Oxidized Starch-Reinforced Aqueous Polymer Isocyanate Cured with High-Frequency Heating.

In this research, an oxidized starch/styrene-butadiene rubber system with high capability of absorbing electromagnetic energy was adopted as the main component, the effect of oxidized starch content on the bonding and mechanical properties of aqueous polymer isocyanate (API) after high-frequency curing was evaluated, and the effect mechanisms were explored by combining thermodynamic tests and material characterization methods. Our findings revealed that the addition of oxidized starch enhanced the mechanical properties of API after high-frequency curing and the increase in the amount of oxidized starch enhanced the improvement effect of high-frequency curing on API bonding and mechanical properties. At 5 wt% oxidized starch, high-frequency curing improved API bonding properties by 18.0% and 17.3% under ambient conditions and after boiling water aging, respectively. An increase in oxidized starch content to 25 wt% increased enhancement to 25.1% and 26.4% for the above conditions, respectively. The enhancement effects of tensile strength and Young's modulus of the API adhesive body were increased from 9.4% and 18.2% to 18.7% and 22.6%, respectively. The potential enhancement mechanism could be that oxidized starch could increase the dielectric loss of API, converting more electromagnetic energy into thermal energy creating more cross-linked structures.

Experimental study on the mechanical and electrochemical properties of aqueous emulsifiable diphenylmethane diisocyanate-modified silicon-carbon composite electrodes.

Aqueous emulsifiable diphenylmethane diisocyanate (EMDI) can form strong chemical bonds with aqueous adhesives due to the large number of isocyanate (-NCO) groups, which can enhance the mechanical performance of the adhesives. Currently, sodium carboxymethyl cellulose (CMC)-styrene butadiene rubber (SBR) emulsion aqueous bonding agents are widely used in the preparation of anode materials for lithium-ion batteries (LIBs). In this study, EMDI was added to a porous silicon-carbon composite electrode prepared from CMC-SBR, and the evolution of the mechanical properties of the electrode with the EMDI content was first investigated via quasi-static uniaxial tensile and interfacial strength tests. Subsequently, the effect of the EMDI content on the electrochemical properties of the electrodes was analysed by electrochemical impedance spectroscopy (EIS) and constant-current (CC) charge/discharge performance tests. Finally, material characterisation of the electrodes was carried out by Fourier transform infrared (FTIR) spectroscopy and specific surface area (Brunauer-Emmett-Teller (BET)) analysis. The results show that the addition of EMDI with a mass ratio of 10-20% to the CMC-SBR binder can enhance the mechanical performance of the active layer and the interfacial performance between the active layer and the current collector of the silicon-carbon composite electrode; simultaneously, EMDI can significantly reduce the electrochemical impedance of the electrode material and improve the capacity retention of the electrode. This study provides a new solution for modifying silicon-carbon composite electrodes and promotes the development of high-performance silicon-carbon electrodes.

Investigation on the mechanical integrity of a PEO-based polymer electrolyte in all-solid-state lithium batteries.

Polyethylene oxide (PEO)-based solid polymer electrolytes (SPEs) have good ionic conductivity and flexibility, and is a key component of all-solid-state lithium batteries (ASSLBs). Therefore, the mechanical integrity of PEO-based SPEs during cell operation needs to be urgently evaluated. Here, we conducted a series of tensile and shear adhesion performance tests on PEO16-LiTFSI electrolyte and LiFePO4 electrode adhesion samples at various temperatures and quenching rates. Based on the interface performance data and the elastic-viscoplastic material model of the PEO-LiTFSI electrolyte, a comprehensive electrochemical-mechanical model was established to analyze the stress in the cell and evaluate the mechanical integrity of the PEO16-LiTFSI electrolyte and SPE/cathode interface. The experimental results show that the adhesion strength of the SPE and cathode decreases significantly with increasing operating temperature and quenching rate. The simulation study indicates that the mechanical properties of the SPE can be fully utilized to a certain extent by increasing the quenching rate. In addition, appropriately increasing the operating temperature helps maintain the mechanical integrity of the SPE during cell operation. However, increasing the quenching rate and operating temperature will reduce the interface bonding properties between the SPE and the cathode, resulting in an increased probability of mechanical failure at the SPE/cathode interface. To suppress this negative effect, a design scheme to maintain the structural integrity of the PEO-based polymer electrolyte is proposed by using the C-rate and the SPE thickness as control parameters, which can assist in engineering design and safe operation of Li/PEO16-LiTFSI/LiFePO4 for ASSLBs.

Effect of the charge rate on the mechanical response of composite graphite electrodes: in situ experiment and mathematical analysis.

