Session S43: Mechanisms of Ionic Conduction and Diffusion in Polymeric Ion Conductors II

Ab initio Molecular Dynamics Study of Proton Transfer Mechanisms through Nano-Confined Structures for Anion Exchange Membranes

The understanding and design of cost-effective and reliable polymer architectures for use as anion-conducting membranes is an important challenge facing emerging electrochemical device technologies. A key ingredient for characterizing the main design principles is embedded in the fundamental understanding of the hydroxide diffusion mechanisms. Recently, nano-confined structures have become a popular tool for exploring the functionalities of anion exchange membranes. As the water structure in nano-confined structures is diverse and controlled by the shape and size of the confined structures, it is suggestive that the hydroxide diffusion mechanism would change dramatically and therefore should be distinct and studied for variant kinds of AEM cells. To this end, we are using the full-atomic scale ab initio molecular dynamics calculations to obtain a molecular level understanding of the mechanisms of hydroxide solvation and transport in different chemical environments to guide the synthesis and experimental materials design. To mimic the complicated AEM environment, our theoretical model contains different graphene sandwich structures to which the cationic groups are attached. The cation groups are then surrounded by water up to experimentally relevant ratios and hydroxide ions.   

Aggregate dynamics in ionomer melts

Melts of ion-containing polymers are desirable as battery materials for their stability. However, the low dielectric constants of polymer backbones drive ion aggregation, which can limit mobility and thus conductivity. Understanding how ion transport is connected to morphology and polymer architecture is critical for designing candidate ionomers. We simulate coarse-grained ionomer melts with explicit counterions, where evenly-spaced charged groups are pendant to the polymer backbones. Previously, we observed that pendant architectures can drive the formation of isolated ionic aggregates (clusters). Here, we use new cluster identification algorithms to show that ion transport in pendant systems is slaved to the collective dynamics of aggregates, where clusters collide, merge, and then separate while exchanging ions. Hopping of isolated ions between clusters does not occur. We extract important aggregate timescales and correlate them with ion mean-squared displacements.

  

Correlation between Ionic Mobility and Microstructure in Block Copolymers. A Coarse-Grained Modeling Study.

Ion transport in a generic model of block copolymers (A-b-B) was studied using Molecular Dynamics simulations. The design parameters of the block copolymer, the ions and the ion-polymer interactions, were all systematically varied to unveil correlations between microstructure and ion mobility (μ). It is found that two key microstructural features had a significant effect on ion transport: the extent of interpenetration between the microdomains (β) and the local fluctuations in the density (ρ) of the polymer matrix. While the β effect has been previously studied in some detail, the effect of density non-homogeneities has received much less attention. To be able to control the local fluctuations in ρ, a polymer design variant is explored that incorporates a second conductive block (A’) that is incompatible with the other two blocks (A’-b-A-b-B). It is found that increasing the fraction of A’ beads, increases the frequency and amplitude of the local ρ depleted regions within the conductive domain, resulting in an increase in μ. More generally, polymer designs are advocated that enhance ion transport by leveraging the natural density and composition fluctuations associated with block copolymer ordering transitions.   

Nano- and Mesoscale Conduction, Diffusion and Electrokinetics in Perfluorosulfonic-Acid Membranes

Perflurosulfonic-acid (PFSA) membrane properties result from transport in a tortuous (mesoscale) network of interconnected, heterogenous, nanoscale hydrophilic domains. Consequently, optimization of membrane performance relies on understanding the emergent phenomena of this multiscale system. Continuum and network modeling elucidate how macroscopic conductivity, water diffusivity, and electrokinetics arise from meso- and nanoscale phenomena of PFSAs.

We predict transport properties of nanoscale hydrophilic domains as a function of water content using concentrated solution theory. A Voronoi resistor network upscales the pore-level description to macroscopic dimensions. Ion-pair formation is accounted for at the nanoscale and network swelling and heterogeneities are included at the mesoscale. This work shows clearly how transport modes couple across length scales and identifies how nonlinearities govern macroscale transport.   

