Free-standing Polymer Electrolyte Research Articles

In situ formation of polymer electrolytes using a phosphazene cross-linker for high-performance lithium-ion batteries

Polymer electrolytes (PEs) have been intensively studied in lithium-ion batteries and the studies have recently focused on improving their electrochemical performances. In this study, an in situ thermal curing method is proposed to prepare polyethylene glycol-based electrolytes using a cyclic phosphazene cross-linker directly onto the lithium anode, endowing the preparation of PEs with improved ion transport across the interfaces and better electrochemical performances than those using free-standing polymer electrolytes. The buildup of the cross-linked electrolyte network by incorporating the phosphazene cross-linker endows the as-prepared PEs exhibiting excellent deformation and thermal stability, a wide electrochemical window, high ionic conductivity (3.07 × 10−3 S cm−1) at room temperature. The in situ-formed, cross-linked electrolyte network with good elasticity and conformal attachment at the electrolyte/electrode interface achieves a high capacity (>160 mAh g−1 at 0.1C rate) and a long cycle life at room temperature (0.2C rate, 93.8% at 250 cycles) in the equipped Li|LiFePO4 cell, making this strategy a promising approach for developing the next-generation lithium-ion batteries.

Scalable Superior Chemical Sensing Performance of Stretchable Ionotronic Skin via a π-Hole Receptor Effect.

Skin-attachable gas sensors provide a next-generation wearable platform for real-time protection of human health by monitoring environmental and physiological chemicals. However, the creation of skin-like wearable gas sensors, possessing high sensitivity, selectivity, stability, and scalability (4S) simultaneously, has been a big challenge. Here, an ionotronic gas-sensing sticker (IGS) is demonstrated, implemented with free-standing polymer electrolyte (ionic thermoplastic polyurethane, i-TPU) as a sensing channel and inkjet-printed stretchable carbon nanotube electrodes, which enables the IGS to exhibit high sensitivity, selectivity, stability (against mechanical stress, humidity, and temperature), and scalable fabrication, simultaneously. The IGS demonstrates reliable sensing capability against nitrogen dioxide molecules under not only harsh mechanical stress (cyclic bending with the radius of curvature of 1 mm and cyclic straining at 50%), but also environmental conditions (thermal aging from -45 to 125 °C for 1000 cycles and humidity aging for 24 h at 85% relative humidity). Further, through systematic experiments and theoretical calculations, a π-hole receptor mechanism is proposed, which can effectively elucidate the origin of the high sensitivity (up to parts per billion level) and selectivity of the ionotronic sensing system. Consequently, this work provides a guideline for the design of ionotronic materials for the achievement of high-performance and skin-attachable gas-sensor platforms.

Free-standing polymer electrolyte for all-solid-state lithium batteries operated at room temperature

This study reports a networked solid polymer electrolyte (N-SPE) containing no solvent, ionic liquid, oligomer, or semisolid additives for lithium-ion batteries (LIBs). The N-SPE comprises a lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt as well as a polymer framework constructed using cage-like polyhedral oligomeric silsesquioxane (POSS) and serving as hubs to network poly(ethylene oxide-co-polypropylene oxide) (P(EO-co-PO)) chains. The networking prevents polymer chain twisting that hinders ion transport. Raman analysis indicates that the POSS hubs improve the dissociation of LiTFSI and localize TFSI− anions. The N-SPE exhibits a low glass transition temperature of −43 °C, a high 25 °C ionic conductivity of 1.1 × 10−4 S cm−1, and a small activation energy of 3.5 kJ mol−1 for ion conduction. The localization of TFSI− results in a high lithium transference number of 0.62, which is determined to be beneficial to Li+ transport. By incorporating the N-SPE into the LiFePO4 cathode and using a free-standing N-SPE membrane, this study assembles a Li|N-SPE|LiFePO4 battery, which delivers a high capacity of 160 mAh g−1 at 25 °C and exhibits excellent charge−discharge cycling stability. The free-standing feature of the N-SPE makes roll-to-roll assembly of LIBs readily scalable for industrial applications.

