A Flexible Nanostructured Paper of a Reduced Graphene Oxide-Sulfur Composite for High-Performance Lithium-Sulfur Batteries with Unconventional Configurations

Strong Capillarity, Chemisorption, and Electrocatalytic Capability of Crisscrossed Nanostraws Enabled Flexible, High-Rate, and Long-Cycling Lithium-Sulfur Batteries

The development of flexible lithium-sulfur (Li-S) batteries with high energy density and long cycling life are very appealing for the emerging flexible, portable, and wearable electronics. However, the progress on flexible Li-S batteries was limited by the poor flexibility and serious performance decay of existing sulfur composite cathodes. Herein, we report a freestanding and highly flexible sulfur host that can simultaneously meet the flexibility, stability, and capacity requirements of flexible Li-S batteries. The host consists of a crisscrossed network of carbon nanotubes reinforced CoS nanostraws (CNTs/CoS-NSs). The CNTs/CoS-NSs with large inner space and high conductivity enable high loading and efficient utilization of sulfur. The strong capillarity effect and chemisorption of CNTs/CoS-NSs to sulfur species were verified, which can efficiently suppress the shuttle effect and promote the redox kinetics of polysulfides. The sulfur-encapsulated CNTs/CoS-NSs (S@CNTs/CoS-NSs) cathode in Li-S batteries exhibits superior performance, including high discharge capacity, rate capability (1045 mAh g^-1 at 0.5 C and 573 mAh g^-1 at 5.0 C), and cycling stability. Intriguingly, the soft-packed Li-S batteries based on S@CNTs/CoS-NSs cathode show good flexibility and stability upon bending.

A densely packed Sb2O3 nanosheet-graphene aerogel toward advanced sodium-ion batteries

As a promising anodic material for rechargeable batteries, Sb2O3 has drawn increasing attention due to its high theoretical capacity and abundant natural deposits. However, poor cyclability and rate performance of Sb2O3 derived from a large volume change during insertion/desertion reactions as well as a sluggish kinetic process restrict its practical application. Herein, we report a facile amorphous-to-crystalline strategy to synthesize a densely packed Sb2O3 nanosheet-graphene aerogel as a novel anode for sodium ion batteries (SIBs). This Sb2O3/graphene composite displays a reversible capacity as high as 657.9 mA h g-1 even after 100 cycles at 0.1 A g-1, along with an excellent rate capacity of 356.8 mA h g-1 at 5.0 A g-1. The superior electrochemical performance is attributed to the synergistic effects of densely packed Sb2O3 nanosheets and graphene aerogel, which serves as both a robust support and stable buffer layer to maintain the structural stability of the nanocomposite, and enhances the electrode kinetics of electrolyte diffusion and electron transfer simultaneously. Hence, this densely-packed two-dimensional Sb2O3 nanosheet-graphene aerogel can be a promising anode material for rechargeable SIBs due to its facile synthesis process and outstanding electrochemical performance.

Aqueous rechargeable zinc/sodium vanadate batteries with enhanced performance from simultaneous insertion of dual carriers

Rechargeable aqueous zinc-ion batteries are promising energy storage devices due to their high safety and low cost. However, they remain in their infancy because of the limited choice of positive electrodes with high capacity and satisfactory cycling performance. Furthermore, their energy storage mechanisms are not well established yet. Here we report a highly reversible zinc/sodium vanadate system, where sodium vanadate hydrate nanobelts serve as positive electrode and zinc sulfate aqueous solution with sodium sulfate additive is used as electrolyte. Different from conventional energy release/storage in zinc-ion batteries with only zinc-ion insertion/extraction, zinc/sodium vanadate hydrate batteries possess a simultaneous proton, and zinc-ion insertion/extraction process that is mainly responsible for their excellent performance, such as a high reversible capacity of 380 mAh g-1 and capacity retention of 82% over 1000 cycles. Moreover, the quasi-solid-state zinc/sodium vanadate hydrate battery is also a good candidate for flexible energy storage device.

