Ion transport in high temperature rotator phase solid electrolytes

<p indent="0mm">Since the commercialization of lithium-ion batteries in the 1990s, lithium-ion batteries have been successfully applied in portable electronics, electric vehicles, and grid energy storage. Although current organic liquid electrolytes have high ionic conductivities, they are inherently flammable, volatile, and prone to leakage. Moreover, severe side reactions and dendrite growth on the surface of the lithium anode during the charge-discharge process can cause safety hazards, which greatly impede their applications in lithium metal batteries. Solid electrolytes, including inorganic solid electrolytes and polymer electrolytes, are regarded as effective alternatives to organic liquid electrolytes for the construction of lithium metal batteries with high energy density and safety. Among them, solid polymer electrolytes offer excellent flexibility, processability, and interfacial compatibility over inorganic solid electrolytes, and they are extraordinarily promising for lithium metal batteries with high energy density and safety. Ideal solid polymer electrolytes should have following features: (1) High ionic conductivity (&gt; <sc>10 ^–4 S cm ^–1) </sc> at room temperature; (2) high lithium ion transference number (~1) to reduce the concentration polarization and improve the rate performance of batteries; (3) intimate contact at the electrode/electrolyte interfaces; (4) wide electrochemical window <sc>(&gt;4.5 V</sc> vs. Li/Li ⁺) to match high-voltage cathodes and improve the energy density of batteries; (5) good mechanical stability to resist processing, buffer electrode volume change and inhibit dendrite growth; (6) good thermal stability to withstand environmental changes. Generally, the ionic conductivity of pure solid polymer electrolytes at room temperature is low <sc>(~10 ^–6 S cm ^–1). </sc> Researchers have tried to improve the ionic conductivities by adjusting the lithium salt concentration, such as developing “polymer-in-salt” solid electrolytes. However, increasing the concentration of lithium salt leads to the deterioration of the mechanical strength. Strategies such as developing novel lithium salts, modifying polymer matrix, and incorporating inorganic fillers into solid polymer electrolytes are proposed to promote ionic conductivities of solid polymer electrolytes. In particular, composite polymer electrolytes, fabricated by dispersing a certain amount of inorganic fillers into solid polymer electrolytes, have improved ionic conductivities without sacrificing their mechanical performances. Poor interfacial property between electrodes and electrolytes is also a critical issue for solid polymer electrolytes. On one hand, poor and uneven solid/solid contacts at the electrode/electrolyte interfaces lead to high resistance and sluggish ionic transport kinetics. Furthermore, the volume change of the positive and negative electrodes in the charge/discharge process deteriorates the interfacial contacts, blocks the ion and electron transport through the interfaces, and greatly reduces the electrochemical reaction kinetics. On the other hand, the electrochemical windows of solid polymer electrolytes are usually narrow <sc>(＜4.5 V).</sc> During cycling, redox reactions are prone to occur at the electrode/electrolyte interfaces, causing battery failure. Solid polymer electrolytes have also poor thermal and mechanical stabilities. Therefore, design and synthesis of polymer-based solid electrolytes with excellent comprehensive performances and construction of fast and stable ion transport channels at the electrolyte/electrode interfaces are of great significance for the successful development of solid-state lithium metal batteries. This paper presents a brief review of the research progress in solid polymer electrolytes from two aspects: Improving the ionic conductivities of solid polymer electrolytes and enhancing the interfacial performance at electrolyte/electrode interfaces. First, targeted optimization strategies on ionic conductivities of solid polymer electrolytes, including constructing continuously aligned ionic transport paths and shortening the ionic transport distance, are summarized. Second, interface optimization strategies, including constructing wetting interfaces and synthesizing asymmetric electrolytes, are presented to reduce the interface resistance and improve the interfacial contact. Finally, perspectives on the development of solid polymer electrolytes and high-performance solid-state lithium metal batteries are discussed, and key research directions and advanced test methods are proposed. This review may provide a comprehensive understanding and further guidance for not only the material design of solid polymer electrolytes, but also the structural design of lithium metal batteries with favorable electrochemical and interfacial performances.

