Electrochemical performance and thermal stability of the electrospun PTFE nanofiber separator for lithium‐ion batteries

Separator is a very important set of lithium‐ion batteries. At present, low porosity and poor thermal stability are two major disadvantages of separator. In this work, we first apply electrospinning method to prepare the Polytetrafluoroethylene (PTFE) nanofiber separator, which has the advantages of electrospinning method and PTFE materials. The effect of the PTFE nanofiber separator is investigated by scanning electron microscope, Capillary Flow Porometer, thermogravimetric–differential scanning calorimeter, linear sweep voltammeter, AC impedance, and charge/discharge cycling tests. The results demonstrate that the PTFE nanofiber separator has a special fiber structure made from PTFE particles gathering one by one along the fibers. Moreover, the PTFE nanofiber separator exhibits several advantages, including suitable pore diameter, uniform pore size distribution, high porosity, and electrolyte uptake, which enhance the ionic conductivity. The thermal stability of the PTFE nanofiber separator is much better than that of the conventional polyolefin separator. The Li/LiCoO₂ cell equipped with PTFE nanofiber separator exhibits excellent rate performance and first charge–discharge specific capacity of 142 and 131 mA h g^?1, respectively, accompanied by relatively stable cycle performance at 0.2 C rate. It is supposed to be a candidate for application in lithium‐ion batteries. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46508.     

Construction of porous coating layer and electrochemical performances of the corresponding modified polyethylene separators for lithium ion batteries

To effectively improve the affinity of polyethylene (PE) separators with liquid electrolyte without causing a serious pore blockage and to develop a more suitable technology for the industrial production process, porous polyvinylidene fluoride (PVDF) layer‐coated PE separators are prepared by the dip‐coating method followed by a dry‐cast process. Different from previous investigations, a less volatile solvent and a relatively volatile nonsolvent are used to yield a preferable pore structure. A brief introduction on the pore formation mechanism during the dry‐cast process is provided. The pore structure of coating layer is found to be successfully controlled by changing evaporation temperature, nonsolvent content, and PVDF concentration. The porous PVDF coating layer‐modified separators show better affinity with liquid electrolyte and thermal stability. Especially, the ionic conductivity of the modified separator/liquid electrolyte system with a suitable porous coating layer on the separator could reach two times as that of PE separator/liquid electrolyte system, and the cell assembled with modified PE separator shows better cycle performances. This modification process is proved to be a facile, controllable, and effective method for PE separator modification. Meanwhile, this work could provide some theoretical and technical guidance for the production process. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41036.     

Poly(vinylidene fluoride)/poly(acrylonitrile) blend fibrous membranes by centrifugal spinning for high‐performance lithium ion battery separators

Fibrous membranes are promising as high‐performance lithium ion battery separators because of high porosity and superior electrolyte uptake. Electrospinning is a popular approach to produce fibrous membranes, but its production rate is very low. As a comparison, mass production of fibrous membranes can be achieved by centrifugal spinning. This study reports fibrous membranes based on poly(vinylidene fluoride)/poly(acrylonitrile) blends by centrifugal spinning and their application as lithium ion battery separators. The blend fibrous membranes have high electrolyte uptake of about 300%, excellent dimensional stability at 180 °C and good mechanical strength over 18 MPa. The coin cells with the blend fibrous membranes as separators show high discharge capacity of 147.7 mAh/g at 0.2 C and superior C‐rate performance. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44515.     

Polyethylene battery separator with auto‐shutdown ability,thermal stability of 220°C,and hydrophilic surface via solid‐state ultraviolet irradiation

To assure the safety of the lithium‐ion battery, the separator is required to have good thermal stability. Because the single‐layer polyethylene (PE) separator can only tolerate a temperature of 130°C, it is seldom employed currently by lithium‐ion battery manufacturers although its cost is low. In this article, we modified PE separator chain structure through solid‐state ultraviolet (UV) irradiation method to achieve a separator with composite structure of ～40% crystallized PE and ～70% gel content. Approximately 40% crystallized PE chains fulfill the task of auto‐shutdown at 130°C through melting and filling the pores. At the same time, the PE separator can maintain integrity till 220°C because of its highly cross‐linked chain structure. Besides, the modified PE separator is hydrophilic with a water contact angle of 33° after UV treatment and is able to absorb more electrolyte. However, the tensile strength and elongation at the break decreased because the cross‐linking network increased the rigidity. Nevertheless, these values still meet the requirements as the separator for lithium‐ion battery. Considering the low cost and easy preparation, current cross‐linked PE separator has potential to be used in lithium‐ion batteries for various applications, including electric vehicles and energy storage purpose. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42169.     

