Fluorinated Covalent Organic Polymers for High Performance Sulfur Cathodes in Lithium–Sulfur Batteries

Abstract

Lithium–sulfur (Li–S) batteries by far offer higher theoretical energy density than that of the commercial lithium-ion battery counterparts, but suffer predominantly from an irreversible shuttling process involving lithium polysulfides. Here, we report a fluorinated covalent organic polymer (F-COP) as a template for high performance sulfur cathodes in Li–S batteries. The fluorination allowed facile covalent attachment of sulfur to a porous polymer framework via nucleophilic aromatic substitution reaction (SNAr), leading to high sulfur content, e.g., over 70 wt %. The F-COP framework was microporous with 72% of pores within three well-defined pore sizes, viz. 0.58, 1.19, and 1.68 nm, which effectively suppressed polysulfide dissolution via steric and electrostatic hindrance. As a result of the structural features of the F-COP, the resulting sulfur electrode exhibited high electrochemical performance of 1287.7 mAh g–1 at 0.05C, 96.4% initial Columbic efficiency, 70.3% capacity retention after 1000 cycles at 0.5C, and robust operation for a sulfur loading of up to 4.1 mgsulfur cm–2. Our findings suggest the F-COP family with the adaptability of SNAr chemistry and well-defined microporous structures as useful frameworks for highly sustainable sulfur electrodes in Li–S batteries.

Cationic Covalent‐Organic Framework as Efficient Redox Motor for High‐Performance Lithium–Sulfur Batteries

“…[52,53] Unfortunately, the performances of batteries with COFs as cathode matrices are barely satisfactory, probably due to the lack of active centers contributing to the fixation of polysulfides. Recent works have focused on introducing functional groups into COFs for chemically bonding with sulfur [54][55][56][57][58] or directly combining the frameworks with multisulfur chains [59] to enhance the interaction between frameworks and polysulfides, which could deliver the batteries with considerable high specific capacity and cycling stability. To date, cationic binders have been widely developed to promote the performance of Li-S batteries.…”

Covalent Organic Frameworks for Next‐Generation Batteries

“…[75,76] The electrochemical performance of the composite electrode material was also better than that of the covalent triazine framework, which was not modified by fluorine atoms. Similarly, Zhang et al [77] and Choi et al [78] use fluorine atoms in a covalent organic framework to fix sulfur via the S N Ar reaction to form CÀ S covalent bonds. They synthesized a new type of fluorine-modified covalent organic framework for use as a composite electrode of the sulfur carrier material, which showed a high sulfur loading and long cycle stability.…”

“…Meanwhile, mainly due to their pore size and large number of electron-rich and electron-deficient species, some COFs used as sulfur host materials can effectively adsorb Li + and S x 2À , which can solve the problems associated with the fast capacity decay and volume expansion of the sulfur composite cathode in LSBs. [71][72][73][74][75][76][77][78][79][80][81][82] In this review, through the classification introduced, COFs can be used as electrode materials for advanced batteries. COFs have potential for use as advanced battery electrode materials, but research on this topic has been insufficient.…”

“…Regarding the applications for next‐generation batteries, we first outline the application progress of COFs in sodium ion batteries (SIBs) and potassium ion batteries (PIBs) . Subsequently, the applications of COFs in lithium‐sulfur batteries (LSBs), lithium metal batteries (LMBs), and multivalent ion batteries (ZIBs, MIBs, AIBs) will be introduced. Finally, the challenges, prospects and improvement strategies of COF electrodes will be discussed and suggested.…”

Covalent Triazine Frameworks Incorporating Charged Polypyrrole Channels for High-Performance Lithium–Sulfur Batteries

“…In order to further increase the sulfur content of organosulfur polymers, perfluoroaryl-elemental sulfur nucleophilic aromatic substitution reaction (SN Ar ) chemistry was introduced. 52,53 The synthesis of the organosulfur polymer called SF-CTF was achieved by the trimerization of perfluorinated aryl cyanides with simultaneous impregnation of sulfur, thus enabling achieving sulfur content >80 wt %. These examples clearly signify the importance of the chemistry of sulfur insertion to achieve a high sulfur content.…”