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.     

Synthesis and characterization of LiFePO4–carbon nanofiber–carbon nanotube composites prepared by electrospinning and thermal treatment as a cathode material for lithium‐ion batteries

Binder‐free LiFePO₄–carbon nanofiber (CNF)–multiwalled carbon nanotube (MWCNT) composites were prepared by electrospinning and thermal treatment to form a freestanding conductive web that could be used directly as a battery cathode without addition of a conductive material and polymer binder. The thermal decomposition behavior of the electrospun LiFePO₄ precursor–polyacrylonitrile (PAN) and LiFePO₄ precursor–PAN–MWCNT composites before and after stabilization were studied with thermogravimetric analysis (TGA)/differential scanning calorimetry and TGA/differential thermal analysis, respectively. The structure, morphology, and carbon content of the LiFePO₄–CNF and LiFePO₄–CNF–MWCNT composites were determined by X‐ray diffraction, high‐resolution transmission electron microscopy, Raman spectroscopy, scanning electron microscopy, and elemental analysis. The electrochemical properties of the LiFePO₄–CNF and LiFePO₄–CNF–MWCNT composite cathodes were measured by charge–discharge tests and electrochemical impedance spectroscopy. The synthesized composites with MWCNTs exhibited better rate performances and more stable cycle performances than the LiFePO₄–CNF composites; this was due to the increase in electron transfer and lithium‐ion diffusion within the composites loaded with MWCNTs. The composites containing 0.15 wt % MWCNTs delivered a proper initial discharge capacity of 156.7 mA h g^?1 at 0.5 C rate and a stable cycle ability on the basis of the weight of the active material, LiFePO₄. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43001.     

Preparation of 4A zeolite coated polypropylene membrane for lithium-ion batteries separator

This study aims to improve wettability and thermal resistance of lithium-ion batteries separators. For this purpose, a commercial polypropylene (PP) separator was coated by 4A zeolite using poly(vinylidene fluoride) as binder and effects of the separators' zeolite content was investigated. All the coated separators showed lower contact angles, higher electrolyte uptakes, and less thermal shrinkages compared to the neat commercial separator. The coated PPA8 separator (zeolite to binder ratio of 8) showed the lowest wettability (contact angle of 0°) and electrolyte uptake (270%) due to its surface porosity resulting from the zeolite particles interstitial cavities as well as their internal cavities. Also, the PPA8 separator ion conductivity was found as 2.25 mS cm⁻¹ and C-rate and cycling performance of its assembled battery were higher compared to those of the commercial PP separator assembled battery. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47841.     

Resin-silica composite nanoparticle grafted polyethylene membranes for lithium ion batteries

To avoid the peeling-off of ceramic nanoparticles (NPs) from polyolefin membranes usually occurred in commercially available ceramic NPs coated polyolefin separators for lithium batteries, we propose a simple one-pot in-situ reaction method to modify commercial polyethylene (PE) separators by surface grafting 3-Aminophenol/formaldehyde (AF)/silica (SiO₂) composite NPs. The AF/SiO₂ composite NPs form self-supporting connected pores on the modified layer of the separator surface, which ensures the transportation of Li⁺. Moreover, the PE@AF/SiO₂ separators has higher electrolyte wettability and compatibility than neat PE separators attributed to the plentiful polar functional groups in the AF/SiO₂ layer and AF/SiO₂ composite NPs, resulting in higher lithium ion transference number (= 0.62) and ionic conductivity (σ = 0.722 mS cm⁻¹). More importantly, the discharge capacity, capacity retention rate and coulombic efficiency (136.2 mA h g⁻¹, 87.9% and 99%, respectively) after 200 cycles of Li|NMC half batteries with PE@AF/SiO₂ separators, are all more excellent than that with the pure PE separator (125 mA h g⁻¹, 83.1% and 85%, respectively). Our results show that the PE@AF/SiO₂ separators obtained by this modification method have higher electrochemical stability in the lithium battery system.     

