Poly Vinylidene Fluoride-hexafluoropropylene Research Articles

Comparison among the performance of LiBOB, LiDFOB and LiFAP impregnated polyvinylidenefluoride-hexafluoropropylene nanocomposite membranes by phase inversion for lithium batteries

This paper describes the physico-chemical and electrochemical properties of polyvinylidenefluoride-hexafluoropropylene (PVdF-HFP) membranes (GPM) prepared by phase inversion technique. Nanocomposite polymer membranes (NCPM) are also prepared by the same technique using AlO(OH)n nanoparticles. The prepared GPM and NCPM are gelled with liquid electrolyte containing three different salts namely, lithium bis(oxalate)borate, lithium fluoroalkylphosphate and lithium difluoro(oxalato)borate. Prepared membranes were subjected to various physico-chemical characterizations likely, mechanical stability, ionic conductivity, morphological studies, surface area and thermal analysis. Electrochemical chemical properties of membranes are evaluated in half-cell configurations (Li/NCPM or GPM/LiFePO4) at room temperature conditions. Galvanostatic cycling profiles clearly indicates the improved performance of chelato borate based anions i.e. BOB and DFOB when compared to fluoroalkyl group (FAP).

Evaporation-induced, close-packed silica nanoparticle-embedded nonwoven composite separator membranes for high-voltage/high-rate lithium-ion batteries: Advantageous effect of highly percolated, electrolyte-philic microporous architecture

Abstract Evaporation-induced, close-packed silica (SiO 2 ) nanoparticle-embedded polyethylene terephthalate (PET) nonwoven composite separator membranes (hereinafter, referred to as “NW-separators”) are fabricated for application in high-voltage/high-rate lithium-ion batteries. The heat-resistant PET nonwoven is employed as a physical support to suppress thermal shrinkage of the NW-separator. A distinctive characteristic of the NW-separator is the well-connected interstitial voids formed between compactly packed SiO 2 nanoparticles adhered by polyvinylidene fluoride–hexafluoropropylene (PVdF–HFP) binders. This allows for the evolution of highly percolated, electrolyte-philic microporous architecture in the NW-separator. In comparison to a commercialized polyethylene (PE) separator, the NW-separator featuring the aforementioned structural uniqueness exhibits substantial improvements in porosity, air permeability, and electrolyte wettability, which contribute to the facile ionic transport and retarded growth of cell impedance during cycling. As a result, superior cell performance is obtained in the NW-separator. Notably, this advantageous effect of the NW-separator on cell performance becomes more pronounced at challenging charge/discharge conditions of high voltages (herein, 4.4 V) and high current densities.

Effect of aging on the ionic conductivity of polyvinylidenefluoride–hexafluoropropylene (PVdF–HFP) membrane impregnated with different lithium salts

The aging towards the ionic conductivity have been studied using of different lithium salts namely, lithium bis(oxalate)borate (LiBOB), lithium difluoro(oxalato)borate (LiDFOB), lithium fluoroalkylphosphate (LiFAP) and LiPF6 in polyvinylidenefluoride–hexafluoropropylene (PVdF–HFP) matrix. The crystallization behavior of LiBOB and LiDFOB has been noticed for the first time during storage of such membranes within the texture of PVdF–HFP matrix. At the same time, such behavior has not been observed in the case of LiFAP and LiPF6 based membranes. The growth of such crystallites would certainly hinder the mobility mechanism of Li+ ions and it has been confirmed by ionic conductivity measurements. The formation of such crystals has been validated through scanning electron microscopic studies.

Application of ployvinylidene fluoride-hexafluoroprorylene in gas diffusion layer of proton exchange membrane fuel cell

The porosity, permeability and other factors of gas diffusion layer (GDL) has a significant impact on the proton exchange membrane fuel cell performance (PEMFC). In this paper, Polytetrafluoroethylene(PTFE) was substituted by polyvinylidene fluoride-hexafluoropropylene(PVDF-HFP) compared with the conventional preparing method of GDL. PVDF-HFP was used in this paper due to it's a semi-crystalline polymer, whose crystalline phase provides good thermal stability while the amorphous phase adds some flexibility, including excellent chemical resistance, thermal resistance and oxidation resistance. Physical properties of PVDF-HFP-based the support layers (SL) involving microstructure, hydrophobicity, gas permeability and performance of PEMFC were investigated. The results show that the GDL based on PVDF-HFP-based the support layers can improve the performance of PEMFC.

