Increased ion conduction in dual cation [sodium][tetraalkylammonium] poly[4-styrenesulfonyl(trifluoromethylsulfonyl)imide-co-ethylacrylate] ionomers

© The Royal Society of Chemistry. Solid-state polymer electrolytes, as an alternative to traditional liquid electrolytes, have been intensively investigated for energy conversion and storage devices. The transport rate of single ions is the key to their high performance. For application in emerging sodium batteries, we have developed three dual-cation polymeric ionomers, which contain bulky tetraalkylammonium ions in addition to the sodium ion. The sizes and relative contents of the ammonium ions vary relative to the sodium ion contents. Comparative studies of ion dynamics, thermal properties, phase behaviours and ionic conductivities were carried out, taking advantage of various spectroscopic and thermal chemistry methods. The ion conductivities of the ionomers are greatly enhanced by the introduction of bulky counterions, as a result of the additional free volume and decreased sodium ion association. Raman spectroscopy and thermal analysis as well as the solid-state nuclear magnetic resonance studies are used to probe the conductivity behaviour.

Synthesis of Sodium Poly[4-styrenesulfonyl(trifluoromethylsulfonyl)imide]-co-ethylacrylate] Solid Polymer Electrolytes

Sodium-based batteries are being considered to replace Li-based batteries for the fabrication of large-scale energy storage devices. One of the main obstacles is the lack of safe and conductive solid Na-ion electrolytes. A Na-ion polymer based on the (4-styrenesulfonyl(trifluromethylsulfonyl) imide anion, Na[STFSI], has been prepared by a radical polymerization process and its conductive properties determined. In addition, a number of multi-component polymers were synthetized by co-reaction of two monomers: Na[STFSI] and ethyl acrylate (EA) at different ratios. The structural and phase characterizations of the polymers were probed by various techniques (DSC, TGA, NMR, GPC, Raman, FTIR and Impedance spectroscopy). Comparative studies with blends of the homopolymers Na[PSTFSI] and poly(ethylacrylate) (PEA) have also been performed. The polymers are all thermally stable up to 300°C and the ionic conductivity of EA copolymers and EA blends are about 1-3 orders of magnitude higher than that of Na[PSTFSI]. The highest conductivity measured at 100°C was found for Na[PSTFSI-blend-5EA] at 7.9 × 10^-9 S cm^-1, despite being well below its Tg. Vibrational spectroscopy indicates interaction between Na⁺ and the EA carbonyl groups, with a concomitant decrease in the sulfonyl interaction, facilitating Na⁺ motion, as well as lowering Tg.

Characterization of superfine wool powder/poly(propylene) blend film

Superfine wool powder was blended and extruded with poly(propylene) (PP) to produce blend pellets, and the extruded pellets were hot-pressed into a blend film. SEM photographs show that the powder could be uniformly incorporated with PP after extrusion. FT-IR spectra shows that no substantial changes occurred in the chemical structure of both PP and wool powder in the blend film. X-Ray diffraction analysis indicates that crystallinity of the blend film was much higher than that of the wool powder and little lower than that of PP. TG-tested results indicate that the thermal stability of the blend film declined with an increase in the powder content. Endothermic peaks of the wool powder in the blend film become more obvious as the powder content increases. Mechanical properties decline greatly with an increase in the wool powder content in the blend film.

Selective hydrogen bonding and hierarchical nanostructures in poly(hydroxyether of bisphenol A)/poly(ε-caprolactone)-block-poly(2-vinyl pyridine) blends

The phase behavior, hydrogen bonding interactions and morphology of poly(hydroxyether of bisphenol A) (phenoxy) and poly(var epsilon-caprolactone)-block-poly(2-vinyl pyridine) (PCL-b-P2VP) were investigated using differential scanning calorimetry (DSC), Fourier transform infrared (FTIR) spectroscopy, optical microscopy and atomic force microscopy (AFM). In this A-b-B/C type block copolymer/homopolymer system, both P2VP and PCL blocks have favorable intermolecular interaction towards phenoxy via hydrogen bonding. However, the hydrogen bonding between P2VP and phenoxy is significantly stronger than that between PCL and phenoxy. Selective hydrogen bonding between phenoxy/P2VP pair at lower phenoxy contents and co-existence of two competitive hydrogen bonding interactions between phenoxy/P2VP and phenoxy/PCL pairs at higher phenoxy contents were observed in the blends. This leads to the formation of a variety of composition dependent nanostructures including wormlike, hierarchical and core–shell morphologies. The blends became homogeneous at 95 wt% phenoxy where both blocks of the PCL-b-P2VP were miscible with phenoxy due to hydrogen bonding. In the end, a model was proposed to explain the microphase morphology of blends based on the experimental results obtained. The swelling of the PCL-b-P2VP block copolymer by phenoxy due to selective hydrogen bonding causes formation of different microphases

Controlled amine functionalization and hydrophilicity of a poly(lactic acid) fabric

The surface of PLA fabric has been successfully modified by pulsed plasma polymerization of heptylamine (PPHA) giving increased hydrophilicity and achieving a sufficient level of primary amine functionality (3.5%) for practical application in biotechnology. This is the first report that the density of primary amine (-NH2) in PPHA, quantified by chemical derivatization, can be controlled by selection of pulsed plasma conditions. The duty cycle and the average RF power were the key parameters for achieving both a higher density of primary amine and increased hydrophilicity.

