Facile Synthesis of a "Two-in-One" Sulfur Host Featuring Metallic-Cobalt-Embedded N-Doped Carbon Nanotubes for Efficient Lithium-Sulfur Batteries

Alloying Strategy Balances the Adsorption-Reduction-Oxidation Process of Sulfur Species Across Wide Temperature Ranges

Transition metal-based catalysts have been demonstrated to effectively anchor and utilize lithium polysulfides (LiPSs), thereby enhancing the capacity of lithium-sulfur batteries (LSBs). However, the immobilized d-band electronic structure of a single transition metal is inadequate for achieving satisfactory adsorption and catalytic conversion. In this study, an alloying strategy is employed to modulate the d-band structure with the aim of achieving the optimal adsorption capacity for LiPSs. For this purpose, cobalt (Co)-nickel (Ni) encapsulated in nitrogen-doped carbon nanotubes as bimetallic catalysts (CoNi/NCNT) are synthesized. The theory calculations and experimental analysis demonstrate that by hybridizing the d-orbitals of Co and Ni, the d-band structure of the CoNi bimetallic is modulated to be at the optimal central position. This configuration leads to the moderate adsorption and detachment of LiPSs on the surface of the catalysts, thereby balancing the "adsorption-reduction-oxidation" process of sulfur (S) species. Therefore, the LSBs with CoNi/NCNT separator are able to achieve good cycling at room temperature (capacity decay rate of 0.086% after 500 cycles at 0.5 C). The modified batteries can achieve excellent cycling performance across a wide temperature range (capacity decay rate of 0.057% after 100 cycles at 0 °C, and 0.34% after 100 cycles at 60 °C).

Modified separators boost polysulfides adsorption-catalysis in lithium-sulfur batteries from Ni@Co hetero-nanocrystals into CNT-porous carbon dual frameworks

In this manuscript, nickel/cobalt bimetallic nanocrystals confining into three-dimensional interpenetrating dual-carbon conductive structure (NiCo@C/CNTs) were successfully manufactured by annealing its core-shell structure (Ni-ZIF-67@ZIF-8) precursor under the high temperature. The results presented that the bimetallic nickel and cobalt nanocrystals with superior catalytic activity could quickly convert solid Li2S/Li2S2into soluble LiPSs and effectively decrease the energy barrier. While the hierarchical CNT-porous carbon dual frameworks can provide quick electron/ion transport because of their large specific surface area and the exposure of enough active sites. When used as the separator modifier for lithium sulfur batteries, the battery properties were significantly improved with high specific capacity, outstanding rate capability, and long-term cycle stability. Specifically, its initial specific capacity can achieve to 1038.51 mAh g-1 at 0.5C. At the high rate of 3C, it still delivers satisfactory discharge capacity of 555 mAhg-1 and the capacity decay rate is only 0.065% per cycle after 1000 cycles at 1C. Furthermore, even exposed to heavy sulfur loading (3.61 mg/cm2), they still maintain promising cycle stability. Therefore, such kinds of MOFs derivative with powerful chemical immobilization and catalytic conversion for polysulfides provides a novel guidance for the modification separator and the potential application in the field of high-performance Li-S batteries.

A Catalytic Electrolyte Additive Modulating Molecular Orbital Energy Levels of Lithium Polysulfides for High-Performance Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries have ultrahigh theoretical specific capacity, but the practical application is hindered by the severe shuttle effect and the sluggish redox kinetics of the intermediate lithium polysulfides (LiPSs). Effectively enhancing the conversion kinetics of LiPSs is essential for addressing these issues. Herein, the redox kinetics of LiPSs are effectively improved by introducing 6-azauracil (6-AU) molecules to the organic electrolyte to modulate the molecular orbital energy level of LiPSs. The 6-AU as a soluble catalyst can form complexes with LiPSs via Li-O bonds. These complexes are liable to transform because of the elevated HOMO and the reduced LUMO energy levels as compared to the dissociative LiPSs, resulting in small energy gaps (Egap) and exhibiting stronger redox activity. Benefiting from the rapid conversion kinetics, the shuttling effect of LiPSs is alleviated to a great extent, so that sulfur utilization is improved and the lithium electrode is protected. In addition, the introduction of 6-AU modulates the deposition behavior of Li2S and eases the coverage of the cathode surface by the insulating Li2S layer. The Li-S battery containing 6-AU provides superior capacity retention of 853 mAh g-1 after 150 cycles at 0.2 C and shows remarkable high-rate performance and retains a specific discharge capacity of 855 mAh g-1 at 5 C. This study accelerates the kinetics of Li-S batteries by tuning the HOMO and LUMO energy levels of LiPSs, which opens an avenue for designing functional electrolyte additives.