The oxygen ionic conductivity of apatite-type and increases with increasing oxygen content. The ion transference numbers, determined by faradaic efficiency measurements at 973-1223 K in air, are close to unity for and and vary in the range 0.96-0.99 for other compositions. Doping of with iron results in an increasing fraction, which was evaluated by Mössbauer spectroscopy and correlates with partial ionic and p-type electronic conductivities, whereas La-stoichiometric apatites stabilize the state. Among the studied materials, the highest ionic and electronic transport is observed for where oxygen interstitials are close neighbors of Si-site cations. Data on transference numbers, total conductivity, and Seebeck coefficient as a function of the oxygen partial pressure confirm that the ionic conduction in Fe-substituted apatites remains dominant under solid oxide fuel cell operation conditions. However, reducing p leads to a drastic decrease in the ionic transport, presumably due to a transition from the prevailing interstitial to a vacancy diffusion mechanism, which is similar to the effect of acceptor doping. Iron additions improve the sinterability of silicate ceramics, increase the n-type electronic conductivity at low and probably partly suppress the ionic conductivity drop. The thermal expansion coefficients of apatite solid electrolytes in air are K−1 at 300-1250 K. © 2004 The Electrochemical Society. All rights reserved.

The development of solid-state batteries (SSBs) is crucial for advancing next-generation battery technology. Compared to conventional lithium-ion batteries using organic liquid electrolytes, SSBs offer higher energy density, improved safety, and wide operating temperature range. Nonetheless, constructing a practical SSB with lithium-metal anode (LMA) and optimal inorganic solid electrolyte (SE) presents significant challenges. In this presentation, we introduce our findings addressing the two primary obstacles impeding the remarkable advancement of SSBs.The electrochemical instability at the interface between LMA and SE is the first persistent issue to overcome. Solid materials constituting SSBs exhibit distinct physical and chemical properties. When these materials, in the form of electrodes or electrolytes, are assembled into an electrochemical cell configuration, they establish solid–solid interfaces within the energy storage system. Achieving electrochemically stable SSBs requires non-reactive interfaces between LMA and SE with conformal contact. Previous studies have predominantly focused on developing interface modification methods to enhance the long-term cycling stability of SSBs. However, such approaches often offer phenomenological solutions without a thorough understanding of the interface dynamics. Additionally, the role of charges within an electric field in the operation of lithium-ion batteries is often overlooked. The LMA-SE interface of SSBs experiences an in situ electric field during the charging process, where the SEs exhibit semiconducting properties distinct from liquid electrolytes. Consequently, identifying the governing mechanism of interface stabilization cannot rely solely on conventional electrochemical techniques or a materials science perspective alone.Interface modeling by tailoring energy band alignment is a fundamental approach to achieving the cycling stability of SSBs. In this study, we used Li1.3Al0.3Ti1.7(PO4)3 (LATP), a representative NASICON-type SE, alongside a Li-metal foil. Although LATP possesses excellent electrochemical stability with a high anodic limit (4.31 V) and small decomposition energy (- 65 meV per atom at 5 V), its direct contact with LMA leads to the formation of a reactive interface having mixed ionic-electronic conducting interphases (MCI). The presence of MCI expedites the reduction of LATP’s metallic constituents by enabling electron transfer across the interface. Given the heightened risk of interfacial instability in the electrochemical cell, controlling interfacial reactions is imperative for constructing SSBs. Herein, we deposited titanium (45 nm) onto the surface of LATP to stabilize the inherently reactive LMA-LATP interface. The pure transition metal interlayer transformed into an ion-conducting crystal structure through electrochemical interactions with Li-metal. This titanium compound self-induced interlayer (TSI) served as a stable Li-ion conductor and electron buffer between LMA and LATP. Lithium symmetric cells having TSI maintained constant overpotential over 1000 cycles and exhibited significantly reduced impedance.Another critical obstacle to realizing practical SSBs is the intrinsic structural instability of SEs. This physical instability shortens the lifespan of SSBs and severely diminishes their electrochemical performance at the typical ambient temperatures of electric vehicle power batteries (25-50 °C). Over the past decade, various solid-state Li-ion conductors have been developed such as Li7La3Zr2O12 (garnet-type oxide SE) and Li6PS5Cl (argyrodite-type sulfide SE). By associating lattice dynamics and ionic transport in these materials, previous studies could improve the ionic conductivity of SE through major substitution with large cations like tantalum or partial substitution with large anions. Recently, antiperovskite-type SEs with the anion-centered unit of Li3AB, where A represents hydroxyl and B represents halogen groups, have attracted attention. The representative composition Li2OHCl is renowned for its facile fabrication process through solid-state reaction, high ionic conductivity (10-3 S cm-1), and excellent compatibility with LMA. Still, significant limitations exist in its use for constructing practical SSBs, as is common with most other types of SEs.We found that substituting trace amounts of aliovalent cations to modify atomic interactions in SEs can significantly enhance the electrochemical reliability of SSBs. This strategy effectively resolves the intrinsically unstable structure of SEs and simultaneously enhances the LMA-SE interfaces. As a proof-of-concept, we substituted lithium in Li2OHCl with metal elements that do not experience crystal field stabilization energy. The resulting modified electrolytes led to prolonged lifespan and superior rate performance in lithium symmetric cells at room temperature. The cells with the modified SEs exhibited a remarkable critical current density exceeding 10.5 mA cm-2 and maintained a stable voltage profile for 3000 hours at 10.0 mA cm-2, whereas the pristine cells achieved a relatively low critical current density of 2.0 mA cm-2 and a short lifespan of 1000 hours at 1.0 mA cm-2. These findings underscore a strong correlation between the physical properties of inorganic materials and the electrochemical stability of SSBs. Finally, we anticipate that the revealing results will contribute to the advancement of high-power SSBs employing solid electrolytes and metallic electrodes.

