Hierarchical NiCo2S4@NiO Core–Shell Heterostructures as Catalytic Cathode for Long‐Life Li‐O2 Batteries

The critical challenges of Li‐O₂ batteries lie in sluggish oxygen redox kinetics and undesirable parasitic reactions during the oxygen reduction reaction and oxygen evolution reaction processes, inducing large overpotential and inferior cycle stability. Herein, an elaborately designed 3D hierarchical heterostructure comprising NiCo₂S₄@NiO core–shell arrays on conductive carbon paper is first reported as a freestanding cathode for Li‐O₂ batteries. The unique hierarchical array structures can build up multidimensional channels for oxygen diffusion and electrolyte impregnation. A built‐in interfacial potential between NiCo₂S₄ and NiO can drastically enhance interfacial charge transfer kinetics. According to density functional theory calculations, intrinsic LiO₂‐affinity characteristics of NiCo₂S₄ and NiO play an importantly synergistic role in promoting the formation of large peasecod‐like Li₂O₂, conducive to construct a low‐impedance Li₂O₂/cathode contact interface. As expected, Li‐O₂ cells based on NiCo₂S₄@NiO electrode exhibit an improved overpotential of 0.88 V, a high discharge capacity of 10 050 mAh g^?1 at 200 mA g^?1, an excellent rate capability of 6150 mAh g^?1 at 1.0 A g^?1, and a long‐term cycle stability under a restricted capacity of 1000 mAh g^?1 at 200 mA g^?1. Notably, the reported strategy about heterostructure accouplement may pave a new avenue for the effective electrocatalyst design for Li‐O₂ batteries.     

Catalytic Polysulfide Conversion and Physiochemical Confinement for Lithium–Sulfur Batteries

The lithium–sulfur (Li–S) battery is widely regarded as a promising energy storage device due to its low price and the high earth‐abundance of the materials employed. However, the shuttle effect of lithium polysulfides (LiPSs) and sluggish redox conversion result in inefficient sulfur utilization, low power density, and rapid electrode deterioration. Herein, these challenges are addressed with two strategies 1) increasing LiPS conversion kinetics through catalysis, and 2) alleviating the shuttle effect by enhanced trapping and adsorption of LiPSs. These improvements are achieved by constructing double‐shelled hollow nanocages decorated with a cobalt nitride catalyst. The N‐doped hollow inner carbon shell not only serves as a physiochemical absorber for LiPSs, but also improves the electrical conductivity of the electrode; significantly suppressing shuttle effect. Cobalt nitride (Co₄N) nanoparticles, embedded in nitrogen‐doped carbon in the outer shell, catalyze the conversion of LiPSs, leading to decreased polarization and fast kinetics during cycling. Theoretical study of the Li intercalation energetics confirms the improved catalytic activity of the Co₄N compared to metallic Co catalyst. Altogether, the electrode shows large reversible capacity (1242 mAh g^?1 at 0.1 C), robust stability (capacity retention of 658 mAh g^?1 at 5 C after 400 cycles), and superior cycling stability at high sulfur loading (4.5 mg cm^?2).     

Facile Synthesis of Crumpled Nitrogen‐Doped MXene Nanosheets as a New Sulfur Host for Lithium–Sulfur Batteries

Crumpled nitrogen‐doped MXene nanosheets with strong physical and chemical coadsorption of polysulfides are synthesized by a novel one‐step approach and then utilized as a new sulfur host for lithium–sulfur batteries. The nitrogen‐doping strategy enables introduction of heteroatoms into MXene nanosheets and simultaneously induces a well‐defined porous structure, high surface area, and large pore volume. The as‐prepared nitrogen‐doped MXene nanosheets have a strong capability of physical and chemical dual‐adsorption for polysulfides and achieve a high areal sulfur loading of 5.1 mg cm^–2. Lithium–sulfur batteries, based on crumpled nitrogen‐doped MXene nanosheets/sulfur composites, demonstrate outstanding electrochemical performances, including a high reversible capacity (1144 mA h g^–1 at 0.2C rate) and an extended cycling stability (610 mA h g^–1 at 2C after 1000 cycles).     