Nano SiO2 particle formation and deposition on polypropylene separators for lithium-ion batteries

The novelty of this work is the formation and deposition of SiO2, as opposed to deposition using commercially available SiO2 powder suspension in the solution, to form ceramic coating on polypropylene (PP) separators for lithium-ion battery. The formation of SiO2 nanoparticles with uniform particle size is accomplished through direct hydrolysis of tetraethyl orthosilicate (TEOS), while the deposition of the formed SiO2 on PP separators was conducted in the same solution containing polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) as binders and acetone as the solvent. The effects of the ceramic coating on the surface morphology, tensile strength, contact angles, electrolyte uptake, thermal shrinkage of the PP separators and the cell performances such as battery rate capability and Coulombic efficiency were investigated. The coated separators show significant reduction in thermal shrinkage and improvement in tensile strength, contact angles, electrolyte uptake and battery performance as compared to the plain PP separator.

Enhancement of electrochemical and thermal properties of polyethylene separators coated with polyvinylidene fluoride–hexafluoropropylene co-polymer for Li-ion batteries

The physical effects of a polyethylene (PE) surface on the electrochemical and thermal properties of a PE separator coated with polyvinylidene fluoride (PVDF)–12 wt% hexafluoropropylene (HFP) co-polymer have been investigated for use in lithium ion batteries. To change the surface property of the PE separator, it is treated using gamma ray irradiation. In comparison with the non-irradiated separator, the irradiated separator shows strong affinity for polar solvents due to the carbonyl band formed during gamma ray irradiation. And the PVDF–12 wt%HFP co-polymer is coated on the both sides of the non-irradiated and irradiated separators using a dip-coating process. It is clearly observed that the ionic conductivity of the irradiated separator coated with PVDF–12 wt%HFP is higher than that of the non-irradiated separator coated with the same one. Based on the activation energy calculation, the ionic conductivity is found to strongly depend on the characteristics of the interfaces between them. In addition, rate discharge property and thermal stability of the irradiated separator coated with PVDF–12 wt%HFP are much better than those of the non-irradiated one. The improvement of the thermal stability could be attributed to cross-linking of the support PE separator using gamma ray irradiation.

Thermal stability of ionic liquid-loaded electrospun poly(vinylidene fluoride) membranes and its influences on performance of electrochromic devices

In this work, the effects of thermal annealing on the structures and properties of an ionic liquid (IL) of 1-butyl-3-methylimidazolium hexafluorophosphate-loaded supercritical carbon dioxide-treated electrospun poly(vinylidene fluoride)-hexafluoropropylene (PVDF-HFP) membrane were studied and compared with a corresponding IL/PVDF-HFP gel membrane. The results show that although the annealing causes slight distortion of the nanofibers as well as some small changes in crystallinity and crystal size, the nanofibrous and microporous morphology of the membrane is not altered and hence the membrane can retain its high optical transparency and ionic conductivity after the annealing, whereas the gel exhibits significantly reduced optical transmittance and ionic conductivity owing to annealing-induced phase separation. The IL/PVDF-HFP electrospun membranes before and after annealing were used as the electrolyte layer in electrochromic devices, respectively. The device with the annealed IL/PVDF-HFP electrospun membrane shows no performance deterioration, demonstrating the potential of such thermally stable electrolyte for electrochromic devices used in harsh environments.

