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.     

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.     

Let It Catch: A Short‐Branched Protein for Efficiently Capturing Polysulfides in Lithium–Sulfur Batteries

Uncovering the key contributions of molecular details to capture polysulfides is important for applying suitable materials that can effectively restrain the shuttle effect in advanced lithium–sulfur batteries. This is particularly true for natural biomolecules with substantial structural and compositional diversities strongly impacting their functions. Here, natural gelatin and zein proteins are first denatured and then adopted for fabrication of nanocomposite interlayers via functionalization of carbon nanofibers. From the results of experiment and molecular dynamic simulations, it is found that the lengths of the sidechains on the two proteins play critical roles. The short‐branched gelatin shows significantly stronger adsorption of polysulfides, as compared with zein comprising many long‐chain residues. The gelatin‐based interlayer, along with its good porous structures/electrical conductivity, greatly suppresses the shuttle effect and yields exceptional electrochemical performance. Furthermore, the implementation of proteins as functional binder additives further supports the finding that gelatin enables stronger polysulfide‐trapping. As a result, high‐loading sulfur cathodes (9.4 mg cm^?2) are realized, which deliver a high average areal capacity of 8.2 mAh cm^?2 over 100 cycles at 0.1 A g^?1. This work demonstrates the importance of sidechain length in capturing polysulfides and provides a new insight in selecting and design of desired polysulfide‐binding molecules.     

Carbon Quantum Dots–Modified Interfacial Interactions and Ion Conductivity for Enhanced High Current Density Performance in Lithium–Sulfur Batteries

Significant progress has achieved for developing lithium–sulfur (Li–S) batteries with high specific capacities and excellent cyclic stability. However, some critical issues emerge when attempts are made to raise the areal sulfur loading and increase the operation current density to meet the standards for various industrial applications. In this work, polyethylenimine‐functionalized carbon dots (PEI‐CDots) are designed and prepared for enhancing performance of the Li–S batteries with high sulfur loadings and operation under high current density situations. Strong chemical binding effects towards polysulfides and fast ion transport property are achieved in the PEI‐CDots‐modified cathodes. At a high current density of 8 mA cm^?2, the PEI‐CDots‐modified Li–S battery delivers a reversible areal capacity of 3.3 mAh cm^?2 with only 0.07% capacity decay per cycle over 400 cycles at 6.6 mg sulfur loading. Detailed analysis, involving electrochemical impedance spectroscopy, cyclic voltammetry, and density functional theory calculations, is done for the elucidation of the underlying enhancement mechanism by the PEI‐CDots. The strongly localized sulfur species and the promoted Li⁺ ion conductivity at the cathode–electrolyte interface are revealed to enable high‐performance Li–S batteries with high sulfur loading and large operational current.     

Popcorn Inspired Porous Macrocellular Carbon: Rapid Puffing Fabrication from Rice and Its Applications in Lithium–Sulfur Batteries

The advancement of electrochemical energy storage is closely bound up with the breakthrough of controllable fabrication of energy materials. Inspired by a popcorn fabrication from corn raw, herein a unique porous macrocellular carbon composed of cross‐linked nano/microsheets by a powerful puffing of rice precursor is described. The rice is directly puffed with a volume enlargement of ≈20 times when it is instantaneously released from a sealed environment with a high pressure of 1.0 MPa at 200 °C. Interestingly, when metal (e.g., Ni) nanoparticles are embedded in the puffed rice derived carbon (PRC), high‐quality PRC/metal composites are achieved with attractive properties of a high electrical conductivity of ≈7.2 × 10⁴ S m^?1, a large porosity of 85.1%, and a surface area of 1492.2 m² g^?1. The PRC/Ni are employed as a host in lithium–sulfur batteries. The designed PRC/Ni/S electrode exhibits a high reversible capacity of 1257.2 mA h g^?1 at 0.2 C, a prolonged cycle life (821 mA h g^?1 after 500 cycles), and enhanced rate capability, much better than other counterparts (PRC/S and rGO/S). The excellent properties are attributed to the advantages of PRC/Ni network with a high electrical conductivity, strong adsorption/blocking ability for polysulfides, and interconnected porous framework.     