Composite sodium p‐toluene sulfonate–polypyrrole–iron anode for a lithium‐ion battery

In this study, p‐toluene sulfonate (TsONa) doped polypyrrole (PPy) was synthesized for an anode in a lithium‐ion battery via a one‐step facile electropolymerization on Fe foil. The obtained TsONa–PPy–Fe composite electrode was investigated with scanning electron microscopy, energy‐dispersive X‐ray spectroscopy, Fourier transform infrared spectroscopy, and galvanostatic charge–discharge profiling. As expected, many irregular microspherical particles of TsONa‐doped PPy formed and combined tightly with the surface of Fe foil. Furthermore, the obtained TsONa–PPy–Fe anode also delivered satisfactory electrochemical performances. For example, the reversible capacity was still about 105–115 mAh/g, even after at least 50 cycles. The high lithium storage activity of PPy and the high conductivity of the TsONa‐doped PPy jointly contributed into the satisfactory electrochemical performances. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44935.     

ForceSpinning of polyacrylonitrile for mass production of lithium‐ion battery separators

We present results on the Forcespinning^® (FS) of Polyacrylonitrile (PAN) for mass production of polymer nanofiber membranes as separators for Lithium‐ion batteries (LIBs). Our results presented here show that uniform, highly fibrous mats from PAN produced using Forcespinning^®, exhibit improved electrochemical properties such as electrolyte uptake, low interfacial resistance, high oxidation limit, high ionic conductivity, and good cycling performance when used in lithium ion batteries compared to commercial PP separator materials. This article introduces ForceSpinning^®, a cost effective technique capable of mass producing high quality fibrous mats, which is completely different technology than the commonly used in‐house centrifugal method. This Forcespinning^® technology is thus the beginning of the nano/micro fiber revolution in large scale production for battery separator application. This is the first time to report results on the cycle performance of LIB‐based polymer nanofiber separators made by Forcespinning^® technology. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 132, 42847.     

A phase separation method toward PPTA–polypropylene nanocomposite separator for safe lithium ion batteries

The building of separators with high thermal stability and security is important for lithium ion batteries. A novel, simple and successive process, which aims at coating poly‐p‐phenylene terephthamide (PPTA) onto commercial polypropylene (PP) separators, has been demonstrated. Without any additional binder, the PPTA nanofiber coating layer sticks to the porous PP separators by physical anchoring, endowing the composite separators with modified wettability toward electrolytes and heat resistance. Meanwhile, migration routes for lithium ions are guaranteed by the porous structure controlled by the self‐assembly of the fibrillar units during the nonsolvent induced phase separation. The cells equipped with the composite separators show better cycling performances. Moreover, the system based on the composite separators shows a sharp drop in ion conductivity after heat treatment at 200 °C for a certain period, indicating the shutdown effect of the composite separators, which can contribute to additional safety of lithium ion batteries. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 134, 46697     

Effects of macromolecular configuration of thermally sensitive binder in lithium‐ion battery

In order to suppress heat generation of nail‐penetrated lithium‐ion battery (LIB) cell, thermally sensitive binders (TSB) based on poly(vinylidene fluoride) (PVDF) and poly(vinylidene fluoride‐co‐hexafluoropropylene) (PVDF‐HFP) were investigated. The testing data showed that with appropriate treatment, TSB could efficiently reduce the peak temperature associated with internal shorting, and did not influence the cycling performance of LIB. The molecular weight of TSB was not a vital factor, while crosslinking was critical. This technology can be used to mitigate thermal runaway of LIB, enabling safe and robust large‐scale energy storage. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45078.     

