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| 2. |
- Gudla, Harish, et al.
(author)
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Importance of the Ion-Pair Lifetime in Polymer Electrolytes
- 2021
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In: Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185 .- 1948-7185. ; 12:35, s. 8460-8464
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Journal article (peer-reviewed)abstract
- Ion pairing is commonly considered as a culprit for the reduced ionic conductivity in polymer electrolyte systems. However, this simple thermodynamic picture should not be taken literally, as ion pairing is a dynamical phenomenon. Here we construct model poly(ethylene oxide)-bis(trifluoromethane)sulfonimide lithium salt systems with different degrees of ion pairing by tuning the solvent polarity and examine the relation between the cation-anion distinct conductivity sigma(d)(+-) and the lifetime of ion pairs tau(+-) using molecular dynamics simulations. It is found that there exist two distinct regimes where sigma(d)(+-) scales with 1/tau(+-) and tau(+-), respectively, and the latter is a signature of longer-lived ion pairs that contribute negatively to the total ionic conductivity. This suggests that ion pairs are kinetically different depending on the solvent polarity, which renders the ion-pair lifetime highly important when discussing its effect on ion transport in polymer electrolyte systems.
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| 3. |
- Eriksson, Therese, 1992-, et al.
(author)
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Carbonyl-Containing Solid Polymer Electrolyte Host Materials : Conduction and Coordination in Polyketone, Polyester, and Polycarbonate Systems
- 2022
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In: Macromolecules. - : American Chemical Society (ACS). - 0024-9297 .- 1520-5835. ; 55:24, s. 10940-10949
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Journal article (peer-reviewed)abstract
- Research on solid polymer electrolytes (SPEs) is now moving beyond the realm of polyethers that have dominated the field for several decades. A promising alternative group of candidates for SPE host materials is carbonyl-containing polymers. In this work, SPE properties of three different types of carbonyl-coordinating polymers are compared: polycarbonates, polyesters, and polyketones. The investigated polymers were chosen to be as structurally similar as possible, with only the functional group being different, thereby giving direct insights into the role of the noncoordinating main-chain oxygens. As revealed by experimental measurements as well as molecular dynamics simulations, the polyketone possesses the lowest glass transition temperature, but the ion transport is limited by a high degree of crystallinity. The polycarbonate, on the other hand, displays a relatively low coordination strength but is instead limited by its low molecular flexibility. The polyester performs generally as an intermediate between the other two, which is reasonable when considering its structural relation to the alternatives. This work demonstrates that local changes in the coordinating environment of carbonyl-containing polymers can have a large effect on the overall ion conduction, thereby also showing that desired transport properties can be achieved by fine-tuning the polymer chemistry of carbonyl-containing systems.
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| 4. |
- Gudla, Harish, et al.
(author)
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Effects of Solvent Polarity on Li-ion Diffusion in Polymer Electrolytes : An All-Atom Molecular Dynamics Study with Charge Scaling
- 2020
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In: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 124:37, s. 8124-8131
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Journal article (peer-reviewed)abstract
- We herein report an all-atom molecular dynamics study on the role of solvent polarity for Li+ diffusion in polymer electrolytes using PEO–LiTFSI (poly(ethylene oxide)–lithium bis(trifluoromethane)sulfonimide) as a model system. By separating the effect of Tg and the effect of solvent polarity in our simulations, we show that the maximum in the diffusion coefficient of Li+ with respect to the dielectric constant of polymer solvent εp is due to transitions in the transport mechanism. In particular, it is found that the frequent interchain hopping involves the coordination of both PEO and TFSI. This optimal solvating ability of PEO at an intermediate value of εp leads to the fast ion conduction. These findings highlight the synergetic effect of solvent polarity and bond polarity on Li-ion diffusion in solid polymer electrolytes.
