Super Selective Separators for Battery Applications

Rechargeable Lithium-sulfur (Li-S) batteries, with a theoretic energy density 5 times that of Lithium-ion batteries (LiBs), exhibit 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. Using highly efficient separators in Li-S batteries can be a solution to these issues. The 3S Battery project (Super Selective Separator for Battery Applications) aims at developing highly efficient separators with nanostructures and desired functions based on transport mechanisms. 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. In this project, separators with the proposed designs will be developed, their performances in Li-S batteries will be evaluated, and the separators' fabrication and upscaling potential will be considered during the development. At the end of the project, 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. The separators should also be adopted for use in other types of metal sulfur batteries for their high selectivity or LiBs for their reduced dendrite formation. The project implementation is based on highly interdisciplinary collaboration between experts at NTNU and SINTEF in nanomaterials, membranes, and batteries. Uppsala University is an international partner. During this reporting year, the 3S Battery project focused on the development of new materials and the design of novel morphologies and nanostructures for efficient Li-S battery separators. Various separator materials with well-defined functionalities were synthesized for improved battery performances to reach the project KPIs. The outcomes of the research activities and some preliminary results are summarized as follows: 1. Two cost-effective 3-D bimetallic MOFs (i.e., Fe-ZIF-8 and UiO-66(Fe/Zr)-NH2) were developed at NTNU and employed as selective materials to fabricate separators with designed functions. The synthesized MOFs contain Fe(II) sites, which enhance sulfur sorption and catalytic conversion of polysulfides when used as the Li-S battery separator materials, thereby significantly improving the battery performance with long cyclic life (up to 2000) and low capacity decay (<0.06%). 2. Separators based on two cobalt-based MOFs with tuned intrinsic properties were also synthesized, structurally characterized, and performance tested, showing an initial capacity as high as 1500 mAhg–1. 3. Cation exchange membranes made from block copolymers (Nexar and sulfonated polyether sulfone) were fabricated as free-standing separators, and their performances as Li-S battery separators were evaluated for further optimization. 4. AOPIM-1 (amidoxime-functionalized PIM (polymer of intrinsic microporosity)) was synthesized and cast onto a porous substrate (e.g., polypropylene (PP), or Celgard) as the selective layer of Li-S battery separators. To optimize the coating layer morphology, alternative substrates were investigated. A highly porous melt-blown PP fabric was selected as the substrate to host the AOPIM-1 coating, resulting in an ideal morphology as a separator, i.e., highly porous material for free Li+ transport and uniform pore size enabling selective elimination of the polysulfide shuttle effect and preventing dendrite formation. AOPIM-1 was also cast as a self-supporting polymer separator without a substrate for further modification to introduce new functionalities. 5. Highly porous nonwoven separators based on electrospinning were developed at SINTEF, including direct electrospinning of PAN (Polyacrylonitrile) embedded with various MOFs and electrospinning of PAN with in-situ growth of MOF (Fe-ZIF-8) by crystallization of precursors?. Hot pressing was applied to optimize the separator pore structures and mechanical properties.

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.