Polypropylene separator coated with a thin layer of poly(lithium acrylate‐co‐butyl acrylate) for high‐performance lithium‐ion batteries

In this paper, poly(lithium acrylate‐co‐butyl acrylate) [P(AALi‐co‐BA)] was synthesized, and a P(AALi‐co‐BA)‐coated polypropylene (PP) separator was prepared by a simple dip‐coating process. In contrast to the conventional thick, dense gel polymer coating layer, a thin P(AALi‐co‐BA) layer was formed on the PP separator, which had less influence on the pore structure of the original PP separator and was beneficial for the migration of lithium ions through the separator. Furthermore, the AALi units in the copolymer could improve the wettability of the separator, while the BA units provided the separator with strong adhesion to the electrodes. As expected, the modified separators showed good wettability, high ionic conductivity, and excellent interface stability. In addition, the cycle stability and rate performance were also improved significantly. This facile, affordable, and effective method has great application potential for the modification of polyolefin‐based separators. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46423.     

Solid polymer electrolyte based on waterborne polyurethane for all‐solid‐state lithium ion batteries

A series of solid polymer electrolytes (SPEs) based on comb‐like nonionic waterborne polyurethane (NWPU) and LiClO₄ are fabricated via a solvent free process. The NWPU‐based SPEs have sufficient mechanical strength which is beneficial to their dimensional stability. Differential scanning calorimetry analysis indicates that the phase separation occurs by the addition of the lithium salt. Scanning electron microscopy and X‐ray diffraction analyses illustrate the good compatibility between LiClO₄ and NWPU. Fourier transform infrared study reveals the complicated interactions among lithium ions with the amide, carbonyl and ether groups in such SPEs. AC impedance spectroscopy shows the conductivity of the SPEs exhibiting a linear Arrhenius relationship with temperature. The ionic conductivity of the SPE with the mass content of 15% LiClO₄ (SPE15) can reach 5.44 × 10^?6 S cm^?1 at 40 °C and 2.35 ×10^?3 S cm^?1 at 140 °C. The SPE15 possesses a wide electrochemical stability window of 0–5 V (vs. Li+/Li) and thermal stability at 140 °C. The excellent properties of this new NWPU‐based SPE are a promising solid electrolyte candidate for all‐solid‐state lithium ion batteries. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134 , 45554.     

Highly selective sulfonated poly(vinylidene fluoride‐co‐hexafluoropropylene)/poly(ether sulfone) blend proton exchange membranes for direct methanol fuel cells

Proton exchange membranes (PEMs) based on blends of poly(ether sulfone) (PES) and sulfonated poly(vinylidene fluoride‐co‐hexafluoropropylene) (sPVdF‐co‐HFP) were prepared successfully. Fabricated blend membranes showed favorable PEM characteristics such as reduced methanol permeability, high selectivity, and improved mechanical integrity. Additionally, these membranes afford comparable proton conductivity, good oxidative stability, moderate ion exchange capacity, and reasonable water uptake. To appraise PEM performance, blend membranes were characterized using techniques such as Fourier transform infrared spectroscopy, AC impedance spectroscopy; atomic force microscopy, and thermogravimetry. Addition of hydrophobic PES confines the swelling of the PEM and increases the ultimate tensile strength of the membrane. Proton conductivities of the blend membranes are about 10^?3 S cm^?1. Methanol permeability of 1.22 × 10^?7cm² s^?1 exhibited by the sPVdF‐co‐HFP/PES10 blend membrane is much lower than that of Nafion‐117. AFM studies divulged that the sPVdF‐co‐HFP/PES blend membranes have nodule like structure, which confirms the presence of hydrophilic domain. The observed results demonstrated that the sPVdF‐co‐HFP/PES blend membranes have promise for possible usage as a PEM in direct methanol fuel cells. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43907.     

Solution‐blown SPEEK/POSS nanofiber–nafion hybrid composite membranes for direct methanol fuel cells

This study aims to develop novel hybrid composite membranes (NHMs) by impregnating Nafion solution into the porous sulfonated poly(ether ether ketone)/polyhedral oligomeric silsesquioxanes (SPEEK/POSS) nanofibers (NFs). The composite membrane was prepared by solution blowing of a mixture of SPEEK/POSS solution. The characteristics of the SPEEK/POSS NFs and the NHMs, including morphology, thermal stability, and performance of membrane as PEMs, were investigated. The performance of NHMs was compared with that of Nafion117 and SPEEK/Nafion composite membranes. Results showed that the introduction of POSS improved the proton conductivity, water swelling, and methanol permeability of membranes. A maximum proton conductivity of 0.163 S cm^?1 was obtained when the POSS content was 6 wt % at 80°C, which was higher than that of Nafion117 and SPEEK/Nafion. NHMs could be used as proton exchange membranes (PEMs) for fuel cell applications. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42843.     