An extrapolation method is usually applied when Ab initio molecular dynamics (AIMD) is applied to studying ionic conductivity in solid-state electrolytes in lithium-ion batteries. As the ions move slowly in solid-state electrolytes, the first-principles method typically involves computationally intensive calculations, and it can take significant time to obtain accurate results. First-principles method is too expensive for the time scale required at room temperature. The classical molecular dynamics method is typically applied to systems containing thousands of atoms and can simulate the system’s behavior over nanoseconds. During the simulation, the positions and velocities of the atoms are updated at discrete time intervals, allowing the system’s behavior to be studied over time. However, its accuracy depends on the empirical force-field libraries. Limited by the computational resource, the previous studies applied the extrapolation method to obtain the room temperature ionic conductivity, which was not accurate because the linear relationship in the Arrhenius equation was not valid in a wide range of temperatures. Deepmd-Kit is a tool that integrates these two different computational approaches. The extrapolation and Deepmd methods were applied to the materials Li10GeP2S12, Li10SiP2S12, Li10GePS12Cl, and Li10SiPS12Cl, respectively. Both methods showed that the lithium ions favor the c direction when diffusing in the LGPS-type solid-state electrolytes. The ionic conductivity is more accurate with the dependent method compared with experiments.

Although solid-state batteries had been suffering from low rate capability due to low ionic conductivities of solid electrolytes, conductivities comparable to non-aqueous liquid electrolytes used in current lithium-ion batteries have been achieved in some recently-developed solid electrolytes. Especially among sulfide systems, the highest conductivity has reached 10−2 S cm−1 [1], which is even higher that of liquid electrolytes when taking the transport number of unity into account. Consequently, ion transport in solid electrolytes can cease from rate-determining; however, it has been replaced by that across interfaces. Ionic conductors often exhibit anomalous ionic conduction at their surface or interface, which is categorized as “nanoionics” taking place in space-charge layers formed with thickness around 10 nm at the interface or surface. It appears in solid-state batteries with sulfide electrolytes as high interfacial resistances to high voltage cathodes, which result in the unimproved power densities of the solid-state batteries in spite of the high ionic conductivities of the employed solid electrolytes. Classical Nernst equation infers that the high potential of cathode makes the space-charge layers lithium-depleted and thus highly-resistive due to the depletion of charge carriers. The high interfacial resistance has been successfully reduced by an interfacial design based on the space-charge model, where an ion-conducting and electron-insulating oxide is interposed at the interface in order to shield the sulfide electrolyte from the high cathode potential and thus act as a buffer against the lithium depletion [2]. The changes in the electrode properties upon the interposition are well explainable by the space-charge layer model [3]. However, because the space-charge layer that lies between solids with only several nanometers in thickness is difficult to access, it has not revealed its presence even before advanced analyses; whereas it has emerged in computations. Electronic structure of LiFePO4 and Li3PS4, which are cathode and solid electrolyte materials, respectively, obtained by the first principles calculation suggests preferential formation of lithium vacancies on the electrolyte side of the LiFePO4/Li3PS4 interface. In addition, when FePO4/Li3PS4 interface is relaxed in order to simulate the interface structure during the battery operation, lithium ions move from the electrolyte to electrode side to bring about lithium depletion on the electrolyte side.

