Host-Guest Regulations in Functionalized Metal and Covalent Organic Frameworks Raise the Performance of Lithium Sulfur Batteries

Lithium sulfur batteries (LSBs) show great promise as next-generation batteries due to their high energy density. However, commercialization is hindered by limited cycle life, fast capacity decay and poor sulfur utilization, primarily due to the intricate phase evolution during battery operation and insulating characteristics of sulfur, leading to uncontrollable sulfur and polysulfide distribution and inefficient conversion kinetics. Therefore, the incorporation of metal and covalent organic frameworks (MOFs and COFs) has been widely employed in LSBs to serve as hosts, enabling the regulation of conversion and diffusion behavior of guest species, including lithium ions, sulfur and polysulfides, within their well-defined nanosized cavities. Nevertheless, pristine frameworks often fail to meet the requisite standards, and framework functionalization offers unique opportunities to tailor desired attributes and facilitate selective host-guest interactions in LSBs. However, a thorough understanding on how to precisely customize the nano-channels with functional groups to promote such interactions remains largely unexplored. In this review, we provide a systematic discussion on how the grafting of functional groups containing various active sites can play a role in host-guest chemistry, and focus on the latest advancements in engineering functionalized MOFs and COFs as charged-species regulators to tackle the problems causing poor LSB electrochemical performance. The concepts of electrophilic and nucleophilic effects are proposed, uncovering the mechanisms of framework functionalization in LSBs and serving as guidance for future developments.