Rechargeable Lithium–sulfur (Li–S) batteries, with a theoretical energy density five times that of Lithium-ion batteries (LiBs), exhibit great potential as a low-cost, sustainable, and high-energy-density alternative to LiBs. However, serious challenges caused by polysulfide shuttling and lithium dendrite formation continue to limit performance by deteriorating capacity retention and cycle stability.
The 3S Battery project (Super Selective Separator for Battery Applications) aims to address these issues through the development of highly efficient separators with nanostructures and selective functionalities based on transport mechanisms. The designed separator should act as a selective channel that effectively suppresses polysulfide migration while facilitating Li? transport with uniform distribution, thereby solving problems caused by both polysulfide shuttling and dendrite formation. Functional coatings can further reactivate trapped sulfur to increase cathode efficiency. At the end of the project, the developed separators are expected to enable Li-S batteries to achieve high performance with the following key performance indicators (KPI): high cycle stability of >2,000 cycles, a capacity decay of ~0.01% per cycle, and an initial discharge capacity of ~1300 mAh g-1 at 0.1 C.
Various separator materials with well-defined functionalities were synthesized for improved battery performances to reach the project KPIs.
• MOF-Based Separators
The project has made significant advances in the use of metal–organic frameworks (MOFs) as selective separator materials, including
(1) Bimetallic MOFs such as Fe-ZIF-8 and UiO-66(Fe/Zr)-NH2, providing Fe(II) sites that enhance sulfur sorption and catalyze polysulfide conversion. These separators exhibited long cyclic life (up to 2000 cycles) with very low capacity decay.
(2) Cobalt-based MOFs with tuned intrinsic properties, demonstrating initial capacities as high as 1500 mAh g?¹.
(3) UiO-66–SO3H coatings synthesized via a one-step functionalization-driven approach. This improved electrolyte wettability, provided abundant chemical binding sites, and enabled stable cycling (72% retention after 850 cycles at 0.5 C, >99% Coulombic efficiency) with high-rate capability (1379–956 mAh g?¹ at 0.1–2 C). They also performed under lean electrolyte and higher sulfur loading conditions (E/S = 7 µL mg?¹).
(4) Mixed-linker UiO-66 (NH2/SO3H) separators, which allow tuning of functional group ratios for shuttle suppression. Cells with varied linker compositions retained 60–70% of capacity after 600 cycles at 0.5 C.
Atomistic modeling (DFT calculations) validated experimental findings, demonstrating strong binding energies between MOFs and lithium polysulfides.
• Polymer and Block Copolymer Separators
Polymeric strategies were also advanced as complementary or alternative separator solutions, including
(1) Polymers of Intrinsic Microporosity (PIMs) in three configurations: (a) pore-filled strategy to integrate PIMs’ high free volume into porous polypropylene Celgard or nonwoven substrates, (b) AOPIM-1 coated as a selective layer on porous support, and (c) self-supporting mixed-matrix membranes with MOFs as nanofillers. These strategies promise highly selective channels for Li? transport while suppressing polysulfide migration (testing on-going).
(2) Block copolymer coatings (Nexar, Pebax) and sulfonated polymer (sPEEK) as non-porous coatings with ion-conducting Li+ transport channels. Pebax-coated separators confirmed functionality, while systematic benchmarking of Nexar and sPEEK against standard separators is underway.
(3) Self-standing sulfonated polyether sulfone (sPES) membranes as separators with strong mechanical integrity, offering a fully polymer-based alternative without the need for a porous substrate. Electrochemical evaluation is ongoing.
• Electrospun Separators
Electrospinning techniques were developed to create highly porous nonwoven separators. PAN-based separators embedded with MOFs or with in-situ MOF growth (Fe-ZIF-8) demonstrated promising structural and electrochemical properties. Hot pressing improved pore structure and mechanical stability. Advanced characterization (TEM, SEM-EDS, XPS) confirmed effective fabrication and functionalization.
The 3S Battery project has made strong progress toward its KPIs, achieving long cycle stability, high initial discharge capacities, and structurally robust separators. Advances in MOF-based coatings, polymeric membranes, and high-loading cathodes are bringing Li–S technology closer to commercial relevance. The outcome of the project has been advanced toward commercialization and Technology Transfer. A Discovery project (HiSep-II) was launched to upscale NTNU’s patented Li–S separator technology for commercialization. Reproducibility studies and upscaling from coin cell to pouch cell are ongoing. The patent is now licensed to a start-up company for co-development.
Rechargeable Lithium-sulfur (Li-S) batteries have a theoretic energy density 5 times that of Lithium-ion batteries (LiBs), showing great potential as a low-cost, sustainable, and high-energy density alternative to the current state-of-the-art LiBs. However, serious problems due to polysulfide shuttling and Li dendrites formation have deteriorated the capacity retention and rate cyclability of Li-S batteries. The 3S Battery project (Super Selective Separator for Battery Applications) is proposed to mitigate these challenges, aiming at developing highly efficient separators for Li-S battery applications. The defined separator should act as a selective channel that effectively suppresses the polysulfides migrating while facilitating the Li+ ion transport with uniform distribution, thereby solving the problems caused by both polysulfide shuttling and dendrite formation. Functional coatings will also reactivate the trapped sulfur to increase cathode efficiency.
3S Battery project will design nanomaterials for the desired functions of Li-S battery separators, then fabricate separators using innovative membrane fabrication techniques, and evaluate the battery performance. The project implementation is based on highly interdisciplinary collaboration between experts at NTNU and SINTEF in nanomaterials, membranes, and batteries. The developed separators are expected to enable Li-S batteries to achieve high performance as indicated by the key performance indicators (KPI), including high cycle stability of >2,000 cycles, corresponding to a capacity decay of ~0.01% per cycle, with an initial discharge capacity of ~1300 mA?h g-1 at a 0.1 C. This performance will be revolutionary and attract industry interest.
Responsible Research and Innovation (RRI) aspects will be included to identify the risks and opportunities stemming from the developed separators. Education and training of competent engineers, especially in battery technology, is also an important part of the project.
