锂离子动力电池高倍率充放电过程中弛豫行为的仿真

基于电化学热耦合模型研究了动力锂离子电池高倍率充放电过程中的弛豫行为, 分析对比了不同充放电机制对电池弛豫行为的影响. 研究发现: 充放电过程中, 欧姆极化是造成电压骤变的主要原因; 而恒流-恒压的充电模式能够缓慢消除欧姆极化, 避免电池电压的骤变; 利用恒流恒压对电池进行充电能够充进更多的电量, 有利于电池性能的完全发挥; 固相锂离子浓度的弛豫时间比液相锂离子浓度的弛豫时间长, 并且在放电后期, 固相扩散的特征时间与液相扩散特征时间的比值不断增大, 固相扩散造成的极化在整个放电过程不可忽略.     

基于电化学模型的锂离子电池多尺度建模及其简化方法

锂离子电池的精确建模和状态估计对于电动汽车电池管理系统非常重要,准二维(P2D)电化学模型由于计算复杂,难以直接应用于电池管理的参数在线估计和实时控制中.本文基于多孔电极理论和浓度理论,提出一种考虑锂离子液相动力学的简化准二维(SP2D)模型.忽略锂离子孔壁流量沿电极厚度方向的变化求解SP2D模型所描述的锂离子电池锂浓度分布,基于锂离子电池电化学平均动力学行为求解固相和液相电势变化,推导出电池电压计算的简化表达式;采用恒流、脉冲以及城市循环工况放电电流对比分析了严格P2D模型与SP2D模型的终端电压和浓度分布.结果表明:SP2D模型在保持较高计算精度的同时,可显著提高计算效率.     

PEMFCs的膜及阴极催化层数值模拟

本文提出了一个质子交换膜燃料电池的膜和阴极催化层的一维非稳态数学模型,模型考虑了电化学反应及反应中的传质过程。本文结合算例分析了燃料电池膜及阴极催化层的性能,结果能验证燃料电池内阻理论。论文结果表明:(1)随着输出电流密度的增大,氧浓度分布不均匀性增大; (2)阴极催化层厚度减小,可提高电池输出电压; (3)电池进口处氧气摩尔浓度增大,可增加电池的输出电压。     

环路热管吸液芯毛细抽吸特性研究

采用计算机记录毛细抽吸量实时变化曲线的方法,研究了环路热管吸液芯对不同工质的毛细抽吸特性,结果表明所有的吸液芯毛细抽吸过程均可分阶段采用玻耳兹曼方程进行描述,其误差在启动阶段和稳健抽吸阶段分别在5%和1%以内。工质的表面张力越大、密度越大,吸液芯的毛细抽吸量就越大;工质的表面张力越大、黏度越小,毛细抽吸速度越大。对孔隙率为45%～56%的吸液芯进行毛细抽吸实验发现,随着孔隙率的增大,抽吸速度和毛细抽吸量均随之增大,可见适当提高吸液芯的孔隙率有助于提高环路热管的性能。     

多孔介质平板通道与空通道的速度分布和温度分布的解析解

建立了平板通道内填充多孔介质和空通道两种情况传热模型,求解动量方程和能量方程的解析解,获得速度分布和温度分布,分析Da数、Bi数、有效导热系数比和孔隙率等参数对温度分布、局部非热平衡程度以及Nu数的影响。计算求解表明,通道内填充多孔介质后流体的速度分布和温度分布更均匀,速度边界层更薄;随着Bi数和有效导热系数比k的增大,θ_s-θ_f的值减小,局部非热平衡程度减弱,Bi越大,θ_s与θ_f越接近,即越接近局部热平衡状态。孔隙率对局部非热平衡程度和Nu数的影响不明显;Nu数随着有效导热系数比k和Da数的减小而增大,随着Bi数和孔隙率的增大而增大,当k1时,变化明显,k1时,变化不明显.     

