Influence of nanoparticle-ion and nanoparticle-polymer interactions on ion transport and viscoelastic properties of polymer electrolytes

Salt Effects on the Mechanical Properties of Ionic Conductive Polymer: A Molecular Dynamics Study

Functional polymers can be used as electrolyte and binder materials in solid-state batteries. This often requires performance targets in terms of both the transport and mechanical properties. In this work, a model ionic conductive polymer system, i.e., poly(ethylene oxide)-LiTFSI, was used to study the impact of salt concentrations on mechanical properties, including different types of elastic moduli and the viscoelasticity with both nonequilibrium and equilibrium molecular dynamics simulations. We found an encouragingly good agreement between experiments and simulations regarding Young's modulus, bulk modulus, and viscosity. In addition, we identified an intermediate salt concentration at which the system shows high ionic conductivity, high Young's modulus, and short elastic restoration time. Therefore, this study laid the groundwork for investigating ionic conductive polymer binders with self-healing functionality from molecular dynamics simulations.

Ion Conductivity in Salt-Doped Polymers: Combined Effects of Temperature and Salt Concentration

We construct a coarse-grained molecular dynamics model based on poly(ethylene oxide) and lithium bis(trifluoromethane)sulfonimide salt to examine the combined effects of temperature and salt concentration on the transport properties. Salt doping notably slows the dynamics of polymer chains and reduces ion diffusivity, resulting in a glass transition temperature increase proportional to the salt concentration. The polymer diffusion is shown to be well represented by a modified Vogel-Fulcher-Tamman (M-VFT) equation that accounts for both the temperature and salt concentration dependence. Furthermore, we find that, at any temperature, the concentration dependence of the conductivity is well described by the product of its infinite dilution value and a correction factor accounting for the reduced segmental mobility with increasing salt concentration. These results highlight the important role of polymer segmental mobility in the salt concentration dependence of ion conductivity for temperatures near and above the glass transition.

Ionic Conductivity and Mechanical Reinforcement of Well-Dispersed Polymer Nanocomposite Electrolytes

Nanoparticles are commonly added to polymer electrolytes to enhance both their mechanical and ion transport properties. Previous work reports significant increases in the ionic conductivity and Li-ion transference in nanocomposite electrolytes with inert, ceramic fillers. The 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 work, we carefully control the inter-silica nanoparticle structure (where each NP has a diameter D = 14 nm) in a model polymer electrolyte system (PEO:LiTFSI). We find that hydrophobically modified silica NPs are stabilized against aggregation in an organic solvent by inter-NP electrostatic repulsion. Favorable NP surface chemistry and a strongly negative zeta potential promote compatibility with PEO and the resulting electrolyte. Upon prolonged thermal annealing, the nanocomposite electrolytes display structure factors with characteristic interparticle spacings determined by particle volume fraction. Thermal annealing and particle structuring yield significant increases in the storage modulus, G^', at 90 °C for the PEO/NP mixtures. We measure the dielectric spectra and blocking-electrode (κ_b) conductivities from -100 to 100 °C, and the Li⁺ current fraction (ρ_Li⁺) in symmetric Li-metal cells at 90 °C. We find that nanoparticles monotonically decrease the bulk ionic conductivity of PEO:LiTFSI at a rate faster than Maxwell's prediction for transport in composite media, while ρ_Li⁺ does not significantly change as a function of particle loading. Thus, when nanoparticle dispersion is controlled in polymer electrolytes, Li⁺ conductivity monotonically, i.e., (κ_bρ_Li⁺), decreases but favorable mechanical properties are realized. These results imply that percolating aggregates of ceramic surfaces, as opposed to physically separated particles, probably are required to achieve increases in bulk, ionic conductivity.

Prediction and Optimization of Ion Transport Characteristics in Nanoparticle-Based Electrolytes Using Convolutional Neural Networks

We develop a convolutional neural network (CNN) model to predict the diffusivity of cations in nanoparticle-based electrolytes and use it to identify the characteristics of morphologies that exhibit optimal transport properties. The ground truth data are obtained from kinetic Monte Carlo (kMC) simulations of cation transport parametrized using a multiscale modeling strategy. We implement deep learning approaches to quantitatively link the diffusivity of cations to the spatial arrangement of the nanoparticles. We then integrate the trained CNN model with a topology optimization algorithm for accelerated discovery of nanoparticle morphologies that exhibit optimal cation diffusivities at a specified nanoparticle loading, and we investigate the ability of the CNN model to quantitatively account for the influence of interparticle spatial correlations on cation diffusivity. Finally, by using data-driven approaches, we explore how simple descriptors of nanoparticle morphology correlate with cation diffusivity, thus providing a physical rationale for the observed optimal microstructures. The results of this study highlight the capability of CNNs to serve as surrogate models for structure-property relationships in composites with monodisperse spherical particles, which can in turn be used with inverse methods to discover morphologies that produce optimal target properties.