An Effective Lithium Sulfide Encapsulation Strategy for Stable Lithium–Sulfur Batteries

With a high theoretical capacity of 1162 mA h g^?1, Li₂S is a promising cathode that can couple with silicon, tin, or graphite anodes for next‐generation energy storage devices. Unfortunately, Li₂S is highly insulating, exhibits large charge overpotential, and suffers from active‐material loss as soluble polysulfides during battery cycling. To date, low‐cost, scalable synthesis of an electrochemically active Li₂S cathode remains a challenge. This work demonstrates that the low conductivity and material loss issues associated with Li₂S cathodes can be overcome by forming a stable, conductive encapsulation layer at the surface of the Li₂S bulk particles through in situ surface reactions between Li₂S and electrolyte additives containing transition‐metal salts. It is identified that the electronic band structure in the valence band region of the thus‐generated encapsulation layers, consisting largely of transition‐metal sulfides, determines the initial charging resistance of Li₂S. Furthermore, among the transition metals tested, the encapsulation layer formed with an addition of 10 wt% manganese (II) acetylacetonate salt proved to be robust within the cycling window, which is attributed to the chemically generated MnS surface species. This work provides an effective strategy to use micrometer‐sized Li₂S directly as a cathode material and opens up new prospects to tune the surface properties of electrode materials for energy‐storage applications.     

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.     

A Polyaniline‐Coated Sulfur/Carbon Composite with an Enhanced High‐Rate Capability as a Cathode Material for Lithium/Sulfur Batteries

Polyaniline‐coated sulfur/conductive‐carbon‐black (PANI@S/C) composites with different contents of sulfur are prepared via two facile processes including ball‐milling and thermal treatment of the conductive carbon black and sublimed sulfur, followed by an in situ chemical oxidative polymerization of the aniline monomer in the presence of the S/C composite and ammonium persulfate. The microstructure and electrochemical performance of the as‐prepared composites are investigated systematically. It is demonstrated that the polyaniline, with a thickness of ≈5–10 nm, is coated uniformly onto the surface of the S/C composite forming a core/shell structure. The PANI@S/C composite with 43.7 wt% sulfur presents the optimum electrochemical performance, including a large reversible capacity, a good coulombic efficiency, and a high active‐sulfur utilization. The formation of the unique core/shell structure in the PANI@S/C composites is responsible for the improvement of the electrochemical performance. In particular, the high‐rate charge/discharge capability of the PANI@S/C composites is excellent due to a synergistic effect on the high electrical conductivity from both the conductive carbon black in the matrix and the PANI on the surface. Even at an ultrahigh rate (10C), a maximum discharge capacity of 635.5 mA h per g of sulfur is still retained for the PANI@S/C composite after activation, and the discharge capacity retention is over 60% after 200 cycles.     

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.     

Nitrogen‐Doped Graphene‐Supported Mixed Transition‐Metal Oxide Porous Particles to Confine Polysulfides for Lithium–Sulfur Batteries

The intricate charge–discharge reactions and bad conductivity nature of sulfur determine the extreme importance of cathode engineering for Li–S batteries. Herein, spinel ZnCo₂O₄ porous particles@N‐doped reduced graphene oxide (ZnCo₂O₄@N‐RGO) are prepared via the combined procedures of refluxing and hydrothermal treatment, consisting of interconnected uniform ZnCo₂O₄ nanocubes with an average size of 5 nm anchored on graphene nanosheets. The as‐obtained composite can act as an inimitable cathode scaffold to suppress the shuttling of polysulfides by chemical confinement of ZnCo₂O₄ and N‐RGO for the first time, as demonstrated by the adsorption energy of ZnCo₂O₄ to Li₂S₄ via the strong chemical bonding between Zn or Co and S. The RGO nanosheets with a relatively high specific surface area provide a good conductive network and structural stability. The introduction of doped N atoms and numerous ZnCo₂O₄ porous nanoparticles can inhibit the transfer of lithium polysulfides between the cathode and anode. Due to the unique structural and compositional features, the as‐obtained hybrid materials with the high sulfur loading of 71% and even 82% still deliver high specific capacity, good rate capability, and enhanced cycling stability with exceptionally high initial Coulombic efficiency, which displays a high utilization of sulfur.     

