Role of Electron-Deficient Imidazoles in Ion Transport and Conductivity in Solid-State Polymer Electrolytes

Abstract

Solid-state polymer electrolytes offer a safer alternative to traditional lithium-ion batteries based on organic electrolytes. The focus is here on imidazole functionalized polymer electrolytes, where the imidazole ligand promotes salt dissolution, while its functionalization allows to tune the dynamic interactions between the cations in solution and the imidazole ligand tethered to the polymer backbone. Although careful choice of polymer backbone and imidazole linker functionality have resulted in polymer electrolytes with increased total ionic conductivities and a boost in the Li + ion contribution, improvements in performance through modifications of the imidazole heterocycle remain underexplored. In this work, we systematically investigate poly(methylsiloxane) (PMS) polymers functionalized with a series of halogen-substituted imidazoles and show that Li + ion conduction can be tuned by electron-deficient heterocycle ligand. When the number of halogen substituents increases, the Li + ion mobility also increases as measured by pulsed-field-gradient NMR and NMR relaxometry. Although beneficial for Li + transport, electron-deficient imidazole ligands result in clustering, as indicated by wide-angle X-ray scattering, and poor salt dissolution, which in turn impedes the overall ionic transport. This work highlights the importance of synthetic design and the necessity for high salt solvation and weak cation-polymer binding to obtain high Li + transport numbers.

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

“…[21][22][23][24] For example, imidazole-and imidazolium-containing polymers which can bind with metals have been synthesized and studied. 25,26 This moiety is particularly interesting due to its wide prevalence in nature and in the human body, and therefore relevance in bioactive applications. 25 In addition, polymer electrolytes containing metal-ligand coordination such as Li + coordinated poly(N-methyl-malonic amide) and Zn 2+ coordinated polyacrylamide have been developed for all-solid-state batteries.…”

Polymer Electrolyte Based on Cyano-Functionalized Polysiloxane with Enhanced Salt Dissolution and High Ionic Conductivity

“…Following from this initial promising result, various halogen-functionalized imidazole side chains have been synthesized and characterized, aiming to weaken the Li + -ligand binding strength. However, the total conductivity was found to decrease, presumably due to insufficient salt dissolution . We thus hypothesize that polar and sterically unhindered ligands offer a more balanced interaction that leads to high salt dissolution but a moderate ligand binding lifetime to achieve high ionic conductivity.…”

“…However, the total conductivity was found to decrease, presumably due to insufficient salt dissolution. 23 We thus hypothesize that polar and sterically unhindered ligands offer a more balanced interaction that leads to high salt dissolution but a moderate ligand binding lifetime to achieve high ionic conductivity. One common ligand used for metal ion coordination is based on a nitrile substituent, 24 with a classic example being the cyanocontaining poly(acrylonitrile) polymer, which can solvate up to 75 wt % of lithium salt.…”

Boronic Ester Transesterification Accelerates Ion Conduction for Comb-like Solid Polymer Electrolytes

“…To our knowledge, compared with cross-linked structures, the brush structures of the comb-like SPEs are more conducive to promoting the movement of chain segments, thus enhancing ionic conduction. − Notably, the enhanced ion conduction seen with SPEs is inadvertently ignored when the boronic ester is introduced into SPEs with dynamic covalent bonds. Most importantly, the grafted segments containing boronic esters endow polyether-based SPEs with high ionic conductivity and t Li + value, and they facilitate the dissociation of lithium salts. − …”