Supramolecular complexation of polysulfides by β-cyclodextrin polymer functionalized graphene hybrid cathode for high-performance lithium-sulfur batteries

Abstract

Lithium-sulfur (Li-S) batteries have currently attracted wide attention due to their high energy density of 2600 Wh kg−1 and low costs. However, commercial applications of Li-S batteries still have been hindered by several issues, such as low sulfur utilization, self discharge, low efficiency, and poor cycling stability mainly caused by shuttle effect. β-Cyclodextrins (β-CDs) are well-known supramolecular hosts, capable of including hydrophobic molecules inside their large cavities. Taking advantage of the remarkable molecular recognition properties of β-CDs, they are thus expected to effectively suppress shuttle effect by dynamically forming supramolecular inclusion complexes with the polysulfides. Here, we introduce a new strategy of using β-cyclodextrin polymers (β-CDPs) as supramolecular receptors chemically grafted onto reduced graphene oxide nanosheets (rGO) to construct ternary supramolecular nanostructure of rGO@CDP@S hybrid cathode for Li-S batteries. Interestingly, this proof-of-concept model system has successfully demonstrated reversible capture and release of lithium polysulfides to confine an unfavorable polysulfide shuttle effect via supramolecular host-guest interaction in Li-S batteries. Furthermore, interaction mechanism between β-CD and polysulfide was deeply investigated at the molecular level. More importantly, the {cyclodextrin/polysulfide} supramolecular inclusion complex was first isolated and fully characterized with a wide range of spectroscopic experiments and computational studies. The cyclodextrins-modified graphene-base sulfur cathode rGO@β-CDP@S exhibits remarkably enhanced electrochemical performances in terms of higher specific capacity (1329 mA h g−1 at 0.1 C), rate capability (387 mA h g−1 at 5.0 C), and excellent cycle life (capacity decay rate of 0.040% and 0.053% per cycle after 1000 cycles at 2.0 and 3.0 C), compared to pristine graphene-coated rGO@S composite. Moreover, it enables good cycling performance of high-sulfur-loading cathodes (S mass-loading of 5.3 mg cm−2) achieving discharge capacity of 560 mAh g−1 (Areal capacities: 3.0 mA h cm-2) at 0.2 C after 100 cycles. The results open up a rational approach to shed new lights on designing new functional molecular materials as cathodes for Li–S batteries.