Graft Copolymer Electrolytes Research Articles

One‐Step Polymerizations Enable Facile Construction and Structural Optimization of Graft Copolymer Electrolytes

AbstractThe development of poly(ethylene oxide) (PEO)‐based solid polymer electrolytes (SPEs) is limited by the semi‐crystalline nature of PEO and the extremely strong EO‐Li+ interactions. To promote the rapid migration of Li+, a one‐step method combining radical polymerization and ring‐opening polymerization catalyzed simultaneously by lithium carboxylate is proposed to construct multi‐component graft copolymer electrolytes (GCPEs) in this study. Tailored macroinitiator with catalytic and initiated sites (PAALi(OH‐Br)) realizes one‐step polymerizations of vinyl monomers and cyclic monomers, and provides GCPEs with poly(ethylene glycol) (PEG) and poly(ε‐caprolactone) (PCL) side chains. The grafted structure of GCPE greatly facilitates the intra‐chain hopping of Li+, resulting in excellent ionic conductivity. The introduction of PCL further improves the tLi+ of GCPE. The three‐component graft copolymer electrolyte constructed by polystyrene (PS), PEO, and PCL exhibits high tensile stress (1.62 MPa), a high ionic conductivity (2.4 × 10−5 S cm−1, 30 °C), and a high tLi+ of 0.47 and high electrochemical stability.

Excellent film-forming, ion-conductive, zwitterionic graft copolymer electrolytes for solid-state supercapacitors

Polymer electrolytes with robust mechanical properties and high ionic conductivity are highly desirable for solid-state energy storage devices. Therefore, in this study, a polymer electrolyte based on a zwitterionic graft copolymer, poly(vinyl alcohol)-graft-poly(sulfobetaine methacrylate) (PVA-g-PSBMA), is developed. Herein, we designed a graft copolymer that utilizes the excellent film-forming nature of PVA main chains and ion-conductive properties of polyzwitterionic PSBMA side chains. The PVA-g-PSBMA electrolytes exhibited microphase separated structures, wherein H3PO4 interacted more strongly with the PSBMA domains. Therefore, the transport of proton ions was facilitated through the well-defined PSBMA-based ion channels, leading to a significant increase in the ionic conductivity. The polymer electrolyte was applied to the electric double-layer capacitor (EDLC) with activated carbon-based electrode. In comparison with the neat conventional PVA-based cell, the solid EDLC with PVA-g-PSBMA zwitterionic graft copolymer electrolytes showed improved specific capacitance (46.6 F g−1), energy density (3.67 Wh kg−1), and power density (186 W kg−1). The improved performance can be attributed to the higher ionic conductivity and better interfacial compatibility of electrode/electrolyte for the PVA-g-PSBMA electrolytes.

High-performance solid-state bendable supercapacitors based on PEGBEM-g-PAEMA graft copolymer electrolyte

We report a solid electrolyte comprising poly(ethylene glycol) behenyl ether methacrylate-g-poly((2-acetoacetoxy)ethyl methacrylate) (PEGBEM-g-PAEMA) graft copolymer for application to solid-state bendable supercapacitors. The graft copolymer was synthesized via free-radical polymerization, and the electrolytes were fabricated by adding 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4). Fourier-transform infrared spectroscopy, differential scanning calorimetry, thermogravimetric analysis, X-ray diffraction, small-angle X-ray scattering, and transmission electron microscopy were employed to investigate the chemical, structural, thermal, and morphological properties of the fabricated electrolytes and the compatibility of the two components. The graft copolymer interacted with EMIMBF4 (the two materials were compatible with each other), and the intruded ionic liquid was well-dispersed through the entire region of the polymer matrix. The fabricated electrolyte maintained its flexible solid film state for EMIMBF4 content up to 200% without leakage problems. The capacitors were manufactured with porous carbon electrodes and a PEGBEM-g-PAEMA/EMIMBF4 electrolyte. The capacitors exhibited a capacitance of 55.5F/g, a power density of 900 W/kg, and an energy density of 25 Wh/kg at 1.0 A/g. The capacitors were measured at various bending angles and exhibited good performance without specific encapsulation. According to the results, the newly synthesized PEGBEM-g-PAEMA graft copolymer is a good candidate for a polymer matrix for electrolytes in all-solid-state bendable/flexible supercapacitors.