All-MXene-Based Integrated Electrode Constructed by Ti3C2 Nanoribbon Framework Host and Nanosheet Interlayer for High-Energy-Density Li-S Batteries

High-energy-density lithium-sulfur (Li-S) batteries hold promise for next-generation portable electronic devices, but are facing great challenges in rational construction of high-performance flexible electrodes and innovative cell configurations for actual applications. Here we demonstrated an all-MXene-based flexible and integrated sulfur cathode, enabled by three-dimensional alkalized Ti₃C₂ MXene nanoribbon (a-Ti₃C₂ MNR) frameworks as a S/polysulfides host (a-Ti₃C₂-S) and two-dimensional delaminated Ti₃C₂ MXene (d-Ti₃C₂) nanosheets as interlayer on a polypropylene (PP) separator, for high-energy and long-cycle Li-S batteries. Notably, an a-Ti₃C₂ MNR framework with open interconnected macropores and an exposed surface area guarantees high S loading and fast ionic diffusion for prompt lithiation/delithiation kinetics, and the 2D d-Ti₃C₂ MXene interlayer remarkably prevents the shuttle effect of lithium polysulfides via both chemical absorption and physical blocking. As a result, the integrated a-Ti₃C₂-S/d-Ti₃C₂/PP electrode was directly used for Li-S batteries, without the requirement of a metal current collector, and exhibited a high reversible capacity of 1062 mAh g^-1 at 0.2 C and enhanced capacity of 632 mAh g^-1 after 50 cycles at 0.5 C, outperforming the a-Ti₃C₂-S/PP electrode (547 mAh g^-1) and conventional a-Ti₃C₂-S on an Al current collector (a-Ti₃C₂-S/Al) (597 mAh g^-1). Furthermore, the all-MXene-based integrated cathode displayed outstanding rate capacity of 288 mAh g^-1 at 10 C and long-life cyclability. Therefore, this proposed strategy of constructing an all-MXene-based cathode can be readily extended to assemble a large number of MXene-derived materials, from a group of 60+ MAX phases, for applications such as various batteries and supercapacitors.

High-Capacity Cathode Material with High Voltage for Li-Ion Batteries

Electrochemical energy storage devices with a high energy density are an important technology in modern society, especially for electric vehicles. The most effective approach to improve the energy density of batteries is to search for high-capacity electrode materials. According to the concept of energy quality, a high-voltage battery delivers a highly useful energy, thus providing a new insight to improve energy density. Based on this concept, a novel and successful strategy to increase the energy density and energy quality by increasing the discharge voltage of cathode materials and preserving high capacity is proposed. The proposal is realized in high-capacity Li-rich cathode materials. The average discharge voltage is increased from 3.5 to 3.8 V by increasing the nickel content and applying a simple after-treatment, and the specific energy is improved from 912 to 1033 Wh kg^-1 . The current work provides an insightful universal principle for developing, designing, and screening electrode materials for high energy density and energy quality.

Dual-Functional Graphene Carbon as Polysulfide Trapper for High-Performance Lithium Sulfur Batteries

The lithium sulfur (Li-S) battery has attracted much attention due to its high theoretical capacity and energy density. However, its cycling stability and rate performance urgently need to improve because of its shuttle effect. Herein, oxygen-doped carbon on the surface of reduced graphene oxide (labeled as ODC/rGO) was fabricated to modify the separators of Li-S batteries to limit the dissolution of the lithium polysulfides. The mesoporous structure in ODC/rGO can not only serve as the physical trapper, but also provide abundant channels for fast ion transfer, which is beneficial for effective confinement of the dissoluble intermediates and superior rate performance. Moreover, the oxygen-containing groups in ODC/rGO are able to act as chemical adsorption sites to immobilize the lithium polysulfides, suppressing their dissolution in electrolyte to enhance the utilization of sulfur cathode in Li-S batteries. As a result, because of the synergetic effects of physical adsorption and chemical interaction to immobilize the soluble polysulfides, the Li-S batteries with the ODC/rGO-coated separator exhibit excellent rate performance and good long-term cycling stability with 0.057% capacity decay per cycle at 1.0 C after 600 cycles.

