Impact of Side Chain Chemistry on Lithium Transport in Mixed Ion–Electron-Conducting Polymers

Abstract

Typical design strategies for mixed ion−electron conduction in polymers have focused on overall ionic conductivity, without specificity for anion vs cation conduction. Here, we demonstrate that side chain chemistry can be used to control Li + conductivity in semiconducting polymers. This design principle is significant for applications that require Li + -specific transport, such as Li-ion batteries. We show that a polythiophene functionalized with an ionic liquid side chain demonstrates higher conductivity and lithium transference than a more commonly studied ether-functionalized P3AT derivative. Poly(3-(6′-(N-methylimidazolium) hexyl)thiophene TFSI − ) (P3HT-Im + TFSI − ) can solvate and conduct ions up to salt concentrations of r = 1.0 (where r = [moles of salt]/[moles of monomer]) while achieving an ionic conductivity of ∼10 −3 S/cm at 80 °C and a lithium transference number of 0.36. On the other hand, poly(3-(methoxyethoxyethoxymethyl)thiophene) (P3MEEMT) shows a peak conductivity of ∼10 −5 S/cm at r = 0.05 and 80 °C, with near-zero lithium transport. This work shows that multiple high dielectric moieties can be used to drive ion conduction in semiconducting polymers, but diffuse, cationic side chains such as imidazolium are preferred for Li-ion conduction.

Conductive Polymers to ‘Wire’ the Microbial Electronics Interface

“…As cation-functionalized P3HT efficiently conducts ions in anhydrous contexts, it is also possible that some molecular permeability remains at the polymer-electrolyte interface that may explain this enhanced reactive area despite our EIS characterization suggesting the loss of this capability in aqueous environments. [16,17] Having determined electroactive areas empirically, we calculated the number electrons transferred per reaction. On screen printed carbon, a one-electron transfer was occurring, while both P3HT 50% Im + and P3HT-Im + supported two-electron transfer.…”

“…Cation-functionalized P3HT polymers are electronically conductive, making it an attractive backbone choice. [16,17] We therefore synthesized poly-3-hexylthiophene (P3HT) functionalized with methylimidazolium side chain termini as a bio-inspired polymer capable of conducting both ions and electrons. Here, we show that flavin electron transfer to these polymers proceeds by an unprecedented and efficient concerted 2-electron mechanism that was previously restricted to biological contexts.…”

A Coacervate-Based Mixed-Conducting Binder for High-Power, High-Energy Batteries

“…Hence, our binder is conductive over the entire potential range of the charge and discharge processes but transitions to an insulator below 3.2 V, which has been suggested to offer protection against overdischarge . The PIL was chosen to be an acrylate functionalized with an imidazolium side chain, as this group is known to have a wide electrochemical stability window, , as well as favorable lithium transport properties owing to its diffuse charge. , These properties indeed translate to the complex, which is found to have an intrinsic ionic conductivity of 6 × 10 –8 S/cm at room temperature and a lithium transference of 0.25, which is noteworthy given the high electronic conductivity of 1 S/cm, as shown in Figures S7–S9. Additionally, Figure S11 shows that the system is electrochemically stable up to 4.5 V vs Li/Li + , more than sufficient for the 2–4 V operating window of LFP.…”

“…This observation is consistent with the fact that fast electronic and ionic conductivities require very different polymer designs. High ionic conductivity is generally associated with both a high dielectric constant and fast segmental motion, while high electrical conductivity generally requires aromaticity combined with a high degree of order. − Hybrid designs, for example incorporating ionically conductive side chains on an electronically conductive backbone, − generally result in a tradeoff between ionic and electronic conduction . In a practical sense, these hybrid materials face hurdles when applied as binders, as increasing the density of ionically conductive side chains drives dissolution in the polar battery electrolyte …”

Ion Size-Dependent Electrochromism in Air-Stable Napthalenediimide-Based Conjugated Polymers

“…In the context of electrochromic devices that operate volumetrically (i.e., the ions penetrate into the bulk of the polymer medium during the doping process), these metrics directly translate to enhanced optical modulation (Δ T %), high coloration efficiencies (CEs), and rapid coloration/bleaching times . However, there are no definitive design principles based on molecular structure for predicting cation transport in CPs, although the formation of TEG-cation chelates has been correlated with reduced cation transport. , Identifying design rules for modulating ion transport will not only benefit ECDs but, more widely, will assist the development of batteries with alternative cation insertion dynamics involving sodium and potassium …”