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.

Quantitative performance analysis of anion-exchange membrane fuel cells with in-plane and through-plane water transport

Effective water management is crucial in anion-exchange membrane fuel cells (AEMFCs) due to water's dual role as a reactant during alkaline oxygen reduction and a product of alkaline hydroxide oxidation. Meanwhile, both hydroxide conduction and water transport in polymer electrolytes are highly dependent on the hydration level. Therefore, a three-dimensional multi-physics model is developed to investigate the critical role of ionic and water transport parameters of the electrolyte on AEMFC performance. Additionally, detailed water distribution analysis is used to reveal its fundamental mechanism. The results reveal that extremely inhomogeneous water distribution and cathode water starvation are the main causes of the poor cell performance. Moreover, increasing membrane water diffusivity, water conversion, and decreasing the electro-osmotic drag coefficient all contribute to better cell performance through improved water balance and cathode water supply. Among these, cell performance is most sensitive to membrane water diffusivity, with a 40 % increase in peak power density when it is increased from 10−10 m2 s−1 to 10−9 m2 s−1. Although high ionic conductivity improves cell performance by directly reducing ohmic impedance, it also results in a more inhomogeneous distribution of water in the whole cell, so that a four-fold increase in ionic conductivity only yields a 62 % increase in peak power density.

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.

Improved Mechanical Strength without Sacrificing Li-Ion Transport in Polymer Electrolytes.

Next-generation batteries demand solid polymer electrolytes (SPEs) with rapid ion transport and robust mechanical properties. However, many SPEs with liquid-like Li+ transport mechanisms suffer a fundamental trade-off between conductivity and strength. Dynamic polymer networks can improve bulk mechanics with minimal impact to segmental relaxation or ionic conductivity. This study demonstrates a system where a single polymer-bound ligand simultaneously dissociates Li+ and forms long-lived Ni2+ networks. The polymer comprises an ethylene oxide backbone and imidazole (Im) ligands, blended with Li+ and Ni2+ salts. Ni2+-Im dynamic cross-links result in the formation of a rubbery plateau resulting in, consequently, storage modulus improvement by a factor of 133× with the introduction of Ni2+ at rNi = 0.08, from 0.014 to 1.907 MPa. Even with Ni2+ loading, the high Li+ conductivity of 3.7 × 10-6 S/cm is retained at 90 °C. This work demonstrates that decoupling of ion transport and bulk mechanics can be readily achieved by the addition of multivalent metal cations to polymers with chelating ligands.

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).

Can Silica Nanoparticles Improve Lithium Transport in Polymer Electrolytes?

Can Silica Nanoparticles Improve Lithium Transport in Polymer Electrolytes?

Proton Diffusion in Liquid 1,2,3-Triazole Studied by Incoherent Quasi-Elastic Neutron Scattering.

Improving the proton transport in polymer electrolytes impacts the performance of next-generation solid-state batteries. However, little is known about proton conductivity in nonaqueous systems due to the lack of an appropriate level of fundamental understanding. Here, we studied the proton transport in small molecules with dynamic hydrogen bonding, 1,2,3-triazole, as a model system of proton hopping in a nonaqueous environment using incoherent quasi-elastic neutron scattering. By using the jump-diffusion model, we identified the elementary jump-diffusion motion of protons at a much shorter length scale than those by nuclear magnetic resonance and impedance spectroscopy for the estimated long-range diffusion. In addition, a spatially restricted diffusive motion was observed, indicating that proton motion in 1,2,3-triazole is complex with various local correlated dynamics. These correlated dynamics will be important in elucidating the nature of the proton dynamics in nonaqueous systems.

Electrotransport Properties of Perfluorinated Cation-Exchange Membranes of Various Thickness.

The present work discusses the influence of the thickness of MF-4SK perfluorinated sulfonic cation-exchange membranes on their electrotransport properties in hydrochloric acid solutions. It is found that diffusion permeability and conductivity are primarily determined with the specific water content of the membranes and increase with their increase. Analysis of the contribution of reverse diffusion through the membrane to the value of the limiting current shows that it can reach 20% for membranes with a thickness of 60 μm. A study of the characteristics of the fuel cell with perfluorinated membranes of different thicknesses shows that the membrane thickness affects both the ohmic resistance of the membrane-electrode assembly and the diffusion limitations of proton transport in polymer electrolytes.

Test procedure for the prediction of water transport in polymer electrolyte fuel cells

Water plays a crucial role for the operation of polymer electrolyte fuel cells. The distribution and state of water in the membrane-electrode-assembly (MEA) not only impacts the performance of the cell, but also its degradation and freeze-start capability. It is therefore important to have a model that precisely predicts the distribution of water across the fuel cell for arbitrary inlet conditions. Unfortunately, water transport coefficients reported in the literature vary by orders of magnitude while operation conditions and other crucial properties of the cell are not precisely known. Furthermore, a large amount of data is simply outdated due to improvements in measurement techniques and setups as well as fuel cell components. In the present work, we have devised a test procedure for water transfer measurements with state of the art MEAs. In parallel, we have derived a semi-empirical model and parametrized it with measured data. The model allows predicting MEA water fluxes and profiles of temperature and concentration over a wide operation range, relevant for passenger as well as heavy duty automotive applications.