Currently, electrolytes used in lithium-ion batteries are flammable and leaky, causing safety problems when used under high temperatures or extreme conditions. Solid electrolytes can fundamentally avoid the occurrence of such issues, at the same time, it can significantly increase the energy density of solid state lithium batteries. Solid electrolytes are classified into inorganic solid electrolytes and solid polymer electrolytes. Although inorganic solid electrolytes have the high mechanical strength and room temperature ionic conductivity, it has poor flexibility and complicated preparation process. In contrast, solid polymer electrolyte has good elasticity and good interface contact, which can overcome many disadvantages of the inorganic solid electrolyte. Among them, polyethylene oxide (PEO) is first proposed as a polymer electrolyte. Due to its excellent mechanical properties, electrochemical stability and thermal stability, it has been a hot topic for polymer solid electrolytes for decades. However, its low room temperature ionic conductivity limits practical application. Given the problems existing in PEO-based polymer electrolytes, several methods for improving ionic conductivity have been proposed. The primary methods include plasticization, the organic-inorganic composite, and polymer blending. Comparing these methods, it can be seen that adding plasticizers to the polymer can effectively increase the conductivity of the electrolyte, but at the same time it will cause a decrease in the mechanical properties, and the safety issues limit its application in lithium-ion batteries. Inorganic fillers can balance the problems of ionic conductivity and mechanical properties, but the construction of a good filler-polymer dispersion system is complicated in preparing such composite electrolytes, design a new type of multi-dimensional the ion transport network also face tremendous challenges. Undoubtedly, in the future research, inorganic fillers will be the dominant means in the modification of composite electrolytes. Compared with the former two modification methods, polymer blending is easy to operate and friendly to industry, but low ionic conductivity at room temperature still exist. To give full play to the advantages of polymer blending, using it as an auxiliary means to improve the performance of composite electrolytes will be the primary development direction of such modification methods. PEO is the most widely studied polymer electrolyte. Even though a large number of modification methods have emerged in its development history, the ionic conductivity at room temperature is still limited to a relatively low level. To meet the needs of the application in lithium-ion batteries, a high operating temperature (50−70℃) is still needed, the biggest problem remains exist. To solve the problems in polymer solid electrolytes, the research directions in the future may mainly focus on: Expanding the modification method of PEO to other polymer electrolyte host materials, broadening the scope of research, and finding the available polymer electrolytes at room temperature. Concerned about the preparation of ultra-thin polymer electrolytes, reducing the ion transmission path to increase the transmission efficiency of lithium ions between positive and negative electrodes. Effectively combines inorganic solid electrolytes and solid polymer electrolytes to give full play to their advantages. In addition, exploring the effect of inorganic filler morphology on the conductivity of the composite electrolyte will also deserve attention. Many one-dimensional, two-dimensional materials can build ion transport networks so that lithium ions can travel rapidly along the path. With the deepening of research, it is expected that PEO-based polymer electrolytes may replace liquid electrolytes and become an essential cornerstone for building the next generation of safe and reliable lithium-ion batteries.

Cellulose-based poly(ionic liquid)s: Correlations between degree of substitution and alkyl side chain length with conductive and morphological properties.