Facile One Pot Polycondensation Method to Synthesize the Crosslinked Polyethylene glycol‐Based Copolymer Electrolytes

Comb‐like polyethylene glycol (PEG)‐based copolymer electrolytes with given crosslinking density are synthesized using PEG as the main chain segments (denoted as MC‐PEG), methoxy polyethylene glycol (MPEG) as side chain segments (denoted as SC‐PEG), and hexamethylene‐1,6‐diisocyanate homopolymer (HDI trimer) as bridging and/or crosslinking agent. In this study, the effects of molecular weight of MC‐PEG and SC‐PEG on Li ion conductivities of the copolymer have been examined. If MC‐PEG and/or SC‐PEG do not crystallize, the ion conductivities of graft copolymer electrolyte increase with either decreasing MC‐PEG or increasing SC‐PEG segment length. The highest ion conductivity of the copolymer electrolyte at room temperature reaches up to 7.7 × 10−4 S cm−1 when the molecular weight of MC‐PEG is 400 g mol−1 and SC‐PEG is 1000 g mol−1 (denoted as GC400‐1000). However, when the MC‐PEG and/or SC‐PEG segments begin to crystallize (i.e., GC400‐2000), where the flexibility of the related PEG molecular chains is largely reduced, the ion conductivity drops by almost two orders comparing to that of GC400‐1000 copolymer electrolyte. Meanwhile, crosslinked GC400‐1000 electrolyte has good thermal and electrochemical stability as well as high mechanical properties, making it suitable for commercial applications in Li ion secondary batteries. image

Synthesis and application of PEGBEM-g-POEM graft copolymer electrolytes for dye-sensitized solar cells

We report a nanostructural amphiphilic graft copolymer electrolyte based on poly(ethylene glycol) behenyl ether methacrylate (PEGBEM) and poly(oxyethylene methacrylate) (POEM) synthesized via a facile free radical polymerization. The synthesis of PEGBEM-g-POEM graft copolymers, and its interaction with an ionic liquid (i.e., 1-methyl-3-propylimidazolium iodide (MPII)), were confirmed by Fourier transform infrared spectroscopy (FT-IR), differential scanning calorimetry (DSC), wide-angle X-ray scattering (WAXS), small-angle X-ray scattering (SAXS), and atomic force microscopy (AFM). The addition of MPII reduced the degree of crystallinity of the graft copolymer, resulting in improved ionic conductivity up to 1.4×10−6S/cm at 25°C. The efficiency of dye-sensitized solar cells (DSSCs) with this graft copolymer electrolyte (3.6%) was higher than those of DSSCs with homopolymer electrolytes (2.3 to 2.5%), indicating the effectiveness of the nanostructural graft copolymer. The higher efficiency resulted from an improved current density (Jsc) due to enhanced polymer flexibility and reduced crystallinity.

Scalable and bendable organized mesoporous TiN films templated by using a dual-functional amphiphilic graft copolymer for solid supercapacitors

An organized mesoporous (om-TiN) film prepared by the nitration of TiO2 templated by a graft copolymer. The solid supercapacitor based on a graft copolymer electrolyte shows a specific capacitance of 266.8 F g−1, the highest value reported for TiN-based electrodes.