气孔对透明陶瓷激光输出性能的影响

定量分析气孔大小和气孔率对陶瓷激光输出性能的影响,建立了气孔分布模型并结合Mie散射和固体激光技术相关理论,讨论气孔散射对陶瓷激光透过率和输出性能的影响。结果表明,气孔率对激光陶瓷的透过率和斜率效率有着决定性的影响。在给定的气孔尺寸分布下,气孔率越高,陶瓷透过率和激光斜率效率越低;在给定的气孔率下,气孔中心尺寸越大,陶瓷样品的透过率和激光斜率效率越低。减小气孔尺寸至0.3 m以下能有效提高激光斜率效率。气孔越大,激光阈值随气孔率增大而上升越快,越不利于实现激光输出。在实际工作中,改进和控制工艺减小气孔尺寸对提高陶瓷的输出性能同样有着重要意义。     

包含液相扩散方程简化的锂离子电池电化学模型

锂离子电池的电化学模型对于电池特性分析和电池管理具有重要意义,但是准二维(P2D)模型复杂度太高,为了在保证模型精度的基础上尽量降低复杂度,本文提出了一种包含液相简化的P2D (LSP2D)模型,该模型首先基于电化学平均动力学将电池端电压化简成为仅耦合固相Li+浓度c_s和液相Li+浓度c_e的方程,然后进一步对表达c_s和c_e演化规律的偏微分方程进行抛物线近似化简,使得最终的模型由多项式组成.COMSOL仿真表明在放电倍率为1C时该模型与单粒子(SP)模型的估算精度和速度相当,但在放电倍率为3C时,该模型的估算时间比P2D模型减少了99.73%,与SP模型相当,估算精度相比SP模型有大幅度提升.     

空心微瓶谐振腔的曲率模型及其传输特性研究

提出了有关空心微瓶谐振腔的曲率变化模型并且研究了曲率对其传输特性的影响。仿真分析了微瓶谐振腔内回音壁模式的分布情况,随着曲率的增加,微瓶谐振腔对回音壁模式的束缚能力越强,使得内部光场能量也相应增加。通过控制熔接机的放电次数制得不同曲率的空心微瓶谐振腔,并且根据理想气体状态方程,推导了制备工艺中曲率模型,研究了空心微瓶谐振腔的曲率随放电次数的变化趋势。实验结果表明,曲率越大的空心微瓶谐振腔品质因子越高,可达到7.26×105,该结论在提升谐振腔的品质因子方面具有重要价值。     

基于柔性通道的圆柱型锂电池热管理实验研究

本文建立了一种基于柔性金属微通道内流动沸腾换热的圆柱型锂离子电池热管理系统。柔性铝微通道结构能够与圆柱型电池很好的接触,并且槽道内工质流动与相变换热可以有效地带走电池放电过程中产生的热量。实验研究了柔性铝槽道板与圆柱型电池表面接触面积和槽道内工质质量流量对电池热性能和电化学性能的影响,并与无冷却结构的电池放电实验结果进行了比较。柔性铝槽道板工质入口质量流量为5.98 kg/h时,电池表面温度和表面温度差最小,电池的输出电压和容量最佳。此外,在考虑热性能、宏观电化学特性、入口质量流量和冷却性能的基础上对实验进行了优化设计。     

辅助放电针对气体间隙直流自击穿特性的影响

为降低气体放电间隙直流条件下自击穿电压分散性,尽可能不影响其自击穿电压,基于环形电极放电间隙,设计了一种在阴极中心植入辅助放电针的辅助放电电极结构。通过电场仿真,研究了辅助放电针直径、长度和顶部倒角对放电间隙场畸变的影响。实验研究了放电间隙无辅助放电针和植入辅助放电针后,其在干燥空气和SF₆气体中的直流自击穿特性。结果表明：辅助放电针直径越小、长度越长,电极环对其电场屏蔽作用越弱,放电间隙场畸变强度越大;辅助放电针对SF₆气体放电间隙直流自击穿电压影响较小,随着场畸变系数的增大,同一气压下干燥空气自击穿电压下降百分数为SF₆气体自击穿电压下降百分数的2～3倍;辅助放电针对直流条件下干燥空气和SF₆气体放电间隙自击穿电压稳定性具有有益作用,分散性减小百分数较无辅助放电针结构均提高约25%。     

