Lithium-sulfur (Li-S) batteries are among the most promising candidates for next-generation energy storage systems, offering a theoretical energy density exceeding 2600 Wh kg⁻¹ and cost advantages over conventional lithium-ion technologies. However, their commercialization is hindered by critical challenges, including rapid capacity fade due to the polysulfide shuttle effect and poor rate capability caused by sluggish lithium ion transport. The separator plays a pivotal role in determining battery performance by mediating ion movement while suppressing undesirable side reactions.
Traditional polyolefin separators, such as Celgard, possess large macropores (>100 nm) that allow unrestricted diffusion of both lithium ions and polysulfide anions. This leads to irreversible loss of active sulfur material, formation of resistive layers on the anode, and significant concentration polarization. While various functional separators have been developed—incorporating nanoporous materials, conductive coatings, or catalytic interlayers—most focus solely on blocking polysulfides without optimizing lithium ion conduction. A truly ideal separator must simultaneously achieve high ion selectivity, excellent ionic conductivity, mechanical robustness, and chemical stability.
In this work, we present a novel approach based on covalent organic frameworks (COFs), which combine intrinsic ordered porosity, low density, and tunable surface chemistry. Specifically, we designed and synthesized a lithium-functionalized COF, TpPa-SO₃Li, featuring aligned nanochannels (~1.2 nm) and uniformly distributed sulfonate groups. These features create a highly selective environment where negatively charged sulfonate sites electrostatically repel polysulfide anions while attracting and facilitating the transport of lithium cations through a hopping mechanism enabled by cation-dipole interactions.
The synthesis begins with interfacial polymerization to form pristine TpPa-SO₃H nanosheets at the interface between aqueous and organic phases. Subsequent lithiation via reaction with lithium acetate replaces protons with lithium ions, yielding TpPa-SO₃Li. The resulting nanosheets exhibit excellent colloidal stability in water, evidenced by a strong Tyndall effect and no aggregation after prolonged storage. TEM and AFM analyses confirm ultrathin (≈1.5 nm), flat, and large-area morphology, ideal for forming continuous, crack-free films.
The TpPa-SO₃Li layer is deposited onto Celgard using a vacuum-assisted self-assembly method, resulting in a uniform coating with a thickness of approximately 230 nm. SEM imaging reveals complete coverage of the original macroporous structure, transforming it into a smooth, compact surface. Cross-sectional analysis confirms consistent layer thickness and integrity. XRD and GISAXS data validate the crystalline order and alignment of the nanochannels within the film, with characteristic peaks at 4.7°, 8.2°, and 26.5° corresponding to (100), (110), and (001) planes.
Functional characterization demonstrates the superior ion-selective properties of the modified separator. Electrolyte contact angle measurements show enhanced wettability (16.5°), indicating favorable interaction between the polar COF framework and electrolyte components. Zeta potential analysis reveals a highly negative surface charge (-71.9 mV), significantly stronger than pristine Celgard (-38.4 mV), confirming the presence of abundant anionic sites. Permeation tests using Li₂S₈ solution clearly show that the TpPa-SO₃Li/Celgard separator effectively blocks polysulfide migration, with no visible color change in the receiving chamber after 24 hours.
Electrochemical evaluation reveals exceptional lithium ion transference number (t₊ = 0.88), far surpassing values for TpPa-SO₃H/Celgard (0.82) and pristine Celgard (0.60). Impedance spectroscopy indicates reduced ionic resistance (2.64 Ω) compared to pristine (1.73 Ω) and TpPa-SO₃H-modified (3.17 Ω) separators, reflecting improved ion transport kinetics. The combination of high selectivity and low resistance enables efficient charge transfer and minimizes polarization.
When integrated into full Li-S cells, the TpPa-SO₃Li/Celgard separator delivers outstanding electrochemical performance. At 0.2 C, the cell achieves an initial discharge capacity of 939.4 mA h g⁻¹ and maintains 78% capacity retention after 100 cycles.NOX4 Antibody supplier Even under high sulfur loading (5.Ganglioside GM1 Antibody custom synthesis 4 mg cm⁻²), the capacity remains stable at 638.PMID:34103688 5 mA h g⁻¹ after 100 cycles, outperforming baseline cells by more than 70%. Rate capability tests reveal a sharp drop at 2 C, attributed to localized polysulfide accumulation and inert layer formation at the cathode. To overcome this, a conductive CNT interlayer was introduced, forming a TpPa-SO₃Li/CNT/Celgard configuration. This hybrid design enables remarkable rate performance, delivering 632.7 mA h g⁻¹ at 4 C and achieving 482.2 mA h g⁻¹ after 400 cycles with minimal decay (0.039% per cycle).
These findings highlight the transformative potential of ion-selective COFs in battery engineering. By precisely tailoring the nanostructure and chemistry of the separator, we achieve simultaneous suppression of polysulfide shuttling and enhancement of lithium ion transport. This dual-functionality paves the way for high-energy, long-cycle-life Li-S batteries suitable for electric vehicles, grid storage, and other demanding applications. The success of TpPa-SO₃Li underscores the importance of molecular-level design in advancing functional materials for sustainable energy technologies.MedChemExpress (MCE) offers a wide range of high-quality research chemicals and biochemicals (novel life-science reagents, reference compounds and natural compounds) for scientific use. We have professionally experienced and friendly staff to meet your needs. We are a competent and trustworthy partner for your research and scientific projects.Related websites: https://www.medchemexpress.com
