Biomimetic Molecule Catalysts to Promote the Conversion of Polysulfides for Advanced Lithium–Sulfur Batteries

Abstract

To overcome the shuttle effect in Li–S batteries, novel biomimetic molecule catalysts are synthesized by grafting hemin molecules to three functionalized carbon nanotube systems (CNTs–COOH, CNTs–OH, and CNTs–NH2). The Li–S battery using the CNTs–COOH@hemin cathode exhibits the optimal initial specific capacity (1637.8 mAh g−1) and cycle durability (up to 1800 cycles). Various in situ characterization techniques, such as Raman spectroscopy, Fourier‐transform infrared reflection absorption spectroscopy, and UV–vis spectroscopy, combined with density functional theory computations are used to investigate the structure–reactivity correlation and the working mechanism in the Li–S system. It is demonstrated that the unique structure of the CNTs‐COOH@hemin composite with good conductivity and adequate active sites resulting from molecule catalyst as well as the strong absorption to polysulfides entrapped by the coordinated Fe(III) complex with FeO bond enables the homogeneous dispersion of S, facilitates the catalysis and conversion of polysulfides, and improves the battery's performance.

Anchoring Polysulfides and Accelerating Redox Reaction Enabled by Fe‐Based Compounds in Lithium–Sulfur Batteries

“…One is that the FeCFeOC composite has a strong chemical affinity for sulfur in the LiPSs, leading to the formation of FeS bond. [30] The other is that the Fe 3+ ions can react with S 6 2− to form a new FeS x species. Furthermore, the binding energy at 163.8 eV is assigned to the high-order Li 2 S n or S 8 , [31] and that at 164.6 eV is attributed to S 8 , indicating that the S 2− in Li 2 S 6 can be oxidized into S 8 .…”

“…However, when discharged to 2.1 V, the high-resolution Fe 2p XPS spectrum of the FeCFeOC/S cathode only shows one broad characteristic peak, and a new FeS bond at 708 eV is formed, which is ascribed to the interaction of Fe 3+ with LiPSs intermediates to form the FeS x species. [30,39] Additionally, the peak position of Fe 3+ shifts toward the higher binding energy, indicating that the electrons transfer from LiPSs to Fe to induce the reduction reaction from Fe 3+ to Fe 2+ and ultimately form the FeS x species and/or Li 2 FeS 2 . [37b,c,40] When the discharge voltage decreases to 1.8 V, the FeS bond disappears, and the peak of metallic Fe at 707.4 eV can be observed, indicating that the FeS x species have been finally converted to Li 2 S and Fe completely.…”

Implanting Cobalt Atom Clusters within Nitrogen‐Doped Carbon Network as Highly Stable Cathode for Lithium–Sulfur Batteries

“…[ 8 ] Thus far, based on the mesoporous carbon acting as a scaffold for hosting sulfur, the two problems of volume change and poor conductivity of sulfur cathode can be efficiently mitigated for Li–S batteries. [ 9 ] Unfortunately, because mesoporous carbons are nonpolar materials, [ 10 ] which are unfavorable for adsorbing the polar polysulfides formed during the charge–discharge process, these carbon materials exhibit a limited capability to impede polysulfide shuttling, [ 11 ] which significantly inhibits their application in cathodes of lithium–sulfur batteries. Therefore, it is important to endow in mesoporous carbons with polarity.…”

Emerging Catalysts to Promote Kinetics of Lithium–Sulfur Batteries

“…In this respect, Yang and co‐workers observed the evolution of LiPSs over different electrodes during discharging progress. [ 98 ] According to the spectra collected in Figure 8c–h, the CNTs–COOH@hemin cathodes usually contained lower‐concentration long‐chain LiPSs species (such as S 8 2− and S 6 2− ), but the concentrations of short‐chain LiPSs species (such as S 3 *− ) were higher than those of the other cathodes. It meant that the electrodes containing CNTs–COOH@hemin possessed the advantage of faster catalytic conversion of long‐chain LiPSs into short‐chain LiPSs.…”