基于遗传算法的锂离子电池等效电路参数识别

结合锂离子电池双极性等效电路模型提出了一种基于遗传算法的参数识别方法,该方法通过指数函数对电路模型中的电阻、电容、恒压源等元件进行有理逼近,根据电池在不同充放电速率下的输出电压特性数据,通过实数编码遗传算法得到最优的函数参数,从而得到最优的电阻、电容,开路电压等电路参数值,针对电池在不同的工作状态,不同的工作参数下的运行数据,系列仿真和实验结果表明该算法原理简明,收敛较快,辨识得到的最优模型其电压输出特性与电池的实际电压输出特性基本吻合,能较精确的反映电池的实际特性,具有较高的辨识精度。     

Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/poly(ethylene terephthalate) composite separator for high-safety lithium-ion batteries

Separators have garnered substantial attention from researchers and developers in regard to their crucial role in the safety of lithium-ion batteries. In this study, a composite separator was prepared by coating cubic Al₂O₃ nanoparticles on non-woven poly(ethylene terephthalate) (PET) via a simple dip-coating process. The basic properties of the Al₂O₃-coated PET non-woven composite separator were characterized by scanning electron microscopy and other specific measurements in respect to its morphology, porosity, electrolyte wettability, and thermal shrinkage as well as its application in lithium-ion batteries. We found that the composite separator has outstanding thermostability, which may improve battery safety. Additionally, by comparison against the commercial Celgard 2500 separator, the proposed composite separator exhibits higher porosity, superior electrolyte wettability, and higher ionic conductivity. More importantly, the lithium-ion battery assembled with this composite separator shows better electrochemical performance (e.g., cycling and discharge C-rate capability) compared to that with the Celgard 2500 separator. The results of this study represent a simple approach to preparing high-performance separators that can be used to enhance the safety of lithium-ion batteries.     

基于扩展单粒子模型的锂离子电池参数识别策略

为了精确识别电动汽车锂离子动力电池的关键状态参数,基于多孔电极理论和浓度理论,建立了一种考虑液相动力学行为的锂离子电池扩展单粒子模型.相较于传统单粒子模型,该模型增加了对负电极表面固体电解质界面膜参数的描述,并考虑了温度和液相浓度变化对锂离子电池关键参数的耦合影响.基于所建立的扩展单粒子模型,提出一种简化的参数灵敏度分析方法和有效的锂电池参数识别策略,用以确定特定工况下的高灵敏度待识别参数,进而利用遗传算法实现参数的优化求解.最后,通过对比分析本文模型和传统单粒子模型的仿真输出电压和相同工况下电池的实验输出电压验证了提出模型和参数识别方法的有效性和可行性,为电池管理系统的健康状态估计提供了理论基础.     

A heatproof separator for lithium-ion battery based on nylon66 nanofibers

The safety of lithium-ion battery is closely related to the anti-shrink and heatproof stability of the separator. In this paper, electrospun nylon66 (PA66) nanofiber-based membrane is used as lithium-ion battery separator, showing good thermodynamics properties via thermogravimetric analysis (TG), thermal shrinkage experiment, and tension test. Furthermore, electrospun nylon66 separator based battery exhibits better safety than the battery applying Celgard commerce separator under the condition of high temperature and severe vibration. Meanwhile, the electrochemical properties of both batteries are nearly identical. These facts prove that the electrospun nylon66 separator is an ideal separator candidate for power lithium-ion battery of electric vehicles. In addition, the nylon66 separator annealed in the air has higher tensile strength and better property of elongation than the unannealed one.     

双相多孔锂离子电池液/固界面的声反射与声透射

采用传递矩阵法,同步联立改进的Biot理论,对含液固界面的双相多孔锂离子电池的超声反射与透射系数进行理论求解。考虑锂离子的摇摆特性对电极力学性能的影响,计算了对应状态下液/固界面的双相多孔锂离子电池声反射及透射系数的角度谱与频率谱。同时,构建了不同荷电状态时含单元锂离子电池的频域仿真模型,以萃取对应的超声反射及透射角度谱及频率谱,并与理论计算结果对比吻合良好。随后,以多单元锂离子电池为例,在不同的荷电状态下,分析了反射与透射系数随斜入射角度、入射波频率的变化关系,并分别指出了其角度谱及频率谱特征点随荷电状态的变化特征,为锂离子电池运行状态的超声无损测量提供了理论基础。     