Molecular Modeling Investigations of Sorption and Diffusion of Small Molecules in Glassy Polymers

With a wide range of applications, from energy and environmental engineering, such as in gas separations and water purification, to biomedical engineering and packaging, glassy polymeric materials remain in the core of novel membrane and state-of the art barrier technologies. This review focuses on molecular simulation methodologies implemented for the study of sorption and diffusion of small molecules in dense glassy polymeric systems. Basic concepts are introduced and systematic methods for the generation of realistic polymer configurations are briefly presented. Challenges related to the long length and time scale phenomena that govern the permeation process in the glassy polymer matrix are described and molecular simulation approaches developed to address the multiscale problem at hand are discussed.

Mechanistic insight into the improved Li ion conductivity of solid polymer electrolytes

Polymer based solid electrolytes (SEs) are envisaged as futuristic components of safer solid state energy devices. But the semi-crystalline nature and slow dynamics of the host polymer matrix are found to hamper the ion transport through the solid polymer network and hence solid state devices are still far beyond the scope of practical application. In this study, we unravel the synergistic roles of Li salt (LiClO₄) and two different polymers – polyethylene oxide (PEO) and polydimethyl siloxane (PDMS), in the Li ion transport through their solid blend based electrolyte. A detailed study using dielectric spectroscopy and thermo-mechanical analysis is conducted to understand the tunability of the PEO chain dynamics with LiClO₄ and the mechanism of hopping of Li ions by forming ion pairs with oxygen dipoles on the PEO backbone is established. Despite the lack of PDMS's capability to solvate ions and promote ion transport directly, its proper mixing within the PEO host matrix is demonstrated to enhance ion transport due to the influence of PDMS on the segmental dynamics of PEO. A detailed molecular dynamics study supported by experimental validation suggests that even inert polymers can affect the dynamics of the active host matrix and increase ion transport, leading to next generation high ionic conductivity solid matrices, and opens new avenues in designing polymer based transparent electrolytes.

The studies shown here prove that both the Li salt and ‘inert-polymer’ mixing have paramount importance in the tunability of Li ion conductivity in solid electrolytes for batteries.

Effect of Polymer Polarity on Ion Transport: A Competition between Ion Aggregation and Polymer Segmental Dynamics

In this work, we use computer simulations to demonstrate that there may be limits to which polymer polarity alone can be used to influence the ionic conductivity of salt-doped polymer electrolytes. Specifically, we use coarse-grained molecular dynamics simulations to probe the effect of the polarity of the polymer electrolyte upon ion mobilities and conductivities of dissolved salts. At low polymer polarities, increasing the polymer dielectric constant reduces ionic aggregation and the resultant correlated ionic motion, and increases the ionic conductivity. At higher polymer polarities, polymer-polymer and polymer-ion interactions slows polymer segmental dynamics, leading to a reduction in the conductivity of the electrolyte. As a consequence, ionic conductivity achieves an optimum at an intermediate polymer polarity.

Perspective: Outstanding theoretical questions in polymer-nanoparticle hybrids

This topical review discusses the theoretical progress made in the field of polymer nanocomposites, i.e., hybrid materials created by mixing (typically inorganic) nanoparticles (NPs) with organic polymers. It primarily focuses on the outstanding issues in this field and is structured around five separate topics: (i) the synthesis of functionalized nanoparticles; (ii) their phase behavior when mixed with a homopolymer matrix and their assembly into well-defined superstructures; (iii) the role of processing on the structures realized by these hybrid materials and the role of the mobilities of the different constituents; (iv) the role of external fields (electric, magnetic) in the active assembly of the NPs; and (v) the engineering properties that result and the factors that control them. While the most is known about topic (ii), we believe that significant progress needs to be made in the other four topics before the practical promise offered by these materials can be realized. This review delineates the most pressing issues on these topics and poses specific questions that we believe need to be addressed in the immediate future.

Modeling the Effect of Polymer Chain Stiffness on the Behavior of Polymer Nanocomposites

Due to their central role in industrial formulations spanning from food packaging to smart coatings, polymer nanocomposites have been the object of remarkable attention over the last two decades. Incorporating nanoparticles (NPs) into a polymer matrix modifies the conformation and mobility of the polymer chains at the NP-polymer interface and can potentially provide materials with enhanced properties as compared to pristine polymers. To this end, it is crucial to predict and control the ability of NPs to diffuse and achieve a good dispersion in the polymer matrix. Understanding how to control the NPs' dispersion is a challenging task controlled by the delicate balance between enthalpic and entropic contributions, such as NP-polymer interaction, NP size and shape, and polymer chain conformation. By performing molecular dynamics (MD) simulations, we investigate the effect of polymer chains' stiffness on the mobility of spherical NPs that establish weak or strong interactions with the polymer. Our results show a sound dependence of the NPs' diffusivity on the long-range order of the polymer melt, which undergoes an isotropic-to-nematic phase transition upon increasing chain stiffness. This phase transition induces a dynamical anisotropy in the nematic phase, with the NPs preferentially diffusing along the nematic director rather than in the directions perpendicular to it. Not only does this tendency determine the NPs' mobility and degree of dispersion in the polymer matrix, but it also influences the resistance to flow of the polymer nanocomposite when a shear is applied. In particular, to assess the role of the chains' conformation on the macroscopic response of our model PNC, we employ reverse nonequilibrium MD to calculate the zero-shear viscosity in both the isotropic and nematic phases, and unveil a plasticizing effect at increasing chain stiffness when the shear is applied along the nematic axis.