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

Abstract

Single-ion-conducting block copolymers are of considerable interest as electrolytes for battery systems, as they eliminate overpotentials due to concentration gradients. In this study, we characterize a library of poly(ethylene oxide) (PEO)-based diblock copolymers where the second block is poly(styrene-4-sulfonyltrifluoromethylsulfonyl)imide with either cation: univalent lithium or divalent magnesium counterions (PEO−PSLiTFSI or PEO−P[(STFSI) 2 Mg]). The PEO chain length is held fixed in this study. Polymers were synthesized in matched pairs that were identical in all aspects except for the identity of the counterion. Using rheology, SAXS, DSC, and AC impedance spectroscopy, we show that the dependence of morphology, modulus, and conductivity on composition in these charged copolymer systems is fundamentally different from uncharged block copolymers. At a given frequency and temperature, the shear moduli of the magnesiated copolymer systems were approximately 3−4 orders of magnitude higher than those of the matched lithiated pair. The shear moduli of all of the lithiated copolymers showed liquid-like rheological features while the magnesiated copolymers did not. All of the lithiated copolymers were completely disordered (homogeneous), consistent with the observed rheological properties. As expected, the moduli of the lithiated copolymers increased with increasing volume fraction of the ion-containing block (ϕ PSTFSI ), and the conductivity decreased with ϕ PSTFSI . However, the magnesiated copolymers followed a distinct trend. We show that this was due to the presence of microphase separation in the regime 0.21 ≤ ϕ PSTFSI ≤ 0.36, and the tendency for microphase separation became weaker with increasing ϕ PSTFSI . The magnesiated copolymer with ϕ PSTFSI = 0.38 was homogeneous. The morphological, rheological, and conductivity properties of these systems are governed by the affinity of the cations for PEO chains; homogeneous systems are obtained when the cations migrate from the ion-containing block to PEO.

Multivalent ion conduction in solid polymer systems

“…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. 85 Additional efforts to improve conductivity and mechanical performance in this area could stem from two approaches.…”

“…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. 85 Additional efforts to improve conductivity and mechanical performance in this area could stem from two approaches. First, since it seems that salt dissociation for single-ion diblock copolymers tends to lead to disordered systems, the incorporation of a third mechanically robust block is necessary to fully recover the original high shear modulus behavior.…”

Cover Feature: All‐Solid‐State Lithium–Organic Batteries Comprising Single‐Ion Polymer Nanoparticle Electrolytes (ChemSusChem 9/2020)

“…Ac omparison of the conductivity-storage modulus plotso f our single-ion NP electrolytes with reported values for representatives ingle-ion electrolytes (Figure 4) demonstratest he significant advances of this study in the area of solid-state electrolytes. [41][42][43][44][45][46] Notably,a lthough our single-ion NP electrolytes are all-organic, they shows uperior mechanical properties and improved Li + ion transport properties in comparison to their organic-inorganic counterparts (Figure 4, shadedi n green). [43,53,54] Hereafter,t oe valuateb attery performance, we focus on the single-ion NP electrolyte comprising 50 wt %o f 20 nm NPs, owing to its optimal properties, as described above.…”

“…CF 3 (SO 2 )N À is bulky and has delocalized electrons, which is expected to facilitate Li + dissociation. [40,44] Furthermore, because the redox intermediates of organicc athode materials are negativelycharged, electrostatic repulsion with the NP surfaces could retard the dissolution of active materials in the electrolyte. As the degree of Li + dissociation and Li + conductivity should be affected by the surface area of the NPs, x- PS@PSTFSI À Li + NPs with three differents izes were prepared by controlling the molar ratio of the reagents.…”

“…[38] Single-ion conductingp olymer electrolytes may resolve these problems, but their conductivities are orders of magnitude lower than those of conventional dual-ion conducting polymere lectrolytes. [39][40][41][42][43][44][45][46] Undoubtedly,t he development of an ew solid-state electrolyte that has high conductivity and ah igh t Li þ value would lead to radicala dvances in lithium-organic battery technologies. Unfortunately,t his topic has not received much attentiona nd examples of high-performance all-solid-state lithium-organic batteries that exhibit mechanicals tability, ah igh specific capacity,a nd good rate capability are extremely challenging.…”