Solid-state Graft Copolymer Electrolytes for Lithium Battery Applications

Battery safety has been a very important research area over the past decade. Commercially available lithium ion batteries employ low flash point (< 80 °C), flammable, and volatile organic electrolytes. These organic based electrolyte systems are viable at ambient temperatures, but require a cooling system to ensure that temperatures do not exceed 80 °C. These cooling systems tend to increase battery costs and can malfunction which can lead to battery malfunction and explosions, thus endangering human life. Increases in petroleum prices lead to a huge demand for safe, electric hybrid vehicles that are more economically viable to operate as oil prices continue to rise. Existing organic based electrolytes used in lithium ion batteries are not applicable to high temperature automotive applications. A safer alternative to organic electrolytes is solid polymer electrolytes. This work will highlight the synthesis for a graft copolymer electrolyte (GCE) poly(oxyethylene) methacrylate (POEM) to a block with a lower glass transition temperature (Tg) poly(oxyethylene) acrylate (POEA). The conduction mechanism has been discussed and it has been demonstrated the relationship between polymer segmental motion and ionic conductivity indeed has a Vogel-Tammann-Fulcher (VTF) dependence. Batteries containing commercially available LP30 organic (LiPF6 in ethylene carbonate (EC):dimethyl carbonate (DMC) at a 1:1 ratio) and GCE were cycled at ambient temperature. It was found that at ambient temperature, the batteries containing GCE showed a greater overpotential when compared to LP30 electrolyte. However at temperatures greater than 60 °C, the GCE cell exhibited much lower overpotential due to fast polymer electrolyte conductivity and nearly the full theoretical specific capacity of 170 mAh/g was accessed.

Optimal phase segregation in graft copolymers

Nanoscale phase segregation in tailored graft copolymer electrolyte is probed by microscopy and small-angle scattering, and modeled by a stochastic technique. Based on stochastic modeling, we describe the percolation and connectivity of the ion-rich phase, and propose boundaries for the graft length and graft density to obtain favorable phase segregation potentially relevant for operation at low levels of hydration. The approach can be adapted for other copolymer systems, and points out new aspects in the structure–property relationships for polymer electrolytes.

Synthesis of proton conducting phosphonic acid-functionalized polyolefins by the combination of ATRP and ADMET

Well-defined copolymers of various compositions possessing polar polyphosphonate brushes precisely placed on an unsaturated polyethylene backbone were obtained by a combination of ADMET and ATRP. Both a macromonomer and macroinitiator approach were explored along with conversion to requisite poly(vinylbenzyl phosphonic acid) (PVBPA)/polyolefin graft copolymers. All polymers were quantitatively deprotected to the corresponding free phosphonic acid-containing PVBPA/polyolefin graft copolymer electrolytes. The thermal behavior and the phase transitions of these materials showed amorphous behavior that was exploited in subsequent proton conductivity experiments. Proton conductivity properties of the PVBPA/polyolefin graft copolymers in anhydrous as well as in humidified conditions reached magnitudes of 10−2 S cm−1 with particular architectures.

Graft copolymer-based lithium-ion battery for high-temperature operation

The use of conventional lithium-ion batteries in high temperature applications (>50 °C) is currently inhibited by the high reactivity and volatility of liquid electrolytes. Solvent-free, solid-state polymer electrolytes allow for safe and stable operation of lithium-ion batteries, even at elevated temperatures. Recent advances in polymer synthesis have led to the development of novel materials that exhibit solid-like mechanical behavior while providing the ionic conductivities approaching that of liquid electrolytes. Here we report the successful charge and discharge cycling of a graft copolymer electrolyte (GCE)-based lithium-ion battery at temperatures up to 120 °C. The GCE consists of poly(oxyethylene) methacrylate-g-poly(dimethyl siloxane) (POEM-g-PDMS) doped with lithium triflate. Using electrochemical impedance spectroscopy (EIS), we analyze the temperature stability and cycling behavior of GCE-based lithium-ion batteries comprised of a LiFePO 4 cathode, a metallic lithium anode, and an electrolyte consisting of a 20-μm-thick layer of lithium triflate-doped POEM-g-PDMS. Our results demonstrate the great potential of GCE-based Li-ion batteries for high-temperature applications.

