Ethylene Oxide Based Polymer Electrolytes Research Articles

Role of Molecular Architecture on Ion Transport in Ethylene oxide-Based Polymer Electrolytes

This work aims to develop a detailed mechanistic understanding of the role of a graft polymer architecture on lithium ion (Li+) transport in poly(ethylene oxide)-based polymer electrolytes. Specifically, we compare Li+ transport in poly(ethylene oxide) (PEO) versus poly(oligo oxyethylene methacrylate) (POEM) polymers doped with lithium bis(trifluoromethanesulfonyl) (LiTFSI) salts, using both experimental electrochemical characterization and molecular dynamics (MD) simulations. Our results indicate that POEM exhibits a range of relaxation processes that cannot be interpreted solely in terms of glass-transition temperature (Tg) effects. Due to its side-chain architecture, the segmental relaxation of POEM is nonuniform across ether oxygens (EOs) and shows a more pronounced sensitivity to temperature above Tg compared to PEO. Moreover, POEM also exhibits a nonuniform Li+ coordination behavior, in which Li+ is primarily solvated by two different chains in POEM, compared to a single chain in PEO. Li+ transport in POEM occurs via two events with distinct characteristic times: a fast intrachain hopping along side chains and a slow interchain hopping between side chains. Taken together, the relaxation processes and ion transport mechanisms identified in POEM provide useful insights into design of more effective solid polymer electrolytes.

Ethylene oxide–based polymer electrolytes with fluoroalkyl moieties for stable lithium metal batteries

In this study, we have developed a new ethylene oxide (EO)-based polymer electrolyte (PE) that contains fluoroalkyl moieties. The developed PE possesses high oxidative decomposition potential as well as adequate ionic conductivity (σ) and favorable mechanical properties for application to Li metal batteries. In particular, we designed a partially fluorinated copolymer, which is improved in conventional polyethylene oxide systems and could be used both as stable electrolyte matrix and electrode binder for high-voltage applications. The results indicated that the stabilization of the EO-based PE during the high-voltage operation of Li metal batteries was significantly improved by the introduction of a fluoroalkyl moiety via simple radical copolymerization. The synthesized copolymer exhibited satisfactory physical stability and unprecedented high oxidation decomposition voltage threshold, which exceeded 4.5 V. Moreover, the copolymer-based PEs presented moderate σ values (> 0.3 mS cm−1 at 60 °C) and retained the intrinsic properties of the EO-based PE.

Effect of temperature on electrochemical performance of ionic liquid based polymer electrolyte with Li/LiFePO4 electrodes

Poly ethylene oxide based polymer electrolytes containing salt lithium bis(trifluoromethylsulfonyl)imide and ionic liquid 1-butyl-3-methyl pyridinium bis(trifluoromethylsulfonyl)imide are synthesized. The prepared polymer electrolytes are found to be thermally stable up to 340–360°C. Ionic conductivity is observed ~2.5×10−5Scm−1 at 25°C and 2.3×10−4Scm−1 at 40°C for 30% IL containing polymer electrolyte. Also, ionic transference number>0.99 and cationic transference number ~0.41 with electrochemical window ~5.2V at 25°C is observed for the electrolyte containing 30% IL. The highest Li+ ion conducting polymer electrolyte is further optimised for battery application. The specific discharge capacity of the prepared cell (Li/polymer electrolyte/LiFePO4) is found to be 106mAhg−1 at 25°C and 160mAhg−1 at 40°C up to 25cycles with 0.1C current rate. The increment in discharge capacity at higher temperature may be due to the better electrode-electrolyte contact. Decrement in cell resistance is also observed at higher temperature.

Towards Aging Resistant Lithium Polymer Batteries for Wide Temperature Applications