Lithium-Ion Exchange Polymer Membranes Limit Migration of Dissolved Polysulfide Species in Li-S Batteries

Conventional lithium-ion (Li-ion) batteries dominate development efforts in electrical vehicles due to their high open circuit voltage, desirable power density, and long cycle life. Despite these advantages, state-of-the-art Li-ion batteries barely meet the energy requirement of EV-based power trains due low energy density (180-200 Wh/kg cell) and high cost. Lithium sulfur (Li-S) batteries are promising for EV applications because they address both these barriers. Li-S batteries exploit a redox couple described by the reaction below, which has an open circuit potential of 2.2 V with respect to Li+/Li: 16 Li+ + S8 + 16e-→ 8 Li2S Assuming full conversion, the theoretical energy density and specific capacity of Li-S batteries is 2500 Wh/kg S and 1675 mAh/g S, respectively.1 In addition, sulfur, the key active cathode material is significantly less expensive than conventional transition metal oxide cathodes used in Li-ion batteries. However, Li-S batteries have not gained acceptance for practical applications because of their poor cycle life. This limitation originates from polysulfide diffusion from the cathode through the electrolyte to the anode which causes parasitic side-reactions and limits capacity and cycle retention. Our work aims to overcome the challenges of polysulfide diffusion by incorporating lithium-ion exchange (LIE) ionomers as polysulfide blocking barriers in Li-S cells. When exposed to electrolyte solvent, these LIE membranes provide a physical barrier that limits polysulfide diffusion, while also delivering high Li+ ion conductivity through swelling in electrolyte solvent. The LIE ionomers can be applied in a variety of ways including as: binders in the electrode formulation, coatings applied directly to the electrode surface, and as freestanding polymer electrolyte membranes. We have also developed a porous, dimensionally stable membrane (DSM) which can be impregnated with LIE ionomers to improve mechanically stability by limiting in-plane (X-Y direction) swelling. This general approach of incorporating LIE ionomers into Li-S cells has achieved improvements in discharge capacity, cycle life, coulombic efficiency, and energy density.

One-Step Process to Fabricate Transparent Polymer Gel Electrolyte for Electrochromic Window

All-solid-state electrochromic device (ECD) has received considerable attention with the development of energy-efficient and high stable smart windows. Polymer electrolyte is necessary for ECD instead of liquid electrolyte to prevent leakage because of its safety and durability in real outdoor condition. Therefore, research has been focused on polymer systems containing PMMA [1], PEO [2], UV-crosslink network [3] and etc. However, several drawbacks i.e. yellowish color, haze, and complex multi-step process still remain. To overcome these problems, we tried to improve optical transparency of polymer electrolyte with a simple and easy fabrication method. In this research, we prepared a transparent and free-standing gel polymer electrolyte with excellent mechanical flexibility and elongation in one-step process. Three-dimensional acrylate crosslinking network has formed directly through UV irradiation under co-presence of the PVdF-HFP and LiClO4 salt. To obtain optimum composition, variables including mono-, di-acrylate oligomers, PVdF-HFP, and plasticizers were controlled. Electrolyte was analyzed with regard to physical and chemical characterization using SEM, FT-IR, DSC, Tensile test, and ion conductivity measurement, etc. In order to fabricate electrochromic device, electrochromic film was deposited by Nano Particle Deposition System (NPDS), which is a facile dry-deposition method to coat ceramic powders using high impact energy when the collisions between particles and substrate occur [4]. Electrochromic performance of the fabricated cell was evaluated by in situ UV-Vis spectroscopy and electrochemical analysis techniques, such as Electrochemical Impedance Spectroscopy(EIS), and Cyclic Voltammetry(CV), etc. Finally, it was confirmed that extremely clear ion conducting membrane was easily fabricated in one-step and successfully applied to ECD at ambient temperature. The transmittance of the polymer electrolyte showed over 80 % and the fabricated ECD cell with polymer electrolyte exhibited its transmittance change greater than 50 % at its wavelength of 800 nm under 3 V which shows almost similar performance of the cell using liquid electrolyte. Therefore, we have developed one-step process to fabricate highly transparent and free-standing polymer electrolyte, to obtain high performance electrochromic device. [1] J. Phys. Chem. B, 2009, 113, 8006–8010 [2] Scientific Reports 2016, 6, 19892 [3] Membranes, 2012, 2, 687-704 [4] Applied Surface Science, 2016, 377, 370–375

