Balancing particle properties for practical lithium-ion batteries

As a state-of-the-art secondary battery, lithium-ion batteries (LIBs) have dominated the consumer electronics market since Sony unveiled the commercial secondary battery with LiCoO₂ as the negative electrode material in the early 1990s. The key to the efficient operation of LIBs lies in the effective contact between the Li-ion-rich electrolyte and the active material particles in the electrode. The particle properties of the electrode materials affect the lithium ion diffusion path, diffusion resistance, contact area with the active material, the electrochemical performance and the energy density of batteries. To achieve satisfied comprehensive performance and of LIBs, it is not only necessary to focus on the modification of materials, but also to balance the properties of electrode material particles. Therefore, in this review, we analyze the influence of particle properties on the battery performance from three perspectives: particle size, particle size distribution, and particle shape. A deep understanding of the effect and mechanism of particles on electrodes and batteries will help develop and manufacture practical LIBs.     

基于统一相场理论的锂电池电极颗粒断裂模拟研究

锂电池充放电过程中, 锂离子的脱出和嵌入会引发电极颗粒的不均匀体积变化和机械应力. 上述锂离子扩散过程诱发的应力与电极颗粒的尺寸大小、截面形状和冲放电速率有关, 可能会导致电极颗粒出现裂缝起裂、扩展甚至断裂等力学失效, 对锂离子电池的容量和循环寿命等性能产生不利影响. 为准确模拟并预测电极颗粒的力学失效过程, 在笔者前期提出的统一相场理论框架内进一步考虑化学扩散、力学变形和裂缝演化等耦合过程, 建立化学–力学耦合相场内聚裂缝模型, 发展相应的多场有限元数值实现算法, 并应用于二维柱状和三维球体锂电池电极颗粒的力学失效分析. 由于同时涵括了基于强度的起裂准则、基于能量的扩展准则以及基于变分原理的裂缝路径判据, 这一模型不仅适用于带初始缺陷电极颗粒的开裂行为模拟, 而且适用于无初始缺陷电极颗粒的损伤破坏全过程分析. 数值计算结果表明, 相场内聚裂缝模型能够模拟锂离子扩散引发的电极颗粒裂缝起裂、扩展、汇聚等复杂演化过程, 可为锂离子电池电极颗粒的力学失效预测和优化设计提供有益的参考.      

Analytical Model and Experimental Verification of the Interfacial Peeling Strength of Electrodes

Background

The interfacial peeling strength of lithium-ion battery electrodes is a very important mechanical property that significantly affects the electrochemical performance of battery cells.

Objective

To characterize the interfacial peeling strength of an electrode, an analytical model based on the energy balance principle is established by considering the state of charge (SOC), the energy release rate, the tensile stiffness, and the peeling angle.

Methods

Uniaxial tensile tests and 180-degree peeling tests are conducted to determine the Young’s modulus and the interfacial peeling strengths of electrodes at different SOCs, respectively. The experimental data serve as a validation of the accuracy of the analytical model.

Results

The interfacial peeling strength of the electrode shows a strong reliance on many factors. Specifically, the interfacial peeling strength increases with the SOC and the energy release, and decreases with the peeling angle. When the tensile stiffness of the active layer equals that of the current collector, the interfacial peeling strength has a maximum value.

Conclusions

By comparing with experimental data of the 180-degree peeling test, the model prediction shows excellent agreement at different SOCs, and the analytical model established in this paper can be used to guide and assess the interfacial properties of electrodes for industry.

     

Particulate modification of lithium-ion battery anode materials and electrolytes

Lithium-ion batteries (LIBs) are considered a rechargeable and commercial energy storage device for electronic equipment such as smartphone and electric vehicles. Despite the prospective future of LIBs, unsatisfied electrochemical properties like reversible capacity, cycle ability and coulombic efficiency still hinder their development. High volume expansion rate, uncontrolled Li dendrite growth and unsatisfied solid electrolyte interphase also occur when LIBs are applied in long-time usage. Numerous modification methods such as exploring high-capacity anode/cathode materials, constructing artificial solid electrolyte interphase and improved conductive binders can be adopted to enhance the performances. Among them, particulate modification for LIBs anode and electrolytes is receiving tremendous attraction in the recent work. The method is composed of changing the morphology and particle size of the active materials, also introduce nano-size additives to the main structure. This review emphasizes on introducing and discussing the modification in following aspects: particulate modification on carbon group IVA element anodes, introduction of additives like transition metal oxide nanoparticles into anode and electrolyte materials, dissipate the influence of Li dendrite growth and ameliorate the performances of solid electrolyte interface. This review hopes to be denoted for the future development of LIBs with the comprehensive understanding on the particulate modification.     