Dual‐Confined Flexible Sulfur Cathodes Encapsulated in Nitrogen‐Doped Double‐Shelled Hollow Carbon Spheres and Wrapped with Graphene for Li–S Batteries

Batteries with high energy and power densities along with long cycle life and acceptable safety at an affordable cost are critical for large‐scale applications such as electric vehicles and smart grids, but is challenging. Lithium–sulfur (Li‐S) batteries are attractive in this regard due to their high energy density and the abundance of sulfur, but several hurdles such as poor cycle life and inferior sulfur utilization need to be overcome for them to be commercially viable. Li–S cells with high capacity and long cycle life with a dual‐confined flexible cathode configuration by encapsulating sulfur in nitrogen‐doped double‐shelled hollow carbon spheres followed by graphene wrapping are presented here. Sulfur/polysulfides are effectively immobilized in the cathode through physical confinement by the hollow spheres with porous shells and graphene wrapping as well as chemical binding between heteronitrogen atoms and polysulfides. This rationally designed free‐standing nanostructured sulfur cathode provides a well‐built 3D carbon conductive network without requiring binders, enabling a high initial discharge capacity of 1360 mA h g^?1 at a current rate of C/5, excellent rate capability of 600 mA h g^?1 at 2 C rate, and sustainable cycling stability for 200 cycles with nearly 100% Coulombic efficiency, suggesting its great promise for advanced Li–S batteries.     

Capture and Catalytic Conversion of Polysulfides by In Situ Built TiO2‐MXene Heterostructures for Lithium–Sulfur Batteries

The detrimental shuttle effect in lithium–sulfur batteries mainly results from the mobility of soluble polysulfide intermediates and their sluggish conversion kinetics. Herein, presented is a multifunctional catalyst with the merits of strong polysulfides adsorption ability, superior polysulfides conversion activity, high specific surface area, and electron conductivity by in situ crafting of the TiO₂‐MXene (Ti₃C₂T_x) heterostructures. The uniformly distributed TiO₂ on MXene sheets act as capturing centers to immobilize polysulfides, the hetero‐interface ensures rapid diffusion of anchored polysulfides from TiO₂ to MXene, and the oxygen‐terminated MXene surface is endowed with high catalytic activity toward polysulfide conversion. The improved lithium–sulfur batteries deliver 800 mAh g^?1 at 2 C and an ultralow capacity decay of 0.028% per cycle over 1000 cycles at 2 C. Even with a high sulfur loading of 5.1 mg cm^?2, the capacity retention of 93% after 200 cycles is still maintained. This work sheds new insights into the design of high‐performance catalysts with manipulated chemical components and tailored surface chemistry to regulate polysulfides in Li–S batteries.     

Solvent‐Mediated Li2S Electrodeposition: A Critical Manipulator in Lithium–Sulfur Batteries

Controlling electrochemical deposition of lithium sulfide (Li₂S) is a major challenge in lithium–sulfur batteries as premature Li₂S passivation leads to low sulfur utilization and low rate capability. In this work, the solvent's roles in controlling solid Li₂S deposition are revealed, and quantitative solvent‐mediated Li₂S growth models as guides to solvent selection are developed. It is shown that Li₂S electrodeposition is controlled by electrode kinetics, Li₂S solubility, and the diffusion of polysulfide/Li₂S, which is dictated by solvent's donicity, polarity, and viscosity, respectively. These solvent‐controlled properties are essential factors pertaining to the sulfur utilization, energy efficiency and reversibility of lithium–sulfur batteries. It is further demonstrated that the solvent selection criteria developed in this study are effective in guiding the search for new and more effective electrolytes, providing effective screening and design criteria for computational and experimental electrolyte development for lithium–sulfur batteries.     