A wide interest is mounting in the field of polymer electrolytes, due to their application in energy efficient devices such as rechargeable batteries, photo-electrochemical cells, electrochromic devices, fuel cells and super capacitors. Polymer electrolytes exhibit unique advantages such as mechanical integrity, wide variety of fabrication methods in desirable size and shape, possibility to fabricate an intimate electrode/electrolyte interface and adapt to a lightweight, leak proof construction, safety and economic packaging structure. Free radical photo-polymerization (UV-curing) can be an interesting alternative process to produce polymer electrolytes for Li-ion batteries. It takes place at ambient temperature: a liquid polyfunctional monomer, containing a proper photo-initiator, forms a cross-linked film upon UV irradiation. It appears highly advantageous, due to its easiness and rapidity in processing, very short time with high efficiency and eco-friendliness as the use of solvent is avoided. In the present work, profoundly ion conducting, self-standing and tack-free ethylene oxide based polymer electrolytes encompassing a room temperature ionic liquid (RTIL) with specific amounts of lithium salt are successfully prepared via a rapid and easily up-scalable UV curing process. All the prepared materials are thoroughly characterized in terms of their physical, chemical and morphological properties, and eventually galvanostatically cycled in lab-scale lithium batteries (LIBs). The appearance of the cross-linked polymer electrolyte is shown in Figure 1A, where the remarkable mechanical abusability is demonstrated. The crosslinking produced by UV irradiation allows the incorporation of higher amount of RTIL (imidazolium, pyrrolidinium etc.) and/or tetraethylene glycol dimethyl ether (tetraglyme) with lithium salt (based on TFSI- ­anion), leading to a material with remarkable morphological characteristics in terms of homogeneity and robustness. FESEM analysis was conducted to characterize the morphology of the photo cured polymer films. A representative top-view is shown in Figure 1B, where the typically wrinkled texture characteristics of highly amorphous, cross-linked polymer electrolytes is evidenced. The bright and dark areas in the image belong to amorphous PEO domains alternated to some residual ordered (semi crystalline) domains, respectively; the wrinkled texture derives from the formation of crosslinking domains between the polymer chains. Similar kind of textures are also present in polymer films kept at temperature under high stress (50 bar for 15 mins at 90 °C) during the preparation process. The UV-cured PEO-based polymer network is able to efficiently hold the RTIL without any leakage. Tensile analysis confirmed that the UV-irradiated membranes showed an average Young’s modulus Eof 0.2 ± 0.05 MPa. If one considers that more than 50 wt. % of ionic liquid is generally incorporated, these are highly satisfying values. The SPE showed the thermal stability up to 375 °C under inert conditions, and such a remarkable result is particularly interesting for application in Li-ion batteries with increased safety. X-ray diffraction analysis (XRD) was used to get further information on the fundamental role of photo curing step in reducing the overall crystallinity of the SPE to nearly fully amorphous state. The SPE exhibited excellent ionic conductivity (>10–4 Scm–1 at room temperature, Figure 1C), electrochemical stability (>5V vs. Li+/Li), and interfacial stability. At 20°C the conductivity value is equal to 2.5×10- 4 Scm- 1. This result is encouraging as it is sufficiently high to allow ambient temperature operation of Li-ion cells. It exceeds 10- 3 Scm- 1 already at 50°C. The Vogel-Tamman-Fulcher (VTF) behavior of the SPE was verified fitting the conductivity data with respect to temperature. The value of activation energy was found to be 8.23 kJ mol- 1. The ability to resist the lithium dendrite nucleation and growth was tested by galvanostatic polarization studies. They showed resistance at current intensity >0.1mAcm-2. The lab-scale Li-polymer cell assembled showed stable charge/discharge characteristics without any capacity (135 mAh g- 1) fading at C/5 current regime (Figure 1D). The overall performance of the SPEs postulates the possibility of effective implementation in the next generation of safe, durable and high energy density secondary all-solid Li-metal polymer batteries working at ambient and/or sub-ambient temperatures. Acknowledgement: MARS-EV project has received funding from the European Union Seventh Framework Program (FP7/2007-2013) under grant agreement n° 609201. Lithops batteries S.r.l. provided the LiFePO4electrode. Figure 1. A) Appearance of the light-cured cross-linked polymer electrolyte, B) FESEM analysis at high magnification of the sample surface; C)Arrhenius plot for ionic conductivity as a function of temperature of the polymer electrolyte membrane (in the inset, the fitting by means of the VTF equation), D) the graph showing the specific capacity vs cycle number, at 50°C and coulombic efficiency. Figure 1

Ionic conductivity, glass transition, and local free volume in poly(ethylene oxide) electrolytes: Single and mixed ion conductors