Design Principles for Optimum Performance of Porous Carbons in Lithium–Sulfur Batteries

A series of experiments is presented that establishes for the first time the role of some of the key design parameters of porous carbons including surface area, pore volume, and pore size on battery performance. A series of hierarchical porous carbons is used as a model system with an open, 3D, interconnected porous framework and highly controlled porosity. Specifically, carbons with surface areas ranging from ≈500–2800 m² g^?1, pore volume from ≈0.6–5 cm³ g^?1, and pore size from micropores (≈1 nm) to large mesopores (≈30 nm) are synthesized and tested. At high sulfur loadings (≈80 wt% S), pore volume is more important than surface area with respect to sulfur utilization. Mesopore size, in the range tested, does not affect the sulfur utilization. No relationship between porosity and long‐term cycle life is observed. All systems fail after 200–300 cycles, which is likely due to the consumption of the LiNO₃ additive over cycling. Moreover, cryo‐scanning transmission electron microscopy imaging of these carbon–sulfur composites combined with X‐ray diffraction (XRD) provides further insights into the effect of initial sulfur distribution on sulfur utilization while also revealing the inadequacy of the indirect characterization techniques alone in reliably predicting distribution of sulfur within porous carbon matrices.     

Simultaneous Cobalt and Phosphorous Doping of MoS2 for Improved Catalytic Performance on Polysulfide Conversion in Lithium–Sulfur Batteries

The lithium–sulfur batteries are susceptible to the loss of sulfur as dissolved polysulfides in the electrolyte and their ensuing redox shutting effect. The acceleration of the conversion kinetics of dissolved polysulfides into the insoluble sulfur and lithium sulfide via electrocatalysis has the appeal of being a root‐cause solution. MoS₂ is the most common electrocatalyst used for this purpose. It is demonstrated that how the effectiveness can be improved by simultaneous cobalt and phosphorus doping of MoS₂ nanotubes (P‐Mo_0.9Co_0.1S₂‐2, containing 1.81 at% of P). Cobalt doping induces the transformation of MoS₂ from 2H phase to metallic 1T phase, which improves the electrical conductivity of the MoS₂. The Co–P coordinated sites on the catalyst surface are highly active for the polysulfide conversion reactions. Consequently, a sulfur cathode with P‐Mo_0.9Co_0.1S₂‐2 can decrease the capacity fade rate from 0.28% per cycle before modification (over 150 cycles at 0.5C rate) to 0.046% per cycle after modification (over 600 cycles at 1C rate). P‐Mo_0.9Co_0.1S₂‐2 also enhances the high rate performance from a capacity of 338 to 931 mAh g^?1 at 6C rate. The results of this study provide the first direct evidence of the beneficial effects of heteroatom codoping of polysulfide conversion catalysts.     

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.     

Fluorine‐Free Noble Salt Anion for High‐Performance All‐Solid‐State Lithium–Sulfur Batteries

Amongst post‐Li‐ion battery technologies, lithium–sulfur (Li–S) batteries have captured an immense interest as one of the most appealing devices from both the industrial and academia sectors. The replacement of conventional liquid electrolytes with solid polymer electrolytes (SPEs) enables not only a safer use of Li metal (Li°) anodes but also a flexible design in the shape of Li–S batteries. However, the practical implementation of SPEs‐based all‐solid‐state Li–S batteries (ASSLSBs) is largely hindered by the shuttling effect of the polysulfide intermediates and the formation of dendritic Li° during the battery operation. Herein, a fluorine‐free noble salt anion, tricyanomethanide [C(CN)₃^?, TCM^?], is proposed as a Li‐ion conducting salt for ASSLSBs. Compared to the widely used perfluorinated anions {e.g., bis(trifluoromethanesulfonyl)imide anion, [N(SO₂CF₃)₂)]^?, TFSI^?}, the LiTCM‐based electrolytes show decent ionic conductivity, good thermal stability, and sufficient anodic stability suiting the cell chemistry of ASSLSBs. In particular, the fluorine‐free solid electrolyte interphase layer originating from the decomposition of LiTCM exhibits a good mechanical integrity and Li‐ion conductivity, which allows the LiTCM‐based Li–S cells to be cycled with good rate capability and Coulombic efficiency. The LiTCM‐based electrolytes are believed to be the most promising candidates for building cost‐effective and high energy density ASSLSBs in the near future.     