Polydopamine hydrophilic modification of polypropylene separator for lithium ion battery

The modified polypropylene (PP) separators with self‐polymerization of dopamine on the surfaces are prepared by a simple solution‐immersion method to improve the interfacial hydrophilic and discharge performance. The contact angle test and the liquid electrolyte uptake capacity test results show that the wettability and the electrolyte‐retention ability of polydopamine‐modified separator are improved significantly. The robust and thin polydopamine layer on the surface also enhances thermal performance and tensile strength of the modified PP separator certified by DSC and tensile strength tests. The ionic conductivity of the modified PP separator is up to 3.08 mS·cm^?1, ～2.5 times of the bare separator. Good discharge capacity retention and C‐rate discharge performance are demonstrated by a 2025 coin half‐cell with the liquid electrolyte‐soaked polydopamine modified PP separator sandwiched between lithium metal anode and LiFePO₄ cathode. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40543.     

Solid‐state single‐ion conducting comb‐like siloxane copolymer electrolyte with improved conductivity and electrochemical window for lithium batteries

Achievement of high conductivity and electrochemical window at ambient temperature for an all‐solid polymer electrolyte used in lithium ion batteries is a challenge. Here, we report the synthesis and characterization of a novel solid‐state single‐ion electrolytes based on comb‐like siloxane copolymer with pendant lithium 4‐styrenesulfonyl (perfluorobutylsulfonyl) imide and poly(ethylene glycol). The highly delocalized anionic charges of ? SO₂? N^(–)? C₄F₉ have a weak association with lithium ions, resulting in the increase of mobile lithium ions number. The designed polymer electrolytes possess ultra‐low glass transition temperature in the range from ?73 to ?54 °C due to the special flexible polysiloxane. Promising electrochemical properties have been obtained, including a remarkably high conductivity of 3.7 × 10^?5 S/cm and electrochemical window of 5.2 V (vs. Li⁺/Li) at room temperature. A high lithium ion transference number of 0.80, and good compatibility with anode were also observed. These prominent characteristics endow the polymer electrolyte a potential for the application in high safety lithium ion batteries. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45848.     

Sulfur/PAN/acetylene black composite prepared by a solution processing technique for lithium–sulfur batteries

A sulfur/poly(acrylonitrile)–PAN/acetylene black–AB composite, comprising sulfur and PAN encapsulated in the pores of AB was prepared by a solution‐based technique with dimethyl sulfoxide as the solvent. The composite was characterized by TGA, X‐ray diffraction, FTIR, Raman, SEM, TEM, and BET studies. The composite exhibited a high discharge capacity of 1330 mAh/g in the first cycle. The AB additive plays multiple roles in the composite, acting as a conducting matrix for electron transport and as a porous framework that adsorbs and retains electrolyte. The presence of PAN along with the porous carbon matrix in the composite provides the necessary resilience to absorb strains due to volume expansion during cycling. The observed improved performance of the composite is primarily attributed to the small size and homogeneous distribution of sulfur in the composite. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46598.     

Flexible freestanding cotton–graphene composites for lithium‐ion batteries

Flexible freestanding cotton–graphene (CGN) composites were prepared by a simple immersion and freeze‐drying method and a thermal annealing process together. The composites had a constant cotton microstructure covered by graphene. The microstructure and morphology of the composites could be easily adjusted through the variation of the thermal annealing temperatures. Electrochemical tests demonstrated that the annealing temperatures had great effects on the electrochemical performances of the obtained composites. The CGN composite annealed at 700 °C exhibited a reversible capacity of 245.2 mAh/g after 100 cycles. Even after it was bent 1000 times, the CGN composite still maintained its superior electrochemical properties. The results suggest that because of its high flexibility and excellent conductive and electrochemical activities, the CGN composites could be used as lithium‐ion battery anode materials on a large scale for corresponding applications. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44727.     