Sulfiphilic Few‐Layered MoSe2 Nanoflakes Decorated rGO as a Highly Efficient Sulfur Host for Lithium‐Sulfur Batteries

Lithium‐sulfur batteries (LSBs) have been regarded as a competitive candidate for next‐generation electrochemical energy‐storage technologies due to their merits in energy density. The sluggish redox kinetics of the electrochemistry and the high solubility of polysulfides during cycling result in insufficient sulfur utilization, severe polarization, and poor cyclic stability. Herein, sulfiphilic few‐layered MoSe₂ nanoflakes decorated rGO (MoSe₂@rGO) hybrid has been synthesized through a facile hydrothermal method and for the first time, is used as a conceptually new‐style sulfur host for LSBs. Specifically, MoSe₂@rGO not only strongly interacts with polysulfides but also dynamically strengthens polysulfide redox reactions. The polarization problem is effectively alleviated by relying on the sulfiphilic MoSe₂. Moreover, MoSe₂@rGO is demonstrated to be beneficial for the fast nucleation and uniform deposition of Li₂S, contributing to the high discharge capacity and good cyclic stability. A high initial capacity of 1608 mAh g^?1 at 0.1 C, a slow decay rate of 0.042% per loop at 0.25 C, and a high reversible capacity of 870 mAh g^?1 with areal sulfur loading of 4.2 mg cm^?2 at 0.3 C are obtained. The concept of introducing sulfiphilic transition‐metal selenides into the LSBs system can stimulate engineering of novel architectures with enhanced properties for various energy‐storage devices.     

Porphyrin‐Derived Graphene‐Based Nanosheets Enabling Strong Polysulfide Chemisorption and Rapid Kinetics in Lithium–Sulfur Batteries

Lithium–sulfur (Li–S) batteries hold great promise as a next‐generation battery system because of their extremely high theoretical energy density and low cost. However, ready lithium polysulfide (LiPS) diffusion and sluggish redox kinetics hamper their cyclability and rate capability. Herein, porphyrin‐derived graphene‐based nanosheets (PNG) are proposed for Li–S batteries, which are achieved by pyrolyzing a conformal and thin layer of 2D porphyrin organic framework on graphene to form carbon nanosheets with a spatially engineered nitrogen‐dopant‐enriched skin and a highly conductive skeleton. The atomic skin is decorated with fully exposed lithiophilic sites to afford strong chemisorption to LiPSs and improve electrolyte wettability, while graphene substrate provides speedy electron transport to facilitate redox kinetics of sulfur species. The use of PNG as a lightweight interlayer enables efficient operation of Li–S batteries in terms of superb cycle stability (cyclic decay rate of 0.099% during 300 cycles at 0.5 C), good rate capability (988 mAh g^?1 at 2.0 C), and impressive sulfur loading (areal capacity of 8.81 mAh cm^?2 at a sulfur loading of 8.9 mg cm^?2). The distinct interfacial strategy is expected to apply to other conversion reaction batteries relying on dissolution–precipitation mechanisms and requiring interfacial charge‐ and mass‐transport‐mediation concurrently.     

In Situ Assembly of 2D Conductive Vanadium Disulfide with Graphene as a High‐Sulfur‐Loading Host for Lithium–Sulfur Batteries

Lithium–sulfur (Li–S) batteries are deemed to be one of the most promising energy storage technologies because of their high energy density, low cost, and environmental benignancy. However, existing drawbacks including the shuttling of intermediate polysulfides, the insulating nature of sulfur, and the considerable volume change of sulfur cathode would otherwise result in the capacity fading and unstable cycling. To overcome these challenges, herein an in situ assembly route is presented to fabricate VS₂/reduced graphene oxide nanosheets (G–VS₂) as a sulfur host. Benefiting from the 2D conductive and polar VS₂ interlayered within a graphene framework, the obtained G–VS₂ hybrids can effectively suppress the polysulfide shuttling, facilitate the charge transport, and cushion the volume expansion throughout the synergistic effect of structural confinement and chemical anchoring. With these advantageous features, the obtained sulfur cathode (G–VS₂/S) can deliver an outstanding rate capability (≈950 and 800 mAh g^?1 at 1 and 2 C, respectively) and an impressive cycling stability at high rates (retaining ≈532 mAh g^?1 after 300 cycles at 5 C). More significantly, it enables superior cycling performance of high‐sulfur‐loading cathodes (achieving an areal capacity of 5.1 mAh cm^?2 at 0.2 C with a sulfur loading of 5 mg cm^?2) even at high current densities.     