Chemical Immobilization Effect on Lithium Polysulfides for Lithium-Sulfur Batteries

Despite great progress in lithium-sulfur batteries (LSBs), great obstacles still exist to achieve high loading content of sulfur and avoid the loss of active materials due to the dissolution of the intermediate polysulfide products in the electrolyte. Relationships between the intrinsic properties of nanostructured hosts and electrochemical performance of LSBs, especially, the chemical interaction effects on immobilizing polysulfides for LSB cathodes, are discussed in this Review. Moreover, the principle of rational microstructure design for LSB cathode materials with strong chemical interaction adsorbent effects on polysulfides, such as metallic compounds, metal particles, organic polymers, and heteroatom-doped carbon, is mainly described. According to the chemical immobilizing mechanism of polysulfide on LSB cathodes, three kinds of chemical immobilizing effects, including the strong chemical affinity between polar host and polar polysulfides, the chemical bonding effect between sulfur and the special function groups/atoms, and the catalytic effect on electrochemical reaction kinetics, are thoroughly reviewed. To improve the electrochemical performance and long cycling life-cycle stability of LSBs, possible solutions and strategies with respect to the rational design of the microstructure of LSB cathodes are comprehensively analyzed.

Cobalt Sulfide Quantum Dot Embedded N/S-Doped Carbon Nanosheets with Superior Reversibility and Rate Capability for Sodium-Ion Batteries

Metal sulfides are promising anode materials for sodium-ion batteries due to their large specific capacities. The practical applications of metal sulfides in sodium-ion batteries, however, are still limited due to their large volume expansion, poor cycling stability, and sluggish electrode kinetics. In this work, a two-dimensional heterostructure of CoS_x (CoS and Co₉S₈) quantum dots embedded N/S-doped carbon nanosheets (CoS_x@NSC) is prepared by a sol-gel method. The CoS_x quantum dots are in situ formed within ultrafine carbon nanosheets without further sulfidation, thus resulting in ultrafine CoS_x particle size and embedded heterostructure. Meanwhile, enriched N and S codoping in the carbon nanosheets greatly enhances the electrical conductivity for the conductive matrix and creates more active sites for sodium storage. As a result, the hybrid CoS_x@NSC electrode shows excellent rate capability (600 mAh g^-1 at 0.2 A g^-1 and 500 mAh g^-1 at 10 A g^-1) and outstanding cycling stability (87% capacity retention after 200 cycles at 1 A g^-1), making it promising as an anode material for high-performance sodium-ion batteries. A CoS_x@NSC//Na_0.44MnO₂ full cell is demonstrated, and it can deliver a specific capacity of 414 mAh g^-1 (based on the mass of CoS_x@NSC) at a current density of 0.2 A g^-1.

Ultrathin Layered SnSe Nanoplates for Low Voltage, High-Rate, and Long-Life Alkali-Ion Batteries

2D electrode materials with layered structures have shown huge potential in the fields of lithium- and sodium-ion batteries. However, their poor conductivity limits the rate performance and cycle stability of batteries. Herein a new colloid chemistry strategy is reported for making 2D ultrathin layered SnSe nanoplates (SnSe NPs) for achieving more efficient alkali-ion batteries. Due to the effect of weak Van der Waals forces, each semiconductive SnSe nanoplate stacks on top of each other, which can facilitate the ion transfer and accommodate volume expansion during the charge and discharge process. This unique structure as well as the narrow-bandgap semiconductor property of SnSe simultaneously meets the requirements of achieving fast ionic and electronic conductivities for alkali-ion batteries. They exhibit high capacity of 463.6 mAh g^-1 at 0.05 A g^-1 for Na-ion batteries and 787.9 mAh g^-1 at 0.2 A g^-1 for Li-ion batteries over 300 cycles, and also high stability for alkali-ion batteries.