Fundamental mechanistic insights into the catalytic reactions of Li─S redox by Co single-atom electrocatalysts via operando methods

Lithium-sulfur batteries represent an attractive option for energy storage applications. A deeper understanding of the multistep lithium-sulfur reactions and the electrocatalytic mechanisms are required to develop advanced, high-performance batteries. We have systematically investigated the lithium-sulfur redox processes catalyzed by a cobalt single-atom electrocatalyst (Co-SAs/NC) via operando confocal Raman microscopy and x-ray absorption spectroscopy (XAS). The real-time observations, based on potentiostatic measurements, indicate that Co-SAs/NC efficiently accelerates the lithium-sulfur reduction/oxidation reactions, which display zero-order kinetics. Under galvanostatic discharge conditions, the typical stepwise mechanism of long-chain and intermediate-chain polysulfides is transformed to a concurrent pathway under electrocatalysis. In addition, operando cobalt K-edge XAS studies elucidate the potential-dependent evolution of cobalt's oxidation state and the formation of cobalt-sulfur bonds. Our work provides fundamental insights into the mechanisms of catalyzed lithium-sulfur reactions via operando methods, enabling a deeper understanding of electrocatalysis and interfacial dynamics in electrical energy storage systems.

Polysulfide Rejection Strategy in Lithium-Sulfur Batteries Using an Ion-Conducting Gel-Polymer Interlayer Membrane

Lithium-sulfur batteries (LiSBs) are emerging as promising alternative to conventional secondary lithium-ion batteries (LiBs) due to their high energy density, low cost, and environmental friendliness. However, preventing polysulfide dissolution is a great challenge for their commercial viability. The present work is focused on preparing a lithium salt and ionic liquid (IL) solution (SIL) impregnated ion (lithium ion)-conducting gel-polymer membrane (IC-GPM) interlayer to prevent polysulfide migration toward the anode by using an electrostatic rejection and trapping strategy. Herein, we introduce an SIL-based freestanding optimized IC-GPM70 (70 wt % SIL) interlayer membrane with high lithium-ion conductivity (2.58 × 10^-3 S cm^-1) along with excellent thermal stability to suppress the migration of polysulfide toward the anode and prevent polysulfide dissolution in the electrolyte. Because of the coulombic interaction, the anionic groups, -CF₂ of the β-phase polymer host PVdF-HFP, TFSI^- anion of IL EMIMTFSI, and anion BOB^- of LIBOB salt, allow hopping of positively charged lithium ions (Li⁺) but reject negatively charged and relatively large-sized polysulfide anions (S_x^-2, 4 <x <8). The cationic group EMIM⁺ of the IL is electrostatically able to attract and trap the polysulfides in the interlayer membrane. Since the shuttle effect of lithium polysulfides in LiSBs has been suppressed by the prepared IC-GPM70 interlayer, the resulting lithium-sulfur cell exhibits significantly higher cycling stability (1200 cycles), rate performance (1343, 1208, 1043, 875, and 662 mAh g^-1 at 0.1C, 0.2C, 0.5C, 1C, and 2C, respectively), and structural integrity during cycling than its counterpart without the IC-GPM70 interlayer. The interlayer membrane has been found to improve the performance and durability of LiSBs, thus making them a viable alternative to conventional LiBs.

Regeneration of Activated Sludge into SiO2-Decorated Heteroatom-Doped Porous Carbon as Advanced Electrodes for Li-S Batteries

The regeneration of harmful activated sludge into an energy source is an important strategy for municipal sludge treatment and recycling. Herein, SiO₂-modified N,S auto-doped porous carbon (NSC@SiO₂) with high conductivity (70 S m^-1) is successfully obtained through a simple calcination method of the activated sludge from wastewater treatment. Further, P-doped NSC@SiO₂ (NSPC@SiO₂) is designed to achieve a higher surface area (891 m² g^-1 vs 624 m² g^-1), a larger pore volume (0.87 cm³ g^-1 vs 0.08 cm³ g^-1), and more carbon defects. Due to its special structure, NSPC@SiO₂ is used as a sulfur host of lithium-sulfur batteries. The results of polysulfide adsorption experiments, S 2p X-ray photoelectron spectra (XPS), Li₂S nucleation experiments, polysulfide symmetric cells, measurement of the galvanostatic intermittent titration (GITT), polarization voltage difference, lithium-ion diffusion rate, and Tafel slope verified that NSPC@SiO₂ greatly improved the adsorption capacity of polysulfides, lowered the barrier to Li₂S formation and the internal resistances of cells, and accelerated Li⁺ ion diffusion and the reaction kinetics of polysulfide conversion, resulting in the excellent performance of polysulfide capture and superior rate performance and cyclic stability. By comparing NSPC@SiO₂ with NSC@SiO₂, a higher initial capacity (1377 mAh g^-1 vs 1150 mAh g^-1 at 0.1C), better rate capacity (912 mAh g^-1 vs 719 mAh g^-1 at 2C), and low capacity decay (0.094% per cycle within 200 cycles) are obtained. Our work provides direction for the treatment, disposal, and resource utilization of activated sludge.