The performance of metal-ion batteries at low temperatures and their fast charge/discharge rates are determined mainly by the electrolyte (ion) transport. Accurate transport properties must be evaluated for designing and/or optimization of lithium-ion and other metal-ion batteries. In this review, we report and discuss experimental and atomistic computational studies on ion transport, in particular, ion diffusion/dynamics, transference number, and ionic conductivity. Although a large number of studies focusing on lithium-ion transport in organic liquids have been performed, only a few experimental studies have been conducted in the organic liquid electrolyte phase for other alkali metals that are used in batteries (such as sodium, potassium, magnesium, etc.). Atomistic computer simulations can play a primary role and predict ion transport in organic liquids. However, to date, atomistic force fields and models have not been explored and developed exhaustively to simulate such organic liquids in quantitative agreement to experimental measurements.

In this paper, we report the investigation on structural, electrical, dielectric properties, and ion dynamics of novel blend polymer electrolyte matrix (PEO-PEI) complexed with sodium hexafluorophosphate salt. All the solid polymer electrolyte films have been synthesized via solution cast method. The SPE films were characterized by the X-ray diffraction, field emission scanning electron microscope, impedance-dielectric spectroscopy, and thermogravimetric analysis. The morphology of the SPE alters with salt addition and confirms the blend polymer complex formation. The FTIR analysis evidenced the complex formation, and increase in the fraction of free ions is achieved. The ionic conductivity exhibits a maximum at the stoichiometric ratio O/Na = 10 and follows the Arrhenius behavior. The fraction of free ions is maximum for the SPE film with the highest ionic conductivity. The optimum electrolyte possesses a voltage stability window of about 4 V, excellent thermal stability up to 380 °C, and high ionic transference number (~ 1). The complex permittivity and conductivity have been simulated in whole frequency window to extract relaxation time and dielectric strength. The dielectric constant and relaxation time exhibited sequentially a maximum and minimum for the SPE film with the highest ionic conductivity. The loss tangent peak shifts toward high frequency with addition of salt, and it infers the faster ion dynamics in the polymer matrix. Then the ion transport parameters number density of charge carriers (n), ion mobility (μ), and diffusion coefficient (D) have been obtained by three methods (FTIR, impedance spectroscopy, and loss tangent method) and are in absolute correlation with the impedance results. An ion transport mechanism has been proposed based on experimental findings.

The temperature dependence of viscosity of the bulk metallic glass Zr41Ti14Cu12.5Ni10Be22.5（Vit1）under constant heating condition was obtained by application of static extension measurement. A few strain rate peaks, which are associated with the glass transition and multistep crystallization, were observed in the temperature-strain rate curve. Above the glass transition temperature, the supercooled liquid of the bulk metallic glass showed the Newtonian rheological characteristic, and its temperature dependences of viscosity in supercooled liquid region can be fitted well with the Vogel-Fulcher-Tammann(VFT) equation. The sample exhibited the fragility parameter D* value of 36, the VFT temperature T0 of 319K and the fragility m value of 30 It was shown that the Vit1 alloy is a strong liquid, similar to sodium silicate liquids. However, the relation between temperature and viscosity deviated from VFT equation below the glass transition temperature.The phenomenon is tentatively interpreted in terms of free-volume model and structure relaxation concept.