The cycling lifespan and coulombic efficiency of lithium-ion batteries are crucial to high C-rate applications. The Li-ion concentration is crucial in determining the mechanical integrity and structural stability of electrodes. In this work, graphite is selected as the working electrode due to its widespread use in the electric vehicle industry. The experimental data have shown that the electrodes with a mass loading of 6.54 mg cm-2 exhibited poor cycling performance and high charge transfer resistance at high charge rates. To explain this phenomenon, an in situ stress measurement system and a C-rate-dependent stress model are established to study the mechanical properties of the composite graphite electrode during the electrochemical process at various C-rates. Moreover, the effect of the Li-ion concentration-dependent modulus and C-rate-dependent partial molar volume is taken into account in the mathematical model. The computational curvature data fit well with the corresponding experimental data, highlighting the importance of considering lithium-ion concentration in mechanical stress. It has been found that stresses along the thickness of the active layer switch between compressive and tensile stresses due to the competition between bending stress and diffusion-induced stress. The stress at the outer surface of the composite graphite electrodes reaches a maximum magnitude of 27.5 MPa at a 1.5C-rate. In contrast, the stress at the interface of the active layer is maximum at a 0.5C-rate due to the existence of more lithium ions. Our study provides a direct insight into the quantitative analysis of electrode stresses at different C-rates.

Enhancing the ionic conductivity and mechanical properties of PEO-based solid electrolytes through thermal pre-stretching treatment.

Pre-stretching as a method for directing polymer crystallization offers a promising solution for addressing the limitations of solid polymer electrolytes in flexible batteries at ambient temperatures. In this study, we have investigated the ionic conductivity, mechanical behaviour, and microstructural and thermal properties of polyethylene oxide (PEO)-based polymer electrolytes with varying pre-strain levels. The results indicate that thermal stretching-induced pre-deformation can significantly increase the through-plane ionic conductivity, in-plane strength, stiffness of solid electrolytes, and cell-specific capacity. However, modulus and hardness decrease for pre-stretched films in the thickness direction. Notably, applying 50-80% pre-strain to the PEO matrix composites through thermal stretching may be preferred for improving the electrochemical cycling performance, as it can increase through-plane ionic conductivity by at least 1.6 times while maintaining compressive stiffness at 80% compared to their unstretched counterparts, while the in-plane strength and stiffness can be boosted by 120-140%. Besides, adding nanoceramics contributes to lithiated PEO exhibiting a higher enhancement coefficient than the pristine sample. This positive effect is because the pre-strain and nano-inorganic filler decrease crystallinity and increase the free volume size of pre-stretched PEO-based electrolytes.

High-Temperature Magnesiothermic Reduction Enables HF-Free Synthesis of Porous Silicon with Enhanced Performance as Lithium-Ion Battery Anode.

Porous silicon-based anode materials have gained much interest because the porous structure can effectively accommodate volume changes and release mechanical stress, leading to improved cycling performance. Magnesiothermic reduction has emerged as an effective way to convert silica into porous silicon with a good electrochemical performance. However, corrosive HF etching is normally a mandatory step to improve the electrochemical performance of the as-synthesized silicon, which significantly increases the safety risk. This has become one of the major issues that impedes practical application of the magnesiothermic reduction synthesis of the porous silicon anode. Here, a facile HF-free method is reported to synthesize macro-/mesoporous silicon with good cyclic and rate performance by simply increasing the reduction temperature from 700 °C to 800 °C and 900 °C. The mechanism for the structure change resulting from the increased temperature is elaborated. A finite element simulation indicated that the 3D continuous structure formed by the magnesiothermic reduction at 800 °C and 900 °C could undertake the mechanical stress effectively and was responsible for an improved cyclic stability compared to the silicon synthesized at 700 °C.

Modeling and Simulation in Capacity Degradation and Control of All-Solid-State Lithium Battery Based on Time-Aging Polymer Electrolyte.

The prediction of electrochemical performance is the basis for long-term service of all-solid-state-battery (ASSB) regarding the time-aging of solid polymer electrolytes. To get insight into the influence mechanism of electrolyte aging on cell fading, we have established a continuum model for quantitatively analyzing the capacity evolution of the lithium battery during the time-aging process. The simulations have unveiled the phenomenon of electrolyte-aging-induced capacity degradation. The effects of discharge rate, operating temperature, and lithium-salt concentration in the electrolyte, as well as the electrolyte thickness, have also been explored in detail. The results have shown that capacity loss of ASSB is controlled by the decrease in the contact area of the electrolyte/electrode interface at the initial aging stage and is subsequently dominated by the mobilities of lithium-ion across the aging electrolyte. Moreover, reducing the discharge rate or increasing the operating temperature can weaken this cell deterioration. Besides, the thinner electrolyte film with acceptable lithium salt content benefits the durability of the ASSB. It has also been found that the negative effect of the aging electrolytes can be relieved if the electrolyte conductivity is kept being above a critical value under the storage and using conditions.