Potential application of microporous structured poly(vinylidene fluoride-hexafluoropropylene)/poly(ethylene terephthalate) composite nonwoven separators to high-voltage and high-power lithium-ion batteries

We demonstrate potential application of a new composite non-woven separator, which is comprised of a phase inversion-controlled, microporous polyvinylidene fluoride-hexafluoropropylene (PVdF-HFP) gel polymer electrolyte and a polyethylene terephthalate (PET) non-woven support, to high-voltage and high-power lithium-ion batteries. In comparison to a commercialized polyethylene (PE) separator, the composite non-woven separator exhibits distinct improvements in microporous structure and liquid electrolyte wettability. Based on the understanding of the composite non-woven separator, cell performances of the separator at challenging charge/discharge conditions are investigated and discussed in terms of ion transport of the separator and AC impedance of the cell. The aforementioned advantageous features of the composite non-woven separator play a key role in providing facile ion transport and suppressing growth of cell impedance during cycling, which in turn contribute to superior cell performances at harsh charge/discharge conditions such as high voltages and high current densities.

Improved performance of polyvinylidenefluoride–hexafluoropropylene based nanocomposite polymer membranes containing lithium bis(oxalato)borate by phase inversion for lithium batteries

Nanocomposite polymer electrolyte membranes were prepared by phase inversion technique in polyvinylidenefluoride–hexafluoropropylene (PVdF–HFP) matrix. These membranes were gelled with 0.5 M LiBOB in EC:DEC (1:1 v/v). These gel polymer membranes (GPMs) were incorporated with nanoparticles of AlO(OH) n and prepared composite polymer membranes (CPMs) also. The a.c. impedance analysis shows that AlO(OH) n filled membrane exhibits conductivity of 1.82 × 10 −3 S cm −1 at ambient temperature. The Li/CPM/LiFePO 4 cell delivered a specific discharge capacity of 158 and 147 mAh g −1 at first and at 20th cycle respectively discharged at C/20 rate. The cell experiences a capacity fade of 0.1 mAh g −1 cycle −1 over the investigated 20 cycles. The studies vindicate that AlO(OH) n filled PVdF–HFP polymer membranes could be the potential material to use as separator cum electrolyte in lithium batteries in conjunction with LiFePO 4 as a counterpart.

Irradiated PVdF-HFP-montmorillonite composite membranes for the application of direct ethanol fuel cells

Montmorillonite supported poly vinylidene fluoride–hexa fluoro propylene composite membranes have been functionalized by an irradiation technique for the application of direct ethanol fuel cells. Structural characterizations of the fabricated membranes are examined by infrared and X-ray photoelectron spectroscopy analyses. The montmorillonite particles reduce porosity and stimulate rough nature to the PVdF-HFP membrane. Thermal stability of the irradiated membranes gets increased by the restricted polymeric segmental motion given via the montmorillonite particles. The ion exchange capacity values purely depend on the grafting degree values whereas water uptake values are influenced by the grafting degree as well as the concentration of montmorillonite particles. Irrespective of the grafting degree values, the adsorbed water molecules dissociate sulfonic acid units in a greater extent and favor ionic conductivity values. Tortuous path ways generated by the montmorillonite particles distort the molecular permeation which in turn results in the lower fuel cross over. By the synergistic combination of irradiation and nanocomposite techniques, demerits of the aforementioned individual techniques have been effectively tackled which in turn results in the extended direct ethanol fuel cell efficiencies of the fabricated fuel cell membranes and emerges its large scale applications.

High proton conductivity and low fuel crossover of polyvinylidene fluoride–hexafluoro propylene–silica sulfuric acid composite membranes for direct methanol fuel cells

Proton exchange membranes were fabricated by directly blending polyvinylidene fluoride–hexafluoro propylene with silica sulfuric acid for the application of direct methanol fuel cells. Morphological and structural characterizations of the prepared membranes were characterized by scanning electron microscopy and infrared spectroscopy, respectively. The performed thermal gravimetric analysis ensured high thermal stability of the prepared membranes. The ion transport and fuel permeation of the fabricated composite membranes were solely governed by the hydrophilic and hydrophobic regions of silica sulfuric acid and host polymer matrix, respectively. The fabricated composite membrane exhibited a maximum fuel cell power density of 43 mW/cm 2 through the high ionic conductivity and low fuel permeability parameters and can be considered as a good contender for direct methanol fuel cells.

