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

Abstract

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.

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

“…Figure c presents the influence of polymer dynamics on the D Li + at 400, 450, and 500 K. A strong correlation can be observed for all systems. Such dependency was also observed in several other SPEs, ,, motivating investigation of strategies to decrease the τ s by lowering the T g of polymer electrolyte.…”

“…In all polymers, the polar functional groups X are five C–C bonds from the backbone. This tether length was found to be the optimal one for the comb-branched EC electrolyte investigated in our previous work. , The LiTFSI salt is used for this study due to its high dissociation constant. In SIC-X electrolytes, the TFSI anion is tethered to the backbone by five C–C bonds, whereas in short-SIC-X systems, the TFSI anion is just one C–C bond away (Scheme ).…”

“…). − Within solid-state electrolytes, compared to ceramics, polymers are easy to fabricate with varied chemistry and architecture and are flexible in nature, which augments their ability to accommodate volume change during the charge–discharge cycles. , The major drawback associated with polymer electrolytes is poor ionic conductivity. Many recent studies focused on strategies to optimize polymer electrolytes by varying chain chemical structure and introducing additives to overcome this issue. − …”

“…Decreasing T g accelerates the segmental dynamics, while higher salt dissociation enhances the Haven ratio (H R ). A combination of both of these effects is believed to be critical to boosting ionic conductivity …”

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

“…Atomistic MD simulations were conducted using Atomistic Polarizable Potential for Liquids, Electrolytes & Polymers (APPLE&P) using an in-house-developed simulation package. , This polarizable force field has been extensively used to investigate polymer electrolytes. − In this force field, each force center has an isotropic polarizability, and the induced point dipoles are computed using the self-consistent approach. We used the Thole screening factor of 0.2 to prevent “polarization catastrophe” .…”