Mechanical Integrity Degradation and Control of All-Solid-State Lithium Battery with Physical Aging Poly (Vinyl Alcohol)-Based Electrolyte.

Ensuring the material durability of an electrolyte is a prerequisite for the long-term service of all-solid-state batteries (ASSBs). Herein, to investigate the mechanical integrity of a solid polymer electrolyte (SPE) in an ASSB upon electrochemical operation, we have implemented a sequence of quasi-static uniaxial tension and stress relaxation tests on a lithium perchlorate-doped poly (vinyl alcohol) electrolyte, and then discussed the viscoelastic behavior as well as the strength of SPE film during the physical aging process. On this basis, a continuum electrochemical-mechanical model is established to evaluate the stress evolution and mechanical detriment of aging electrolytes in an ASSB at a discharge state. It is found that the measured elastic modulus, yield stress, and characteristic relaxation time boost with the prolonged aging time. Meanwhile, the shape factor for the classical time-decay equation and the tensile rupture strength are independent of the aging history. Accordingly, the momentary relaxation modulus can be predicted in terms of the time-aging time superposition principle. Furthermore, the peak tensile stress in SPE film for the full discharged ASSB will significantly increase as the aging proceeds due to the stiffening of the electrolyte composite. It may result in the structure failure of the cell system. However, this negative effect can be suppressed by the suggested method, which is given by a 2D map under different lithiation rates and relative thicknesses of the electrolyte. These findings can advance the knowledge of SPE degradation and provide insights into reliable all-solid-state electrochemical device applications.

Mechanical Integrity of Conductive Carbon-Black-Filled Aqueous Polymer Binder in Composite Electrode for Lithium-Ion Battery.

The mechanical stability of aqueous binder and conductive composites (BCC) is the basis of the long-term service of composite electrodes in advanced secondary batteries. To evaluate the stress evolution of BCC in composite electrodes during electrochemical operation, we established an electrochemical-mechanical model for multilayer spherical particles that consists of an active material and a solid-electrolyte-interface (SEI)-enclosed BCC. The lithium-diffusion-induced stress distribution was studied in detail by coupling the influence of SEI and the viscoelasticity of inorganic-filler-doped polymeric bonding material. It was found that tensile hoop stress plays a critical role in determining whether a composite electrode is damaged or not-and circumferential cracks may primarily initiate in BCC, rather than in other electrode components. Further, the peak tensile stress of BCC is at the interface with SEI and does not occur at full lithiation due to the relaxation nature of polymer composite. Moreover, mechanical damage would be greatly misled if neglecting the existence of SEI. Finally, the structure integrity of the binder and conductive system can be effectively improved by (1) increasing the carbon black content as much as possible in the context of meeting cell capacity requirements-it is greater than 27% and 50% for sodium alginate and the mixtures of carboxy styrene butadiene latex and sodium carboxymethyl cellulose, respectively, for composite graphite anode; (2) reducing the elastic modulus of SEI to less than that of BCC; (3) decreasing the lithiation rate.

Towards a Deeper Understanding of Creep and Physical Aging Behavior of the Emulsion Polymer Isocyanate.

Information of the relaxation behaviors of polymer film is crucial to judge the durability of emulsion polymer isocyanate (EPI) as a structural adhesive for bonding timber-based products. A sequence of tensile creep tests and free volume evaluation of the cured EPI adhesive films during isothermal condition were carried out by dynamic mechanical analysis and positron annihilation lifetime spectroscopy, respectively. It is the first time to explore the creep response and physical aging of the EPI film, as well as associated microstructural evolution. The results indicate that the creep characteristics of the glassy EPI coating intimately depend upon the crosslinker and elapsed time, and the ideal momentary creep master curve can be constructed in terms of modified horizontal shifting method. Furthermore, the relaxation process is found to be dominated by vacancy diffusion mechanism. In addition, increasing the polymeric isocyanate content can significantly enhance the resistance to creep deformation of EPI films, but also accelerate the physical aging process. Due to a higher packing degree of pure polymer films, the EPI films with aqueous emulsified isocyanate exhibit much better relaxation resistance compared to that with general isocyanate crosslinker.

Effect of Conductive Carbon Black on Mechanical Properties of Aqueous Polymer Binders for Secondary Battery Electrode.

