Session Y19: Ion and Thermal Transport in Polymers

Diffusive charge transport in high-valency redox-active polymer solutions

Redox-active polymers (RAPs) are a promising material for energy storage in flow batteries due their large size preventing detrimental redox material crossover and adjustable molecular chemistry and architecture for optimized performance. There has been a recent effort to understand the physics governing charge diffusion in RAPs and we have used simulations and theory to show how a variety of molecular charge transport mechanisms affect diffusive motion in higher valency RAP solutions with explicit salt by employing a full Coulombic potential. Our coarse-grained model of RAP solutions employs Brownian dynamics for polymer motion and a kinetic Monte Carlo update steps for the charge hopping dynamics. We perform these simulations for single chains and multi-chain systems where we show how a various transport mechanisms interplay, including the intra-polymer transport of charge via self-exchange and polymer segmental motions, as well as hopping due to inter-polymer collisions and translational diffusion of the chains. We also investigated the effect of salt on the radius of gyration and the effect of varying the intra and inter charge hopping rates representative of the Co, Fe, and Ru based RAPs of our experimental counter parts. Additionally, we included a surface in our model to investigate electrode-polymer charge transport.   

Influence of anion chemistry on ion transport in single ion conducting polymer electrolytes

Single ion conducting (SIC) polymer electrolytes exhibit highly selective lithium ion conduction, crucial for eliminating safety issues in lithium-ion batteries. In this work, we report results from atomistic molecular dynamics simulations focused on elucidating the influence of different anion chemistries on ionic conductivity in SIC copolymer electrolytes. For these electrolyte systems (synthesized by our experimental collaborators), the polymer backbone was anchored with polyethylene glycol (PEG) chains and lithiated anionic moieties. We consider derivatives of three different anions: acrylic acid, Acrylamido methylpropane sulfonic acid (AMPS), and bis(trifluoromethane)sulfonamide (TFSI). Our results suggest that with increasing anion content, the Li⁺ transport mechanism transitions from vehicular, in which Li⁺ diffuses together with the polyanion, to a combination of hopping and rearrangement of neighboring ion clusters. At a fixed anion content, the ion-polymer coordination behavior and the ion cluster morphologies are observed to be significantly different for the different anion chemistries considered in this study, thus rationalizing the significant influence of anion choice on the observed transport characteristics.   

Single Sodium-Ion Conducting Polymer Electrolyte Gels with Gyroidal Networks

Multiblock copolymers containing polar blocks of a sodium sulfonate group and nonpolar polyethylene blocks (precisely 12, 18, or 23 carbons) self-assemble into gyroid morphologies at an elevated temperature. Here, we place a single alkene group in the middle of each 18-carbon polyethylene block to enable a crosslinking reaction for maintaining the gyroid networks at ambient conditions. The morphological transitions of unsaturated polyethylene sodium sulfonates (uPES18Na) involve layered, gyroid, and hexagonally packed cylinder morphologies where gyroid morphologies exist at 100 – 140 °C. Crosslinked polyethylene sodium sulfonates (xPES18Na) are evident by the loss of C=C bonds and higher gel content with increasing reaction time. The X-ray scattering data show that xPES18Na exhibits less ordered gyroids than the uPES18Na. The xPES18Na of crosslinked gyroid networks can be swollen with a polar solvent, and the correlation between the morphologies and ionic conductivities will be discussed.   

Assessing Effective Medium Theories for Conduction through Lamellar Grains

A numerical finite-difference model was developed for 2D transient diffusion in lamellar structures. The focus was heat transfer, but it could be applied to mass, momentum, and electron/ion transfer as well. The control volume contains two phases. The phases have different transport properties and are arranged in grains that constitute units of coherent orientation. The effect of grain size, grain boundaries, and phase contrast on apparent transport properties was evaluated by examining a progression of increasingly complex structures. The numerical model had good agreement with analytical expressions for homogeneous, parallel, and perpendicular structures. Effective medium theories (EMT) predict apparent transport properties in the limit of many, small, randomly oriented grains. The impact of grain size and contrast on EMT prediction accuracy was investigated. The numerical model quantitatively shows EMT predictions grow poorer with increasing phase contrast. Moreover, structural specifics such as grain boundary connectivity have more significant impact in large grains. Thus, larger variability from one structure to another is observed in large grains than in small grains. Specifically, standard deviation decreased by an order of magnitude on going from 2 grains to 15 grains. This numerical approach provides some insight into regimes in which EMT approximations are appropriate and can be extended to 3D (which increases connectivity of transport pathways) and other types of structures.   