Most of the ceramic ionic conductors typically conduct one ion. For example, AgI conducts Ag+ ion, Sodium β”-alumina conducts Na+ ion and Yttria-stabilized zirconia (YSZ) conducts O2- ions. Some selected materials such as alkaline earth cerates or zirconates conduct both protons and O2- ions. In such materials also, conductivity of one species is not independent of the other species. Thus, temperature and atmosphere determine the total conductivity. All of such materials are single phase materials. We have been investigating two phase materials such that one phase conducts one ion (Na+) while the other phase conducts another ion (O2-). The materials in such cases is a composite with the two phases forming an interpenetrating network. The volume ratios of the two phases can be adjusted as necessary. Since the two phases essentially constitute a mixture, conduction of one species occurring through one phase is independent of the conduction of the other species through the second phase. Yet, the condition of electroneutrality is applicable globally across multiple phases. Transport equations can then be written down in the usual manner but accounting for different volume fractions for the two phases. A mixed Na+ ion and O2- ion two phase conductor is fabricated by first making a sintered dense composite containing α-alumina and YSZ such that both phases are contiguous. YSZ and α-alumina are immiscible. This allows the fabrication of fine-grained, dense, strong composites. Subsequently, the sample is packed in Na-β”-alumina + YSZ powder and heat treated in air over a range of temperatures from 1200oC to 1400oC. During this process, α-alumina is converted into Na-β”-alumina by the incorporation of Na2O into α-alumina by coupled transport in which Na+ transports through the formed Na-β”-alumina and O2- ions transport through YSZ. The reaction front over time sweeps through the sample forming a two phase composite of Na-β”-alumina + YSZ. Such a material is now capable of conducting two ions; Na+ and O2-. Materials capable of transporting other alkali ions can be made by an ion exchange process in molten salts. For example, by ion exchanging in lithium salts, it is possible to make mixed Li-ion and O2- ion conducting materials. The oxygen ion conductivity is negligible at low temperatures. Thus, the materials at room temperature are predominantly alkali ion conductors with possible use in all solid state lithium ion and sodium ion batteries. Another mixed ion conductor was made by a similar process using a composite of Ga2O3 and YSZ. It was similarly packed in Na-β”-alumina powder and heat treated. In this case a two phase mixture containing YSZ and sodium zirconium gallate formed, the latter with a chemical formula of Na0.7Ga4.7Zr0.3O8 with a monoclinic structure. This compound is isostructural with Na0.7Ga4.7Ti0.3O8 which is known to be one dimensional sodium ion conductor. Measurement of ionic conductivity on Na0.7Ga4.7Zr0.3O8 + YSZ showed it to be a mixed Na+ ion and O2- ion conductor. Sodium ion conductivity of Na0.7Ga4.7Zr0.3O8 is much lower than that of Na-β”-alumina. The activation energy for Na+ ion transport is much lower than for O2- ion transport in YSZ. As a result, the two phase mixture is predominantly a Na+ ion conductor at low temperatures and an O2- ion conductor at elevated temperatures. Both Na-β”-alumina + YSZ and Na0.7Ga4.7Zr0.3O8 + YSZ are mixed Na+ ion and O2- ion conductors. This allows one two explore electrochemical transport of two species and examine the applicability of Goldman-Hotchkin-Katz trype of an equation, routinely used in aqueous electrochemistry, for solid-state systems. Preliminary results of experiments conducted by maintaining different chemical potentials of Na2O at the two electrodes and different chemical potentials of oxygen (by testing in a fuel cell mode) will be described. Possible degradation of Na-β”-alumina in reducing atmosphere will also be addressed. Acknowledgements: This work was supported by the National Science Foundation under Grant Number DMR-1407048.

The Li[FSA]-[C2C1im][FSA] (FSA–: bis(fluorosulfonyl)amide and C2C1im+: 1-ethyl-3-methylimidazolium) ionic liquids have been studied as electrolytes for Li secondary batteries, though their thermal, physical, and electrochemical properties have not been systematically characterized. In this study, the thermal and transport properties of Li[FSA]-[C2C1im][FSA] ionic liquids as a function of the Li[FSA] molar fraction and temperature, in view of their operation at both room and intermediate temperatures. Differential scanning calorimetric analysis revealed that this system has a wide liquid-phase temperature range from Li[FSA] fractions of 0.0 to 0.4 and indicated the existence of the Li[C2C1im][FSA]2 line compound. Single-crystal X-ray diffraction analysis was used to determine the crystal structure of Li[C2C1im][FSA]2, which consists of Li+ octahedrally coordinated by six O atoms originating from four FSA– anions. The temperature dependences of the viscosity and ionic conductivity were fitted by the Vogel–Tammann–Fulcher equation, and the viscosity and molar ionic conductivity were connected by the fractional Walden rule. Lithium–metal deposition/dissolution efficiency decreased with increasing measurement temperature and decreasing Li[FSA] fraction. Aluminum corrosion at positive potentials was investigated by a potential step method, which revealed that the stability of an aluminum electrode was improved at high Li[FSA] fractions at 298 K, and the corrosion-limit potential decreased at elevated temperatures.

