Ionic Conductivity, Salt Partitioning, and Phase Separation in High-Dielectric Contrast Polyether Blends and Block Polymer Electrolytes

Abstract

Small-molecule battery electrolytes are composed of mixtures of high-polarity and low-viscosity solvents at compositions that optimize ionic conductivity. In this work, we examined analogous polymer blends composed of one component with rapid segmental dynamics to provide low viscosity and another with a high dielectric constant (ε) to enhance ion dissociation. We investigated the inherent tradeoff between polymer polarity and segmental dynamics limiting ionic conductivity through the analysis of ionic conductivity of electrolytes containing lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) with different polarity hosts: poly(allyl glycidyl ether) (PAGE) (ε ≈ 9), poly[(cyanoethyl glycidyl ether)-co-(n-butyl glycidyl ether)] [P(CEGE-co-nBGE)] (ε ≈ 24), and poly(cyanoethyl glycidyl ether) (PCEGE) (ε ≈ 36). Two high-polarity-contrast polymer blends, PAGE/P(CEGE-co-nBGE)/LiTFSI and PAGE/PCEGE/LiTFSI, were prepared. While PAGE/PCEGE/LiTFSI blends were immiscible at all compositions, PAGE/P(CEGE-co-nBGE)/LiTFSI blends were miscible at LiTFSI concentrations above r = 0.065. The immiscibility of PAGE/PCEGE/LiTFSI blends imposed a negative deviation in ionic conductivity from a calculated linear average of the two single-polymer electrolytes. This negative deviation was decreased in magnitude to less than 10% in miscible PAGE/P(CEGE-co-nBGE)/LiTFSI blends between 30 and 90 °C. To understand the changes in the effective interaction parameter in the presence of LiTFSI, we investigated the disordered-state small-angle X-ray scattering (SAXS) of a diblock polymer, PAGE-b-PCEGE, across a range of LiTFSI concentrations. By fitting SAXS profiles of this copolymer using the random phase approximation and an adjustable contrast model, we found that the effective interaction parameter decreased monotonically as the LiTFSI concentration increased. At low concentrations, LiTFSI was primarily solvated in the high-polarity PCEGE-rich phase.

Mixed-Polarity Copolymers Based on Ethylene Oxide and Cyclic Carbonate: Insights into Li-Ion Solvation and Conductivity

“…The localization of salt in the low-polarity material increases the effective dielectric constant of that phase, thereby decreasing the contrast with the high-polarity, salt-free block. This is in contrast to what has been recently observed in high-polarity contrast block copolymers electrolytes consisting of poly­(ally glycidyl ether), a low-polarity/low-viscosity component, and poly­(cyanoethyl glycidyl ether), a high-polarity component . In this case, the high-polarity phase has a strong preference for the salt and does primarily solvate Li + at low salt concentrations, which increases the polarity contrast and in turn the immiscibility between the blocks.…”

“…This is in contrast to what has been recently observed in high-polarity contrast block copolymers electrolytes consisting of poly(ally glycidyl ether), a low-polarity/low-viscosity component, and poly-(cyanoethyl glycidyl ether), a high-polarity component. 5 In this case, the high-polarity phase has a strong preference for the salt and does primarily solvate Li + at low salt concentrations, which increases the polarity contrast and in turn the immiscibility between the blocks. Finally, we measured the ionic conductivity of the BCP r = 0.05 LiTFSI system.…”

“…Inspired by the success of mixing small-molecule electrolytes, recent work has attempted to apply this concept to SPEs to enhance ionic conductivity . A coarse-grain MD simulation study demonstrated that miscible blends of polymers with contrasting polarities may exhibit greater ionic conductivity than their polymer counterparts .…”

“…7 Inspired by the success of mixing small-molecule electrolytes, recent work has attempted to apply this concept to SPEs to enhance ionic conductivity. 5 A coarse-grain MD simulation study demonstrated that miscible blends of polymers with contrasting polarities may exhibit greater ionic conductivity than their polymer counterparts. 8 The increased ionic conductivity comes from an increase in charge carrier concentration, since the high-polarity component in blends promote ionic dissociation.…”

“…However, the practical use of a polyether SPE in a Li + battery requires improvements in several material properties, with insufficient ionic conductivity of PEO at room temperature being the most significant one. In liquid electrolyte systems, high ionic conductivity is commonly achieved by mixing a high-polarity, high-viscosity solvent (such as ethylene carbonate, EC, ε s = 89.8) with a low-polarity, low-viscosity solvent (such as dimethyl carbonate, DMC, ε s = 3.1). − These electrolyte mixtures exhibit higher ionic conductivity than that of either solvent alone, owing to the high dielectric constant of EC and the low viscosity of DMC …”