Phase Engineering of Defective Copper Selenide toward Robust Lithium-Sulfur Batteries

The shuttling of soluble lithium polysulfides (LiPS) and the sluggish Li-S conversion kinetics are two main barriers toward the practical application of lithium-sulfur batteries (LSBs). Herein, we propose the addition of copper selenide nanoparticles at the cathode to trap LiPS and accelerate the Li-S reaction kinetics. Using both computational and experimental results, we demonstrate the crystal phase and concentration of copper vacancies to control the electronic structure of the copper selenide, its affinity toward LiPS chemisorption, and its electrical conductivity. The adjustment of the defect density also allows for tuning the electrochemically active sites for the catalytic conversion of polysulfide. The optimized S/Cu_1.8Se cathode efficiently promotes and stabilizes the sulfur electrochemistry, thus improving significantly the LSB performance, including an outstanding cyclability over 1000 cycles at 3 C with a capacity fading rate of just 0.029% per cycle, a superb rate capability up to 5 C, and a high areal capacity of 6.07 mAh cm^-2 under high sulfur loading. Overall, the present work proposes a crystal phase and defect engineering strategy toward fast and durable sulfur electrochemistry, demonstrating great potential in developing practical LSBs.

CS-CNTs homojunctions prepared byin situgrowth of carbon nanotubes on the surface of porous carbon spheres for lithium-sulfur batteries

Lithium-sulfur battery is expected to become a new generation of commercial battery owing to its ultra-high theoretical specific capacity, low-cost, and environmental benign. However, the inherent insulation of sulfur and the shuttle effect of lithium polysulfide between electrodes limit the application of lithium-sulfur battery. In order to solve these problems, we focus on the design of carbon-sulfur composite structure. Herein, CS-CNTs homojunctions featured with the carbon nanotubes (CNTs)in situgrown on carbon sphere (CS) is designed and synthesized by simple polymerization and heat treatment. The composites of CS with interconnected pore networks and CNTs with high conductivity not only offer a conductive framework to promote fast electron transmission, but also provide a larger space to load sulfur and effectively capture polysulfides. The CS-CNTs@S cathode shows better electrochemical performance compared with CS-CPs@S and CS@S. The first discharge specific capacity is 1053 mAh g^-1at 0.1 C. After 200 cycles, the specific capacity still remains at 427 mAh g^-1.

Cerium Oxysulfide with O-Ce-S Bindings for Efficient Adsorption and Conversion of Lithium Polysulfide in Li-S Batteries

Understanding of adsorption and kinetic conversion of polysulfide lithium (LiPSs) in Li-S batteries is quite crucial for the design of efficient effective sulfur carriers. Herein, based on the possible interactions with LiPSs, Ce₂O₂S with unique O-Ce-S bindings is proposed to be used as a promising carrier additive and a 2D Ce₂O₂S/C composite is synthesized via a one-facile NaCl-template method and subsequent sulfuration under 700 °C. The 2D Ce₂O₂S/C exhibits a stronger adsorption capability than CeO₂/C through the adsorption test for Li₂S₆. Combined with XPS and DFT results, the superiority is mainly originated from the formation of S-S and Li-S bonds between LiPSs and the lattice S on the surface of Ce₂O₂S. The 2D Ce₂O₂S/C composite also exhibits a better catalytic ability than CeO₂ according to the change of the free energies of the polysulfides during the discharge process, which coincides with the lower oxidation potential for Li₂S₂/Li₂S transition by cyclic voltammetry. Resultantly, the cathodes using the Ce₂O₂S/C composite as a carrier manifest an enhanced rate and cycling performances. Hence, our work paves a phenomenon wherein Ce₂O₂S with O-Ce-S bindings is more beneficial to improve the cycling stability of Li-S batteries than CeO₂ containing single Ce-O bonds, which may be also suitable for other kinds of metallic sulfur oxide compounds.

