Interwoven NiCo2O4Nanosheet/Carbon Nanotube Composites as Highly Efficient Lithium−Sulfur Cathode Hosts

Trace Metal Impurities Induce Differences in Lithium-Sulfur Batteries

Carbon nanotubes (CNTs) with exceptional conductivity have been widely adopted in lithium-sulfur (Li-S) batteries. While trace metal impurities in CNTs have demonstrated electrocatalytic activity in various catalytic processes, their influence on sulfur electrocatalysis in Li-S batteries has been largely overlooked. Herein, we reveal that the trace metal impurities content in CNTs significantly improves the specific capacity and cycling performance of Li-S batteries by analyzing both our own results and previous literature with CNTs as the sulfur hosts. Even under lean electrolyte conditions (E/S ratio of 5 μL mgs-1), we demonstrate that a small content of metal impurities in CNTs (∼2 wt %) could account for a 14.3% increase in specific capacity and a 14.1% increase in capacity retention under a high sulfur loading of 3.5 mg cm-2. The electron transfer from confined metal catalysts within CNTs leads to electron accumulation at the carbon interface, facilitating electron donation to adsorbed sulfur species and lowering the energy barrier for Li2S formation.

Crystalline Multi-Metallic Compounds as Host Materials in Cathode for Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) battery is one of the most promising next-generation rechargeable batteries. Lots of fundamental research has been done for the problems during cycling like capacity fading and columbic efficiency reducing owing to severe diffusion and migration of polysulfide intermediates. In the early stage, a wide variety of carbon materials are used as host materials for sulfur to enhance electrical conductivity and adsorb soluble polysulfides. Beyond carbon materials, metal based polar compounds are introduced as host materials for sulfur because of their strong catalytic activity and adsorption ability to suppress the shuttle effect. In addition, relatively high density of metal compounds is helpful for increasing volumetric energy density of Li-S batteries. This review focuses on crystalline multi-metal compounds as host materials in sulfur cathodes. The multi-metal compounds involve not only transition metal composite oxides with specific crystalline structures, binary metal chalcogenides, double or complex salts, but also the metal compounds doped or partially substituted by other metal ions. Generally, for the multi-metal compounds, microstructure and morphologies in micro-nano scale are very significant for mass transfer in electrodes; moreover, adsorption and catalytic ability for polysulfides make fast kinetics in the electrode processes.

Interconnected NiCo2O4 nanosheet arrays grown on carbon cloth as a host, adsorber and catalyst for sulfur species enabling high-performance Li-S batteries

Li-S batteries are a promising next-generation electrochemical energy-storage system due to their high energy density, as well as the abundance and low cost of sulfur. However, the low conductivity of sulfur and Li2S/Li2S2, as well as the dissolution and shuttling of intermediate lithium polysulfides, is a great challenge for high-performance Li-S batteries. Herein, interconnected NiCo2O4 nanosheet arrays grown on carbon cloth (CC) are applied as the cathode (S/NiCo2O4/CC) in Li-S batteries for accommodating sulfur. The obtained cathode shows high conductivity, high dispersion of sulfur species and excellent polysulfide adsorption and catalytic properties. As a result, significantly higher specific capacity (1480 vs. 1048 mA h g-1 at 0.1C) and greatly enhanced rate performance (624 vs. 215 mA h g-1 at 2C) are obtained for the S/NiCo2O4/CC cathode in comparison to S/CC. Further, the S/NiCo2O4/CC cathode demonstrates a low capacity decay of 0.060% per cycle over 400 cycles at 0.5C.

Self-Supported and Flexible Sulfur Cathode Enabled via Synergistic Confinement for High-Energy-Density Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries have attracted much attention in the field of electrochemical energy storage due to their high energy density and low cost. However, the "shuttle effect" of the sulfur cathode, resulting in poor cyclic performance, is a big barrier for the development of Li-S batteries. Herein, a novel sulfur cathode integrating sulfur, flexible carbon cloth, and metal-organic framework (MOF)-derived N-doped carbon nanoarrays with embedded CoP (CC@CoP/C) is designed. These unique flexible nanoarrays with embedded polar CoP nanoparticles not only offer enough voids for volume expansion to maintain the structural stability during the electrochemical process, but also promote the physical encapsulation and chemical entrapment of all sulfur species. Such designed CC@CoP/C cathodes with synergistic confinement (physical adsorption and chemical interactions) for soluble intermediate lithium polysulfides possess high sulfur loadings (as high as 4.17 mg cm^-2 ) and exhibit large specific capacities at different C-rates. Specially, an outstanding long-term cycling performance can be reached. For example, an ultralow decay of 0.016% per cycle during the whole 600 cycles at a high current density of 2C is displayed. The current work provides a promising design strategy for high-energy-density Li-S batteries.

A Collaboratively Polar Conductive Interface for Accelerating Polysulfide Redox Conversion

In order to alleviate the inferior cycle stability of the sulfur cathode, a self-assembled SnO₂-doped manganese silicate nanobubble (SMN) is designed as a sulfur/polysulfide host to immobilize the intermediate Li₂S _x, and nitrogen-doped carbon (N-C) is coated on SMN (SMN@C). The exquisite N-C conductive network not only provides sufficient free space for the volume expansion during the phase transition of solid sulfur into lithium sulfide but also reduces R_ct of SMN. During cycling, the soluble polysulfide could be fastened by the silicate with an oxygen-rich functional group and heteronitrogen atoms through chemical bonding, enabling a confined shuttle effect. The synergistic effect between N-C and SMN could also effectively facilitate the interconversion between lithium polysulfides and Li₂S, reducing the potential barrier and accelerating the redox kinetics. With an areal sulfur loading of 2 mg/cm², the S-SMN@C cathodes demonstrate a high initial capacity of 1204 mA·h/g at 0.1 C, and an outstanding cycle stability with a capacity fading rate of 0.0277%, ranging from the 2nd cycle to the 1000th cycle at 2 C.