Ethylene oxide based polymer electrolytes which exhibit single-ion (cation Li+ or anion ClO4−) and mixed ion (from the dissociation of LiClO4 salt) conduction were studied by employing positron lifetime annihilation spectroscopy (PALS) and conductivity (σ) measurements in the temperature range between 170 and 370 K. We present experimental evidence for the validity of (i) the linear expansion of local free volume from PALS, (ii) the Vogel–Tammann–Fulcher (VTF) law for σ, and (iii) the Cohen–Turnbull equation that relates σ to the local free volume. These were found to be valid in the temperature range above the end (or freezing) temperature of the glass transition, Tge≈1.06TgPALS≈1.18TgDSC [TgPALS and TgDSC are the Tg’s from PALS and differential scanning calorimetry (DSC), respectively]. From VTF fits to σ we obtained a Vogel temperature of T0≈TgDSC and pseudoactivations energies of B=3.7–5.7 kJ/mol. These parameters disagree with many data published in the literature but are in perfect agreement with the free volume experiments. Moreover, we found T0=TgPALS−(20–28) K and TgDSC=TgPALS−(25–27) K. Indications for the existence of two relaxation processes near Tg were observed in the free volume expansion curves, which were attributed to the motion of free polymer segments and those interacting with ions. The discrepancy between TgDSC and TgPALS can be attributed to the two-phase microseparation of the polymer electrolytes; DSC responds mainly to the polymer segments in the ion-depleted regions while PALS responds to the polymer segments in the ion-rich regions. From the Cohen–Turnbull plots the critical hole volume required for an elementary jump of an ion was estimated to be γν*≈1 nm3 and was found to be independent of the type of ion. This shows that each type of ionic conductivity is associated with the same segmental mobility. Below TgePALS the conductivity is larger than expected from the (extrapolated) VTF law, but smaller than displayed in the frozen-in free volume.

A comparison of ionic conductivitybehaviour in various ethylene oxidebased polymer electrolytes

Glass transition temperatures T g and ionic conductivities σ have been measured as a function of salt concentration m for LiCF 3SO 3 complexes of three types of ethylene oxide (EO) based materials: (i) low molecular weight poly(ethylene oxide)s, viz. tetraglyme, PEG300 and PEG600 and end-esterified PEG300, (ii) higher molecular weight amorphous polymer electrolytes, poly(methoxy polyethylene glycol monomethacrylate) (PEM), methyl oxyethylene extended poly(propylene oxide) (Me-xPPO), and (iii) some [12]and [13]-crown-4 polymethacrylates (CEP). The variation of σ with temperature T has been fitted to the VTF equation σ=AT −1/2exp{−B/(T−T o)}. It is found (a) that the salt dependent behaviour of all the amorphous PEO materials is very similar but very different from that of the CEPs; (b) that (T g−T o) is related to B in an approximately linear manner for all salted systems independent of polymer type; (c) that for PEO systems, where the formation of ionic cross links is insignificant, T o is independent of salt concentration, but that for the crown ether polymers in which ionic cross links are expected, T o increases with m; and (d) that for the crown ether polymers, A/ m and B vary weakly with m and that the A/ m and B values for all the ethylene oxide based materials appear to converge to common high values at high salt concentrations. This is compatible with the idea that the solvated cations, incorporated intrahelically, cause the PEO to form crown like configurations.

Influence of water absorption on poly–ethylene oxide-based polymer electrolytes complexed with ammonium, sodium and magnesium perchlorates

The influence of water absorption on polyethylene oxide and its complexes with NH4ClO4, NaClO4 and Mg(ClO4)2 (for O/M=10 and 3) has been studied. A quantitative estimation of water absorption kinetics has been carried out. The changes in microstructure and thermal behaviour due to water absorption have been studied using X-ray diffraction and thermogravimetric/differential thermal analyses. The electrical conductivity of different films has been measured as a function of relative humidity and explained in terms of the generation of free mobile ions due to water absorption. © 1998 Chapman & Hall

Investigations on poly ethylene oxide based polymer electrolyte complexed with AgNO 3

The polymer electrolyte PEO+AgNO 3 has been investigated by IR, optical absorption, DTA, electrical conductivity, mobility, and transport number measurements. The material is a mixed (ionic + electronic) conductor. The ionic conductivity is due to Ag + and anionic movement. The material properties change with time, mainly associated with an increased electronic conductivity due to reduction of Ag + in PEO lattice. The maximum electrical conductivity has been found to be ∼3 × 10 −7s cm −1 for the composition Ag + EO = 0.014 to 0.085.