Understanding How Metal-Ligand Coordination Enables Solvent Free Ionic Conductivity in PDMS

Abstract

Ionically conductive polymers are commonly made of monomers containing high polarity moieties to promote high ion dissociation, like poly(ethylene oxide) (PEO), polyvinylidene difluoride (PVDF), poly(vinyl alcohol) (PVA). However, the glass transition temperature ($T_g$) of these polymers are relatively high, and therefore yields a glassy state at room temperature and limits the mechanical flexibility of the material. Although polydimethylsiloxane (PDMS) has many attractive physical and chemical properties, including low glass transition temperature, mechanical flexibility, and good biocompatibility, its low dielectric constant suppresses ion dissociation. In this paper, we overcome this shortage by functionalizing the PDMS with ligands that can form labile coordination with metal ions, which greatly promotes the ion dissociation and improves the ionic conductivity by orders of magnitude. By combining an experimental study with a fully atomistic molecular dynamics simulation, we systematically investigated the ion transport mechanisms in this low $T_g$, low intrinsic conductivity material.

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

“…Meanwhile, the local segmental relaxations and solvation site rearrangements that govern Li + transport are only moderately hindered. While it has been previously shown that total conductivity is unaffected by mixed-salt dynamic network formation, 20 here it is confirmed that Li + conductivity is also unaffected. Furthermore, it is demonstrated here that even a simple dynamic network comprising one type of ligand functionality and two types of metal cations is a straightforward approach to partially decoupling bulk mechanics from Li + conductivity.…”

“…One dynamic network Li + conductor studied by Mackanic et al 19 uses a hydrogen bonding network to improve toughness and PEO-based backbones to improve conductivity. Similarly, Zhang et al 20 blended a pyridyl amine-functionalized siloxane polymer with Li + and Cu 2+ salts to form a "mixed salt" electrolyte that simultaneously solvates Li + and improves the shear modulus through Cu−pyridyl interactions. More generally, our group has established that conductivity and modulus can be independently tuned via metal−ligand coordination chemistry.…”