Anion π-π Stacking for Improved Lithium Transport in Polymer Electrolytes.

Polymer electrolytes (PEs) with excellent flexibility, processability, and good contact with lithium metal (Li°) anodes have attracted substantial attention in both academic and industrial settings. However, conventional poly(ethylene oxide) (PEO)-based PEs suffer from a low lithium-ion transference number (TLi+), leading to a notorious concentration gradient and internal cell polarization. Here, we report two kinds of highly lithium-ion conductive and solvent-free PEs using the benzene-based lithium salts, lithium (benzenesulfonyl)(trifluoromethanesulfonyl)imide (LiBTFSI) and lithium (2,4,6-triisopropylbenzenesulfonyl)(trifluoromethanesulfonyl)imide (LiTPBTFSI), which show significantly improved TLi+ and selective lithium-ion conductivity. Using molecular dynamics simulations, we pinpoint the strong π-π stacking interaction between pairs of benzene-based anions as the cause of this improvement. In addition, we show that Li°∥Li° and Li°∥LiFePO4 cells with the LiBTFSI/PEO electrolytes present enhanced cycling performance. By considering π-π stacking interactions as a new molecular-level design route of salts for electrolyte, this work provides an efficient and facile novel strategy for attaining highly selective lithium-ion conductive PEs.

Mechanistic insights into water transport in polymer electrolyte fuel cells with a variation of cell temperature and relative humidity of inlet gas elucidated by operando synchrotron X-ray radiography

Excessive liquid water in the gas diffusion layer (GDL) of polymer electrolyte fuel cells (PEFCs) is known to degrade their performance. PEFCs for automotive applications are required to work over a broad temperature range and over a wide range of relative humidity (RH) levels of the gas in their channels. Time-resolved operando synchrotron X-ray radiography was used to understand the effect of temperature and RH on the condensation and transport of water in PEFCs. The results show that the type of condensation and transport to the channel can be classified into four categories on the basis of the transient behavior of the liquid water distribution. The first category is concurrent liquid and vapor transport, the second is liquid-transport dominated, the third is vapor-only transport, and the fourth is accumulation-only near the ribs. We propose the possibility of oversaturation in the GDL, which is usually not considered in numerical calculations of the water distribution in PEFCs. We also show the possibility that experimental results are inconsistent with continuous water distribution in the in-plane direction, which is predicted under the assumption that the GDL structure is uniform and that capillary pressure transports liquid water in the GDL.

Ionic conductivity study of Camphor Sulphonic Acid doped PMMA-PVC blends as polyelectrolyte

Blends of PMMA and PVC were synthesized using film cast and doped with Camphor Sulphonic Acid (CSA). In order that the mechanical properties do not deteriorate, plasticizers were not added. The amorphous nature of the doped samples was confirmed through XRD and ac electrical conductivity of these blends was studied as a function of frequency in the range 100 Hz to 1 MHz at eight different temperatures from 30 °C to100 °C. The nonlinearity observed indicates that the ion transport in polymer electrolyte is dependent on polymer segmental motion. The slight increase in conductivity with temperature was due to the hopping mechanism between the coordinating sites and the linear variation of conductivity with frequency for all the temperatures and the samples and was attributed to the polarization of electrodes.

Impact of Charged Surfaces on the Structure and Dynamics of Polymer Electrolytes: Insights from Atomistic Simulations

Polymer electrolytes are intensely investigated for use as solid electrolytes in next-generation lithium-ion and lithium-metal batteries. However, little is known about the structural and dynamical properties of polymer electrolytes close to electrode surfaces. Here, a PEO-Li[NTf2] polymer electrolyte, confined between two oppositely charged graphite-like electrodes, is studied via molecular dynamics simulations. Three different surface charges of σS = 0, ±0.5 and ± 1e nm–2 are considered. Upon charging, a very strong and component-specific layering is observed. Only for the highest surface charge, lithium ions get desolvated and come into direct contact with the negative electrode. The layer structure goes along with the emergence of free energy barriers, which lead to a reduction of the lithium-ion dynamics, as quantified by spatially resolved mean-square displacements, corrected for a drift component. Interchain transfers that are known to be very important for long-range lithium-ion transport in polymer electrolytes play no significant role in transitions of lithium ions between different layers.

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.

Water Domain Enabled Transport in Polymer Electrolytes for Lithium-Ion Batteries

Widespread commercial adoption of polymer electrolytes for lithium-ion batteries has been hindered by subpar transport properties, namely, ionic conductivities of <1 mS/cm at room temperature and s...

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.

Molecular Analysis of Ion Transport in Polymer Electrolyte of All Solid-State Lithium Ion Battery

Molecular Analysis of Ion Transport in Polymer Electrolyte of All Solid-State Lithium Ion Battery

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.