To predict the cyclic stability of secondary battery electrodes, the mechanical behaviors of polymer binders and conductive composites (BCC) is of great significance. In terms of uniaxial tension, tensile stress relaxation, and bonding strength tests, the present study encompasses a systematic investigation of the mechanical properties of two typical aqueous binders with different contents of Super-S carbon black (SS) under a liquid electrolyte. Meanwhile, the microstructure of cured film and the surface morphology of the bonding interface are investigated in detail. When the weight ratio of SS increases from 0% to 50%, the cured BCC films manifest a higher ratio of tensile strength to modulus and a shorter characteristic relaxation time. Moreover, suitable loadings of SS can improve the tensile shear strength and remarkably reduce the percentage of interface failure of aqueous polymer-bonded Cu current collector. Nevertheless, an excess of carbon black amount cannot maintain its enhancing effect and can even impair the adhesive layer. Finally, a sodium alginate-based polymer composite holds much more superior mechanical properties than the mixture of sodium carboxymethyl cellulose and styrene-butadiene rubber at the same content of carbon black. Noticeably, the two kinds of aqueous polymer doped by 50 wt % of SS exhibit the best adhesive properties.

A Coupled Thermodynamic Model for Transport Properties of Thin Films during Physical Aging.

A coupled diffusion model based on continuum thermodynamics is developed to quantitatively describe the transport properties of glassy thin films during physical aging. The coupled field equations are then embodied and applied to simulate the transport behaviors of O2 and CO2 within aging polymeric membranes to validate the model and demonstrate the coupling phenomenon, respectively. It is found that due to the introduction of the concentration gradient, the proposed direct calculating method on permeability can produce relatively better consistency with the experimental results for various film thicknesses. In addition, by assuming that the free volume induced by lattice contraction is renewed upon CO2 exposure, the experimental permeability of O2 within Matrimid® thin film after short-time exposure to CO2 is well reproduced in this work. Remarkably, with the help of the validated straightforward permeability calculation method and free volume recovery mechanism, the permeability behavior of CO2 is also well elucidated, with the results implying that the transport process of CO2 and the variation of free volume are strongly coupled.

Revealing the Mechanical Properties of Emulsion Polymer Isocyanate Film in Humid Environments.

Knowledge of the mechanical behaviors of polymer film in humid environments is of great significance for predicting the long-term performance of emulsion polymer isocyanate (EPI) as a high-performance wood adhesive. A tri-copolymer latex was cross-linked by the general polymeric methylene diisocyanate (p-MDI) and aqueous emulsified isocyanate (EMDI) at different loadings for preparing EPI. Furthermore, a series of uniaxial tension tests under different relative humidity (RH) were carried out on cured EPI samples before and after post-curing treatment, and the corresponding chemical structure, as well as the microstructure of polymers, was investigated in detail. In addition, a constitutive equation was formulated to calculate the viscoelastic characteristics of the adhesive layer. The results indicate that the EPI films reveal various kinds of intrinsic deformation as RH increases, and the tensile rupture stress and stiffness would obviously decrease, even at cross-linker weight ratios of up to 20%. Furthermore, the moisture resistance could be markedly improved by increasing the isocyanate content and post-cure. Importantly, EMDI-cross-linked film not only exhibits much better mechanical properties than that containing p-MDI at 0⁻80% RH, but is also more sensitive to post-cure. Finally, the derived viscoelastic model could efficiently track moisture-dependent stress-strain curves of EPI films, and the obtained relaxation time further reveals the influence mechanism of isocyanate and post-cure on the mechanical response of the cured polymer under moist conditions.

Analysis of lithium ion concentration and stress in the solid electrolyte interphase on the graphite anode.

One of the critical challenges in advancing lithium ion battery performance is increasing mechanical stability of the solid electrolyte interphase (SEI) layers. Our work aims at developing a mathematical model to study the lithium ion concentration and stress in the SEI on the graphite anode. The main influence factors on the SEI stress have been thoroughly investigated. We find that the ion transportation of the SEI has the underlying effects on the maximum stress in the graphite active layer, especially at a high charging rate. The physical properties of the SEI should be taken into account to obtain an accurate anode stress. The tensile SEI stress along the hoop direction is dominant, and should be regarded as the leading cause of mechanical failure for the SEI. Moreover, the peak stress in the SEI is independent of the charging rate, but can be effectively reduced by rationally designing geometric and material properties of anode components by: (1) decreasing modulus of the SEI itself; (2) enhancing tensile stiffness of the current collector; and (3) making the ratio of anode radius to thickness larger than ten.