Lithium-Ion Transport in Poly(ionic liquid) Diblock Copolymer Electrolytes: Impact of Salt Concentration and Cation and Anion Chemistry

Abstract

Styrene-based poly(ionic liquid) (PIL) diblock copolymers and their analogous PIL homopolymers were synthesized in this study with various covalently attached cations (methylimidazolium (MIm + ) and methylpyrrolidinium (MPyr + )) and counteranions (bis(trifluoromethanesulfonyl)imide (TFSI − ) and bis(fluorosulfonyl)imide (FSI − )). Solid polymer electrolytes (SPEs) were prepared by mixing the polymer with the corresponding salts (Li + TFSI − and Li + FSI − ) under various salt concentrations r = [Li + ]/[PIL] (mol/mol) = 0.1−0.8. The impacts of lithium salt concentration and cation/anion chemistry were explored in regards to electrochemical, morphological, transport, and physical properties. The results show that the SPE with the MIm + /FSI − ion pair has the lowest PIL T g (−7 °C), ca. 1−3 orders of magnitude higher conductivity compared to other SPEs as well as high electrochemical stability (lithium−metal stripping-plating). SPEs with the FSI − anion exhibit an ion-hopping-dominated transport mechanism and similar ion conductivities compared to their analogous PIL homopolymer SPEs at the same salt concentrations. The negative transference number of the SPE with the MIm + /TFSI − ion pair at a high salt concentration indicates the formation of larger anion-rich clusters and results in lower conductivity. This work reveals the impact of cation/anion chemistries on salt-doped PIL block polymers, which may enable new highly stable SPEs for lithium batteries.

Nanostructured Ionic Separator Formed by Block Copolymer Self-Assembly: A Gateway for Alleviating Concentration Polarization in Batteries

“…The morphology of this ionic separator was studied by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) using ruthenium tetroxide as the staining agent for the PSLiTFSI microphase (Figure ). − The bright stripes seen in Figure represent PSLiTFSI lamellae. The domain spacing, the center-to-center distance between adjacent PSLiTFSI lamellae, was measured to be around 18 nm by using the line scan across the box indicated in the figure.…”

Thermodynamics and Structure–Property Relationships of Charged Block Polymers

“…[91] Although the measured conductivity was high, Li transference was negative in all cases, implying that conductivity was heavily dominated by anion mobility. [91,92] Regardless, these studies all suggests that charge confinement into percolating domains has great potential for improving transport properties beyond current theoretical limits. The four aforementioned studies .…”

“…[ 90 ] More recent work compared Li conductivity trends between styrene‐based IL‐functional BPEs and the charged homopolymer counterparts, which were strikingly similar, indicating a σ norm ∼ 1. [ 91 ] Although the measured conductivity was high, Li transference was negative in all cases, implying that conductivity was heavily dominated by anion mobility. [ 91,92 ] Regardless, these studies all suggests that charge confinement into percolating domains has great potential for improving transport properties beyond current theoretical limits.…”

“…[ 91 ] Although the measured conductivity was high, Li transference was negative in all cases, implying that conductivity was heavily dominated by anion mobility. [ 91,92 ] Regardless, these studies all suggests that charge confinement into percolating domains has great potential for improving transport properties beyond current theoretical limits. The four aforementioned studies involved ILs which transport ions through a segmental motion‐assisted, ion hopping mechanism wherein the ion hopping mode improves with respect to decreasing backbone–backbone distance while the segmental motion mode remains correlated to T g .…”

Proton Conducting Sulfonated Poly(Ionic Liquid) Block Copolymers

“…Specifically, two categories of ionomers that have attracted particular attention in electrochemical energy applications are sulfonated block copolymers and poly­(ionic liquid) block copolymers (PILBCPs). Sulfonated block copolymers (containing sulfonic acid in one block) have been intensively explored as proton conducting membranes and ionomers for PEMFCs. − More recently, PILBCPs (containing IL chemistry in one block) have been designed and applied as solid polymer electrolytes for batteries and hydroxide conducting membranes and ionomers for alkaline fuel cells. − Conjoining these two materials into one, that is, sulfonated PIL block copolymers (S-PILBCPs), where one block consists of IL chemistry and the other contains sulfonic acid, is of significant interest to provide a new material platform with desired properties such as high proton conductivity (from the sulfonated block) and IL-philicity (from the PIL). Protons could be transported through the hydrophilic sulfonated domain and O 2 could diffuse through the hydrophobic PIL/IL domain to react with the electrons at the triple phase boundary, resulting in enhanced Pt utilization.…”