Morphology Effects on Diffusion of Selective Penetrants in Block Copolymers from Constrained Random Walk Model and Molecular Dynamics Simulations

Block copolymers, in which ions dissolve in and diffuse through one microphase while the other provides mechanical strength, are used in transport applications. Their transport properties are sensitive to the morphology; the dependence of diffusion or conductivity on morphology was previously predicted based on the fraction of orientations of the morphology that allow conduction versus homopolymer diffusion or conductivity. However, factors such as segregation strength, volume fraction, and molecular weight also vary across studies and affect the results. Here, we predict diffusion based on numerical results of random walks confined by surfaces. Coarse-grained molecular dynamics (MD) simulations are then used to investigate influence of degree of segregation, where ions are mapped onto selective penetrants. Through a sample with randomly oriented grains, the diffusion would be ~33% for cylinders and ~67% for lamellae versus that of homopolymer. However, the results for the inner portion of gyroid are intermediate between these values and depend on monomer volume fraction, showing that the gyroid phase may not perform better than lamellae for transport applications, in contrast to what has been expected.   

Coarse-Grained Molecular Dynamics of Ionic Transport in Polymer Electrolyte

Solid-state conducting polymers have become promising candidates in the search of low cost and safe electrolyte materials. However, current solid polymer electrolytes only operate under limited conditions with relatively low ion conductivity. The improvement of polymer properties requires a deeper understanding of the mechanisms of ionic transport in polymer networks, where modeling and computation can play an important role. In this work, we develop a coarse-grained molecular dynamics (CGMD) model with parameters calibrated by full atomistic simulations. Our model captures the conformational evolution of polymer chains during the transport process, for both self-doped and salt-doped polymers. The correlations between the motion of ions and the structure of the polymer network are revealed using advanced machine learning algorithms. In addition, to quantify the electrolyte performance, the lithium-ion diffusivity and conductivity are extracted from the simulations. Via a high throughput computation approach, we screen the effects of both operation conditions and intrinsic polymer properties on the conduction of the polymer electrolyte, providing useful guidance for designing future polymer electrolyte materials.   

Effect of Ion-Polymer Solvation Strength on Ion Diffusion in Model Diblock Copolymers

Salt-doped block copolymers, with one microphase domain that dissolves ions (allowing for ion conduction) and another that provides mechanical strength, are of interest as safe, robust battery electrolytes, among other applications. These materials are challenging to model due to the need to represent both strong, local ion correlations and much longer length scale features of the overall microphase separated morphology. Additionally, the microphase domains have significantly different local dielectric strengths and, relatedly, ions are strongly selectively solvated in the higher dielectric microphase; it is not clear how/whether the major physical impacts of these chemical features can be captured using simple, generic models that can easily access the time and length scales of interest (e.g. without resorting to atomistic simulations with polarizability). We work towards such a coarse-grained model, including only radially symmetric pairwise interactions. The model is implemented within a fluids density functional theory framework and in molecular dynamics simulations. Besides bonding interactions, Lennard-Jones potentials (less favorable for unlike monomers), and the Coulomb potential between ions (the strength of which is set by the higher dielectric microphase where the vast majority of ions exist), we include a phenomenological solvation potential for interactions with ions. The solvation potential is of form S/r⁴, where S can be different for the different ion-ion and ion-monomer interactions (this drives ions to the higher dielectric microphase). This form gives the proper scaling of solvation energy with ion size, among other advantages. In contrast to prior work without strong solvation, we find conditions under which the microphase domain spacing increases monotonically as salt is added, and we find that ion diffusion increases with polymer length for diblock copolymers but decreases for homopolymers.   

Mechanisms of charge diffusion in polyethylene oxide based electrolytes: insights from molecular dynamics simulations

The future of a number of technologies requires significant improvements in energy storage techniques. Batteries with a higher energy density, which are safer, more efficient, less expensive, and greener, are essential. In this context, lithium-ion (Li-ion) batteries based on a solid polymeric electrolyte are promising candidates, being much safer and less expensive than current liquid-based technologies. Designing optimal polymer/salt mixtures for battery applications requires us to gain a deeper understanding of the fundamental ionic transport mechanisms at play in these systems. Here, through classical molecular dynamics simulations, we investigate charge diffusion in systems consisting of a mixture of polyethylene oxide (PEO) and lithium bis(trifluoromethane)sulfonimide (LiTFSI) salt. In particular, we take into account the effect of ionic interactions, and show that at typical salt concentrations, there is a strong departure from the infinite dilution limit. We also investigate how specific chemical modifications of the polymer chain can influence charge transport, and provide guidelines for the design of efficient PEO-based polymer electrolytes.   