Quasi solid-state dye-sensitized solar cells based on P(VDF-co-CTFE) graft copolymer electrolytes

A graft copolymer of poly(vinylidene fluoride-co-chlorotrifluoroethylene) with poly(oxyethylene methacrylate), i.e. P(VDF-co-CTFE)-g-POEM was synthesized via atom transfer radical polymerization (ATRP) using CTFE unit as a macroinitiator. This graft copolymer was complexed with metal salt (LiI) and ionic liquid (1-methyl,3-propyl imidazolium iodide, MPII) to produce a polymer electrolyte. Low molecular weight oligomer, poly(ethylene glycol) dimethyl ether (PEGDME, 500 g/mol) was additionally introduced to enhance an ionic conductivity and cell performances. Coordinative interactions and microphase-separated structures were investigated using FT-IR spectroscopy, X-ray diffraction (XRD) and transmission electron microscopy (TEM). The maximum energy conversion efficiency of the dye-sensitized solar cell (DSSC) employing P(VDF-co-CTFE)-g-POEM, PEGDME and Li + MPII reached 4.2% at 100 mW/cm 2.

Proton-conducting properties of the membranes based on poly(vinyl phosphonic acid) grafted poly(glycidyl methacrylate)

Proton-conducting properties of the graft copolymer electrolytes were examined throughout this work. The homopolymers poly(glycidyl methacrylate), PGMA and poly(vinyl phosphonic acid), PVPA were synthesized by free-radical polymerizations of the monomers glycidyl methacrylate, GMA, and vinyl phosphonic acid, VPA, respectively. The graft copolymers were produced by grafting of PVPA onto PGMA via ring opening of ethylene oxide groups. To examine the influence of the concentration of VPA on the proton conductivity, several graft copolymers were produced at various stoichiometric ratios with respect to monomer repeat units. The materials were characterized by FT-IR and 1H NMR spectroscopy and the thermal properties were studied by thermogravimetry (TG) and differential scanning calorimetry (DSC). The TGA results demonstrated that the samples are thermally stable up to at least 150 °C. The proton conductivities of humidified and dry samples were studied via impedance spectroscopy. In the anhydrous state, the proton conductivity of P(GMA)-graft-P(VPA) 10 was 5 × 10 − 5 S/cm at 150 °C. The proton conductivity of the same material increased with the humidity content and reached to 0.03 S/cm at 80 °C under 50% of RH, which approached to that of Nafion ® at the same humidification level.

Synthesis and characterization of poly(ether sulfone) grafted poly(styrene sulfonic acid) for proton conducting membranes

Two-step synthesis of proton-conducting poly(ether sulfone) (PES) graft copolymer electrolyte membrane is proposed. Fridel Craft alkylation reaction was used to introduce chloromethyl pendant group onto the PES polymer backbone. Later on, atom transfer radical polymerization (ATRP) was applied to synthesize a series of poly(ether sulfone) grafted poly(styrene sulfonic acid) (PES-g-PSSA). Successful chloromethyl substitution and grafting of the pendant group was characterized by the 1H-NMR and elemental analysis. Electrochemical properties such as ion exchange capacity (IEC), water uptake and proton conductivity increased with increasing PSSA contents. Thermal gravimetric analysis (TGA) showed the thermal stability of membranes up to 270 °C. Proton conductivity for maximum amount of grafting was 0.00297 S/cm.

Proton-conducting composite membranes from graft copolymer electrolytes and phosphotungstic acid for fuel cells

New organic–inorganic composite membranes based on poly(vinylidene fluoride-co-chlorotrifluoroethylene)-graft-poly(styrene sulfonic acid) [P(VDF-co-CTFE)-g-PSSA] with embedded phosphotungstic acid (PWA) were prepared. Fourier transform infrared spectra indicated the existence of a specific interaction between P(VDF-co-CTFE)-g-PSSA graft copolymer and PWA particles. PWA nanoparticles were well confined in the polymeric matrix up to 20 wt.%, above which they started to be extracted from the matrix, as revealed by scanning electron microscope analysis. Accordingly, Young’s modulus of membranes also increased with PWA concentration up to 20 wt.%, above which it continuously decreased. Upon incorporation of PWA nanoparticles, the proton conductivity of composite membranes slightly decreased from 0.042 to 0.035 S/cm at room temperature up to 20 wt.%, presumably due to strong interaction between the sulfonic acids of graft copolymer and PWA nanoparticles. The characterization by thermal gravimetric analysis demonstrated the enhancement of thermal stabilities of the composite membranes with increasing concentration of PWA.

