High Performance MOF Separator for Li-S Battery using Langmuir-Blodgett Technology

Lithium-sulfur (Li-S) batteries have been widely emphasized as alternatives to commercial lithium-ion batteries not only for their higher theoretical capacity (1,675 mAh/g) but also for abundance of sulfur in nature and being safe. However, the shuttle effect of lithium polysulfide limits long cycle life of batteries, and separators have been modified to effectively control reaction intermediates. For example, polymer-based separators have been coated with oxide or carbon in order to enhance filtration via pore or electrical attraction/repulsion. In particular, metal-organic frameworks (MOFs) as coating materials have advantages of porosity and electrochemical property which can be easily adjusted by the composition of metal clusters and ligands. However, most traditional coating techniques such as chemical vapor deposition (CVD) and vacuum filtration coating methods require heating process and produce relatively thick layer (at least microscale), thereby limiting its applicability.<br/>Herein, we synthesized two types of MOF (i.e. MOF-5 and IRMOF-3) nanosheets using Langmuir-Blodgett (LB) technique to functionalize Li-S battery separator. A modified separator was uniformly formed (3.27 Å for R_a; roughness average) by depositing five molecular monolayer films (total thickness of ~47.5 Å) at room temperature, while zinc acetate dihydrate and (2-amino-)terephthalic acid were used as MOF precursors. To note, polysulfide permeation test in H-cell revealed that a LB-coated MOF separator had comparable degree of filtration to a slurry-coated MOF separator despite of its thin thickness (c.a. 4,000 times thinner than slurry-coated one). Moreover, Li-S battery with a IRMOF-3 coated separator exhibited 1.13 times higher cycle stability (retention of 54.26% after 100 cycles) compared to one with non-coated separator (retention of 48.83% after 100 cycles). This suggests that heteroatom (i.e. nitrogen in IRMOF-3) and pore structure of MOF coating layer contribute to enhanced filtration ability of separators. Consequently, we can improve the cycle stability of Li-S batteries by applying a nanoscale MOF modification layer to separators via LB, which can further pave a way for commercialization of Li-S batteries.