Double-Shelled Nanocages with Cobalt Hydroxide Inner Shell and Layered Double Hydroxides Outer Shell as High-Efficiency Polysulfide Mediator for Lithium-Sulfur Batteries

Core-Shell Structure and Interaction Mechanism of γ-MnO2 Coated Sulfur for Improved Lithium-Sulfur Batteries

Lithium-sulfur batteries have attracted worldwide interest due to their high theoretical capacity of 1672 mAh g^-1 and low cost. However, the practical applications are hampered by capacity decay, mainly attributed to the polysulfide shuttle. Here, the authors have fabricated a solid core-shell γ-MnO₂ -coated sulfur nanocomposite through the redox reaction between KMnO₄ and MnSO₄ . The multifunctional MnO₂ shell facilitates electron and Li⁺ transport as well as efficiently prevents polysulfide dissolution via physical confinement and chemical interaction. Moreover, the γ-MnO₂ crystallographic form also provides one-dimensional (1D) tunnels for the Li⁺ incorporation to alleviate insoluble Li₂ S₂ /Li₂ S deposition at high discharge rate. More importantly, the MnO₂ phase transformation to Mn₃ O₄ occurs during the redox reaction between polysulfides and γ-MnO₂ is first thoroughly investigated. The S@γ-MnO₂ composite exhibits a good capacity retention of 82% after 300 cycles (0.5 C) and a fade rate of 0.07% per cycle over 600 cycles (1 C). The degradation mechanism can probably be elucidated that the decomposition of the surface Mn₃ O₄ phase is the cause of polysulfide dissolution. The recent work thus sheds new light on the hitherto unknown surface interaction mechanism and the degradation mechanism of Li-S cells.

Polysulfide-Scission Reagents for the Suppression of the Shuttle Effect in Lithium-Sulfur Batteries

Lithium-sulfur batteries have become an appealing candidate for next-generation energy-storage technologies because of their low cost and high energy density. However, one of their major practical problems is the high solubility of long-chain lithium polysulfides and their infamous shuttle effect, which causes low Coulombic efficiency and sulfur loss. Here, we introduced a concept involving the dithiothreitol (DTT) assisted scission of polysulfides into lithium-sulfur system. Our designed porous carbon nanotube/S cathode coupling with a lightweight graphene/DTT interlayer (PCNTs-S@Gra/DTT) exhibited ultrahigh cycle-ability even at 5 C over 1100 cycles, with a capacity degradation rate of 0.036% per cycle. Additionally, the PCNTs-S@Gra/DTT electrode with a 3.51 mg cm^-2 sulfur mass loading delivered a high initial areal capacity of 5.29 mAh cm^-2 (1509 mAh g^-1) at current density of 0.58 mA cm^-2, and the reversible areal capacity of the cell was maintained at 3.45 mAh cm^-2 (984 mAh g^-1) over 200 cycles at a higher current density of 1.17 mA cm^-2. Employing this molecule scission principle offers a promising avenue to achieve high-performance lithium-sulfur batteries.

Preparation of NiCoP Hollow Quasi-Polyhedra and Their Electrocatalytic Properties for Hydrogen Evolution in Alkaline Solution

Double metal phosphide (NiCoP) with hollow quasi-polyhedron structure was prepared by acidic etching and precipitation of ZIF-67 polyhedra and further phosphorization treatment with NaH₂PO₂. The morphology and microstructure of NiCoP quasi-polyhedron and its precursors were investigated by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and a micropore and chemisorption analyzer. Electrocatalytic properties were examined by typical electrochemical methods, such as linear sweep voltammetry, cyclic voltammetry, chronoamperometry, and electrochemical impedance spectroscopy in 1.0 M KOH aqueous solution. Results reveal that, compared with CoP hollow polyhedra, NiCoP hollow quasi-polyhedra exhibit better electrochemical properties for hydrogen evolution with a low onset overpotential of 74 mV and a small Tafel slope of 42 mV dec^-1. When the current density is 10 mA cm^-2, the corresponding overpotential is merely 124 mV, and 93% of its electrocatalytic activity can be maintained for 12 h. This indicates that NiCoP with hollow quasi-polyhedron structure, bimetallic merit, and low cost may be a good candidate as electrocatalyst in the practical application of hydrogen evolution.

Three-Dimensional Carbon Nanotubes Forest/Carbon Cloth as an Efficient Electrode for Lithium-Polysulfide Batteries

The development of a three-dimensionally flexible, large-surface area, high-conductivity electrode is important to improve the low conductivity and utilization of active materials and restrict the shuttle of long-chain polysulfides in Li-polysulfide batteries. Herein, we constructed an integrated three-dimensional carbon nanotube forest/carbon cloth electrode with heteroatom doping and high electrical conductivity. The as-constructed electrode provides strong trapping on the polysulfide species and fast charge transfer. Therefore, the Li-polysulfide batteries with as-constructed electrodes achieved high specific capacities of ∼1200 and ∼800 mA h g^-1 at 0.1 and 1 C, respectively. After 300 cycles at 0.5 C, a specific capacity of 623 mA h g^-1 was retained.

A novel compact cathode using sponge-like RANEY® nickel as the sulfur immobilizer for lithium–sulfur batteries

A novel sulfur immobilizer named RANEY® nickel is introduced for lithium–sulfur batteries for the first time.