A series of solid biopolymer electrolytes (SBEs) based carboxymethyl cellulose doped with different weight percentage of ammonium fluoride, NH4F were prepared by the solution cast technique. The ionic conductivity was investigated using electrical impedance spectroscopy in the frequency range of 5 0Hz – 1 MHz. The highest ionic conductivity of 2.68 × 10−7 Scm−1 (9 wt.% NH4F) was obtained at ambient temperature. The ionic mobility and diffusion coefficient of charge carriers were identified to influence the enhancement of ionic conductivity in SBE samples. The exponent s of SBE samples were determined using Joncher's Universal Power Law. The conduction mechanism of the most conductive SBE sample can be represented by quantum mechanical tunnelling model.

The morphology and ionic conductivity of poly(1‐n‐alkyl‐3‐vinylimidazolium)‐based homopolymers polymerized from ionic liquids were investigated as a function of the alkyl chain length and counterion type. In general, X‐ray scattering showed three features: (i) backbone‐to‐backbone, (ii) anion‐to‐anion, and (iii) pendant‐to‐pendant characteristic distances. As the alkyl chain length increases, the backbone‐to‐backbone separation increases. As the size of counterion increases, the anion‐to‐anion scattering peak becomes apparent and its correlation length increases. The X‐ray scattering features shift to lower angles as the temperature increases due to thermal expansion. The ionic conductivity results show that the glass transition temperature (Tg) is a dominant, but not exclusive, parameter in determining ion transport. The Tg‐independent ionic conductivity decreases as the backbone‐to‐backbone spacing increases. Further interpretation of the ionic conductivity using the Vogel–Fulcher–Tammann equation enabled the correlation between polymer morphology and ionic conductivity, which highlights the importance of anion hoping between adjacent polymer backbones. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012

Rechargeable lithium-ion batteries take advantage of solid-state ionic conductors, such as solid polymer electrolytes (SPEs) [1, 2]. Despite the large technological potential of solvent-free SPEs, they suffer from a serious problem: a relatively low ionic conductivity. The addition of ionic liquids (ILs) to SPEs appears as a possible solution due to the enhancement of electrochemical stability (~4-6 V) and ionic conductivity (~10-3 – 10-2 S.cm-1) [3].ILs has two important features for their specific use in battery applications: high concentration of ions and high ion mobility at room temperature. The use of PVDF polymer and its copolymers together with ILs as SPEs still has to be thoroughly explored.This work shows the thermal and electrochemical properties of SPEs based on poly(vinylidene fluoride-co-trifluoroethylene), P(VDF-TrFE), and specific amounts (4wt% to 32wt%) of the ionic liquid N,N,N-trimethyl-N-(2-hydroxyethyl)ammonium bis(trifluoromethylsulfonyl)imide, [N1 1 1 2(OH)][NTf2]. The SPE was prepared by thermaly induced phase separation (TIPS).The addition of ILs in P(VDF-TrFE) affects the microstructure, thermal stability and ionic conductivity of the polymer membrane.The ionic conductivity increases as the ionic liquid (IL) content increases with a maximum value at room temperature of 1.7x10-5 S.cm-1for an IL composition of 32wt%. The temperature behavior of the ionic conductivity follows the Arrhenius equation and the electrical response of the membrane is well represented by a Randles modified equivalent circuit. The results prove that the developed SPEs are adequate for energy storage devices. Acknowledgments This work was supported by FEDER through the COMPETE Program and by the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Project PEST-C/FIS/UI607/2011, PEST-C/QUI/UI0686/2011, PEST-OE/EQB/LA0004/2011, projects PTDC/CTM-NAN/121274/2010 and PTDC/QUI-QUI/117340/2010, grants SFRH/BD/68499/2010 (C.M.C.) and SRFH/BD/90366/2012 (R.L.) and FCT Investigator grant (J.M.S.S.E.). The authors also thank funding from Matepro –Optimizing Materials and Processes”, ref. NORTE-07-0124-FEDER-000037”, co-funded by the “Programa Operacional Regional do Norte” (ON.2 – O Novo Norte), under the “Quadro de Referência Estratégico Nacional” (QREN), through the “Fundo Europeu de Desenvolvimento Regional” (FEDER). The authors thank Solvay for kindly supplying the high quality materials.