Anhydrous proton conducting membranes based on crosslinked graft copolymer electrolytes

A comb-like copolymer consisting of a poly(vinylidene fluoride- co-chlorotrifluoroethylene) backbone and poly(hydroxy ethyl acrylate) side chains, i.e. P(VDF- co-CTFE)- g-PHEA, was synthesized through atom transfer radical polymerization (ATRP) using CTFE units as a macroinitiator. Successful synthesis and a microphase-separated structure of the copolymer were confirmed by proton nuclear magnetic resonance ( 1H NMR), FT-IR spectroscopy, and transmission electron microscopy (TEM). This comb-like polymer was crosslinked with 4,5-imidazole dicarboxylic acid (IDA) via the esterification of the –OH groups of PHEA and the –COOH groups of IDA. Upon doping with phosphoric acid (H 3PO 4) to form imidazole–H 3PO 4 complexes, the proton conductivity of the membranes continuously increased with increasing H 3PO 4 content. A maximum proton conductivity of 0.015 S/cm was achieved at 120 °C under anhydrous conditions. In addition, these P(VDF- co-CTFE)- g-PHEA/IDA/H 3PO 4 membranes exhibited good mechanical properties (765 MPa of Young's modulus), and high thermal stability up to 250 °C, as determined by a universal testing machine (UTM) and thermal gravimetric analysis (TGA), respectively.

Proton conducting membranes based on poly(vinyl chloride) graft copolymer electrolytes

AbstractThe direct preparation of proton conducting poly(vinyl chloride) (PVC) graft copolymer electrolyte membranes using atom transfer radical polymerization (ATRP) is demonstrated. Here, direct initiation of the secondary chlorines of PVC facilitates grafting of a sulfonated monomer. A series of proton conducting graft copolymer electrolyte membranes, i.e. poly(vinyl chloride)‐g‐poly(styrene sulfonic acid) (PVC‐g‐PSSA) were prepared by ATRP using direct initiation of the secondary chlorines of PVC. The successful syntheses of graft copolymers were confirmed by 1H‐NMR and FT‐IR spectroscopy. The images of transmission electron microscopy (TEM) presented the well‐defined microphase‐separated structure of the graft copolymer electrolyte membranes. All the properties of ion exchange capacity (IEC), water uptake, and proton conductivity for the membranes continuously increased with increasing PSSA contents. The characterization of the membranes by thermal gravimetric analysis (TGA) also demonstrated their high thermal stability up to 200°C. The membranes were further crosslinked using UV irradiation after converting chlorine atoms to azide groups, as revealed by FT‐IR spectroscopy. After crosslinking, water uptake significantly decreased from 207% to 84% and the tensile strength increased from 45.2 to 71.5 MPa with a marginal change of proton conductivity from 0.093 to 0.083 S cm−1, which indicates that the crosslinked PVC‐g‐PSSA membranes are promising candidates for proton conducting materials for fuel cell applications. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Single-step synthesis of proton conducting poly(vinylidene fluoride) (PVDF) graft copolymer electrolytes

The single-step synthesis of proton conducting poly(vinylidene fluoride) (PVDF) graft copolymer electrolytes is demonstrated. The graft copolymers of PVDF backbone with poly(sulfopropyl methacrylate) (PVDF-g-PSPMA) and poly(styrene sulfonic acid) (PVDF-g-PSSA) were synthesized using PVDF as a macroinitiator for atom transfer radical polymerization (ATRP). 1H NMR and FT-IR spectroscopy show that the “grafting from” method using ATRP was successful and the maximum grafting degrees were 35 and 25wt% for PVDF-g-PSPMA and PVDF-g-PSSA, respectively. The IEC values were 0.63 and 0.45meq/g, the water uptakes were 46.8 and 33.4wt% and the proton conductivities were 0.015 and 0.007S/cm at room temperature, for PVDF-g-PSPMA and PVDF-g-PSSA, respectively. Both membranes exhibited excellent thermal stability up to around 350°C, verified by thermal gravimetric analysis (TGA).

