a negative electrode in various aqueous electrolytes. Pseudo-capacitive properties of the LiCoO₂/AC electrochemical capacitor were determined by cyclic voltammetry, charge–discharge test, and electrochemical impedance measurement. The charge storage mechanism of the LiCoO₂-positive electrode in aqueous electrolyte was discussed, too. The results showed that the potential range, scan rate, species of aqueous electrolyte, and current density had great effect on capacitive properties of the hybrid capacitor. In the potential range of 0–1.4 V, it delivered a discharge specific capacitance of 45.9 Fg^–1 (based on the active mass of the two electrodes) at a current density of 100 mAg^–1 in 1 molL^–1 Li₂SO₄ aqueous electrolyte. The specific capacitance remained 41.7 Fg^–1 after 600 cycles.     

Structure and electrochemical performance of LiCoO2 cathode material in different voltage ranges

LiCoO₂ sample prepared by high-temperature solid state calcination shows a typical hexagonal structure with a single phase and fine particle size distribution. The high-voltage electrolyte with additive fluoroethylene carbonate (FEC) has been used. Electrochemical results show that the initial discharge capacities of the prepared LiCoO₂ cathode are 157.7, 169.5, 191.0, and 217.5 mAh g^?1 in the voltage ranges of 3.0–4.3, 3.0–4.4, 3.0–4.5, and 3.0–4.6 V, respectively. The capacity increases, while the initial coulombic efficiency and capacity retention decrease with increasing the charge cutoff voltage. The capacity retention is only 10.4 % after 200 cycles at 1C rate in the voltage range of 3.0–4.6 V. X-ray diffraction measurements confirm structural changes of the layered material in the different voltage ranges. A phase transition from the O3 to the H1-3 phase can be observed when LiCoO₂ is charged above 4.5 V. The AC impedance analysis indicates that the resistances (R _(sf+b), R _ct) of the prepared LiCoO₂ rapidly increase when the cell is charged to higher voltage. The amount of dissolved Co into the electrolyte also greatly increases with increasing the charge cutoff voltage.     

Role of mesopores on the electrochemical performance of LiCoO<Subscript>2</Subscript> composite cathodes for lithium ion batteries

Mesoporous carbon (MC) was utilized to increase the mesoporosity of LiCoO₂ composite cathode. Graphite powder (GP) was chosen as a standard of comparison because of its very low mesoporosity. Compared with MC, GP has similar particle size, lower specific surface area, and higher electronic conductivity. Acetylene black (AB) exists in the form of chains of nanoparticles. With all other factors held constant, the mixture of AB and MC (ABMC)-loaded LiCoO₂ composite cathode (ABMC cathode) was superior to the mixture of AB and GP (ABGP)-loaded LiCoO₂ composite cathode (ABGP cathode). The reason is described as follows. Both GP and MC form a conductive network with AB chains. ABGP cathode has higher electronic conductivity than ABMC cathode. But the ionic conductivity of the ABMC cathode is more easily enhanced than the ABGP cathode because the former has much greater mesoporosity. In addition, the mesopores absorb and retain electrolyte solution and then provide buffer lithium ions for quick electrochemical reactions, so shortening the lithium ion transfer path in the composite cathode.