Ion Transport Mechanisms in Lamellar Phases of PS-PEO Block Copolymer Electrolytes Doped with LiPF6 Salts.

We use a multiscale simulation strategy to elucidate, at an atomistic level, the mechanisms underlying ion transport in the lamellar phase of polystyrene-polyethylene oxide (PS-PEO) block copolymer (BCP) electrolytes doped with LiPF6 salts. We compare the results obtained for ion transport in the phase separated BCP melts to those for salt-doped PEO homopolymer melts. The ions were found to exhibit slower dynamics in both the block copolymer and in the PEO phase of the BCP melt compared to those in pure PEO melt. Such results are shown to arise from slower segmental dynamics in the BCP melt and the coordination characteristics of the ions. Further, polymer backbone-ion residence times analyzed as a function of distance from the interface show that ions have a larger residence time near the interface compared to that near the bulk of lamella, and demonstrates the influence of glassy PS blocks and phase segregation on ion transport properties. Ion transport modes in the BCP melt were also studied and were compared with those in pure PEO melts.   

Physical Insights into Ion Conduction in Polymeric Systems

Polymerized ionic liquids (PolyILs) are promising candidates for a broad range of applications. However, relatively low ionic conductivity limits their current use, and fundamental understanding of mechanisms controlling ion transport in PolyILs is critical for their rational design. In this talk, we present analysis of several PolyILs with different chemical structures and ion size. Combining results of dielectric spectroscopy with differential scanning calorimetry and nuclear magnetic resonance we analyzed the charge and mass transport of several different PolyILs. Based on Random Barrier Model, we demonstrate that conductivity relaxation provides information on ion diffusion and the magnitude of cross-correlations effects between ionic motions. We propose a model that estimates ionic diffusivity from characteristic times of the conductivity relaxation process and ionic concentration [1].

1. C. Gainaru, et al., J. Phys. Chem. B 120, 11074 (2016)
  

Effect of Composition and Morphology on Ion Transport in Ternary Polymer-Polymer-Salt Blend Electrolytes

Conventional battery electrolytes typically employ a blend of high polarity and low viscosity liquid organic hosts, each of which have some intrinsic ionic conductivity at a given salt concentration. Naively, one may assume that the conductivities of the blend electrolytes will be an average of the intrinsic conductivities weighted by each parent host’s volume fraction. However, experiments have shown that the ionic conductivities of these blend electrolytes display a positive deviation relative to this simple mixing rule. We hypothesize that similar physics, in which there will be marked positive departure from this simple mixing rule, may manifest in blend polymer electrolytes. The strength of these deviations may depend on various factors, such as the phase stability of the blend. Using fully atomistic molecular dynamics, we explore ion transport in poly(glycidyl ether)-based, ternary polymer-polymer-salt blend electrolytes and seek to correlate underlying micro-/nanoscopic blend morphology to observed changes in ionic transport.   

Ionic conductivity in cationic polythiophenes

Recently, there has been considerable interest in developing materials that combine electronic with ionic conduction. Herein, we designed novel single ion conductors, based on cationic polythiophenes. They consist of a polythiophene backbone, a side group with a cation covalently bonded and a mobile anion. We report on the ionic conductivity (with Dielectric Spectroscopy) as a function of backbone molecular weight, side group length, and anion size. The side group length varies from four to ten methylene units and increases the room temperature conductivity by four orders of magnitude (internal plasticization). The anion size (anionic radii from 0.19 nm to 0.44 nm) affects both the structure (lamellar to amorphous by increasing anion radius as evidenced by WAXS) and the measured conductivity. The dc-conductivity increases by six orders of magnitude by increasing anion size at ambient temperature. As a result, conductivities as high as 2 × 10^-3 S/cm could be measured at high temperatures. Differences in dc - conductivity are discussed in terms of changes in glass temperature that is increasing with increasing anion size. On top of this, there is a weak dependence of conductivity on the side group length and anion size as evidenced by the different fragilities