Characterization of 10% Ballistic Gelatin to Evaluate Temperature, Aging and Strain Rate Effects

Ballistic gelatin is widely used as a soft tissue simulant in physical surrogates for the human body to evaluate penetrating impacts and, more recently to evaluate blunt impact and blast loading effects on soft tissues. It is known that the properties of gelatin are sensitive to temperature and aging time, but this has not previously been quantified. The mechanical properties of 10% ballistic gelatin were measured using a compression test apparatus with temperature controlled platens to maintain the sample temperature at a fixed level. Penetration testing was undertaken using a standard BB impact test to assess the effect of aging. The gelatin was found to be within calibration after 3?days (72?h of aging), based on the standard penetration test. The material properties were evaluated using the stress at failure, strain at failure and material stiffness as characterized by the Neo-Hookean constitutive model. The stress at failure and material stiffness increased with decreasing temperature and increasing strain rate, as expected, while the strain at failure remained relatively constant for the test conditions considered (1 to 23??C, strain rate from 0.01 to 1.0?s^?1). The study showed that the penetration resistance was consistent after 72?h of aging, while the mechanical study demonstrated increasing failure stress and stiffness with decreasing failure strain at longer aging times, suggesting that these effects offset one another so that the penetration resistance remains relatively constant. The primary contribution of this study was to show the importance of temperature and aging time, through mechanical and penetration testing, to achieve appropriate and consistent response from ballistic gelatin.     

Arterial Wall Stiffening in Caveolin-1 Deficiency-Induced Pulmonary Artery Hypertension in Mice

Background

Pulmonary artery hypertension (PAH) is a complex disorder that can lead to right heart failure. The generation of caveolin-1 deficient mice (CAV-1^?/?) has provided an alternative genetic model to study the mechanisms of pulmonary hypertension. However, the vascular adaptations in these mice have not been characterized.

Objective

To determine the histological and functional changes in the pulmonary and carotid arteries in CAV-1^?/? induced PAH.

Methods

Pulmonary and carotid arteries of young (4–6 months old) and mature (9–12 months old) CAV-1^?/? mice were tested and compared to normal wild type mice.

Results

Artery stiffness increases in CAV-1^?/? mice, especially the circumferential stiffness of the pulmonary arteries. Increases in stiffness were quantified by a decrease in circumferential stretch and transition strain, increases in elastic moduli, and an increase in total strain energy at physiologic strains. Changes in mechanical properties for the pulmonary artery correlated with increased collagen content while changes in the carotid artery correlated with decreased elastin content.

Conclusions

We demonstrated that an increase in artery stiffness is associated with CAV-1 deficiency-induced pulmonary hypertension. These results improve our understanding of arterial remodeling in PAH.

     

Real-Time Stress Measurement in SiO<Subscript>2</Subscript> Thin Films during Electrochemical Lithiation/Delithiation Cycling

Oxide coatings have been shown to improve the cyclic performance of high-energy density electrode materials such as Si. However, no study exists on the mechanical characterization of these oxide coatings. Here, thin film SiO₂ electrodes are cycled under galvanostatic conditions (at C/9 rate) in a half-cell configuration with lithium metal foil as counter/reference electrode, with 1 M LiPF₆ in ethylene carbonate, diethyl carbonate, dimethyl carbonate solution (1:1:1, wt%) as electrolyte. Stress evolution in the SiO₂ thin film electrodes during electrochemical lithiation/delithiation is measured in situ by monitoring the substrate curvature using a multi-beam optical sensing method. Upon lithiation SiO₂ undergoes extensive inelastic deformation, with a peak compressive stress of 3.1 GPa, and upon delithiation the stress became tensile with a peak stress of 0.7 GPa. A simple plane strain finite element model of Si nanotube coated with SiO₂ shell was developed to understand the mechanical response of the core-shell type microstructures under electrochemical cycling; measured stress response was used in the model to represent SiO₂ constitutive behavior while Si was treated as an elastic-plastic material with concentration dependent mechanical properties obtained from the literature. The results reported here provide insights and quantitative understanding as to why the highly brittle SiO₂ coatings are able to sustain significant volume expansion (300%) of Si core without fracture and enhance cyclic performance of Si reported in the literature. Also, the basic mechanical properties presented here are necessary first step for future design and development of durable Si/SiO₂ core shell structures or SiO₂-based electrodes.     