Rational Design of a Dual‐Function Hybrid Cathode Substrate for Lithium–Sulfur Batteries

A unique 3D hybrid sponge with chemically coupled nickel disulfide‐reduced graphene oxide (NiS₂‐RGO) framework is rationally developed as an effective polysulfide reservoir through a biomolecule‐assisted self‐assembly synthesis. An optimized amount of NiS₂ (≈18 wt%) with porous nanoflower‐like morphology is uniformly in situ grown on the RGO substrate, providing abundant active sites to adsorb and localize polysulfides. The improved polysulfide adsorptivity from sulfiphilic NiS₂ is confirmed by experimental data and first‐principle calculations. Moreover, due to the chemical coupling between NiS₂ and RGO formed during the in situ synthesis, the conductive RGO substrate offers a 3D electron pathway to facilitate charge transfer toward the NiS₂‐polysulfide adsorption interface, triggering a fast redox kinetics of polysulfide conversion and excellent rate performance (C/20–4C). Therefore, the self‐assembled hybrid structure simultaneously promotes static polysulfide‐trapping capability and dynamic polysulfide‐conversion reversibility. As a result, the 3D porous sponge enables a high sulfur content (75 wt%) and a remarkably high sulfur loading (up to 21 mg cm^?2) and areal capacity (up to 16 mAh cm^?2), exceeding most of the reported values in the literature involving either RGO or metal sulfides/other metal compounds (sulfur content of <60 wt% and sulfur loading of <3 mg cm^?2).     

Electrochemically Stable Rechargeable Lithium–Sulfur Batteries with a Microporous Carbon Nanofiber Filter for Polysulfide

As a primary component in lithium–sulfur (Li–S) batteries, the separator may require a custom design in order to facilitate electrochemical stability and reversibility. Here, a custom separator with an activated carbon nanofiber (ACNF)‐filter coated onto a polypropylene membrane is presented. The entire configuration is comprised of the ACNF filter arranged adjacent to the sulfur cathode so that it can filter out the freely migrating polysulfides and suppress the severe polysulfide diffusion. Four differently optimized ACNF‐filter‐coated separators have been developed with tunable micropores as an investigation into the electrochemical and engineering design parameters of functionalized separators. The optimized parameters that are verified by electrochemical and microstructural analyses require the coated ACNF filter to possess the following: (i) a porous architecture with abundant micropores, (ii) small micropore sizes, and (iii) high electrical conductivity and effective electrolyte immersion. It is found that the ACNF20‐filter‐coated separator demonstrates an overall superior boost in the electrochemical utilization (discharge capacity: 1270 mA h g^?1) and polysulfide retention (capacity fade rate: 0.13% cycle^?1 after 200 cycles). These results show that the modified thin‐film‐coating technique is a viable approach to designing ultratough ACNF‐filter‐coated separators with outstanding mechanical strength and flexibility as an advanced component in Li–S cells.     

A Lithium–Sulfur Cell Based on Reversible Lithium Deposition from a Li2S Cathode Host onto a Hostless‐Anode Substrate

The development of lithium–sulfur batteries necessitates a thorough understanding of the lithium‐deposition process. A novel full‐cell configuration comprising an Li₂S cathode and a bare copper foil on the anode side is presented here. The absence of excess lithium allows for the realization of a truly lithium‐limited Li–S battery, which operates by reversible plating and stripping of lithium on the hostless‐anode substrate (copper foil). Its performance is closely tied to the efficiency of lithium deposition, generating valuable insights on the role and dynamic behavior of lithium anode. The Li₂S full cell shows reasonable capacity retention with a Coulombic efficiency of 96% over 100 cycles, which is a tremendous improvement over that of a similar lithium‐plating‐based full cell with LiFePO₄ cathodes. The exceptional robustness of the Li₂S system is attributed to an intrinsic stabilization of the lithium‐deposition process, which is mediated by polysulfide intermediates that form protective Li₂S and Li₂S₂ regions on the deposited lithium. Combined with the large improvements in energy density and safety by the elimination of a metallic lithium anode, the stability and electrochemical performance of the lithium‐plating‐based Li₂S full cell establish it as an important trajectory for Li–S battery research, focusing on practical realization of reversible lithium anodes.