Highly Solvating Electrolytes for Lithium–Sulfur Batteries

There is a critical need to evaluate lithium–sulfur (Li–S) batteries with practically relevant high sulfur loadings and minimal electrolyte. Under such conditions, the concentration of soluble polysulfide intermediates in the electrolyte drastically increases, which can alter the fundamental nature of the solution‐mediated discharge and thereby the total sulfur utilization. In this work, an investigation into various high donor number (DN) electrolytes that allow for increased polysulfide dissolution is presented, and the way in which this property may in fact be necessary for increasing sulfur utilization at low electrolyte and high loading conditions is demonstrated. The solvents dimethylacetamide, dimethyl sulfoxide, and 1‐methylimidazole are holistically evaluated against dimethoxyethane as electrolyte co‐solvents in Li–S cells, and they are used to investigate chemical and electrochemical properties of polysulfide species at both dilute and practically relevant conditions. The nature of speciation exhibited by lithium polysulfides is found to vary significantly between these concentrations, particularly with regard to the S₃^?? species. Furthermore, the extent of the instability in conventional electrolyte solvents and high DN solvents with both lithium metal and polysulfides is thoroughly investigated. These studies establish a basis for future efforts into rationally designing an optimal electrolyte for a lean electrolyte, high energy density Li–S battery.     

Fabrication of N‐doped Graphene–Carbon Nanotube Hybrids from Prussian Blue for Lithium–Sulfur Batteries

Hybrid nanostructures containing 1D carbon nanotubes and 2D graphene sheets have many promising applications due to their unique physical and chemical properties. In this study, the authors find Prussian blue (dehydrated sodium ferrocyanide) can be converted to N‐doped graphene–carbon nanotube hybrid materials through a simple one‐step pyrolysis process. Through field emission scanning electron microscopy, transmission electron microscopy, X‐ray diffraction, Raman spectra, atomic force microscopy, and isothermal analyses, the authors identify that 2D graphene and 1D carbon nanotubes are bonded seamlessly during the growth stage. When used as the sulfur scaffold for lithium–sulfur batteries, it demonstrates outstanding electrochemical performance, including a high reversible capacity (1221 mA h g^?1 at 0.2 C rate), excellent rate capability (458 and 220 mA h g^?1 at 5 and 10 C rates, respectively), and excellent cycling stability (321 and 164 mA h g^?1 at 5 and 10 C (1 C = 1673 mA g^?1) after 1000 cycles). The enhancement of electrochemical performance can be attributed to the 3D architecture of the hybrid material, in which, additionally, the nitrogen doping generates defects and active sites for improved interfacial adsorption. Furthermore, the nitrogen doping enables the effective trapping of lithium polysulfides on electroactive sites within the cathode, leading to a much‐improved cycling performance. Therefore, the hybrid material functions as a redox shuttle to catenate and bind polysulfides, and convert them to insoluble lithium sulfide during reduction. The strategy reported in this paper could open a new avenue for low cost synthesis of N‐doped graphene–carbon nanotube hybrid materials for high performance lithium–sulfur batteries.     

Long‐Life,High‐Rate Lithium–Sulfur Cells with a Carbon‐Free VN Host as an Efficient Polysulfide Adsorbent and Lithium Dendrite Inhibitor

Lithium–sulfur (Li‐S) batteries are a promising next‐generation energy‐storage system, but the polysulfide shuttle and dendritic Li growth seriously hinder their commercial viability. Most of the previous studies have focused on only one of these two issues at a time. To address both the issues simultaneously, presented here is a highly conductive, noncarbon, 3D vanadium nitride (VN) nanowire array as an efficient host for both sulfur cathodes and lithium‐metal anodes. With fast electron and ion transport and high porosity and surface area, VN traps the soluble polysulfides, promotes the redox kinetics of sulfur cathodes, facilitates uniform nucleation/growth of lithium metal, and inhibits lithium dendrite growth at an unprecedented high current density of 10 mA cm^?2 over 200 h of repeated plating/stripping. As a result, VN‐Li||VN‐S full cells constructed with VN as both an anode and cathode host with a negative to positive electrode capacity ratio of only ≈2 deliver remarkable electrochemical performance with a high Coulombic efficiency of ≈99.6% over 850 cycles at a high 4 C rate and a high areal capacity of 4.6 mA h cm^?2. The strategy presented here offers a viable approach to realize high‐energy‐density, safe Li‐metal‐based batteries.     