To Promote the Catalytic Conversion of Polysulfides Using Ni-B Alloy Nanoparticles on Carbon Nanotube Microspheres under High Sulfur Loading and a Lean Electrolyte

Despite their high theoretical energy density, the application of lithium-sulfur batteries is seriously hindered by the polysulfide shuttle and sluggish kinetics, especially with high sulfur loading and under low electrolyte usage. Herein, to facilitate the conversion of lithium polysulfides, nickel-boron (Ni-B) alloy nanoparticles, dispersed uniformly on carbon nanotube microspheres (CNTMs), are used as sulfur hosts for lithium-sulfur batteries. It is demonstrated that Ni-B alloy nanoparticles can not only anchor polysulfides through Ni-S and B-S interactions but also exhibit high electrocatalytic capability toward the conversion of intermediate polysulfide species. In addition, the intertwined CNT microspheres provide an additional conductive scaffold in response to the fast electrochemical redox. The enhanced redox kinetics is beneficial to improve the specific capacity and cycling stability of the sulfur cathode, based on the fast conversion of lithium polysulfides and effective deposition of the final sulfide products. Conclusively, the S/Ni-B/CNTM composite delivers a high specific capacity (1112.7 mAh g_s^-1) along with good cycle performance under both high sulfur loading (8.3 mg cm^-2) and a lean electrolyte (3 μL mg_s^-1). Consequently, this study opens up a path to design new sulfur hosts toward lithium-sulfur batteries.

Lithium-Sulfur Batteries: Advances and Trends

A review with 132 references. Societal and regulatory pressures are pushing industry towards more sustainable energy sources, such as solar and wind power, while the growing popularity of portable cordless electronic devices continues. These trends necessitate the ability to store large amounts of power efficiently in rechargeable batteries that should also be affordable and long-lasting. Lithium-sulfur (Li-S) batteries have recently gained renewed interest for their potential low cost and high energy density, potentially over 2600 Wh kg−1. The current review will detail the most recent advances in early 2020. The focus will be on reports published since the last review on Li-S batteries. This review is meant to be helpful for beginners as well as useful for those doing research in the field, and will delineate some of the cutting-edge adaptations of many avenues that are being pursued to improve the performance and safety of Li-S batteries.

Conversion of Co Nanoparticles to CoS in Metal-Organic Framework-Derived Porous Carbon during Cycling Facilitates Na2S Reactivity in a Na-S Battery

Room-temperature sodium-sulfur batteries have attracted wide interest due to their high energy density and high natural abundance. Polysulfide dissolution and irreversible Na₂S conversion are challenges to achieving high battery performance. Herein, we utilize a metal-organic framework-derived Co-containing nitrogen-doped porous carbon (CoNC) as a catalytic sulfur cathode host. A concentrated sodium electrolyte based on sodium bis(fluorosulfonyl)imide, dimethoxyethane, and bis(2,2,2-trifluoroethyl) ether is used to mitigate polysulfide dissolution. We tune the amount of Co present in the CoNC carbon host by acid washing. Significant improvement in reversible sulfur conversion and capacity retention is observed with a higher Co content in CoNC, with 600 mAh g^-1 and 77% capacity retention for CoNC and 261 mAh g^-1 and 56% capacity retention for acid-washed CoNC at cycle 50 at 80 mAh g^-1. Post-mortem X-ray photoelectron spectroscopy, transmission electron microscopy, and selected area electron diffraction suggest that CoS is formed during cycling in place of Co nanoparticles and CoN₄ sites. Raman spectroscopy suggests that CoS exhibits a catalytic effect on the oxidation of Na₂S. Our findings provide insights into understanding the role Co-based catalysts play in sulfur batteries.

Efficient Polysulfide Redox Enabled by Lattice-Distorted Ni3Fe Intermetallic Electrocatalyst-Modified Separator for Lithium-Sulfur Batteries

Exploring efficient electrocatalysts for lithium-sulfur (Li-S) batteries is of great significance for the sulfur/polysulfide/sulfide multiphase conversion. Herein, we report nickel-iron intermetallic (Ni₃Fe) as a novel electrocatalyst to trigger the highly efficient polysulfide-involving surface reactions. The incorporation of iron into the cubic nickel phase can induce strong electronic interaction and lattice distortion, thereby activating the inferior Ni phase to catalytically active Ni₃Fe phase. Kinetics investigations reveal that the Ni₃Fe phase promotes the redox kinetics of the multiphase conversion of Li-S electrochemistry. As a result, the Li-S cells assembled with a 70 wt % sulfur cathode and a Ni₃Fe-modified separator deliver initial capacities of 1310.3 mA h g^-1 at 0.1 C and 598 mA h g^-1 at 4 C with excellent rate capability and a long cycle life of 1000 cycles at 1 C with a low capacity fading rate of ∼0.034 per cycle. More impressively, the Ni₃Fe-catalyzed cells exhibit outstanding performance even at harsh working conditions, such as high sulfur loading (7.7 mg cm^-2) or lean electrolyte/sulfur ratio (∼6 μL mg^-1). This work provides a new concept on exploring advanced intermetallic catalysts for high-rate and long-life Li-S batteries.