Ion Transport In Polymer Electrolytes Research Articles

Charge Scaling in Classical Force Fields for Lithium Ions in Polymers.

Polymer electrolytes are of interest for applications in energy storage. Molecular simulations of ion transport in polymer electrolytes have been widely used to study the conductivity in these materials. Such simulations have generally relied on classical force fields. A peculiar feature of such force fields has been that in the particular case of lithium ions (Li+), their charge must be scaled down by approximately 20% to achieve agreement with experimental measurements of ion diffusivity. In this work, we present first-principles calculations that serve to justify the charge-scaling factor and van der Waals interaction parameters for Li+ diffusion in poly(ethylene glycol) (PEO) with bistriflimide (TFSI-) counterions. Our results indicate that a scaling factor of 0.79 provides good agreement with DFT calculations over a relatively wide range of Li+ concentrations and temperatures, consistent with past reports where that factor was adjusted by trial and error. We also show that such a scaling factor leads to diffusivities that are in quantitative agreement with experimental measurements.

Dynamic density functional theory of polymers with salt in electric fields.

We present a dynamic density functional theory for modeling the effects of applied electric fields on the local structure of polymers with added salt (polymer electrolytes). Time-dependent equations for the local electrostatic potential and volume fractions of polymer, cation, and anion of added salt are developed using the principles of linear irreversible thermodynamics. For such a development, a field theoretic description of the free energy of polymer melts doped with salts is used, which captures the effects of local variations in the dielectric function. Connections of the dynamic density functional theory with experiments are established by relating the three phenomenological Onsager's transport coefficients of the theory to the mutual diffusion of electrolyte, ionic conductivity, and transference number of one of the ions. The theory is connected with a statistical mechanical model developed by Bearman and Kirkwood [J. Chem. Phys. 28, 136 (1958)] after relating the three transport coefficients to friction coefficients. The steady-state limit of the dynamic density functional theory is used to understand the effects of dielectric inhomogeneity on the phase separation in polymer electrolytes. The theory developed here provides not only a way to connect with experiments but also to develop multi-scale models for studying connections between local structure and ion transport in polymer electrolytes.

Percolating coordinated ion transport cells in polymer electrolytes to realize room-temperature solid-state lithium metal batteries

Polymer electrolytes, notable for their good mechanical properties, superior processability, and high electrochemical stability, are promising for high-energy-density lithium metal batteries. However, the ionic conduction of polymer electrolytes is seriously constrained by either the entanglement of polymer chains or binding of anions, particularly at ambient temperatures, making their practical application impossible. Herein, we employ a high-salt-concentration strategy with poly(vinylene carbonate) to construct a percolating network by linking coordinated ion transport cells. The cells, consisting of lithium-ion at the core surrounded by coordinated poly(vinylene carbonate), N,N-dimethylformamide, or incompletely bonded bis(trifluoromethanesulfonyl)imide, ensure an efficient coordination and de-coordination process for ion transport. Thus, a high rate of Li+ transport is realized, achieving an ionic conductivity of 0.82 mS cm−1 at 30 °C. Consequently, the formulated solid-state lithium metal batteries with the poly(vinylene carbonate) electrolyte enable superior stability in cycling under a wide temperature range (0–60 °C), high working voltage (4.5 V), and high mass load (>10 mg cm−2). This simple strategy for creating an ion-percolating network by linking coordinated ion transport cells not only offers new insights into understanding the mechanism for ion transport in polymer electrolytes but also paves the way for the application of solid-state lithium metal batteries.

Ion Transport in Polymer Electrolytes: Building New Bridges between Experiment and Molecular Simulation.

