Aminomethyl-Functionalized Carbon Nanotubes as a Host of Small Sulfur Clusters for High-Performance Lithium-Sulfur Batteries

Unexpected spontaneous dynamic oxygen migration on carbon nanotubes

Combining density functional theory (DFT) calculations and ab initio molecular dynamics (AIMD) simulations, we show that oxygen functional groups exhibit unexpected spontaneous dynamic behaviors on the interior surface of single-walled carbon nanotubes (SWCNTs). The hydroxyl and epoxy migrations are achieved by the C-O bond breaking/reforming reactions or the proton transfer reaction between the neighboring epoxy and hydroxyl groups. It is demonstrated that the spontaneous dynamic characteristic is attributed to the sharply reduced energy barrier less than or comparable to thermal fluctuations. We also observe a stable intermediate state with a dangling C-O bond, which permits the successive migration of the oxygen functional groups. However, on the exterior surface of SWCNTs, it is difficult for the oxygen groups to migrate spontaneously because there are relatively high energy barriers, and the dangling C-O bond prefers to transform into the more stable epoxy configuration. The spontaneous oxygen migration is further confirmed by the oxygen migration process using DFT calculations and AIMD simulations at room temperature. Our work provides a new understanding of the behavior of oxygen functional groups at interfaces and gives a potential route to design new carbon-based dynamic materials.

Recent Progress in High-Performance Lithium Sulfur Batteries: The Emerging Strategies for Advanced Separators/Electrolytes Based on Nanomaterials and Corresponding Interfaces

Lithium-sulfur (Li-S) batteries, possessing excellent theoretical capacities, low cost and nontoxicity, are one of the most promising energy storage battery systems. However, poor conductivity of elemental S and the "shuttle effect" of lithium polysulfides hinder the commercialization of Li-S batteries. These problems are closely related to the interface problems between the cathodes, separators/electrolytes and anodes. The review focuses on interface issues for advanced separators/electrolytes based on nanomaterials in Li-S batteries. In the liquid electrolyte systems, electrolytes/separators and electrodes system can be decorated by nano materials coating for separators and electrospinning nanofiber separators. And, interface of anodes and electrolytes/separators can be modified by nano surface coating, nano composite metal lithium and lithium nano alloy, while the interface between cathodes and electrolytes/separators is designed by nano metal sulfide, nanocarbon-based and other nano materials. In all solid-state electrolyte systems, the focus is to increase the ionic conductivity of the solid electrolytes and reduce the resistance in the cathode/polymer electrolyte and Li/electrolyte interfaces through using nanomaterials. The basic mechanism of these interface problems and the corresponding electrochemical performance are discussed. Based on the most critical factors of the interfaces, we provide some insights on nanomaterials in high-performance liquid or state Li-S batteries in the future.

Carbon nanotubes assembled on porous TiO2 matrix doped with Co3O4 as sulfur host for lithium-sulfur batteries

Advanced design and fabrication of high performance sulfur cathodes with improved conductivity and chemical adsorption towards lithium polysulfides (LiPS) are crucial for further development of Li-S batteries. Hence, we designed a TiO₂/Co₃O₄-CNTs composite derived from Ti-MOF (MIL-125) as the host matrix for sulfur cathode. The polar nature of metal oxides (TiO₂, Co₃O₄) creates the adsorptive sites in the composite and leads to an efficient chemical capture of LiPS. The CNTs ensure the contact between S/Li₂S and the host material with high conductivity, enhanced charge transfer and fast electrochemical kinetics. At the same time, the CNTs strengthen the stability of the electrode material. Consequently, the as-prepared TiO₂/Co₃O₄-CNTs composite showed excellent electrochemical performance. The cell with S-TiO₂/Co₃O₄-CNTs delivers an initial specific capacity of 1270 mAh g^-1 at 0.2 C and high rate performance with a capacity of 603 mAh g^-1 at 3 C.