Integration of Graphene, Nano Sulfur, and Conducting Polymer into Compact, Flexible Lithium-Sulfur Battery Cathodes with Ultrahigh Volumetric Capacity and Superior Cycling Stability for Foldable Devices

Lithium-sulfur batteries, as one of the most promising next-generation batteries, attract tremendous attentions due to their high energy density and low cost. However, their practical application is hindered by their short cycling life and low volumetric capacity. Herein, compact, flexible, and free-standing films with a sandwich structure are designed simply by vacuum filtration, in which nanosulfur is homogenously coated by graphene and poly(3,4-ethylene-dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). This unique hierarchical structure not only provides a highly conductive network and intimate contacts between nanosulfur and graphene/PEDOT:PSS for effective charge transportation, but also offers synergistic physical restriction and chemical confinement of dissoluble intermediate lithium polysulfides during electrochemical processes. Therefore, these conductive compact films, used directly as cathodes, show the highest reversible volumetric capacity of 1432 Ah L^-1 at 0.1 C and 1038 Ah L^-1 at 1 C, and excellent cycling stability with a minimal decay rate of 0.04% per cycle over 500 cycles at 1 C. Meanwhile, remarkable rate performance with a high capacity of 701 mAh g^-1 at 4 C is also achieved. Soft-packaged batteries based on this flexible cathode are further fabricated and demonstrate excellent mechanical and electrochemical properties with little capacity decay under folded state, highlighting the practical application of our deliberately designed electrode in a flexible power system.

Molecularly Imprinted Polymer Enables High-Efficiency Recognition and Trapping Lithium Polysulfides for Stable Lithium Sulfur Battery

Using molecularly imprinted polymer to recognize various target molecules emerges as a fascinating research field. Herein, we applied this strategy for the first time to efficiently recognize and trap long-chain polysulfides (Li₂S_x, x = 6-8) in lithium sulfur battery to minimize the polysulfide shuttling between anode and cathode, which enables us to achieve remarkable electrochemical performance including a high specific capacity of 1262 mAh g^-1 at 0.2 C and superior capacity retention of over 82.5% after 400 cycles at 1 C. The outstanding performance is attributed to the significantly reduced concentration of long-chain polysulfides in electrolyte as evidenced by in situ UV/vis spectroscopy and Li₂S nucleation tests, which were further confirmed by density functional theory calculations. The molecular imprinting is demonstrated as a promising approach to effectively prevent the free diffusion of long-chain polysulfides, providing a new avenue to efficiently recognize and trap lithium polysulfides for high-performance lithium sulfur battery with greatly suppressed shuttle effect.

A Dual-Function Na2 SO4 Template Directed Formation of Cathode Materials with a High Content of Sulfur Nanodots for Lithium-Sulfur Batteries

The sulfur content in carbon-sulfur hybrid using the melt-diffusion method is normally lower than 70 wt%, which greatly decreases the energy density of the cathode in lithium-sulfur (Li-S) batteries. Here, a scalable method inspired by the commercialized production of Na₂ S is used to prepare a hierarchical porous carbon-sulfur hybrid (denoted HPC-S) with high sulfur content (≈85 wt%). The HPC-S is characterized by the structure of sulfur nanodots naturally embedded in a 3D carbon network. The strategy uses Na₂ SO₄ as the starting material, which serves not only as the sulfur precursor but also as a salt template for the formation of the 3D carbon network. The HPC-S cathode with such a high sulfur content shows excellent rate performance and cycling stability in Li-S batteries because of the sulfur nanoparticles, the unique carbon framework, and the strong interaction between them. The production method can also be readily scaled up and used in practical Li-S battery applications.

High-Capacity and Ultrafast Na-Ion Storage of a Self-Supported 3D Porous Antimony Persulfide-Graphene Foam Architecture