A Li2S‐TiS2‐Electrolyte Composite for Stable Li2S‐Based Lithium–Sulfur Batteries

Li₂S is a fully lithiated sulfur‐based cathode with a high theoretical capacity of 1166 mAh g^?1 that can be coupled with lithium‐free anodes to develop high‐energy‐density lithium–sulfur batteries. Although various approaches have been pursued to obtain a high‐performance Li₂S cathode, there are still formidable challenges with it (e.g., low conductivity, high overpotential, and irreversible polysulfide diffusion) and associated fabrication processes (e.g., insufficient Li₂S, excess electrolyte, and low reversible capacity), which have prevented the realization of high electrochemical utilization and stability. Here, a new cathode design composed of a homogeneous Li₂S‐TiS₂‐electrolyte composite that is prepared by a simple two‐step dry/wet‐mixing process is demonstrated, allowing the liquid electrolyte to wet the powder mixture consisting of insulating Li₂S and conductive TiS₂. The close‐contact, three‐phase boundary of this system improves the Li₂S‐activation efficiency and provides fast redox‐reaction kinetics, enabling the Li₂S‐TiS₂‐electrolyte cathode to attain stable cyclability at C/7 to C/3 rates, superior long‐term cyclability over 500 cycles, and promising high‐rate performance up to 1C rate. More importantly, this improved performance results from a cell design attaining a high Li₂S loading of 6 mg cm^?2, a high Li₂S content of 75 wt%, and a low electrolyte/Li₂S ratio of 6.     

Designing Safe Electrolyte Systems for a High‐Stability Lithium–Sulfur Battery

Safety, nontoxicity, and durability directly determine the applicability of the essential characteristics of the lithium (Li)‐ion battery. Particularly, for the lithium–sulfur battery, due to the low ignition temperature of sulfur, metal lithium as the anode material, and the use of flammable organic electrolytes, addressing security problems is of increased difficulty. In the past few years, two basic electrolyte systems are studied extensively to solve the notorious safety issues. One system is the conventional organic liquid electrolyte, and the other is the inorganic solid‐state or quasi‐solid‐state composite electrolyte. Here, the recent development of engineered liquid electrolytes and design considerations for solid electrolytes in tackling these safety issues are reviewed to ensure the safety of electrolyte systems between sulfur cathode materials and the lithium‐metal anode. Specifically, strategies for designing and modifying liquid electrolytes including introducing gas evolution, flame, aqueous, and dendrite‐free electrolytes are proposed. Moreover, the considerations involving a high‐performance Li⁺ conductor, air‐stable Li⁺ conductors, and stable interface performance between the sulfur cathode and the lithium anode for developing all‐solid‐state electrolytes are discussed. In the end, an outlook for future directions to offer reliable electrolyte systems is presented for the development of commercially viable lithium–sulfur batteries.     

Freestanding and Sandwich‐Structured Electrode Material with High Areal Mass Loading for Long‐Life Lithium–Sulfur Batteries

Freestanding cathode materials with sandwich‐structured characteristic are synthesized for high‐performance lithium–sulfur battery. Sulfur is impregnated in nitrogen‐doped graphene and constructed as primary active material, which is further welded in the carbon nanotube/nanofibrillated cellulose (CNT/NFC) framework. Interconnected CNT/NFC layers on both sides of active layer are uniquely synthesized to entrap polysulfide species and supply efficient electron transport. The 3D composite network creates a hierarchical architecture with outstanding electrical and mechanical properties. Synergistic effects generated from physical and chemical interaction could effectively alleviate the dissolution and shuttling of the polysulfide ions. Theoretical calculations reveal the hydroxyl functionization exhibits a strong chemical binding with the discharge product (i.e., Li₂S). Electrochemical measurements suggest that the rationally designed structure endows the electrode with high specific capacity and excellent rate performance. Specifically, the electrode with high areal sulfur loading of 8.1 mg cm^?2 exhibits an areal capacity of ≈8 mA h cm^?2 and an ultralow capacity fading of 0.067% per cycle over 1000 discharge/charge cycles at C/2 rate, while the average coulombic efficiency is around 97.3%, indicating good electrochemical reversibility. This novel and low‐cost fabrication procedure is readily scalable and provides a promising avenue for potential industrial applications.