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.     

Sulfonated poly(styrene)s‐PBIOO® blend membranes: Thermo‐oxidative stability and conductivity

Low‐cost polymers poly(styrene) and poly(α‐methylstyrene) have been sulfonated followed by blending with PBIOO® (30 wt % sulfonated ionomer, 70 wt % PBIOO). At this polymer ratio the sulfonated ionomer served as the macromolecular acidic cross‐linker which led to enhancement of the PBIOO stability. Both membrane types were treated with Fenton's Reagent to investigate their resistance to oxidation and radical attack. Indeed, the blend membranes showed enhanced stability in oxidative conditions compared to the pure PBIOO membranes. Furthermore, the sulfonated poly(α‐methylstyrene)‐PBIOO blend membrane showed less weight loss during and after Fenton's Test than the corresponding poly(styrene sulfonic acid)‐PBIOO membrane. Assuming all the characteristics of the blend membrane before and after the Fenton's Test, we concluded for a partial degradation of both sulfonated poly(styrene)s, whereas they remain in the blend membrane matrix due to the acid‐base crosslinking. Thus, since the sulfonated poly((α‐methyl)styrene)‐PBIOO blend membranes conserved their integrity even after Fenton's Test they can be regarded as potential low‐cost high‐T fuel cell membranes. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39889.     

Mitigating thermal runaway of lithium‐ion battery by using thermally sensitive polymer blend as cathode binder

Thermally sensitive binder (TSB) is developed as an internal safety mechanism of lithium‐ion battery (LIB). The TSB is a polymer blend of poly(vinylidene fluoride) (PVDF) and poly(vinylidene fluoride‐co‐hexafluoropropylene) (PVDF‐HFP). Compared with regular PVDF binder, the softening and swelling of TSB are more pronounced when temperature is above 110 °C. With the TSB, the cycling performance of LIB cell is not affected; upon nail penetration, the heat generation rate is significantly reduced. The crystallinity of TSB is an important factor. This technology may lead to the development of thermal‐runaway‐mitigating LIB cells. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45737.     

Evolution of mechanical properties of polypropylene separator in liquid electrolytes for lithium‐ion batteries

Knowledge of the mechanical behaviors of polymeric separators immersed in liquid electrolytes is of great significance for predicting the long‐term performance of lithium batteries with high performance and safety. In terms of tensile tests, heating shrinkage, and dynamic mechanical analysis as well as the essential work of fracture method, the study reported here encompasses a systematic investigation of the mechanical properties of a typical commercial polypropylene separator in mixtures of ethylene carbonate and dimethyl carbonate and lithium hexafluorophosphate (LiPF₆), comparing with the results in ionic liquid (IL) electrolyte composed of lithium tetrafluoroborate (LiBF₄) and 1‐butyl‐3‐methylimidazolium tetrafluoroborate (BMIBF₄) and dry condition. It has been found that liquid electrolytes have obvious negative effect on the dimensional stability at elevated temperature and mechanical properties, especially on crack resistance of the polymer separator. LiBF₄‐BMIBF₄ has much smaller damage on the strength, Young's modulus and fracture toughness of separator than the organic solution except the dynamic modulus at high temperature. Notably, the maximum tensile stress, Young's modulus and the reciprocal of relaxation time of the polymer separator are linearly dependent with strain rate under quasi‐static condition, and the relaxation time has clarified the coupling effect mechanism of liquid electrolyte and loading rate. Moreover, the non‐dimensional viscoelastic constitute equation could perfectly track the tensile behavior of wet and dry separators at different strain rate, and a property model could well characterize the temperature‐dependent storage modulus of polymer separators from rubbery to viscous state. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46441.