The key challenge for high-performance sodium-ion batteries is the exploitation of appropriate electrode materials with a long cycling stability and high rate capability. Here, we report Sb₂S₅ nanoparticles (∼5 nm) uniformly encapsulated in three-dimensional (3D) porous graphene foam, which were fabricated by a facile hydrothermal coassembly strategy, as a high-performance anode material for sodium-ion batteries. The as-prepared composite can be directly used as electrodes without adding a binder or current collector, exhibiting outstanding electrochemical performance with a high reversible capacity (845 mA h g^-1 at 0.1 A g^-1), ultralong cycling life (91.6% capacity retention after 300 cycles at 0.2 A g^-1), and exceptional rate capability (525 mA h g^-1 at 10.0 A g^-1). This is attributed to fast Na⁺ ion diffusion from the ultrasmall nanoparticles and excellent electric transport between the active material and 3D porous graphene, which also provide an effective strategy for anchoring the nanoparticles. Experimental results show that the Sb₂S₅ undergoes a reversible reaction of Sb₂S₅ + 16Na ↔ 5Na₂S + 2Na₃Sb during sodiation/desodiation. Moreover, a full cell with Na₃(VO_0.5)₂(PO₄)₂F₂/C cathode and the as-prepared composite anode was assembled, displaying high output voltage (∼2.2 V) with a stable capacity of 828 mA h g^-1 for anode material (with 100 cycles at 0.1 A g^-1), showing the potential for practical application.

Graphene: a promising 2D material for electrochemical energy storage

Graphene, with unique two-dimensional form and numerous appealing properties, promises to remarkably increase the energy density and power density of electrochemical energy storage devices (EESDs), ranging from the popular lithium ion batteries and supercapacitors to next-generation high-energy batteries. Here, we review the recent advances of the state-of-the-art graphene-based materials for EESDs, including lithium ion batteries, supercapacitors, micro-supercapacitors, high-energy lithium-air and lithium-sulfur batteries, and discuss the importance of the pore, doping, assembly, hybridization and functionalization of different nano-architectures in improving electrochemical performance. The major roles of graphene are highlighted as (1) a superior active material, (2) ultrathin 2D flexible support, and (3) an inactive yet electrically conductive additive. Furthermore, we address the enormous potential of graphene for constructing new-concept emerging graphene-enabled EESDs with multiple functionalities of lightweight, ultra-flexibility, thinness, and novel cell configurations. Finally, future perspectives and challenges of graphene-based EESDs are briefly discussed.

Freestanding reduced graphene oxide-sulfur composite films for highly stable lithium-sulfur batteries

Freestanding reduced graphene oxide-sulfur (rGO-S) composite films were fabricated by combining solution infiltration of sulfur into solvated rGO films with freeze-drying. Such rGO-S composite films can directly serve as the cathodes of lithium-sulfur (Li-S) batteries. The nanostructured architecture of rGO-S composite films considerably improved the cycling stability of Li-S batteries.

Graphitic Nanocarbon-Selenium Cathode with Favorable Rate Capability for Li-Se Batteries

A well-organized selenium/carbon nanosheets nanocomposite(Se/CNSs) is prepared by confining chain-like Se_n molecules in hierarchically micromesoporous carbon nanosheets. A unique two-dimensional morphology and high graphitization degree of carbon nanosheets benefits fast Li⁺/e^- access to the active Se, which guarantees a high utilization of Se during the(de)lithiation process. Besides, the chain-like Se molecules confined in the carbon matrix could alleviate the shuttle effect of polyselenides and promise a stable electrochemistry. Therefore, the resultant Se/CNSs delivers a highly reversible capacity, a long cycle life and favorable rate capabilities. Furthermore, a Li-Se pouch cell built from a metallic Li anode and the as-prepared Se/CNSs cathode exhibits an excellent electrochemical performance, demonstrating the potential of Se/CNSs in serving future energy storage devices with high energy density.

A reduced graphene oxide@sulfur nanocomposite as a high-capacity host matrix for advanced lithium–sulfur batteries

Reduced graphene oxide@sulfur nanocomposite as a high-capacity host matrix was prepared and characterized for advanced lithium–sulfur batteries.

A polymer enhanced sulfur/graphene aerogel as a no-slurry cathode for lithium–sulfur batteries

We presented a facile no-slurry method to synthesize a three-dimensional polyvinylpyrrolidone–sulfur/graphene aerogel composite, which can be pressed into flexible sheets and directly used as a cathode without any additives.

A review of flexible lithium–sulfur and analogous alkali metal–chalcogen rechargeable batteries

This review summarizes recent progress in flexible Li–S and analogous alkali metal–chalcogen batteries, including flexible chalcogen cathodes, flexible alkali metal anodes, flexible solid-state electrolytes, and flexible battery prototypes.