Preparation and characterization of poly(vinylidene fluoride)-13X zeolite mixed matrix membranes for lithium ion batteries' separator with enhanced performance

13X zeolite was hydrothermally synthesized and poly(vinylidene fluoride) (PVDF)/13X zeolite particles mixed matrix membranes were prepared using phase inversion method as the lithium-ion battery separator. Hydrophilic and porous 13X zeolite loading impacts on the critical separator properties of morphology, wettability, electrolyte uptake, and high temperatures dimensional stability were investigated using scanning electron microscopy, contact angle, and thermal shrinkage analysis. Electrolyte uptake of the 13X zeolite particles loaded PVDF separators increased and also the incorporation facilitated the lithium ions migration (ion conductivity) due to the Lewis acidity of their structure. The 8 wt% 13X zeolite loaded separator (S2) revealed higher porosity (~+20%), electrolyte uptake (+80%), ion conductivity (+80%), and thermal shrinkage (~−47% at 165°C). C-rate capability and cycle performance of a cell battery assembled using the S2 separator considerably improved compared with those of the assembled by the neat PVDF and commercial polypropylene separators.     

Poly(urethane acrylate)‐based gel polymer films for mechanically stable,transparent, and highly conductive polymer electrolyte applications

Polymer electrolytes are attractive for the applications in conventional electrochemical devices and emerging flexible devices. In this study, we developed a poly(urethane acrylate)‐based gel polymer electrolyte with excellent mechanical stability, optical transparency, and a high ionic conductivity. These polymer electrolytes showed excellent dimensional stability and an elastomer‐like behavior with a Shore A hardness in the range of 20–40. The optical transmittance values of these polymers films were over 80% in the visible range. Their ionic conductivities were controlled via changes in the concentration of the linker, dimethylol propionic acid (DMPA), and the lithium salt incorporated into the polymer. The maximum ionic conductivity reached 3.7 mS/cm at room temperature (～23 °C) when the DMPA/poly(ethylene glycol) molar ratio was 0.25, and the ionic conductivity was found to be proportional to the salt concentration. We believe that these polymer electrolytes will be useful in various electrochemical applications where flexibility, high ionic conductivity, and transparency in the electrolytes are necessary. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45009.     

Enhanced mechanical strength and conductivity of PVFM based membrane and its supporting polymer electrolytes

Polyvinyl formal based polymer electrolyte membranes are prepared via the optimized phase inversion method with poly(ethylene oxide) (PEO) blending. The physical properties of blend membranes and the electrochemical properties of corresponding gel polymer electrolytes (GPEs) are characterized by field emission scanning electron microscopy, X‐ray diffraction, differential scanning calorimetry, mechanical strength test, electrolyte uptake test, AC impedance spectroscopy, cyclic voltammetry, and galvanostatic charge–discharge test. The comparative study shows that the appearance of PEO obviously enhances the tensile strength of membranes and the ionic conductivity of corresponding GPEs. When the weight ratio of PEO is 30%, the tensile strength of membrane achieves 12.81 MPa, and its GPE shows high ionic conductivity of 2.20 × 10⁻³ S cm⁻¹, wide electrochemical stable window of 1.9–5.7 V (vs. Li/Li⁺), and good compatibility with LiFePO₄ electrode. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41839.     