Ionic Liquid-Based Polymer Electrolytes via Surfactant-Assisted Polymerization at the Plasma-Liquid Interface.

We first report an innovative method, which we refer to as interfacial liquid plasma polymerization, to chemically cross-link ionic liquids (ILs). By this method, a series of all-solid state, free-standing polymer electrolytes is successfully fabricated where ILs are used as building blocks and ethylene oxide-based surfactants are employed as an assisted-cross-linking agent. The thickness of the films is controlled by the plasma exposure time or the ratio of surfactant to ILs. The chemical structure and properties of the polymer electrolyte are characterized by scanning electron microscopy (SEM), Fourier transformation infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR) spectroscopy, X-ray photoelectron spectroscopy (XPS), differential scanning calorimetry (DSC), and electrochemical impedance spectroscopy (EIS). Importantly, the underlying polymerization mechanism of the cross-linked IL-based polymer electrolyte is studied to show that fluoroborate or halide anions of ILs together with the aid of a small amount of surfactants having ethylene oxide groups are necessary to form cross-linked network structures of the polymer electrolyte. The ionic conductivity of the obtained polymer electrolyte is 2.28 × 10(-3) S·cm(-1), which is a relatively high value for solid polymer electrolytes synthesized at room temperature. This study can serve as a cornerstone for developing all-solid state polymer electrolytes with promising properties for next-generation electrochemical devices.

Physicochemical Properties of PEMA-MgTf2-EMITFSI Polymer Electrolytes

The present work was carried out to investigate the effects of MgTf2 and EMITFSI ionic liquid on the physicochemical properties of PEMA based electrolytes. Free-standing polymer electrolyte films consisting of poly(ethyl methacrylate), magnesium triflate salt and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquid were prepared at different weight percentage ratios (wt%) using the solution casting technique. The interactions between the electrolyte constituents were confirmed using Fourier transform infrared spectroscopy. The conductivity of the films was influenced by the content of the doping salt and ionic liquid. The conductivity behavior was then correlated with the results of differential scanning calorimetry. The highest ionic conductivity of 1.72×10-6 S cm-1 at room temperature and 1.97×10-4 S cm-1 at 100 °C was achieved for the sample containing 20 wt% of salt and 30 wt% of ionic liquid. The sample exhibited a wide electrochemical potential stability window between -4.19 to +3.74 V.

Effect of ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide on the properties of poly(glycidyl methacrylate) based solid polymer electrolytes

A free standing polymer electrolytes films, containing poly(glycidyl methacrylate) (PGMA) as the polymer host, lithium perchlorate (LiClO4), and ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl) imide [Bmim][TFSI] as a plasticizer was successfully prepared via the solution casting method. The XRD analysis revealed the amorphous nature of the electrolyte. ATR-FTIR and thermal studies confirmed the interaction and complexation between the polymer host and the ionic liquid. The maximum ionic conductivity of the solid polymer electrolyte was found at 2.56 × 10–5 S cm–1 by the addition of 60 wt % [Bmim][TFSI] at room temperature and increased up to 3.19 × 10–4 S cm–1 at 373 K, as well as exhibited a transition of temperature dependence of conductivity: Arrhenius-like behavior at low and high temperatures.