Clarification of the dispersion mechanism of three typical chemical dispersants in lithium-ion battery (LIB) slurry

This paper mainly clarified the dispersion mechanism of three typical chemical dispersants which are polyethylene glycol octylphenyl ether (Triton X-100, T-100), polyethylene pyrrolidone (PVP) and carboxymethyl cellulose (CMC) within lithium-ion battery (LIB) slurry. Initially, the optimum amounts of T-100, PVP and CMC are selected from 0%, 0.5%, 1.5% and 2.5% by evaluating the impedance of LIB slurry in the case of adding each typical chemical dispersant with EIS method. Moreover, the impedance spectrum of three different slurry samples which are PVDF-NMP solution, LiCoO₂ slurry and Carbon Black (CB) slurry with the optimum amount of each dispersant are also investigated. After using SEM and C element distribution images of LIB slurry to verify the correctness of the dispersion mechanism of each dispersant, it is concluded that the dispersion CMC with its optimum amount 1.5% is the best one to promote the formation of conductive paths and CB-coated LiCoO₂ network structure within LIB slurry, which has the considerably potential to improve the performance of LIB.     

Research progress in degradation mechanism of silicon anode materials for lithium-ion batteries

硅负极材料由于具有非常高的理论比容量,使之成为锂离子电池极具前景的负极替代材料,然而,硅负极材料在充放电过程中会发生非常大的体积变形,这会引起活性材料的破坏失效,严重影响其电化学循环性能,成为制约其在锂离子电池领域广泛应用的最大瓶颈,本文介绍了硅负极材料的不同结构形态及其在充放电过程中电化学性能的退化机理,并综述了充放电过程中的力学性能演化、相关理论分析、数值模拟计算等方面的最新国际研究进展,展望了硅负极材料力学失效方面的研究重点,     

A Technique for Dynamic Tensile Testing of Human Cervical Spine Ligaments

An experimental study is undertaken to examine the dynamic stress–strain characteristics of ligaments from the human cervical spine (neck). Tests were conducted using a tensile split Hopkinson bar device and the engineering strain rates imposed were of the order of 10²∼10³/s. As ligaments are extremely soft and pliable, specialized test protocols applicable to Hopkinson bar testing were developed to facilitate acquisition of reliable and accurate data. Seven primary ligaments types from the cervical spines of three male cadavers were subjected to mechanical tests. These yielded dynamic stress–strain curves which could be approximated by empirical equations. The dynamic failure stress/load, failure stain/deformation, modulus/stiffness, as well as energy absorption capacity, were obtained for the various ligaments and classified according to their location, the strain rate imposed and the cadaveric source. Compared with static responses, the overall average dynamic stress–strain behavior foreach type of ligament exhibited an elevation in strength but reduced elongation.     

Strain Evolution in Lithium Manganese Oxide Electrodes

Lithium manganese oxide, LiMn₂O₄ (LMO) is a promising cathode material, but is hampered by significant capacity fade due to instability of the electrode-electrolyte interface, manganese dissolution into the electrolyte and subsequent mechanical degradation of the electrode. In this work, electrochemically-induced strains in composite LMO electrodes are measured using the digital image correlation (DIC) technique and compared with electrochemical impedance spectroscopy (EIS) measurements of surface resistance for different scan rates. Distinct, irreversible strain variations are observed during the first delithiation cycle. The changes in strain and surface resistance are highly sensitive to the electrochemical changes occurring during the first cycle and correlate with prior reports of the removal of the native surface layer and the formation of cathode-electrolyte interface layer on the electrode surface. A large capacity fade is observed with increasing cycle number at high scan rates. Interestingly, the total capacity fade scales proportionately to the strain generated after each lithiation and delithiation cycle. The simultaneous reduction in capacity and strain is attributed to chemo-mechanical degradation of the electrode. The in situ strain measurements provide new insight into the electrochemical-induced volumetric changes in LMO electrodes with progressing cycling and may provide guidance for materials-based strategies to reduce strain and capacity fade.     

Investigation of the Effects of Environmental Fatigue on the Mechanical Properties of GFRP Composite Constituents Using Nanoindentation

Background

Fatigue failure criteria for fibre reinforced polymer composites used in the design of marine structures are based on the micromechanical behaviour (e.g. stiffness properties) of their constituents. In the literature, there is a lack of information regarding the stiffness degradation of fibres, polymer matrix and fibre/matrix interface regions affected by environmental fatigue.

Objective

The aim of present study is to characterize the stiffness properties of composite constituents using the nanoindentation technique when fatigue failure of composites is due to the combined effect of sea water exposure and cyclic mechanical loads.