In Situ Reactive Synthesis of Polypyrrole-MnO2 Coaxial Nanotubes as Sulfur Hosts for High-Performance Lithium-Sulfur Battery

Lithium-sulfur batteries are considered as a promising candidate for high energy density storage applications. However, their specific capacity and cyclic stability are hindered by poor conductivity of sulfur and the dissolution of redox intermediates. Here, we design polypyrrole-MnO₂ coaxial nanotubes to encapsulate sulfur, in which MnO₂ restrains the shuttle effect of polysulfides greatly through chemisorption and polypyrrole serves as conductive frameworks. The polypyrrole-MnO₂ nanotubes are synthesized through in situ polymerization of pyrrole using MnO₂ nanowires as both template and oxidization initiator. A stable Coulombic efficiency of ∼98.6% and a decay rate of 0.07% per cycle along with 500 cycles at 1C-rate are achieved for S/PPy-MnO₂ ternary electrodes with 70 wt % of S and 5 wt % of MnO₂. The excellent trapping ability of MnO₂ to polysulfides and tubular structure of polypyrrole with good flexibility and conductivity are responsible for the significantly improved cyclic stability and rate capability.

A Cooperative Interface for Highly Efficient Lithium-Sulfur Batteries

A cooperative interface constructed by "lithiophilic" nitrogen-doped graphene frameworks and "sulfiphilic" nickel-iron layered double hydroxides (LDH@NG) is proposed to synergistically afford bifunctional Li and S binding to polysulfides, suppression of polysulfide shuttles, and electrocatalytic activity toward formation of lithium sulfides for high-performance lithium-sulfur batteries. LDH@NG enables high rate capability, long lifespan, and efficient stabilization of both sulfur and lithium electrodes.

A sulfur host based on titanium monoxide@carbon hollow spheres for advanced lithium-sulfur batteries

Lithium–sulfur batteries show advantages for next-generation electrical energy storage due to their high energy density and cost effectiveness. Enhancing the conductivity of the sulfur cathode and moderating the dissolution of lithium polysulfides are two key factors for the success of lithium–sulfur batteries. Here we report a sulfur host that overcomes both obstacles at once. With inherent metallic conductivity and strong adsorption capability for lithium-polysulfides, titanium monoxide@carbon hollow nanospheres can not only generate sufficient electrical contact to the insulating sulfur for high capacity, but also effectively confine lithium-polysulfides for prolonged cycle life. Additionally, the designed composite cathode further maximizes the lithium-polysulfide restriction capability by using the polar shells to prevent their outward diffusion, which avoids the need for chemically bonding all lithium-polysulfides on the surfaces of polar particles.

The promise of lithium-sulfur batteries with higher energy densities than lithium-ion is hindered by the insulating nature of sulfur and dissolution of polysulfides. Here the authors design titanium monoxide/carbon hollow nanospheres that overcome both obstacles, enabling improved electrochemical properties.

Good Low-Temperature Properties of Nitrogen-Enriched Porous Carbon as Sulfur Hosts for High-Performance Li-S Batteries

Despite the increased attention devoted to exploring cathode construction based on various nitrogen-enriched carbon scaffolds at room temperature, the low-temperature behaviors of Li-S cathodes have yet to be studied. Herein, we demonstrate the good low-temperature electrochemical performances of nitrogen-enriched carbon/sulfur composite cathodes. Electrochemical evaluation indicates that a reversible capacity of 368 mAh g(-1) (0.5 C) over 100 cycles is achieved at -20 °C. After returning to 25 °C, a capacity of 620 mAh g(-1) (0.5 C) is achieved over 350 cycles with a low-capacity attenuation rate (0.071% per cycle) and an initial capacity of 1151 mAh g(-1) (0.1C). This positive electrochemical property was speculated to result from the good surface chemistry of the various amine groups in the nitrogen-enriched carbon materials with enhanced polysulfide immobilization.

COF-Net on CNT-Net as a Molecularly Designed, Hierarchical Porous Chemical Trap for Polysulfides in Lithium-Sulfur Batteries

The hierarchical porous structure has garnered considerable attention as a multiscale engineering strategy to bring unforeseen synergistic effects in a vast variety of functional materials. Here, we demonstrate a "microporous covalent organic framework (COF) net on mesoporous carbon nanotube (CNT) net" hybrid architecture as a new class of molecularly designed, hierarchical porous chemical trap for lithium polysulfides (Li2Sx) in Li-S batteries. As a proof of concept for the hybrid architecture, self-standing COF-net on CNT-net interlayers (called "NN interlayers") are fabricated through CNT-templated in situ COF synthesis and then inserted between sulfur cathodes and separators. Two COFs with different micropore sizes (COF-1 (0.7 nm) and COF-5 (2.7 nm)) are chosen as model systems. The effects of the pore size and (boron-mediated) chemical affinity of microporous COF nets on Li2Sx adsorption phenomena are theoretically investigated through density functional theory calculations. Benefiting from the chemical/structural uniqueness, the NN interlayers effectively capture Li2Sx without impairing their ion/electron conduction. Notably, the COF-1 NN interlayer, driven by the well-designed microporous structure, allows for the selective deposition/dissolution (i.e., facile solid-liquid conversion) of electrically inert Li2S. As a consequence, the COF-1 NN interlayer provides a significant improvement in the electrochemical performance of Li-S cells (capacity retention after 300 cycles (at charge/discharge rate = 2.0 C/2.0 C) = 84% versus 15% for a control cell with no interlayer) that lies far beyond those accessible with conventional Li-S technologies.