Closely packed SiO 2 nanoparticles/poly(vinylidene fluoride-hexafluoropropylene) layers-coated polyethylene separators for lithium-ion batteries

In an effort to improve thermal shrinkage and electrochemical performance of a separator for a lithium-ion battery, we develop a new composite separator by introducing ceramic coating layers onto both sides of a polyethylene (PE) separator. The ceramic coating layers are comprised of SiO 2 nanoparticles and polyvinylidene fluoride-hexafluoropropylene (PVdF-HFP) binders. In comparison to the dense structure of conventional nanocomposite coating layers, the ceramic coating layers are featured with close-packed SiO 2 nanoparticles, which affords a well-developed porous structure, i.e. highly connected interstitial voids formed between the nanoparticles. On the basis of this structural understanding of the composite separators, the effects of ceramic coating layers on the separator properties are investigated as a function of SiO 2 powder size. Owing to the existence of the heat-resistant SiO 2 coating layers, the composite separators show significant reduction in thermal shrinkage than the pristine PE separator. More intriguingly, in comparison to the large-sized (=530 nm) SiO 2, the small-sized (=40 nm) SiO 2 offers a large number of SiO 2 nanoparticles in the ceramic coating layers, high porosity contributing to facile ion transport, and small increase in the cell impedance, which consequently allows substantial improvements in cell performances as well as thermal shrinkage of the separator.

Effect of microporous structure on thermal shrinkage and electrochemical performance of Al 2O 3/poly(vinylidene fluoride-hexafluoropropylene) composite separators for lithium-ion batteries

In a bid to develop a separator with improved thermal shrinkage that critically affects internal short-circuit failures of lithium-ion batteries, we demonstrate a new approach, which is based on introducing microporous composite coating layers onto both sides of a polyethylene (PE) separator. The composite coating layers consist of alumina (Al 2O 3) nanoparticles and polyvinylidene fluoride-hexafluoropropylene (PVdF-HFP) gel polymer electrolytes. The microporous structure of the composite coating layers is considered a crucial factor governing the thermal shrinkage and electrochemical performance of the separators. The microporous structure is determined by controlling the phase inversion, specifically the solvent–nonsolvent miscibility, in the coating solutions. To quantitatively identify the effect of the solvent–nonsolvent miscibility, three different nonsolvents are chosen in the decreasing order of solubility parameter ( δ) difference against the solvent (acetone, δ = 20 MPa 1/2), which are respectively water ( δ = 48 MPa 1/2), butyl alcohol ( δ = 29 MPa 1/2), and isopropyl alcohol ( δ = 24 MPa 1/2). The microporous structure of the composite coating layers becomes more developed with the increase of not only the nonsolvent content but also the solubility parameter difference between acetone and nonsolvent. Based on this observation, we investigate the influence of the morphological variations on the thermal shrinkage and electrochemical performance of the composite separators.

A study on the role of BaTiO3 in lithum bis(perfluoroethanesulfonyl)imide-based PVDF-HFP nanocomposites

Lithium bis(perfluoroethanesulfonyl)imide (BETI; guest species)-based polyvinylidenefluoride-hexafluoropropylene (PVDF-HFP) (host matrix) polymer nanocomposites (PNC) films by loading barium titanate (BaTiO3) as a filler in ascending proportions with plasticizer (mixture of EC + DMC) while keeping host and guest content as constants has been investigated by employing AC impedance, thermal, X-ray diffraction (XRD), phase morphology, and Fourier transform infrared (FTIR) studies. The ionic conductivity measurements on these PNC show that 2.5% BaTiO3-loaded polymer nanocomposites (PNC) showed mitigation in magnitude of the conductivity compared with that of 0 wt.% loaded PNC; but increase in conductivity is noted thereafter with increase in filler content of up to 7.5 wt.%. The higher conductivity is observed for 7.5% filler-loaded membrane. The XRD study identifies suppression of polymer phase associated with (200) plane. The SEM image illustrates inhomogeneity in surface morphologies for PNCs with the filler dispersed. The thermal profile registers the endothermic changes associated with polymer host indicating a varying heat of fusion ∆Hm with filler increase. FTIR studies confirm possible interaction between various constituents of the PNCs.