Nanostructure, interactions, and conductivities of polymer electrolytes comprising silver salt and microphase‐separated graft copolymer

AbstractSilver polymer electrolytes were prepared by blending silver salt with poly(oxyethylene)9 methacrylate)‐graft‐poly(dimethyl siloxane), POEM‐g‐PDMS, confining silver salts within the continuous ion‐conducting POEM domains of microphase‐separated graft copolymer. AgClO4 polymer electrolytes exhibited their maximum conductivity at high silver concentrations as well as higher ionic conductivities than AgCF3SO3 electrolytes. The difference in conductivities of the two electrolytes was investigated in terms of the differences in the interactions of silver ions with ether oxygen of POEM and, hence, with the anions of salts. Upon the addition of salt in graft copolymer, the increase of Tg in AgClO4 was higher than that in AgCF3SO3 electrolytes. Analysis of an extended configuration entropy model revealed that the interaction of ether oxygen/AgClO4 was stronger than that of ether oxygen/AgCF3SO3 whereas the interaction of Ag+/ClO4− was weaker than that of Ag+/CF3SO3−. These interactions are supported by the anion vibration mode of FT‐Raman spectroscopy. It is thus concluded that the higher ionic conductivity of AgClO4 electrolytes was mostly because of higher concentrations of free ions, resulting from their strong ether oxygen/silver ion and weak silver ion/anion interactions. A small angle X‐ray scattering study also showed that the connectivity of the POEM phase was well developed to form nanophase morphology and the domain periodicities of graft copolymer electrolytes monotonically increased with the increase of silver concentration up to critical concentrations, after which the connectivity was less developed and the domain spacings remained invariant. This is attributed to the fact that silver salts are spatially and selectively incorporated in conducting POEM domains as free ions up to critical concentrations, after which they are distributed in both domains as ion pairs without selectivity. The increase of domain d‐spacing in AgClO4 electrolytes was larger than that in AgCF3SO3, which again results from high concentrations of free ions in the former. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1018–1025, 2007

Polarization in Cells Containing Single-Ion Graft Copolymer Electrolytes

The single-ion conductor, -incorporated poly[ methacrylate-ran-lithium methacrylate]-graft-poly(dimethyl siloxane), P(OEM--LiMA)--PDMS, was used as an electrolyte in cells containing a lithium anode and a thin-film vanadium oxide cathode. Cycle testing revealed an unanticipated high polarization which resulted in a significant drop in capacity compared to that of cells constructed with conventional salt-doped electrolytes produced by the addition of a lithium salt to an uncharged electrolyte poly( methacrylate-graft-polydimethyl siloxane. Impedance spectra obtained from a vanadium oxide symmetric cell fitted with a single-ion electrolyte showed a large resistance associated with the cathode/electrolyte interface. Further battery testing illustrated that the polarization remained even when lithium triflate was added to the single-ion electrolyte. In contrast, cells consisting of a lithium anode, single-ion electrolyte, and an alloying cathode showed no rise in polarization over what is found in similar cells constructed with a salt-doped electrolyte. These observations are consistent with the hypothesis that diffusion of lithium ions into the bulk vanadium oxide may be coulombically hindered by single-ion electrolytes.

Synthesis and Characterization of Single-Ion Graft Copolymer Electrolytes

A microphase-separating single-ion conductor, poly[ methacrylate-ran-lithium methacrylate]-graft-poly(dimethyl siloxane), P(OEM--LiMA)--PDMS, was prepared by lithiating a precursor polymer synthesized by free radical methods using commercially available macromonomers. This material possessed a low conductivity, stemming from high ion-pairing interactions that severely restricted the number of charge carriers available for conduction. Subsequent conversion of the LiMA units via the addition of , a Lewis acid, resulted in a 2 orders-of-magnitude rise in conductivity, a gain that could be attributed to a large increase in the number of mobile cations. By blending this material with uncharged POEM--PDMS, the room-temperature conductivity was optimized to . With a lithium transference number of unity, these materials exhibit higher dc-measured conductivities at elevated currents than their salt-doped counterparts and are electrochemically stable to .