Relationship between Ion Dissociation, Melt Morphology, and Electrochemical Performance of Lithium and Magnesium Single-Ion Conducting Block Copolymers

Abstract

Single-ion conducting block copolymers, such as poly(ethylene oxide)-b-poly[(styrene-4-sulfonyltrifluoromethylsulfonyl)imide lithium] (PEO−P[(STFSI)Li]), represent an exciting new class of materials capable of improving the performance of solid-state batteries with metal anodes. In this work, we report on the synthesis and characterization of a matched set of lithiated (PEO−P[(STFSI)Li]) and magnesiated (PEO−P[(STFSI) 2 Mg]) single-ion conducting diblock copolymers. We measure the temperature dependence of ionic conductivity, and through analysis using the Vogel−Tamman− Fulcher (VTF) relation, demonstrate that ion dissociation is significantly lower for all PEO−P[(STFSI) 2 Mg] samples when compared to their PEO−P[(STFSI)Li] counterparts. The VTF parameter characterizing the activation barrier to ion hopping was similar for both cations, but the VTF prefactor that reflects effective charge carrier concentration was higher in the lithiated samples by an order of magnitude. We study the melt morphology of the single-ion conducting block copolymers using temperature-dependent X-ray scattering and use the mean-field theory of Leibler to extract the effective Flory−Huggins interaction parameter (χ) for PEO/P[(STFSI)Li] and PEO/P[(STFSI) 2 Mg] from the X-ray scattering data. We demonstrate a linear relationship between the charge-concentration-related VTF parameter and the parameter quantifying the enthalpic contribution to χ. It is evident that ion dissociation and block copolymer thermodynamics are intimately coupled; ion dissociation in these systems suppresses microphase separation.

Dependence of Morphology, Shear Modulus, and Conductivity on the Composition of Lithiated and Magnesiated Single-Ion-Conducting Block Copolymer Electrolytes

“…This is consistent with our previous work wherein we concluded that the effective Flory−Huggins interaction parameter, χ, between PEO and PSLiTFSI is negative. 18 The scattering profiles of the magnesiated copolymers are shown in Figure 5b. Sample PEO−P[(STFSI) 2 Mg](9.5−3.6) with ϕ PSTFSI = 0.21 exhibits a broad well-defined peak at q = q* = 0.330 nm −1 .…”

Multivalent ion conduction in solid polymer systems

“…In these systems, the anion is tethered to the PS block in the copolymer; upon incorporation of the PEO block, the cation tends to dissociate from the anion and reside in the PEO-solvating domains. 84 Unfortunately, for Li + single-ion conducting BCPs, cation dissociation and subsequent solvation by the PEO block results in favorable interactions between the two blocks and a lack of microphase separation, eliminating mechanical property improvements until a third incompatible block is added, such as PS without tethered anions. 84 Unlike the Li + polymer, the Mg 2+ -containing polymer shows weak phase separation, which imparts multiple orders of magnitude improvement to the shear modulus of the resulting films.…”

“…84 Unfortunately, for Li + single-ion conducting BCPs, cation dissociation and subsequent solvation by the PEO block results in favorable interactions between the two blocks and a lack of microphase separation, eliminating mechanical property improvements until a third incompatible block is added, such as PS without tethered anions. 84 Unlike the Li + polymer, the Mg 2+ -containing polymer shows weak phase separation, which imparts multiple orders of magnitude improvement to the shear modulus of the resulting films. 85 However, this phase separation likely stems from the incomplete dissociation of the Mg 2+ cation from the tethered anion, and thus overall ionic conductivity is substantially reduced due to the lower concentration of mobile cations.…”

Poly(ethylene oxide)‐based block copolymer electrolytes for lithium metal batteries

“…(e)), PEO‐ b ‐P(LiSTFSI) (Fig. (f)), P(LiMTFSI)‐ b ‐PEO‐ b ‐P(LiMTFSI) (LiMTFSI: lithium 1‐[3‐(methacryloyloxy)‐propylsulfonyl]‐1‐(trifluoromethanesulfonyl)imide) (Figs (g) and (h)) and POEM‐ b ‐P(LiMTFSI) (Fig. (i)) .…”