Methods

In the present study, the nanoindentation technique was used to characterize the stiffness properties of composite constituents where the effects of neighbouring phases, material pile up and viscoplasticity properties of the polymer matrix are corrected by finite element simulation.

Results

The use of finite element simulation in conjunction with nanoindentation test data, results in more accurate estimation of projected indented area which is required for measuring the properties of composite constituents. In addition, finite element simulation provides a greater understanding of the stress transfer between composite constituents during the nanoindentation process.

Conclusions

Results of nanoindentation testing on the composite microstructure of environmentally fatigue failed composite test coupons establish a strong link to the stiffness degradation of the fiber/matrix interface regions, verifying the degradation of composite constituents identified by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) analysis.

     

Revealing the correlation between structural evolution and long-term cyclability of the LiNi0.5Co0.2Mn0.3O2/artificial graphite pouch cells at various rates

In recent years, researches on improving high-voltage performance of lithium-ion batteries incorporating LiNi_0.5Co_0.2Mn_0.3O₂ (NCM523) and artificial graphite (AG) have been widely reported. However, limited attentions have been paid to understand the effects and influence mechanisms of charge and discharge rates and charge limit currents on cyclability of NCM523/AG cells. Herein, a ∼1.9 Ah NCM523/AG pouch cell is employed, whose electrochemical and structural evolutions after 800 cycles at various rates are comprehensively investigated. We find that cycling performances are strongly influenced by charge rate, followed by limit current and discharge rate. The cell charged at a high rate and cell charged until reaching a low limit current both exhibit low capacity retentions compared to the cell discharged at a high rate. Possible failure reasons are analyzed by advanced characterizations. Results reveal that NCM523 cathodes of the cells deteriorated early experience severe transition metal dissolution, lattice distortion, and partial phase transformation. Meanwhile, the deposited transition metals on AG anodes catalyze the electrolyte consumption, lithium plating and active area loss. Finally, these side reactions notably increase cell impedance and electrochemical polarization. Undoubtedly, these findings clearly outline the challenges and optimization direction for high-rate NCM523/AG cells.     

A Dugdale model based geometrical amplifier enables the measurement of separation-to-failure for a cohesive interface

Regardless of all kinds of different formulae used for the traction-separation relationship in cohesive zone modeling, the peak traction σ_m and the separation-to-failure δ₀ (or equivalently the work-to-separation Γ) are the primary parameters which control the interfacial fracture behaviors. Experimentally, it is hard to determine those quantities, especially for δ₀, which occurs in a very localized region with possibly complicated geometries by material failure. Based on the Dugdale model, we show that the separation-to-failure of an interface could be amplified by a factor of L/r_p in a typical peeling test, where L is the beam length and r_p is the cohesive zone size. Such an amplifier makes δ₀ feasible to be probed quantitatively from a simple peeling test. The method proposed here may be of importance to understanding interfacial fractures of layered structures, or in some nanoscale mechanical phenomena such as delamination of thin films and coatings.     

State-of-Charge and Deformation-Rate Dependent Mechanical Behavior of Electrochemical Cells

The state-of-charge and deformation-rate dependent mechanical behavior of cylindrical lithium-ion battery cells was investigated. The research revealed that both state of charge and deformation rates affected the stiffness of the battery cells. Battery mechanical failure load was only weakly dependent on the state of charge. For the deformation-rate dependency on the mechanical integrity of battery cells, the battery mechanical failure load was either decreased significantly at high state of charge or decreased slightly at low state of charge as deformation rate increased. For the correlation between mechanical integrity and electrical failure, the displacement at the battery mechanical failure load coincided with a voltage drop. However, at high state of charge, premature and incomplete voltage drops were observed before the definite final voltage drop. No such premature voltage drop was observed in low state-of-charge specimens. The results of this research may be used as a reference for the design of impact and damage tolerant electric vehicle battery systems.     

Thermal analyses of LiCoO2 lithium-ion battery during oven tests

A three dimensional thermal abuse model for graphite/LiPF₆/LiCoO₂ batteries is established particularly for oven tests. To investigate the influence of heat release condition and oven temperature on battery thermal behaviors, we perform a series of simulations with respect to a unit cell during oven thermal abuses of various oven temperatures and under various heat release conditions. Simulation results enable detailed analyses to thermal behaviors of batteries. It is found that during oven thermal abuse processes that do not get into thermal runaway, the negative electrode is the maximum heat generation rate zone; during oven thermal abuse processes that do get into thermal runaway, the positive electrode is the maximum heat generation rate zone. The positive-solvent reaction is found to be the major heat generation source causing thermal runaway. It is also found that the heat release condition and the oven temperature are combined to dictate thermal behaviors of the battery. The critical oven temperature that causes thermal runaway rises if the heat release condition is better and the critical heat release coefficient that can effectively restrain the occurrence of thermal runaway increases with the increase of oven temperature.     