Preparation and Characterization of Porous PVdF-HFP/clay Nanocomposite Membranes

Polyvinylidene fluoride-hexafluoropropylene (PVdF-HFP) / clay nanocomposite membranes were prepared by phase inversion method through controlling retention time to apply for a lithium ion secondary batteries. Increased membrane porosity with macrovoids was observed at increasing retention time. Partially intercalated structures of PVdF-HFP/clay nanocomposite membranes were confirmed by X-ray diffraction (XRD) analysis. PVdF-HFP membranes containing various kind of clay showed the increase of membrane modulus compared to the pristine PVdF-HFP membrane.

A novel poly(vinylidene fluoride-hexafluoropropylene)/poly(ethylene terephthalate) composite nonwoven separator with phase inversion-controlled microporous structure for a lithium-ion battery

We demonstrate a novel and facile approach to fabrication of a new composite nonwoven separator for a lithium-ion battery, which comprises a phase inversion-controlled, microporous polyvinylidene fluoride-hexafluoropropylene (PVdF-HFP) gel polymer electrolyte and a polyethylene terephthalate (PET) nonwoven support. The thermally stable PET nonwoven is chosen as a mechanical support and contributes to improving the thermal shrinkage of the composite nonwoven separator. The microporous PVdF-HFP gel polymer electrolyte serves as a pore size controller for the composite nonwoven separator. In order to provide a theoretical basis for this approach, an investigation of the phase diagram for coating solutions consisting of PVdF-HFP, solvent (acetone), and nonsolvent (water) is preceded. Based on this understanding of the phase behavior, the effects of phase inversion, more specifically, the water content in the coating solutions, on the morphology evolution of the composite nonwoven separators are identified. The phase inversion-governed microporous structures of the composite nonwoven separators play a crucial role in determining electrochemical performances of cells. The composite nonwoven separator is expected to be a promising alternative to a commercialized polyethylene (PE) separator, particularly in next-generation lithium-ion batteries necessitating superior battery safety and performance.

Al2O3/PVdF_HFP 세라믹코팅층의 미세기공구조가 리튬이차전지용 복합분리막의 열 안정성 및 전기화학특성에 미치는 영향

양극 (cathodes)과 음극 (anodes)이 서로 물리적으로 닿게되는 내부 단락 (internal short-circuit) 현상은 리튬이차전지 안전성 (safety) 이슈의 주요 원인으로 고려되고 있으며, 분리막 (separators)의 열 안정성 (thermal stability)에 의해 크게 영향을 받는 것으로 알려져 있다. 본 연구에서는 기존 폴리올레핀 (polyolefin) 계열 분리막에 비해 열 안정성이 현저히 개선된 세라믹 복합막 구조의 신규 분리막을 개발하여, 전지 내부 단락 발생을 억제하고자 하였다. 본 연구의 복합분리막은 알루미나 (<TEX>$Al_2O_3$</TEX>) 나노입자와 PVdF-HFP (polyvinylidene fluoride-hexafluoropropylene) 바인더로 구성된 세라믹 코팅층을, 폴리에틸렌 (polyethylene, PE) 분리막 양면에 도입시킴으로써 제조되었다. 세라믹 코팅층의 모폴로지는 코팅용액의 상전이 (phase inversion) 현상 제어를 통해 결정되었으며, 비용매 (물) 함량이 증가함에 따라 기공크기 및 기공구조가 보다 더 발달되었다. 이러한 세라믹 코팅층의 기공구조 변화는 복합분리막의 열 안정성 및 전기화학특성에 큰 영향을끼치는 것으로 관찰되었으며, 이를 상전이 현상 관점에서 체계적으로 해석하였다. The internal short-circuit between cathodes and anodes has been known to be a critical concern for the safety failures of lithium-ion batteries, which is strongly influenced by the thermal stability of separators. In this study, to effectively suppress the internal short-circuit failures, we developed a new composite separator with the improved thermal stability compared to conventional polyolefin-based separators. The composite separators were prepared by introducing a ceramic coating layer (<TEX>$Al_2O_3$</TEX>/PVdF-HFP) onto both sides of a polyethylene (PE) separator. The microporous structure of ceramic coating layers is determined by controlling the phase inversion of coating solutions and becomes more developed with the increase of nonsolvent (water) content. This structural change of ceramic coating layers was observed to greatly affect the thermal stability as well as the electrochemical performance of composite separators, which was systematically discussed in terms of phase inversion.