ConspectusPolymer electrolytes constitute a promising type of material for solid-state batteries. However, one of the bottlenecks for their practical implementation lies in the transport properties, often including restricted Li+ self-diffusion and conductivity and low cationic transference numbers. This calls for a molecular understanding of ion transport in polymer electrolytes in which molecular dynamics (MD) simulation can provide both new physical insights and quantitative predictions. Although efforts have been made in this area and qualitative pictures have emerged, direct and quantitative comparisons between experiment and simulation remain challenging because of the lack of a unified theoretical framework to connect them.In our work, we show that by computing the glass transition temperature (Tg) of the model system and using the normalized inverse temperature 1000/(T - Tg + 50), the Li+ self-diffusion coefficient can be compared quantitatively between MD simulations and experiments. This allows us to disentangle the effects of Tg and the polymer dielectric environment on ion conduction in polymer electrolytes, giving rise to the identification of an optimal solvating environment for fast ion conduction.Unlike Li+ self-diffusion coefficients and ionic conductivity, the transference number, which describes the fraction of current carried by Li+ ions, depends on the boundary conditions or the reference frame (RF). This creates a non-negligible gap when comparing experiment and simulation because the fluxes in the experimental measurements and in the linear response theory used in MD simulation are defined in different RFs. We show that by employing the Onsager theory of ion transport and applying a proper RF transformation, a much better agreement between experiment and simulation can be achieved for the PEO-LiTFSI system. This further allows us to derive the theoretical expression for the Bruce-Vincent transference number in terms of the Onsager coefficients and make a direct comparison to experiments. Since the Bruce-Vincent method is widely used to extract transference numbers from experimental data, this opens the door to calibrating MD simulations via reproducing the Bruce-Vincent transference number and using MD simulations to predict the true transference number.In addition, we also address several open questions here such as the time-scale effects on the ion-pairing phenomenon, the consistency check between different types of experiments, the need for more accurate force fields used in MD simulations, and the extension to multicomponent systems. Overall, this Account focuses on building new bridges between experiment and simulation for quantitative comparison, warnings of pitfalls when comparing apples and oranges, and clarifying misconceptions. From a physical chemistry point of view, it connects to concentrated solution theory and provides a unified theoretical framework that can maximize the power of MD simulations. Therefore, this Account will be useful for the electrochemical energy storage community at large and set examples of how to approach experiments from theory and simulation (and vice versa).

Molecular Dynamics Study of Ion Transport in Polymer Electrolytes of All-Solid-State Li-Ion Batteries.

Atomistic analysis of the ion transport in polymer electrolytes for all-solid-state Li-ion batteries was performed using molecular dynamics simulations to investigate the relationship between Li-ion transport and polymer morphology. Polyethylene oxide (PEO) and poly(diethylene oxide-alt-oxymethylene), P(2EO-MO), were used as the electrolyte materials, and the effects of salt concentrations and polymer types on the ion transport properties were explored. The size and number of LiTFSI clusters were found to increase with increasing salt concentrations, leading to a decrease in ion diffusivity at high salt concentrations. The Li-ion transport mechanisms were further analyzed by calculating the inter/intra-hopping rate and distance at various ion concentrations in PEO and P(2EO-MO) polymers. While the balance between the rate and distance of inter-hopping was comparable for both PEO and P(2EO-MO), the intra-hopping rate and distance were found to be higher in PEO than in P(2EO-MO), leading to a higher diffusivity in PEO. The results of this study provide insights into the correlation between the nanoscopic structures of ion solvation and the dynamics of Li-ion transport in polymer electrolytes.

Role of free volumes and segmental dynamics on ion conductivity of PEO/LiTFSI solid polymer electrolytes filled with SiO2 nanoparticles: a positron annihilation and broadband dielectric spectroscopy study.

The limited ionic conductivity of polymer electrolytes is a major issue for their industrial application. Enhancement of ionic conductivity in the poly(ethylene oxide), PEO, based electrolyte has been achieved by loading passive nanofillers such as SiO2 nanoparticles (NPs). To investigate the role of modifications in free volume characteristics and the polymer chain dynamics induced by the loading of passive fillers on the ionic conductivity of the PEO based ternary electrolyte, a systematic investigation has been carried out using positron annihilation and broadband dielectric spectroscopy. As a result of interfacial interactions, the loading of SiO2 NPs alters the semi-crystalline morphology of PEO resulting in a higher crystallinity at lower loadings due to the surface confinement of PEO chains, and the formation of smaller PEO crystallites at higher loadings due to interparticle nanoconfinement. These modifications are accompanied by a decrease in free volume fraction at the lowest loading (0.5 wt%) followed by an increase at higher loadings (≥2.0 wt%). The Almond-West formalism considering two different universalities in different temperature and frequency ranges has been used to explain the ion-conduction process at different NP loadings. The Li ion conductivity is observed to be maximum for a 5.0 wt% loading of SiO2 NPs. The enhancement in ionic conductivity is observed to be directly correlated with the free volume characteristics and segmental dynamics of the PEO matrix, confirming their role in ion transport in polymer electrolytes.

