Sandwiching Sulfur into the Dents Between N, O Co-Doped Graphene Layered Blocks with Strong Physicochemical Confinements for Stable and High-Rate Li-S Batteries

A Hybrid Structure to Improve Electrochemical Performance of SiO Anode Materials in Lithium-Ion Battery

The wide utilization of lithium-ion batteries (LIBs) prompts extensive research on the anode materials with large capacity and excellent stability. Despite the attractive electrochemical properties of pure Si anodes outperforming other Si-based materials, its unsafety caused by huge volumetric expansion is commonly admitted. Silicon monoxide (SiO) anode is advantageous in mild volume fluctuation, and would be a proper alternative if the low initial columbic efficiency and conductivity can be ameliorated. Herein, a hybrid structure composed of active material SiO particles and carbon nanofibers (SiO/CNFs) is proposed as a solution. CNFs, through electrospun processes, serve as a conductive skeleton for SiO nanoparticles and enable SiO nanoparticles to be uniformly embedded in. As a result, the SiO/CNF electrochemical performance reaches a peak at 20% the mass ratio of SiO, where the retention rate reaches 73.9% after 400 cycles at a current density of 100 mA g-1, and the discharge capacity after stabilization and 100 cycles are 1.47 and 1.84 times higher than that of pure SiO, respectively. A fast lithium-ion transport rate during cycling is also demonstrated as the corresponding diffusion coefficient of the SiO/CNF reaches ~8 × 10-15 cm2 s-1. This SiO/CNF hybrid structure provides a flexible and cost-effective solution for LIBs and sheds light on alternative anode choices for industrial battery assembly.

Heteroatom Codoped Graphene: The Importance of Nitrogen

Although graphene has exceptional properties, they are not enough to solve the extensive list of pressing world problems. The substitutional doping of graphene using heteroatoms is one of the preferred methods to adjust the physicochemical properties of graphene. Much effort has been made to dope graphene using a single dopant. However, in recent years, substantial efforts have been made to dope graphene using two or more dopants. This review summarizes all the hard work done to synthesize, characterize, and develop new technologies using codoped, tridoped, and quaternary doped graphene. First, I discuss a simple question that has a complicated answer: When can an atom be considered a dopant? Then, I briefly discuss the single atom doped graphene as a starting point for this review's primary objective: codoped or dual-doped graphene. I extend the discussion to include tridoped and quaternary doped graphene. I review most of the systems that have been synthesized or studied theoretically and the areas in which they have been used to develop new technologies. Finally, I discuss the challenges and prospects that will shape the future of this fascinating field. It will be shown that most of the graphene systems that have been reported involve the use of nitrogen, and much effort is needed to develop codoped graphene systems that do not rely on the stabilizing effects of nitrogen. I expect that this review will contribute to introducing more researchers to this fascinating field and enlarge the list of codoped graphene systems that have been synthesized.

Carbon-Nitride-Based Materials for Advanced Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries are promising candidates for next-generation energy storage systems owing to their high energy density and low cost. However, critical challenges including severe shuttling of lithium polysulfides (LiPSs) and sluggish redox kinetics limit the practical application of Li-S batteries. Carbon nitrides (C_xN_y), represented by graphitic carbon nitride (g-C₃N₄), provide new opportunities for overcoming these challenges. With a graphene-like structure and high pyridinic-N content, g-C₃N₄ can effectively immobilize LiPSs and enhance the redox kinetics of S species. In addition, its structure and properties including electronic conductivity and catalytic activity can be regulated by simple methods that facilitate its application in Li-S batteries. Here, the recent progress of applying C_xN_y-based materials including the optimized g-C₃N₄, g-C₃N₄-based composites, and other novel C_xN_y materials is systematically reviewed in Li-S batteries, with a focus on the structure-activity relationship. The limitations of existing C_xN_y-based materials are identified, and the perspectives on the rational design of advanced C_xN_y-based materials are provided for high-performance Li-S batteries.

Constructing Binder- and Carbon Additive-Free Organosulfur Cathodes Based on Conducting Thiol-Polymers through Electropolymerization for Lithium-Sulfur Batteries

Herein, the concept of constructing binder- and carbon additive-free organosulfur cathode was proved based on thiol-containing conducting polymer poly(4-(thiophene-3-yl) benzenethiol) (PTBT). The PTBT featured the polythiophene-structure main chain as a highly conducting framework and the benzenethiol side chain to copolymerize with sulfur and form a crosslinked organosulfur polymer (namely S/PTBT). Meanwhile, it could be in-situ deposited on the current collector by electro-polymerization, making it a binder-free and free-standing cathode for Li-S batteries. The S/PTBT cathode exhibited a reversible capacity of around 870 mAh g^-1 at 0.1 C and improved cycling performance compared to the physically mixed cathode (namely S&PTBT). This multifunction cathode eliminated the influence of the additives (carbon/binder), making it suitable to be applied as a model electrode for operando analysis. Operando X-ray imaging revealed the remarkable effect in the suppression of polysulfides shuttle via introducing covalent bonds, paving the way for the study of the intrinsic mechanisms in Li-S batteries.

