Influence of Functional Groups on Li-Ion Transport in Dual-Ion vs Single-Ion Conducting Comb-Branched Polymer Electrolytes

Solid polymer electrolytes, SPEs, are a safer choice for Li-ion batteries compared with highly volatile and flammable organic solvents. However, poor ionic conductivity and transference number are the biggest hurdles for their commercialization. In a quest to enhance these properties, we employed atomistic molecular dynamics (MD) simulations to investigate dual-ion and single-ion conducting (SIC) comb-branched polymers with different functional groups. The variations in the functional group structure instigate differences in segmental polymer dynamics, the ability to dissociate lithium salt, and dynamic coupling between ions and polymer, all of which collectively impact the ionic conductivity and transference number. We investigate correlations among these parameters to reveal the Li-ion transport mechanisms and examine the impact of these molecular scale characteristics on the ionic conductivity. In SICs, we found that the Li-ion dynamics is slow due to multichain ion coordination, which is absent when anions are not covalently attached to the polymer. Even with the transference number close to unity in SIC electrolytes, the sluggish ion dynamics results in lower Li-ion conductivity compared to the dual-ion conducting electrolytes. Such a trade-off behavior in SICs encourages ideas to maintain the transference number while improving the Li-ion dynamics

Li⁺ Transport in Ethylene Carbonate Based Comb-Branched Solid Polymer Electrolyte: A Molecular Dynamics Simulation Study

Solid polymer electrolytes (SPEs) have the potential to resolve safety issues, be compatible with high-voltage cathode materials, and allow flexible designs of Li-ion batteries. Due to the limited Li+ transference number, a high degree of crystallinity at room temperature, and instability toward oxidation, polyether-based SPEs have been limited in batteries with the high-voltage cathodes and Li-metal anodes. Low ionic conductivity remains one of the biggest challenges for all types of SPE. Furthermore, the understanding of Li+ transport mechanisms and the related correlations with polymer structure are limited. In this study, extensive atomistic molecular dynamics simulations employing polarizable force field were conducted for a series of poly­(alkyl ethylene carbonate) comb-branched architectures doped with lithium bis­(trifluoromethane)­sulfonimide salt. By studying systems with systematic variance in the polymer structure, the Li+ transport mechanisms have been investigated through structural and dynamical correlations of cation local environments. The molecular-scale insights into the Li+ transport allow proposing principles for the design of comb-branched SPEs with improved conductivity

Adsorption Mechanism of Perfluorooctanoate on Cyclodextrin-Based Polymers: Probing the Synergy of Electrostatic and Hydrophobic Interactions with Molecular Dynamics Simulations

Contamination of natural water resources by per- and polyfluorinated alkyl substances (PFAS) has affected millions of people around the world and emphasized the need for development of novel and effective adsorbent materials. We demonstrate how atomistic molecular dynamics (MD) simulations can be used to provide molecular scale insight into the role of electrostatic and hydrophobic interactions on the adsorption of the perfluorooctanoate (PFOA) surfactant, a prominent longer-chain PFAS, on a polymer-based network in water. Specifically, the adsorption of ammonium perfluorooctanoate salt has been investigated on the β-cyclodextrin (CD) network cross-linked with decafluorobiphenyl linkers as an example of an absorbent material that has already demonstrated efficient PFAS adsorption. Examination of pairwise interactions reveals the importance of the dual pronged adsorption mechanism involving both electrostatic and hydrophobic interactions. The adsorption of ammonium counterions on the CD segments facilitates attraction of the anionic headgroup of the PFOA surfactant, while fluorinated linkers provide an additional hydrophobic attraction for the PFOA tail as well as higher affinity of the network toward PFOA in comparison with hydrocarbons. These competing interactions result in PFOA adsorption primarily outside of the CD cavity with the PFOA tail mostly interacting with fluorinated linkers. We demonstrate that simulations using “what if” scenarios are a powerful approach to infer the role of different interactions in the adsorption of PFAS

Selection of Polymer Segment Species Matters for Electrolyte Properties and Performance in Lithium Metal Batteries

Control of homogeneous lithium deposition governs prospects of advanced cell development and practical applications of high-energy-density lithium metal batteries. Polymer electrolytes are thus explored and employed to mitigate the growth of high-surface-area lithium species while enhancing the reversibility of the lithium reservoir upon cell cycling. Herein, an in-depth understanding of the distribution of membrane properties and lithium deposition behavior affected by the selection of polymer segment species is derived. It is demonstrated that severely localized lithium deposits featuring needle-like morphologies may be readily observed when electrostatic fields (or partial charges) and the amount of Li+ coordinators of the primary and secondary polymer segment species appear rather dissimilar, leading to a sudden cell failure at early stages of cell operation. In comparison, employment of optimized copolymer electrolytes enables superior cell performance at 1C even with thicker cathodes (6.3 mg cm–2). Additionally, the improvement of cell-cycling stability due to enhancement of similarity of dipole moments and partial charge distributions among copolymer segments are also demonstrated for different polymer systems, contributing to avoidance of undesired lithium protrusions, also reflecting a viable concept for the design of future copolymer electrolytes