Glass Transition Temperature and Ion Binding Determine Conductivity and Lithium–Ion Transport in Polymer Electrolytes

Polymer electrolytes with high Li+-ion conductivity provide a route toward improved safety and performance of Li+-ion batteries. However, most polymer electrolytes suffer from low ionic conduction and an even lower Li+-ion contribution to the conductivity (the transport number, t+), with the anion typically transporting over 80% of the charge. Here, we show that subtle and potentially undetected associations within a polymer electrolyte can entrain both the anion and the cation. When removed, the conductivity performance of the electrolyte can be improved by almost 2 orders of magnitude. Importantly, while some of this improvement can be attributed to a decreased glass transition temperature, Tg, the removal of the amide functional group reduces interactions between the polymer and the Li+ cations, doubling the Li+ t+ to 0.43, as measured using pulsed-field-gradient NMR. This work highlights the importance of strategic synthetic design and emphasizes the dual role of Tg and ion binding for the development of polymer electrolytes with increased total ionic conductivity and the Li+ ion contribution to it.

Influence of Host Polarity on Correlating Salt Concentration, Molecular Weight, and Molar Conductivity in Polymer Electrolytes.

We use coarse-grained molecular dynamics simulations to study the effect of salt concentration and host polymer molecular weight on ion transport in polymer electrolytes. We find that increasing salt concentration or molecular weight similarly slows polymer dynamics across a wide range of host polarities, and that the resulting relaxation times display a correlation to the product of the salt concentration and polymer molecular weight. However, we find that molar conductivity only decreases with polymer dynamics at high polarities but is uncorrelated with the latter at low polarities. We attribute such differences to the variation in ionic aggregation between high and low polarity electrolytes. At low polarity, ionic dissociation significantly increases with molecular weight and salt concentration, offsetting the slowdown in polymer dynamics and yielding the observed insensitivity of molar conductivity. However, at high polarity, ions are mostly dissociated, independent of either molecular weight or salt concentration, thereby strongly coupling molar conductivity to polymer dynamics.

Nanothin film conductivity measurements reveal interfacial influence on ion transport in polymer electrolytes

Nanoscale interfacial zone limits ion transport properties of polymer electrolytes.

Review of Recent Nuclear Magnetic Resonance Studies of Ion Transport in Polymer Electrolytes.

Current and future demands for increasing the energy density of batteries without sacrificing safety has led to intensive worldwide research on all solid state Li-based batteries. Given the physical limitations on inorganic ceramic or glassy solid electrolytes, development of polymer electrolytes continues to be a high priority. This brief review covers several recent alternative approaches to polymer electrolytes based solely on poly(ethylene oxide) (PEO) and the use of nuclear magnetic resonance (NMR) to elucidate structure and ion transport properties in these materials.

Improving the Lithium Ion Transport in Polymer Electrolytes by Functionalized Ionic-Liquid Additives: Simulations and Modeling

We present a theoretical study combining molecular dynamics (MD) simulations with an analytical lithium ion transport model [Maitra and Heuer, Phys. Rev. Lett. 2007, 98, 227802] to highlight a novel strategy to increase the lithium mobility in polymer electrolytes based on poly(ethylene oxide) (PEO). This is achieved by using a pyrrolidinium-based ionic liquid (IL) where the cation has been chemically functionalized by a short oligoether side chain [von Zamory et al., Phys. Chem. Chem. Phys. 2016, 18(31), 21539] as an additive. Since the oligoether moieties at the pyrrolidinium cations form pronounced coordinations to the lithium ions for sufficiently long side chains, the ions can be detached from the PEO backbone. In this way, a fundamentally new lithium ion transport mechanism is established (shuttling mechanism), in which the lithium dynamics is decoupled from the polymer dynamics, the latter typically being slow under experimental conditions. Based on our simulations, we incorporate this novel mechanism into our existing model, which accurately reproduces the observed lithium dynamics. We demonstrate that the use of oligoether-functionalized IL additives significantly increases the lithium diffusivity. Finally, we show that for experimentally relevant electrolytes containing long polymer chains, an even stronger increase of the lithium mobility can be expected.