Thermal, vibrational, and dielectric studies on PVP/LiBF4+ionic liquid [EMIM][BF4]-based polymer electrolyte films

Free-standing polymer electrolyte membranes based on poly(vinyl) pyrrolidone (PVP)/salt(LiBF4) having different amounts of ionic liquid (IL) [EMIM][BF4] were prepared and characterized by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), Fourier transform infrared (FT-IR) spectroscopy, and alternating current (AC) impedance spectroscopic techniques. The DSC results show a shift in Tm of PVP with salt/or IL content. TGA and DTGA (first derivative of TGA) results give evidence of the presence of uncomplexed PVP, PVP/salt, and PVP/IL complexes. Signatures of these entities are also present in the dielectric spectra. Complexation of PVP with salt and IL has been confirmed by FT-IR analysis. Electrical conductivity as a function of temperature has been studied for PVP/LiBF4/IL [EMIM][BF4]. Role of IL in changing phase transition, conductivity, and dielectric relaxation frequency has been discussed.

“Cut and Stick” Rubbery Ion Gels as High Capacitance Gate Dielectrics

A free-standing polymer electrolyte called an ion gel is employed in both organic and inorganic thin-film transistors as a high capacitance gate dielectric. To prepare a transistor, the free-standing ion gel is simply laid over a semiconductor channel and a side-gate electrode, which is possible because of the gel's high mechanical strength.

Studies of Poly(Ethyl Methacrylate) Complexed with Ammonium Trifluoromethanesulfonate

Free standing polymer electrolyte films comprising of ammonium trifluoromethane sulfonate in poly(ethyl methacrylate) were prepared and characterized. The structural and electrical properties of the polymer electrolytes were investigated by X-ray diffraction and a.c. impedance spectroscopy, respectively. The formation of polymer-salt complex has been confirmed by Fourier transform infrared spectroscopy study. Conductivity of the polymer electrolytes increased with salt content. The highest ionic conductivity in the order of 10-5 S cm-1 at room temperature was achieved for the system with 35 wt% of ammonium salt. The temperature dependence of conductivity obeyed the Vogel-Tammam-Fulcher relation. The activation energy has been calculated from the VTF formalism. The ionic transference number of the mobile ions estimated by Wagner’s polarization method was close to unity for the highest conducting sample implying that the conductivity was contributed by ions which was expected to be protons.

PVDF-HFP/PVC Blend Based Lithium Ion Conducting Polymer Electrolytes

Flexible free standing polymer electrolyte films have been successfully prepared using a blend of poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) and poly(vinyl chloride) (PVC) doped with lithium perchlorate (LiClO4). The conductivity of the film was influenced by salt concentration and the degree of crystallinity. An optimum room temperature conductivity obtained was 2.10 x 10-4 S cm-1 for PVDF-HFP/PVC containing 35 wt.% LiClO4. The temperature-dependent conductivity of the polymer films exhibited VTF-type behaviour.

Proton-Conducting Interpenetrating Polymer Network with Reduced Methanol Permeability

A freestanding polymer electrolyte membrane was developed having low permeability to methanol while maintaining high permeability to water and proton conductivity. A two-polymer composite formed an interpenetrating polymer network (IPN) composed of proton-conducting 2-acrylamido-2-methyl propanesulfonic acid and a second polymer, poly(vinyl alcohol), serving as a methanol barrier. Ion conductivity and methanol permeability were controlled by adjusting polymer ratios and the extent of cross-linking of the two polymers. Compared to Nafion, the IPNs had up to fifteen times greater selectivity for water over methanol and proton conductivity about an order of magnitude lower. © 2004 The Electrochemical Society. All rights reserved.

A poly(vinylidene fluoride)-based gel electrolyte membrane for lithium batteries

Highly conducting, free standing polymer electrolyte membranes were prepared by gelification of an ethylene carbonate–propylene carbonate lithium methide solution in a poly(vinylidene fluoride)–chlorotrifluoroethylene copolymer matrix. The electrochemical characterization showed that these membranes have a room temperature conductivity of the order of 0.5×10 −3 S cm −1 , an anodic break-down voltage exceeding 4.7 V vs. Li and good compatibility with the lithium metal electrode. These properties make these membranes of interest for the development of lithium metal and lithium ion batteries.