Despite extensive studies on nanoporous membranes for regulating lithium-ion (Li⁺) flux in lithium (Li)-metal batteries, the pore size design has largely focused on very small (< 5 nm) or extremely large (> 20 nm) dimensions, overlooking the intermediate pore size range. This gap, particularly between 5 and 15 nm, has limited exploration of critical Li⁺ transport phenomena and their impact on improving cell performance. Here, we developed robust and free-standing polymeric films with three-dimensional (3D) continuous nanoporous channels, precisely tuned to pore diameters ranging from 5 to 14 nm and immobilized sulfonate groups. Our systematic investigations revealed how pore size and immobilized anionic groups correlated with Li⁺ conductivity and battery performance. Notably, sulfonate-functionalized channels promoted Li⁺ conductivity significantly within this optimal pore range compared to non-functionalized counterparts. In an ether-based electrolyte with 1 M lithium bis(fluorosulfonyl)imide (LiFSI), the Li⁺ conductivity peaked at a pore diameter of 10 nm. Furthermore, the mobility of Li⁺ was approximately 4.4 times faster than FSI⁻, resulting in reducing interfacial resistance and promoting uniform Li deposition. The sulfonated nanoporous membrane in Li|LiFePO₄ full cells with an N/P ratio of 2.3 delivered excellent cycling stability over 1000 cycles while retaining approximately 80 % of the initial capacity.
We developed a block polymer-based synthetic route to sulfonated porous composites (SPCs) with precisely controlled nanopore size. By reducing the pore size to <4 nm and introducing a high density of surface sulfonic acid, the permeation of vanadium ions was effectively suppressed. We employed a polymerization-induced microphase separation (PIMS) process, in which a polyethylene fiber mat impregnated with a liquid polymerization mixture was spontaneously transformed into a fiber-reinforced and cross-linked block polymer membrane. Selective etching and sulfonation then produced the target composite membrane. In a vanadium redox flow battery (VRFB) cell, an SPC with 3.6 nm-sized mesopores, 109 m2 g–1 of specific surface area, and 0.3 mL g–1 of mesoporosity outperformed a Nafion 212 membrane of similar thickness, providing higher proton conductivity and much lower vanadium permeability. Thanks to the composite reinforcement, the membrane demonstrated remarkably enhanced mechanical stability. The SPC membrane could be successfully operated up to 300 cycles. Compared with Nafion 212, the SPC exhibited higher energy efficiencies (EEs) and higher discharge capacity retention. These results suggest the promise of block polymer-based permselective membranes in advanced battery applications.
Partially sulfonated amphiphilic poly(arylene ether sulfone)s (PSPAESs) were synthesized by one-step nucleophilic aromatic substitution copolymerization. A 4-fluoro-4′-hydroxydiphenyl sulfone potassium salt was used as a hydrophobic monomer, and 5-((4-fluorophenyl)sulfonyl)-2-hydroxybenzenesulfonic acid as a hydrophilic monomer bearing a sulfonic acid group was synthesized from the hydrophobic monomer via selective sulfonation. 1H and 13C nuclear magnetic resonance spectroscopy analysis of PSPAESs indicated formation of statistical amphiphilic copolymers with control over the degree of sulfonation by varying the feed. Dynamic light scattering and transmission electron microscopy analysis indicated that PSPAESs self-assembled into spherical micelles in aqueous solutions. Interestingly, the micellar solution of PSPAESs prepared by dialysis was found to grow Cu2S nanowires on a Cu grid under ambient conditions. Formation of Cu2S nanowires on various substrates including a Si wafer and graphene was demonstrated in the presence of Cu and a sulfur source. UV-vis spectroscopy and X-ray photoelectron spectroscopy data suggests PSPAESs assist dissolution of metallic Cu into Cu(II) enabling the formation of Cu2S nanowires.