Chemically Specific Dynamic Bond Percolation Model for Ion Transport in Polymer Electrolytes

We introduce a coarse-grained approach for characterizing the long-timescale dynamics of ion diffusion in general polymer electrolytes using input from short molecular dynamics trajectories. The approach includes aspects of the dynamic bond percolation model [J. Chem. Phys. 1983, 79, 3133−3142] by treating ion diffusion in terms of hopping transitions on a fluctuating lattice. We extend this well-known approach by using short (i.e., 10 ns) molecular dynamics (MD) trajectories to predict the distribution of ion solvation sites that comprise the lattice and to predict the rate of hopping among the lattice sites. This yields a chemically specific dynamic bond percolation (CS-DBP) model that enables the description of long-timescale ion diffusion in polymer electrolytes at a computational cost that makes feasible the screening of candidate materials. We employ the new model to characterize lithium-ion diffusion properties in six polyethers that differ by oxygen content and backbone stiffness: poly(trimethylen...

Effect of Nanoparticles on Ion Transport in Polymer Electrolytes

Using all atom molecular dynamics and trajectory-extending kinetic Monte Carlo simulations, we study the influence of Al2O3 nanoparticles on the transport properties of ions in polymer electrolytes composed of poly(ethylene oxide) (PEO) melt solvated with LiBF4 salt. We observe that the mobility of Li+ cations and BF4– anions and the overall conductivity decrease upon addition of nanoparticles. Our results suggest that the nanoparticles slow the dynamics of polymer segments near their surfaces. Moreover, the preferential interactions of the ions with the nanoparticles are seen to lead to an enhancement of ion concentration near the particle surfaces and a further reduction in the polymer mobilities near the surface. Together, these effects are seen to increase the residence times of Li+ cations near the polymer backbone in the vicinity of the nanoparticles and reduce the overall mobility and conductivity of the electrolyte. Overall, our simulation results suggest that both the nanoparticle-induced changes in polymer dynamical properties and the interactions between the nanoparticles and ions influence the conductivity of the electrolyte.

Ionic Conductivity and Self-Assembly in Poly(isoprene-b-ethylene oxide) Electrolytes Doped with LiTf and EMITf

Diblock copolymers of poly(isoprene-b-ethylene oxide), PI-b-PEO (IEO), are employed as templates for the development of nanostructured polymer electrolytes by salt-doping. For this purpose, lithium triflate (CF3SO3Li, LiTf) and 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (CF3SO3–C6H11N2, EMITf) salts were separately introduced at various [EO]:[salt] ratios. The local structure, nanodomain morphology and ion dynamics of the resulted block copolymer electrolytes were investigated by infrared spectroscopy, X-ray scattering, differential scanning calorimetry and dielectric spectroscopy. The structural investigation revealed strong effects of both LiTf and EMITf salt addition to the copolymer nanodomain morphology. These include transitions between different ordered nanophases and an increased domain spacing. The latter was independent of the kind of salt providing the possibility of studying ion transport under identical nanodomain sizes but in the presence of different interactions. Ionic conductivity in the two systems was fundamentally different. In IEO/EMITf ionic conductivity was much higher and comparable to the PEO/EMITf case. However, in IEO/LiTf, ion conductivity was reduced by a factor of a 100 relative to the PEO/LiTf case. This reflects combined effects of increased interaction parameter and of preferential wetting of electrodes. These results suggest ways for manipulating ion transport in polymer electrolytes.