High ion and lower molecular transportation of the poly vinylidene fluoride–hexa fluoro propylene hybrid membranes for the high temperature and lower humidity direct methanol fuel cell applications

Poly vinylidene fluoride–hexa fluoro propylene hybrid membranes are prepared by directly blending the copolymer with different sizes of acidified stannous oxide particles. Steric repulsion between the hydrophobic host polymer and hydrophilic acidified particles generates cavities in the hybrid membranes. The infrared and energy dispersive spectroscopic studies confirmed the structural characterization of the hybrid membranes. The high acid moieties of stannous oxide particles conscript higher ion channels whereas the hygroscopic property favors high water molecules retention and provokes the self-humidification of the hybrid membranes. Though high ionic conductivities are achieved for the hybrid membranes, inferior methanol permeabilities are also maintained due to the hydrophobic units and methanol resistive behavior of host polymer and stannous oxide particles, respectively. By the combined efforts of high ion and lower molecular transportation, direct methanol fuel cells performance is facilitated for the fabricated hybrid membranes and influences its potential application in fuel cells field.

Enhanced breakdown strength of multilayered films fabricated by forced assembly microlayer coextrusion

There is a need in electronic systems and pulsed power applications for capacitors with high energy density. From a material standpoint, capacitive energy density improves with increasing dielectric constant and/or breakdown strength. Current state-of-the-art polymeric capacitors are, however, limited in that their dielectric constant is low (2–4). Our approach to improve polymer film capacitors is to combine, through microlayer coextrusion, two polymers with complementary properties: one with a high breakdown strength (polycarbonate) and one with a high dielectric constant (polyvinylidene fluoride-hexafluoropropylene). As opposed to the monolith controls, multilayered films with various numbers of layers and compositions subjected to a pulsed voltage exhibit treeing patterns that hinder the breakdown process. Consequently, substantially enhanced breakdown strengths are measured in the mutilayered films. It is further shown, by varying the overall film thickness, that the charge at the tip of the needle electrode is a key parameter that controls treeing. Based on the acquired data, a breakdown mechanism is formulated to explain the increased dielectric strengths. Using the understanding gained from these systems, selection and optimization of future layered structures can be carried out to obtain additional property enhancements.

Lithium fluoroalkylphosphate based novel composite polymer electrolytes (NCPE) incorporated with nanosized SiO 2 filler

This paper describes the preparation and characterization of polyvinylidene fluoride–hexafluoropropylene (PVdF–HFP) based nanocomposite polymer electrolyte (NCPE). For the first of its kind lithium fluoroalkylphosphate (LiPF 3(CF 3CF 2) 3) was incorporated as electrolyte salt in the polymer skeleton. Ethylene carbonate and diethyl carbonate mixture (1:1, wt/wt) was used as a plasticizing agent and SiO 2 nanoparticle as filler. The NCPE membranes were characterized by a.c. impedance, Scanning electron microscope, Differential scanning calorimetry, Fourier transform infrared and fluorescence studies. An electrolyte with 2.5 wt% SiO 2 exhibited a conductivity of 1.16 mS cm −1 at ambient temperature. It was found that filler contents above 2.5 wt% rendered the membranes less conducting. Activation energy and percentage of crystallinity has also been calculated.