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| 5. |
- Gudla, Harish
(author)
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Using Molecular Dynamics Simulations to Explore Critical Property Relationships in Polymer Electrolytes : Polarity, Coordination, Ionic transport, Ion-pairing, and Ion-ion Correlations
- 2022
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Doctoral thesis (other academic/artistic)abstract
- While ion transport in solid polymer electrolytes (SPEs) has been explored for decades, there still remains controversies about its fundamental properties, often correlated with gaps between experimental and computational studies. Using molecular dynamics simulations to understand the complex transport mechanisms and also to fill these gaps is the main goal of this thesis. This is achieved by critically examining the relationships between different properties in SPE systems: polarity, coordination, ion-pairing, and ion-ion correlations, which highly influence the ionic transport mechanism. Firstly, the relation between polarity, ion-pairing, and ion-ion correlations was explored. The solvent polarity (εp) of poly(ethylene oxide) (PEO) doped with LiTFSI system is modulated using a charge scaling method. When separating the effects of solvent polarity and glass transition temperature, a maximum in the Li-ion diffusion coefficient with respect to εp is observed. This is attributed to the transitions in the transport mechanisms and an optimal solvating ability of Li-ion at intermediate values of εp. The solvent polarity also plays a critical role in the formation of charge-neutral ion pairs, which is commonly considered detrimental for ionic conductivity. The relation between cation−anion distinct conductivity and the lifetime of ion pairs was thereby examined, where it is found that short-lived ion pairs actually contribute positively to the ionic conductivity. Moreover, the origins of the recently observed negative transference numbers were scrutinized. A strong dependence of the reference frame in the estimation of the transference numbers is found, which explains observed differences between experiments and computations. Secondly, the role of coordination chemistry and its influence on ion transport mechanisms and conduction properties in SPEs was studied. The change in the cation coordination with both polymers and anions was used to study the dominant transport mechanisms at different molecular weights and salt concentrations for PEO and a polyester-based SPE, which shows that essentially very little true hopping occurs in these materials. In this context, the coordination and ionic transport properties of three resemblant carbonyl-coordinating polymers are also investigated: polyketones, polyesters, and polycarbonates. The extra main-chain oxygens for the latter polymers are shown to decrease the electrostatic energy between Li-ion and the carbonyl group, and the cationic transference numbers are thus found to be increasing as the coordination strength decrease.
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| 6. |
- Shao, Yunqi, et al.
(author)
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Transference Number in Polymer Electrolytes : Mind the Reference-Frame Gap
- 2022
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In: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 144:17, s. 7583-7587
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Journal article (peer-reviewed)abstract
- The transport coefficients, in particular the transference number, of electrolyte solutions are important design parameters for electrochemical energy storage devices. The recent observation of negative transference numbers in PEO-LiTFSIunder certain conditions has generated much discussion about its molecular origins, by both experimental and theoretical means.However, one overlooked factor in these efforts is the importance of the reference frame (RF). This creates a non-negligible gapwhen comparing experiment and simulation because thefluxes in the experimental measurements of transport coefficients and in thelinear response theory used in the molecular dynamics simulation are defined in different RFs. In this work, we show that, byapplying a proper RF transformation, a much improved agreement between experimental and simulation results can be achieved.Moreover, it is revealed that the anion mass and the anion-anion correlation, rather than ion aggregates, play a crucial role for thereported negative transference numbers
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| 7. |
- Unge, Mikael, et al.
(author)
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Electronic conductivity of polymer electrolytes : electronic charge transport properties of LiTFSI-doped PEO
- 2020
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In: Physical Chemistry, Chemical Physics - PCCP. - : ROYAL SOC CHEMISTRY. - 1463-9076 .- 1463-9084. ; 22:15, s. 7680-7684
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Journal article (peer-reviewed)abstract
- The electronic structure of poly(ethyleneoxide) with and without a common electrolyte lithium bis(trifluoromethane)sulfonimide salt is calculated from first principles. Introducing the salt into the polymer electrolyte significantly reduces the band gap, down to 0.6 eV. Thus, this will have a significant impact on the leakage currents in polymer electrolytes used in all-solid-state batteries.
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| 8. |
- Zhang, Chao, et al.
(author)
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2023 Roadmap on molecular modelling of electrochemical energy materials
- 2023
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In: Journal of Physics. - : Institute of Physics Publishing (IOPP). - 2515-7655. ; 5:4
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Journal article (peer-reviewed)abstract
- New materials for electrochemical energy storage and conversion are the key to the electrification and sustainable development of our modern societies. Molecular modelling based on the principles of quantum mechanics and statistical mechanics as well as empowered by machine learning techniques can help us to understand, control and design electrochemical energy materials at atomistic precision. Therefore, this roadmap, which is a collection of authoritative opinions, serves as a gateway for both the experts and the beginners to have a quick overview of the current status and corresponding challenges in molecular modelling of electrochemical energy materials for batteries, supercapacitors, CO2 reduction reaction, and fuel cell applications.
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| 9. |
- Zhang, Chao, et al.
(author)
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Molecular Dynamics Simulations of the Ionic Conductivity: : From Aqueous Electrolytes to Polymer Electrolytes
- 2019
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Conference paper (other academic/artistic)abstract
- Lithium batteries are electrochemical devices which involve multiple time-scale and length-scale to achieve its optimal performance and safety requirement. In terms of the electrolyte which serves as the ionic conductor, a molecular-level understanding of the corresponding transport phenomenon is crucial for the rational design. In this talk, we will report our recent progress on molecular dynamics simulations of ionic conductivities in different types of electrolytes. These include: 1) LiCl solution which is a prototype system for strongly dissociated 1-1 electrolytes; 2) Alkaline electrolyte NaOH where the conduction mechanism involves the interchange of chemical and hydrogen bonds; 3) Polymer electrolytes in which both bond polarity and solvent polarity can play important roles. An outlook on using machine-learning as a multi-scale modelling tool for investigating ion transport and dielectric properties will be also given.
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