Li+ Ion Transport in Polymer Electrolytes Based on a Glyme-Li Salt Solvate Ionic Liquid

ABSTRACT Polymer electrolytes (PEs) have served as the focus of intensive research as new ion-conducting materials, especially for lithium battery applications. A new strategy to develop fast lithium-conducting PEs is reported here. The thermal, ionic transport, and electrochemical properties of polymer solutions in a glyme-Li salt solvate ionic liquid, [Li(G4) 1 ][TFSA], composed of an equimolar mixture of lithium bis(trifluoromethanesulfonyl) amide (Li[TFSA]) and tetraglyme (G4), were characterized. Poly(ethylene oxide) (PEO), poly(methyl methacrylate) (PMMA), and poly(butyl acrylate) (PBA) were combined with [Li(G4) 1 ][TFSA] in order to explore the effects of polymer structure on the properties. The self-diffusion coefficient ratio of the glyme and Li + ions ( D G / D Li ) was investigated to evaluate the stability of the complex (solvate) cations. The D G / D Li values suggested that the [Li(G4) 1 ] + complex cations underwent a ligand exchange reaction between G4 and PEO in the PEO-based solution, whereas the cations remained stable ( D G / D Li = 1) in the PMMA- and PBA-based solutions. The robustness of the [Li(G4) 1 ] + complex cations in the PMMA- and PBA-based solutions was reflected in high weight-loss temperature, greater Li transference number, and high oxidative stability. Owing to the lower glass transition temperature and low affinity towards Li + ions, the PBA-based solutions yielded superior lithium transport properties (ionic conductivity of 10 −4 ∼10 −3 Scm −1 and Li transference number as high as 0.5) among the investigated polymer solutions.

Ionic Transport in Polymer Electrolytes Based on PEO and the PMImI Ionic Liquid: Effects of Salt Concentration and Iodine Addition

We find a strong impact of ion pairing on ionic transport in potential Grätzel-cell electrolytes based on poly(ethylene oxide) (PEO) and 1-propyl-3-methylimidazolium iodide (PMImI). Furthermore, the addition of free iodine enhances both mass and charge transport, which can be explained by the reduced pair-formation tendency of the bulky triiodide ion. These results arise from conductivity and diffusion measurements on amorphous complexes with EO/PMImI molar ratios of 20 and 30 and their evaluation in a comprehensive ion-transport model. In particular, the charge diffusivity D(σ) was compared with the PMIm diffusivity D(cat)* determined by pulsed-field-gradient nuclear magnetic resonance and the iodine diffusivity D(an)* obtained from radiotracer depth profiling. Simultaneous fitting of these diffusion coefficients in complexes with and without iodine additive yields best values for the model parameters. The results characterize not only the mobility of free ions and pairs as a function of temperature and composition but also the degree of ion pairing.

Decoupling of Ionic Transport from Segmental Relaxation in Polymer Electrolytes

We present detailed studies of the relationship between ionic conductivity and segmental relaxation in polymer electrolytes. The analysis shows that the ionic conductivity can be decoupled from segmental dynamics and the strength of the decoupling correlates with the fragility but not with the glass transition temperature. These results call for a revision of the current picture of ionic transport in polymer electrolytes. We relate the observed decoupling phenomenon to frustration in packing of rigid polymers, where the loose local structure is also responsible for the increase in their fragility.

New insights from variable-temperature and variable-pressure studies into coupling and decoupling processes for ion transport in polymer electrolytes and glasses