Vanadium Nitride Quantum Dots/Holey Graphene Matrix Boosting Adsorption and Conversion Reaction Kinetics for High-Performance Lithium-Sulfur Batteries

Lithium-sulfur batteries (LSBs) have been considered as potential next-generation energy storage systems due to their high specific energy of 2600 Wh kg^-1 and 2800 Wh L^-1. Nevertheless, the practical application of LSBs still faces several hazards, including the shuttle effect of soluble lithium polysulfides, low electrical conductivities of solid sulfur and lithium sulfides, and large volume expansion during charge/discharge cycles. To address this critical challenge, we innovatively proposed facile synthesis of nanostructured VN quantum dots (VNQD)/holey graphene matrix for stabilizing the sulfur cathode by simultaneously promoting the trapping, anchoring, and catalyzing efficiencies of both LiPSs and Li₂S. Benefiting from abundant edge catalytic sites of VNQD, in-plane nanopores of graphene, and high electrical conductivity, the sulfur host not only provides high adsorption capability toward soluble polysulfides, strong binding ability for anchoring solid Li₂S, and their rapid conversion kinetics but also contributes abundant sulfur storage sites and efficient transport pathways for lithium ions (Li⁺) and electrons. Consequently, the sulfur cathode exhibits high initial capacities of 1320 mAh g^-1, high rate capability (850 mAh g^-1 @ 4 mA cm^-2), and high capacity retention of 99.95% per cycle after 500 cycles, providing a feasible solution for the practical utilization of shuttle-free Li-S batteries.

Graphene-Based Nanomaterials as the Cathode for Lithium-Sulfur Batteries

The global energy crisis and environmental problems are becoming increasingly serious. It is now urgent to vigorously develop an efficient energy storage system. Lithium-sulfur batteries (LSBs) are considered to be one of the most promising candidates for next-generation energy storage systems due to their high energy density. Sulfur is abundant on Earth, low-cost, and environmentally friendly, which is consistent with the characteristics of new clean energy. Although LSBs possess numerous advantages, they still suffer from numerous problems such as the dissolution and diffusion of sulfur intermediate products during the discharge process, the expansion of the electrode volume, and so on, which severely limit their further development. Graphene is a two-dimensional crystal material with a single atomic layer thickness and honeycomb bonding structure formed by sp2 hybridization of carbon atoms. Since its discovery in 2004, graphene has attracted worldwide attention due to its excellent physical and chemical properties. Herein, this review summarizes the latest developments in graphene frameworks, heteroatom-modified graphene, and graphene composite frameworks in sulfur cathodes. Moreover, the challenges and future development of graphene-based sulfur cathodes are also discussed.

A Natural Polymer Captor for Immobilizing Polysulfide/Polyselenide in Working Li-SeS2 Batteries

SeS₂ has become a promising cathode material owing to its enhanced electrical conductivity over sulfur and higher theoretical specific capacity than selenium; however, the working Li-SeS₂ batteries have to face the practical challenges from the severe shuttling of soluble dual intermediates of polysulfide and polyselenide, especially in high-SeS₂-loading cathodes. Herein, a natural organic polymer, Nicandra physaloides pectin (NPP), is proposed to serve as an effective polysulfide/polyselenide captor to address the shuttling issues. Informed by theoretical calculations, NPP is competent to provide a Lewis base-based strong binding interaction with polysulfides/polyselenides via forming lithium bonds, and it can be homogeneously deposited onto a three-dimensional double-carbon conductive scaffold to finally constitute a polysulfide/polyselenide-immobilizing interlayer. Operando spectroscopy analysis validates the enhanced polysulfide/polyselenide trapping and high conversion efficiency on the constructed interlayer, hence bestowing the Li-SeS₂ cells with ultrahigh rate capability (448 mAh g^-1 at 10 A g^-1), durable cycling lifespan (≈ 0.037% capacity attenuation rate per cycle), and high areal capacity (> 6.5 mAh cm^-2) at high SeS₂ loading of 15.4 mg cm^-2. Importantly, pouch cells assembled with this interlayer exhibit excellent flexibility, decent rate capability with relatively low electrolyte-to-capacity ratio, and stable cycling life even under a low electrolyte condition, promising a low-cost, viable design protocol toward practical Li-SeS₂ batteries.

Efficient Catalytic Conversion of Polysulfides by Biomimetic Design of "Branch-Leaf" Electrode for High-Energy Sodium-Sulfur Batteries

Rechargeable room temperature sodium-sulfur (RT Na-S) batteries are seriously limited by low sulfur utilization and sluggish electrochemical reaction activity of polysulfide intermediates. Herein, a 3D "branch-leaf" biomimetic design proposed for high performance Na-S batteries, where the leaves constructed from Co nanoparticles on carbon nanofibers (CNF) are fully to expose the active sites of Co. The CNF network acts as conductive "branches" to ensure adequate electron and electrolyte supply for the Co leaves. As an effective electrocatalytic battery system, the 3D "branch-leaf" conductive network with abundant active sites and voids can effectively trap polysulfides and provide plentiful electron/ions pathways for electrochemical reaction. DFT calculation reveals that the Co nanoparticles can induce the formation of a unique Co-S-Na molecular layer on the Co surface, which can enable a fast reduction reaction of the polysulfides. Therefore, the prepared "branch-leaf" CNF-L@Co/S electrode exhibits a high initial specific capacity of 1201 mAh g^-1 at 0.1 C and superior rate performance.