The Role of Ion Size and Electronic Character in Zwitterionic Electrolytes for Transport of Lithium-Metal Ions

Long-range Li-ion transport in most solid polymeric electrolytes occurs via a vehicular mechanism that couples polymer segmental dynamics to electrolyte performance. Consequently, the majority of successful polymer electrolyte candidate materials are extensively plasticized, representing a tradeoff between desirable mechanical properties and electrochemical functionality. We have recently demonstrated zwitterionic polymer electrolytes (PZIs) that transport Li+ ions via a superionic mechanism that enables high Lithium salt conductivitities (10-3 S/cm) and transport numbers (t+~0.67) despite modest glass transition temperatures (>0°C), however the chemical design features that contribute to this mechanism are poorly understood. In this talk I will develop design rules for fast ion transport in PZIs investigating the factors of tethered ion size and electronic character. These investigations lead to the discovery of universalities connecting the design of PZIs and their small molecule ZI counterparts.   

Impact of Side Chain Chemistry on Lithium Transport in Mixed Ion-Electron Conducting Polymers

Mixed ion-electron conducting polymers (MIECs) have recently been reported to significantly improve rate capability when used as binders in Lithium-ion battery cathodes. While it is widely known that the transport of Li+ is of utmost importance for battery functionality, this property has yet to be well characterized in MIECs. Here, we show that two thiophene derivates functionalized with chemically distinct ion conducting side chains, Poly{3-[6’-(N-methylimidazolium) hexyl] thiophene} (P3HT-Im+) and Poly[3-(methoxyethoxyethoxymethyl) thiophene] (P3MEEMT), show contrasting lithium solvation and transport properties. P3HT-Im+ can solvate and conduct ions up to a molar concertation of r=1.0 ([moles of salt]/[moles of monomer]), achieving an ionic conductivity of  ~10^–3 S/cm at 80°C, and a lithium transference number of 0.36. On the other hand, P3MEEMT shows a peak conductivity of ~10^–5 S/cm at r=0.05 and 80°C, with 0 lithium transport. This work shows that multiple high dielectric moieties can be used to impart ion conduction in semiconducting polymers, but diffuse, cationic side chains such as imidazolium are ideal for lithium conduction.    

Effect of Polymer Topology on Microstructure, Dynamics, Rheology, and Ionic conductivity in PEO based electrolytes

Microstructure, dynamics, and ionic conductivity of neat linear polymers have been extensively studied for solid polymer electrolytes in the past. In this work, we used poly (ethylene oxide) of various architectures (linear, stars and hyper-branched) and lithium contents to explore the effects of chain topology and molar ratio of the salt on glass transition, crystallization, mechanical properties, and ionic conductivity of PEO based electrolytes. XRD results suggest that room temperature crystallinity is suppressed more for the non-linear PEOs compared to the linear chains due to high degree of branching and completely diminished with lithium content exceeding 0.08 (Li/EO ratio) for all PEO topologies. Independent of polymer architecture, DSC results showed increase in T_g values that is similar in all electrolytes, suggesting a similar slowing down of alpha relaxation. Electrochemical impedance spectroscopy results indicate that the ionic conductivity in electrolytes with non-linear PEO topologies is higher than their linear analogues. Finally, the rheology of the electrolytes shows a decrease in the viscosity when non-linear PEO architectures are employed in comparison to the linear PEO.     

Phonon Catalysis

Over the last decade the Atomistic Simulation & Energy (ASE) research group has developed two methods for modeling phonons, and their interactions via molecular dynamics (MD) simulations, namely Green-Kubo Modal Analysis (GKMA) and Interface Conductance Modal Analysis (ICMA). These methods use the normal modes of a system, which are computed in the harmonic limit, as a basis set for decomposing the heat flow in a system, in a very general way. They can be applied to any form of solid or rigid molecule. These techniques allow one to compute how much each individual normal mode/phonon in a given system contributes to thermal transport. However, the same approach can also be extended to other properties to determine the extent to which specific phonons contribute to other phenomena. For example, the same modal analysis techniques used in GKMA and ICMA can be used to determine which modes/phonons are responsible for a chemical reaction, a chemical transformation, ion/mass diffusion, or a phase change etc. Furthermore, once the modes that primarily contribute to a particular phenomenon are identified, they can in concept be externally excited to accelerate the transformation, which is a phenomenon the ASE group has termed “phonon catalysis”. This talk will show a first example of this phenomenon observed in MD simulations, whereby a Li-ion conductor has several specific phonons excited and its diffusivity increases by 4-6 orders of magnitude. Importantly, the modes responsible for the diffusion are excited to the same temperature that corresponds with the observed diffusivity, but the bulk temperature of the material remains effectively unchanged. This first example shows what may be possible with targeted phonon excitation, as phonon catalysis may be a new approach to stimulating or even suppressing chemical transformations in real world applications (e.g., a solid oxide fuel cell that operates at room temperature, with the transport and reaction kinetics of 1000°C).   