A fresh analysis of literature data shows how the influences of temperature and pressure on ion transport and structural relaxation in glass-forming systems may be combined within the framework of ‘master plots’ based on the equation E A = M · V A , to reveal new insights into coupling and decoupling effects in a wide range of systems. E A,σ and V A,σ are, respectively, instantaneous activation energies and volumes for ionic conductivity and the parameter, M σ , is a corresponding ‘process modulus’. For structural relaxations occurring at the glass transition, the appropriate modulus is given by M s = T g · d P/d T g . We can now identify typical behaviour patterns for fragile liquids on the one hand, and typical inorganic glasses on the other. Thus, the parameters, M σ and M s , for fragile systems such as molten Ca(NO 3) 2:KNO 3 (CKN) or a typical polymer electrolyte such as a complex of LiCF 3SO 3 in PPG, are found to remain constant over a wide range of temperatures down to T g , despite changes in the temperature (and pressure) dependences of the ionic conductivities, as indicated for example by a return to Arrhenius behaviour in the case of CKN, or by so-called Stickel plots and changes in the VTF parameters for the polymer electrolytes. If E* and V* are activation energies and volumes assigned to elementary steps, when again E* = M · V*, we can go further and identify the microscopic processes driving forward structural relaxation. In the case of inorganic glasses, where usually we find the decoupling index R τ ≈ 10 12, we identify two distinct decoupling paradigms represented by strong and fragile systems respectively, where in both cases the activation volumes for ion transport are very similar to the corresponding ionic volumes. In the former case (typified by the strongly cross-linked silicate and aluminosilicate systems), the negative activation volumes for structural relaxation (negative values of d T g /d P) are clearly indicative of a ‘water-like’ behaviour attributable to the collapse of the network under pressure. On the other hand, for the more fragile fast-ion conducting silver iodomolybdate glass, the experimental results show that M s (at T g ) ≈ M σ (in glass), implying some recoupling of structural relaxation to ion transport. Arguments based on the dynamic structure model lead us to predict that a similar close link should exist between M s (at T g ) and M σ in the relatively fragile lithium and sodium borate glasses, thus highlighting the need for more information concerning the effects of pressure on the glass transition temperatures of common inorganic glasses.

The Use of Radiotracer Diffusion to Investigate Ionic Transport in Polymer Electrolytes: Examples, Effects, and Their Evaluation

Abstract The paper highlights some remarkable results obtained by applying the radiotracer diffusion (RTD) technique to the study of ionic transport in salt-in-polymer electrolytes. The technique is based on the determination of a radiotracer depth profile by serial sectioning following isothermal diffusion annealing. Unlike alternative methods, RTD is able to measure the self-cation and self-anion diffusivity even for systems dilute in salt. Another unique feature is the capability to investigate foreign-ion diffusion at extremely low concentration levels. Combined with DC conductivity data, RTD may provide a virtually complete picture of mass and charge transport in solid-like polymer electrolytes (SPEs). The paper describes the special SPE-related procedures used in the RTD experiments and their analysis. The advantages of the method will be demonstrated with selected examples of self- and foreign-ion diffusion in prototype SPE systems. We also present prominent examples of RTD dealing with the effects of salt precipitation and oxide nano-particles used as dispersed filler material.

Synthesis and application as polymer electrolyte of hyperbranched polyether made by cationic ring‐opening polymerization of 3‐{2‐[2‐(2‐hydroxyethoxy)ethoxy]ethoxymethyl}‐3′‐methyl‐oxetane

AbstractA novel cyclic ether monomer 3‐{2‐[2‐(2‐hydroxyethoxy)ethoxy]ethoxy‐methyl}‐3′‐methyloxetane (HEMO) was prepared from the reaction of 3‐hydroxymethyl‐3′‐methyloxetane tosylate with triethylene glycol. The corresponding hyperbranched polyether (PHEMO) was synthesized using BF3·Et2O as initiator through cationic ring‐opening polymerization. The evidence from 1H and 13C NMR analyses revealed that the hyperbranched structure is constructed by the competition between two chain propagation mechanisms, i.e. active chain end and activated monomer mechanism. The terminal structure of PHEMO with a cyclic fragment was definitely detected by MALDI‐TOF measurement. A DSC test implied that the resulting polyether has excellent segment motion performance potentially beneficial for the ion transport of polymer electrolytes. Moreover, a TGA assay showed that this hyperbranched polymer possesses high thermostability as compared to its liquid counterpart. The ion conductivity was measured to reach 5.6 × 10−5 S/cm at room temperature and 6.3 × 10−4 S/cm at 80 °C after doped with LiTFSI at a ratio of Li:O = 0.05, presenting the promise to meet the practical requirement of lithium ion batteries for polymer electrolytes. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 3650–3665, 2006