非晶合金剪切带动力学行为研究

剪切带是一种材料塑性变形高度局域化的变形模式, 广泛存在于非晶体系的形变中, 控制着这些无序体系失稳、灾难性断裂行为.传统的非晶体系如岩石, 胶体, 玻璃和聚合物等因较差的力学性能以及过于复杂的结构而不利于剪切带的实验研究. 近几十年来, 非晶合金的出现极大丰富了剪切带的研究, 推进了对剪切带的认识. 通过大量非晶合金中剪切带的实验和理论研究, 人们发现剪切带行为具有空间不均匀性和时间不连续性的特征, 表现出复杂的动力学特征, 和自然界以及物理系统中许多复杂体系的动力学行为相似.同时, 剪切带的性质尤其是其动力学行为对非晶合金的宏观力学行为和性能有重要的影响, 对理解这类材料的微观变形机理也起着重要的作用.本文结合团队近年来在非晶合金剪切带行为方面的研究结果, 对剪切带的运动行为和物理机制进行介绍, 包剪切带间歇性运动行为、以及间歇性运动在表征其动力学性质中的作用以及物理机制, 以及剪切带的自组织临界行为、物理机制等.最后对非晶合金剪切带行为研究中亟需解决的问题进行了总结和展望.      

Fracture of bicrystal metal/ceramic interfaces: A study via the mechanism-based strain gradient crystal plasticity theory

Two continuum mechanical models of crystal plasticity theory namely, conventional crystal plasticity theory and mechanism-based crystal plasticity theory, are used to perform a comparative study of stresses that are reached at and ahead of the crack tip of a bicrystal niobium/alumina specimen. Finite element analyses are done for a stationary crack tip and growing cracks using a cohesive modelling approach. Using mechanism-based strain gradient crystal plasticity theory the stresses reached ahead of the crack tip are found to be two times larger than the stresses obtained from conventional crystal plasticity theory. Results also show that strain gradient effects strongly depend on the intrinsic material length to the size of plastic zone ratio (l/R₀). It is found that the larger the (l/R₀) ratio, the higher the stresses reached using mechanism-based strain gradient crystal plasticity theory. An insight into the role of cohesive strength and work of adhesion in macroscopic fracture is also presented which can be used by experimentalists to design better bimaterials by varying cohesive strength and work of adhesion.     

冰在低温下的单轴压缩力学行为和破坏机制

利用带有低温装置的Instron5848材料实验机和分离式Hopkinson压杆装置（SHPB）,在-10℃、-20℃和-30℃温度下,对多晶冰进行了应变率为10-4~102S-1范围内的单轴压缩力学性能实验,分析了实验结果的可靠性和有效性。研究发现：冰的压缩强度具有明显的温度和应变率敏感性,随应变率的增大、温度的降低而提高;压缩强度与应变率对数呈线性关系,应变率的升高会增强降温对压缩强度的强化效应。在研究的应变率和温度范围内,冰主要有径向膨胀、纵向劈裂和整体破碎三种破坏模式,裂尖能量得不到及时释放、冰体内氢键强度和裂纹滑移摩擦阻力增大是导致冰破坏模式不同和压缩强度增大的原因。     

Comparison testings between two high-temperature strain measurement systems

An experimental evaluation was conducted at NASA Lewis Research Center to compare and contrast the performance of a newly developed resistance strain gage, the PdCr temperature-compensated wire strain gage, to that of a conventional high-temperature extensometry. The evaluation of the two strain measurement systems was conducted through the application of various thermal and mechanical loading spectra using a high-temperature thermomechanical uniaxial testing system equipped with quartz lamp heating. The purpose of the testing was not only to compare and contrast the two strain sensors but also to investigate the applicability of the PdCr strain gage to the testing environment typically employed when characterizing the high-temperature mechanical behavior of structural materials. Strain measurement capabilities to 800°C were investigated with a nickel base superalloy IN100 substrate material, and application to titanium matrix composite (TMC) materials was examined with the SCS-6/Ti-15-3 [0]₈ system. PdCr strain gages installed by three attachment techniques—namely, flame spraying, spot welding and rapid infrared joining—were investigated.