Lithium ion mobility in poly(vinyl alcohol) based polymer electrolytes as determined by 7Li NMR spectroscopy

Solvent-free polymer electrolytes based on poly(vinyl alcohol) (PVA) and LiCF₃SO₃ have shown relatively high conductivities (10⁻⁸-10⁻⁴ S cm⁻¹), with Arrhenius temperature dependence below the differential scanning calorimeter (DSC) glass transition temperature (343 K). This behaviour is in stark contrast to traditional polymer electrolytes in which the conductivity reflects VTF behaviour. ⁷Li nuclear magnetic resonance (NMR) spectroscopy has been employed to develop a better understanding of the conduction mechanism. Variable temperature NMR has indicated that, unlike traditional polymer electrolytes where the linewidth reaches a rigid lattice limit near T_g, the lithium linewidths show an exponential decrease with increasing temperature between 260 and 360 K. The rigid lattice limit appears to be below 260 K. Consequently, the mechanism for ion conduction appears to be decoupled from the main segmental motions of the PVA. Possible mechanisms include ion hopping, proton conduction or ionic motion assisted by secondary polymer relaxations.

Glass transition and free volume behaviour of poly(acrylonitrile)/LiCF3SO3 polymer-in-salt electrolytes compared to poly(ether urethane)/LiClO4 solid polymer electrolytes

Measurements of the glass transition temperature (T_g) and free volume behaviour of poly(acrylonitrile) (PAN) and PAN/lithium triflate (LiTf), with varying salt composition from 10 to 66 wt% LiTf, were made by positron annihilation lifetime spectroscopy (PALS). Addition of salt from 10 to 45 wt% LiTf resulted in an increase in the mean free volume cavity size at room temperature (r.t.) as measured by the orthoPositronium (oPs) pickoff lifetime, τ₃, with little change in relative concentration of free volume sites as measured by oPs pickoff intensity, I₃. The region from 45 to 66 wt% salt displayed no variation in relative free volume cavity size and concentration. This salt concentration range (45 wt%<[LiTf]<66 wt%) corresponds to a region of high ionic conductivity of order 10⁻⁵ to 10⁻⁶ S cm⁻¹ at T_g as measured by PALS. A percolation phenomenon is postulated to describe conduction in this composition region. Salt addition was shown to lower the T_g as measured by PALS; T_g was 115°C for PAN and 85°C for PAN/66 wt% LiTf. The T_g and free volume behaviour of this polymer-in-salt electrolyte (PISE) was compared to a poly(ether urethane)/LiClO₄ where the polymer is the major component, i.e. traditional solid polymer electrolyte (SPE). In contrast to the PISE, the T_g of the SPE was shown to increase with increasing salt concentration from 5.3 to 15.9 wt%. The relative free volume cavity size and concentration at r.t. were shown to decrease with increasing salt concentration. Ionic conductivity in this SPE was of order 10⁻⁵ S cm⁻¹ at r.t., which is over 60°C above T_g, 10⁻⁸ S cm⁻¹ at 25°C above T_g, and conductivity was not measurable at T_g.

Cycling performance of lithium metal polymer cells assembled with ionic liquid and poly(3-methyl thiophene)/carbon nanotube composite cathode

A poly(3-methylthiophene) (PMT)/multi-walled carbon nanotube (CNT) composite is synthesized by in situ chemical polymerization. The PMT/CNT composite is used as an active cathode material in lithium metal polymer cells assembled with ionic liquid (IL) electrolytes. The IL electrolyte consists of 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4) and LiBF4. A small amount of vinylene carbonate is added to the IL electrolyte to prevent the reductive decomposition of the imidazolium cation in EMIBF4. A porous poly(vinylidene fluoride-co-hexafluoropropylene) (P(VdF-co-HFP)) film is used as a polymer membrane for assembling the cells. Electrochemical properties of the PMT/CNT composite electrode in the IL electrolyte are evaluated and the effect of vinylene carbonate on the cycling performance of the lithium metal polymer cells is investigated. The cells assembled with a non-flammable IL electrolyte and a PMT/CNT composite cathode are promising candidates for high-voltage–power sources with enhanced safety.