Bridging the length scales in ionic separations via data-driven science and machine learning

Selective ionic separations is important to wastewater remediation, mineral recovery, and bio-based products produced from organic acids derived from biomass. Central to achieving selective ionic separations is understanding how ion-exchange membranes' composition and microstructure affect ion partitioning coefficients and ionic conductivity. Our lab has engaged in thin film and bulk membrane measurements to generate structure-property relationships that enable selective ionic separations. These experiments are complemented with molecular dynamics and quantum calculations to provide further insights to ionic selectivity. The talk will briefly conclude with our future approach to advanced selective ionic separations using machine learning that captures data from molecular simulations, materials experiments, and device-level demonstrations to guide future materials design and ionic separation platforms for more effective and energy efficient ionic separations.    

Superionic Li-ion Transport in a Single-Ion Conducting Polymer Blend Electrolyte

Single-ion conducting polymers (SICs) are promising candidates for the next generation of safer polymer electrolytes, due to their stability and high transference number. However, the conductivity in SICs is often limited by the mobility of the polymer backbone due to the ionic coupling. We present polymer blend electrolytes, consisting of a new single Li-ion conducting polymer blended with poly(ethylene oxide) (PEO). Dielectric relaxation spectroscopy is used to probe the ion transport properties and segmental dynamics of these systems and X-ray scattering is used to evaluate the morphology. The PEO associates with the ionic aggregates of the SIC, forming a miscible blend with pathways that promote ion transport. At high PEO content, ionic conductivity greater than 10^-5 S/cm and 10^-4 S/cm is achieved at 90°C and 130°C, respectively. A comparison of conductivity and polymer relaxation times shows that the high PEO content blends exhibit superionic transport, in which there is some decoupling of the Li-ion from the backbone. This superionic transport is not commonly found in single Li-ion conductors at temperatures with a mobile polymer, and thus this work presents a critical step for establishing design rules of superionic transport in SICs.   

Effect of Thermal Aging on Nanoparticle Structure in Polymer Nanocomposite Electrolytes

Nanoparticles are commonly added to polymer electrolytes in order to concomitantly enhance their mechanical and ion transport properties. Previous work has shown significant increases in the ionic conductivity and Li-ion transference in nanocomposite electrolytes with inert, ceramic fillers. Mechanistic understanding of this property enhancement, however, assumes nanoparticle dispersion states--namely, well-dispersed or percolating aggregates--that are seldom quantified using small-angle scattering. In this report, we study the effect of thermal aging on the structure of a model nanocomposite electrolyte system--PEO:LiTFSI with 14 nm SiO₂ NPs--using small-angle x-ray scattering (SAXS). In agreement with similar studies on numerous polymer nanocomposite systems, we find a structure factor with a characteristic interparticle spacing that appears upon prolonged annealing. In parallel, we measure the ionic conductivity and dielectric relaxations of these materials. By combining these two techniques--SAXS and impedance spectroscopy--we hope to determine the effect of nanoparticle structure on the Li-ion conductivity of polymer electrolytes.   

Thermal transport in polymers: Intra- versus intermolecular energy transport

The thermal transport of polymer materials is important for various applications such as e.g. packaging. Long chain molecules provide different pathways of energy transport due to the distinct microscopic interactions, i.e., stiff, covalently bonded backbone interactions versus soft, nonbonded interactions, such as van der Waals (vdW) forces. The rate of intramolecular energy transport along the molecular backbone is higher than that of intermolecular transport. This correlation between energy-transport rate and macromolecular configuration opens opportunities for tailoring the thermal conductivity of a material.

We devise a simple model that accounts for the fast bonded and slow nonbonded, diffusive energy transport. We show how the model parameters can be quantitatively obtained from molecular simulation and validate the model by studying both, atomistic all-atom representations of polymer materials and a universal, coarse-grained model [1]. Applications to the thermal transport of lamella-forming diblock copolymers are discussed and compared to experiments that image heat flux with sub-10 nm resolution [2].