Enhanced Conductivity via Homopolymer-Rich Pathways in Block Polymer-Blended Electrolytes

Abstract

The optimization of ionic conductivity and lithium-ion battery stability can be achieved by independently tuning the ion transport and mechanical robustness of block polymer (BP) electrolytes. However, the ionic conductivity of BP electrolytes is inherently limited by the covalent attachment of the ionically conductive block to the mechanically robust block, among other factors. Herein, the BP electrolyte polystyrene-block-poly(oligo-oxyethylene methacrylate) [PS-b-POEM] was blended with POEM homopolymers of varying molecular weights. The incorporation of a higher molecular weight homopolymer additive (α > 1 state) promoted a "dry brush-like" homopolymer distribution within the BP self-assembly and led to higher lithium salt concentrations in the more mobile homopolymer-rich region, increasing overall ionic conductivity relative to the "wet brush-like" (α < 1 state) and unblended composites, where α is the molecular weight ratio between the POEM homopolymer and the POEM block in the copolymer. Neutron and X-ray reflectometry (NR and XRR, respectively) provided additional details on the lithium salt and polymer distributions. From XRR, the α > 1 blends showed increased interfacial widths in comparison to their BP (unblended) or α < 1 counterparts because of the more central distribution of the homopolymer. This result, paired with NR data that suggested even salt concentrations across the POEM domains, implied that there was a higher salt concentration in the homopolymer POEM-rich regions in the dry brush blend than in the wet brush blend. Furthermore, using 7 Li solid-state nuclear magnetic resonance spectroscopy, we found a temperature corresponding to a transition in lithium mobility (T Li mobility ) that was a function of blend type. T Li mobility was found to be 39 °C above T g in all cases. Interestingly, the ionic conductivity of the blended BPs was highest in the α > 1 composites, even though these composites had higher T g s than the α < 1 composites, demonstrating that homopolymer-rich conducting pathways formed in the α > 1 assemblies had a larger influence on conductivity than the greater lithium ion mobility in the α < 1 blends.

Intrinsic Ion Transport Properties of Block Copolymer Electrolytes

“…A more plausible explanation is that the volume fraction of conducting material that should be considered is less than the total volume fraction of PEO-LiTFSI in the system. It has been previously suggested that PEO-LiTFSI in SEO exhibits a lower ionic mobility only locally near the polystyrene interface, while far from the interface, the PEO within the block copolymer should behave identically to the homopolymer PEO. ,− Here, we consider a two-layer model in which the PEO domain is partitioned into an inactive region near the domain interface with no contribution to the conductivity and an active region, which exhibits the same conductivity as homopolymer PEO at the same salt concentration. Using the two-layer model, we can define an “active volume fraction” (ϕ active ) of PEO within the block copolymer that contributes to the measured conductivity given by The inactive PEO phase (ϕ inactive ) in the SEO has a corresponding volume fraction given by We report ϕ active and ϕ inactive for each salt concentration in Table , where the value of ϕ active is derived from the average value of σ n across the measured temperature range.…”

Segmental and interfacial dynamics quantitatively determine ion transport in solid polymer composite electrolytes

“…[16][17][18] Moreover, the preference of ion transport along interface rather than entering interior of ceramics, was further proved by the high conductivity under large volume fraction of interfaces. 19 In addition, new twists ranging from copolymerization, [20][21][22] chain network, 23,24 polymer blend, 25 and nano-porous structure, 19,26 to ionic side group 27 and plasticizer, 28,29 bring promising improvements. Releasing the potential of solid composite electrolytes requires mastering mechanism of ion transport in such materials.…”

Ionic Conductivities of Broad Dispersity Lithium Salt-Doped Polystyrene/Poly(ethylene oxide) Triblock Polymers

“…15,20 Since such high molar mass materials are difficult to synthesize and process, other investigations focused on the properties of different block polymer architectures including triblock, 21,22 star, 23 brush, 24,25 and hyperbranched materials. 26 More recently, Morris et al 27 and Xie et al 28 investigated microphase separated block polymer/homopolymer blends to further improve these "dry" electrolyte conductivities. Collectively, these studies establish that chain architecture, the presence of PEO homopolymer, and selected polymer end-group functionalities 29 influence the observed conductivities.…”