Estimation of temperature distribution of LiFePO<Subscript>4</Subscript> lithium ion battery during charge–discharge process

Thermal issues of lithium ion batteries are key factors affecting the safety, operational performance, life, and cost of the battery. An electrochemical–thermal coupling model based on thermoelectrochemical basic data was established to investigate the thermal behavior of LiFePO₄ lithium ion battery. In this paper, the finite element method was used for simulation of temperature field distribution inside battery during charge–discharge process, and the influence of the charge–discharge rate and ambient temperature on the distribution of temperature field was summarized. The results showed that the highest temperature of battery was recorded at the junction of negative and separator during charge–discharge process. At a low discharge current, the modeling results agreed well with the experimental data. When the ambient temperature was 303.15 K, the maximum temperatures inside the battery were 304.60, 304.83, 306.55, and 309.96 K for 0.1, 0.2, 0.5, and 1.0 C charge–discharge rates, respectively. If the ambient temperature increased to 323.15 K, the maximum temperatures were increased by 24.96, 27.91, 33.18, and 32.59 K for 0.1, 0.2, 0.5, and 1.0 C charge–discharge rates, respectively, and the homogenous temperature field distribution inside the battery was worse.     

Vinyl ethylene carbonate as an electrolyte additive for high-voltage LiNi<Subscript>0.4</Subscript>Mn<Subscript>0.4</Subscript>Co<Subscript>0.2</Subscript>O<Subscript>2</Subscript>/graphite Li-ion batteries

Vinyl ethylene carbonate (VEC) is investigated as an electrolyte additive to improve the electrochemical performance of LiNi_0.4Mn_0.4Co_0.2O₂/graphite lithium-ion battery at higher voltage operation (3.0–4.5 V) than the conventional voltage (3.0–4.25 V). In the voltage range of 3.0–4.5 V, it is shown that the performances of the cells with VEC-containing electrolyte are greatly improved than the cells without additive. With 2.0 wt.% VEC addition in the electrolyte, the capacity retention of the cell is increased from 62.5 to 74.5 % after 300 cycles. The effects of VEC on the cell performance are investigated by cyclic voltammetry(CV), electrochemical impedance spectroscopy(EIS), x-ray powder diffraction (XRD), energy dispersive x-ray spectrometry (EDS), scanning electron microscopy (SEM), and attenuated total reflectance-Fourier transform infrared (ATR-FTIR). The results show that the films electrochemically formed on both anode and cathode, derived from the in situ decomposition of VEC at the initial charge–discharge cycles, are the main reasons for the improved cell performance.     

Analysis on the capacity degradation mechanism of a series lithium-ion power battery pack based on inconsistency of capacity

The lithium-ion battery has been widely used as an energy source. Charge rate, discharge rate, and operating tem- perature are very important factors for the capacity degradations of power batteries and battery packs. Firstly, in this paper we make use of an accelerated life test and a statistical analysis method to establish the capacity accelerated degradation model under three constant stress parameters according to the degradation data, which are charge rate, discharge rate, and operating temperature, and then we propose a capacity degradation model according to the current residual capacity of a Li-ion cell under dynamic stress parameters. Secondly, we analyze the charge and discharge process of a series power battery pack and interpret the correlation between the capacity degradations of the battery pack and its charge/discharge rate. According to this cycling condition, we establish a capacity degradation model of a series power battery pack under inconsistent capacity of cells, and analyze the degradation mechanism with capacity variance and operating temperature difference. The comparative analysis of test results shows that the inconsistent operating temperatures of cells in the series power battery pack are the main cause of its degradation; when the difference between inconsistent temperatures is narrowed by 5 ℃, the cycle life can be improved by more than 50%. Therefore, it effectively improves the cycle life of the series battery pack to reasonably assemble the batteries according to their capacities and to narrow the differences in operating temperature among cells.     

光伏发电系统充放电控制策略研究

本对光伏发电系统中影响蓄电池寿命各因素进行了简单分析,并在此基础上提出了一种新的充放电控制策略。实验结果证明该控制策略能较显提高蓄电池的放电容量。