A Molecular Engineering Approach: Multi-Functional Binders Design for Lithium-Sulfur Battery

Abstract

Lithium-sulfur (Li-S) batteries have attracted great attention as the next-generation batteries due to their high theoretical gravimetric energy density of ~2510 Wh/kg in comparison to ~400 Wh/kg for lithium-ion (Li-ion) batteries. Besides, sulfur has advantages, including abundant reserves, low price, and environmental friendliness. However, the practical application of Li-S batteries is hindered by several challenges of sulfur cathode such as dissolution of lithium polysulfides (LiPSs) in the electrolyte, the cracks issue caused by volume change of active materials during cycling, and poor conductivity, which reduce the capacity and cycle life of practical cells significantly. Even worse, these challenges become more severe when fabricating high-loading sulfur cathode, which is the path to the commercial application of Li-S batteries. In this doctoral dissertation, I have developed a “bottom-up” molecular engineering strategy to achieve high-loading, long-cycle-life sulfur cathode based on a multi-functional binders design. To confine LiPSs, Chapter 2 demonstrates the design and synthesis of novel binder PENDI based on redox active naphthalene diimide (NDI), which confines LiPSs through chemical interactions and redox mediation effect. The detailed redox mediation mechanism of PENDI to reduce the accumulation of LiPSs is proposed, providing a promising strategy to prevent the dissolution of LiPSs via an organic redox mediator. Chapter 3 presents the strategy to mitigate cracks by reversibly crosslinking PENDI with triPy crosslinker based on pi-pi interaction. The resulting PENDI/triPy supramolecular network demonstrates not only enhanced mechanical properties, but also tunable self-healing properties. Besides, a strategy to decouple macro-scale properties from the covalent structure of supramolecular polymers is reported to tune both mechanical and self-healing properties over a wide range, without changing the structure of polymer components. Building upon Chapter 2 and Chapter 3, Chapter 4 integrates the molecular designs into sulfur cathode and details the optimization of high-loading, long-cycle-life sulfur cathodes by mechanically stabilizing sulfur cathode with PENDI-based binders. By utilizing triPy crosslinker and PVDF additive, both the toughness and stiffness of PENDI-based binders are dramatically improved, enabling the fabrication of sulfur cathode with high areal discharge capacity (> 4.1 mAh/cm2, C/10 rate) and high cathode discharge capacity (> 10 mAh).