An ultrathin double-layer covalent organic framework/zwitterionic microporous polymer functional separator for high-performance lithium-sulfur battery

The development of efficient lithium-sulfur (Li-S) battery depends on a well-designed functional separator that can effectively limit the transport of polysulfides while facilitating the migration of lithium ions. Herein, a multifunctional separator is fabricated with commercial polypropylene membrane (PP) as bottom porous support, a covalent organic framework (COF) film as gutter layer, and an ultrathin zwitterionic microporous polymer (QTB-SA) layer as functional barrier. In such a configuration, the COF gutter layer plays a role in adjusting the surface property of PP support without obviously increasing the transport resistance of lithium ions, while the QTB-SA functional barrier enables the formation of confined ionic channels due to its intrinsic microporosity and zwitterionic nature. Thus, selective ion transport concerning this separator can be expected owing to the availability of electrostatic interaction with lithium polysulfides. The experimental results show that the QTB-SA/COF/PP-based Li-S battery possesses high prolonged cycling behavior (capacity decay rate of 0.087 % per cycle at 1C over 500 cycles) and attractive rate capability up to 5C. Moreover, the Li-S battery can deliver a decent areal capacity of 3.4 mAh cm−2 after 100 cycles with a raised sulfur loading (4.5 mg cm−2).