New all solid‐state polymer electrolyte based on epoxy resin and ionic liquid for high temperature applications

Flexible epoxy network loaded with high amount of ionic liquid (IL) 1‐decyl‐3‐methylimidazolium bromide ([DMIM]Br) has been reported by using a mixture of polyol amine as curing agent. The IL presents good electrochemical response even at 170 °C, as no evidence of redox reactions was observed. The incorporation of as high as 50 wt % of this IL within the epoxy matrix resulted in solid and flexible electrolyte with good thermal stability below 180 °C, as measured by thermogravimetric analysis and ionic conductivity of around 10^?6 S cm^?1 at room temperature and higher than 10^?3 S cm^?1 at high temperature. This electrolyte presented a prodigious potential for applications in electrochemical devices at high temperature like batteries and supercapacitors, and the flexibility of this solid electrolyte persist at low temperature because of its low glass transition temperature. Furthermore, leakage problems were not observed. Thereby, impedance spectroscopy and cyclic voltammetry were performed to characterize the electrochemical properties. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45838.     

Durability and performance of polystyrene‐b‐poly(vinylbenzyl trimethylammonium) diblock copolymer and equivalent blend anion exchange membranes

Anion exchange membranes (AEM) are solid polymer electrolytes that facilitate ion transport in fuel cells. In this study, a polystyrene‐b‐poly(vinylbenzyl trimethylammonium) diblock copolymer was evaluated as potential AEM and compared with the equivalent homopolymer blend. The diblock had a 92% conversion of reactive sites with an IEC of 1.72 ± 0.05 mmol g^?1, while the blend had a 43% conversion for an IEC of 0.80 ± 0.03 mmol g^?1. At 50°C and 95% relative humidity, the chloride conductivity of the diblock was higher, 24–33 mS cm^?1, compared with the blend, 1–6 mS cm^?1. The diblock displayed phase separation on the length scale of 100 nm, while the blend displayed microphase separation (～10 μm). Mechanical characterization of films from 40 to 90 microns thick found that elasticity and elongation decreased with the addition of cations to the films. At humidified conditions, water acted as a plasticizer to increase film elasticity and elongation. While the polystyrene‐based diblock displayed sufficient ionic conductivity, the films' mechanical properties require improvement, i.e., greater elasticity and strength, before use in fuel cells. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41596.     

Highly efficient fire retardant behavior,thermal stability,and physicomechanical properties of rigid polyurethane foam based on recycled poly(ethylene terephthalate)

This study focused on the burning phenomena, thermal stability, and physicomechanical properties of polyurethane foam based on recycled poly(ethylene terephthalate) (RPUF) with and without halogen-free flame retardants (FRs). Flammability behavior and associated mechanisms were studied by cone calorimetry, LOI, UL 94, FTIR, TGA, FE-SEM, and XPS. The results of cone calorimeter testing indicate improved FR performance with notable reductions in peak heat release rate (~39.1%), peak CO production (~61.7%), and peak CO₂ production (~43.0%). LOI values significantly increase, up to 29.5–47.1%, in the presence of FRs, and V-0 ratings are attained even at a rather low loading of FR (6.07 wt%). Meanwhile, the RPUF completely burns to the holder clamp with a low LOI value (17%), and it do not pass the UL94 HB standard. The addition of FRs notably improves the residual char of RPUF, indicating that FRs contributed to the formation of a barrier layer to protect RPUF during degradation. The comparison between experimentally determined TGA results and calculated values provides support for the effect of FRs on the thermal degradation behavior of RPUF. Sorption isotherm experiments of RPUF/FR systems show low moisture absorptivity and a weak hysteresis effect due to strong intermolecular bonds between RPUF and added FRs. The compression test, density, and morphology of foam samples are also discussed.     

Preparation and characterization of core–shell structure Fe3O4/C nanoparticles with unique stability and high electrochemical performance for lithium-ion battery anode material

Core–shell structure carbon coating Fe₃O₄ nanoparticles are prepared by a two-step method. The crystalline structure and the electrochemical performance of the prepared samples are investigated. The results indicate that a uniform and continuous carbon layer is formed on the surface of Fe₃O₄ nanoparticles. The core–shell structure Fe₃O₄/C nanoparticles show a high initial discharge capacity of 1546 mAh g⁻¹ and a specific stable discharge capacity of about 800 mAh g⁻¹ at 0.5 C with no noticeable capacity fading up to 100 cycles.