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

Current Status and Future Prospects of Metal-Sulfur Batteries

Lithium-sulfur batteries are a major focus of academic and industrial energy-storage research due to their high theoretical energy density and the use of low-cost materials. The high energy density results from the conversion mechanism that lithium-sulfur cells utilize. The sulfur cathode, being naturally abundant and environmentally friendly, makes lithium-sulfur batteries a potential next-generation energy-storage technology. The current state of the research indicates that lithium-sulfur cells are now at the point of transitioning from laboratory-scale devices to a more practical energy-storage application. Based on similar electrochemical conversion reactions, the low-cost sulfur cathode can be coupled with a wide range of metallic anodes, such as sodium, potassium, magnesium, calcium, and aluminum. These new "metal-sulfur" systems exhibit great potential in either lowering the production cost or producing high energy density. Inspired by the rapid development of lithium-sulfur batteries and the prospect of metal-sulfur cells, here, over 450 research articles are summarized to analyze the research progress and explore the electrochemical characteristics, cell-assembly parameters, cell-testing conditions, and materials design. In addition to highlighting the current research progress, the possible future areas of research which are needed to bring conversion-type lithium-sulfur and other metal-sulfur batteries into the market are also discussed.

TiO2 and Co Nanoparticle-Decorated Carbon Polyhedra as Efficient Sulfur Host for High-Performance Lithium-Sulfur Batteries

Metal organic frameworks (MOFs)-derived porous carbon is proposed as a promising candidate to develop novel, tailorable structures as polysulfides immobilizers for lithium-sulfur batteries because of their high-efficiency electron conductive networks, open ion channels, and abundant central ions that can store a large amount of sulfur and trap the easily soluble polysulfides. However, most central ions in MOFs-derived carbon framework are encapsulated in the carbon matrix so that their exposures as active sites to adsorb polysulfides are limited. To resolve this issue, highly dispersed TiO₂ nanoparticles are anchored into the cobalt-containing carbon polyhedras that are converted from ZIF-67. Such a type of TiO₂ and Co nanoparticles-decorated carbon polyhedras (CCo/TiO₂ ) provide more exposed active sites and much stronger chemical trapping for polysulfides, hence improving the sulfur utilization and enhancing reaction kinetics of sulfur-containing cathode simultaneously. The sulfur-containing carbon polyhedras decorated with TiO₂ nanoparticles (S@CCo/TiO₂ ) show a significantly improved cycling stability and rate capability, and deliver a discharge capacity of 32.9% higher than that of TiO₂ -free S@CCo cathode at 837.5 mA g^-1 after 200 cycles.

"One-for-All" strategy to design oxygen-deficient triple-shelled MnO2 and hollow Fe2O3 microcubes for high energy density asymmetric supercapacitors

Intrinsically poor conductivity, sluggish ion transfer kinetics, and limited specific area are the three main obstacles that confine the electrochemical performance of metal oxides in supercapacitors. Engineered hollow metal oxide nanostructures can effectively satisfy the increasing power demand of modern electronics. In this work, both triple-shelled MnO₂ and hollow Fe₂O₃ microcubes have been synthesized from a single MnCO₃ template. The oxygen vacancies are introduced in both the positive and negative electrodes through a facile method. The oxygen vacancies can not only improve the conductivity and facilitate ion diffusion but also increase the electrode/electrolyte interfaces and electrochemically active sites. Consequently, both the oxygen-deficient triple-shelled MnO₂ and hollow Fe₂O₃ exhibit larger capacitance and rate capability than the samples without oxygen vacancies. Moreover, due to the matchable specific capacitance and potential window between the positive and negative electrodes, the asymmetric supercapacitor exhibits high specific capacitance (240 F g^-1), excellent energy density of 133 W h kg^-1 at 1176 W kg^-1, excellent power density (23 529 W kg^-1 at 73 W h kg^-1), and high cycling stability (90.9% after 5000 cycles). This strategy is highly reproducible in oxide-based electrodes, which have the potential to meet the requirements of practical application.

Synergistic Effect of Co-Ni Hybrid Phosphide Nanocages for Ultrahigh Capacity Fast Energy Storage

Rational design of metal compounds in terms of the structure/morphology and chemical composition is essential to achieve desirable electrochemical performances for fast energy storage because of the synergistic effect between different elements and the structure effect. Here, an approach is presented to facilely fabricate mixed-metal compounds including hydroxides, phosphides, sulfides, oxides, and selenides with well-defined hollow nanocage structure using metal-organic framework nanocrystals as sacrificial precursors. Among the as-synthesized samples, the porous nanocage structure, synergistic effect of mixed metals, and unique phosphide composition endow nickel cobalt bimetallic phosphide (NiCo-P) nanocages with outstanding performance as a battery-type Faradaic electrode material for fast energy storage, with ultrahigh specific capacity of 894 C g-1 at 1 A g-1 and excellent rate capability, surpassing most of the reported metal compounds. Control experiments and theoretical calculations based on density functional theory reveal that the synergistic effect between Ni and Co in NiCo-P can greatly increase the OH- adsorption energy, while the hollow porous structure facilitates the fast mass/electron transport. The presented work not only provides a promising electrode material for fast energy storage, but also opens a new route toward structural and compositional design of electrode materials for energy storage and conversion.

Uniform Mesoporous MnO2 Nanospheres as a Surface Chemical Adsorption and Physical Confinement Polysulfide Mediator for Lithium-Sulfur Batteries

The actual implementation of lithium-sulfur batteries is hindered by inferior cyclic stability and poor coulombic efficiency stemming from the notorious shuttling of soluble polysulfide intermediates. Herein, uniform mesoporous MnO₂ nanospheres were prepared using a facile self-assembly and room-temperature reaction method. As a sulfur carrier of sulfur cathodes, the versatile architecture of MnO₂ not only provides powerful chemical adsorption to anchor polysulfide intermediates on the large polar surface area but also restrains them within the nanopores by physical confinement. The mesoporous MnO₂-stabilized sulfur cathode demonstrates a high initial reversible capacity of 1349.3 mA h g^-1 and a capacity fading rate of 0.073% at 1.0 C over 500 cycles. Furthermore, a reversible areal capacity of 2.5 mA h cm^-2 was achieved with stable cycling performance at a sulfur content of 80.7%. Our work offers a facile method to build efficient sulfur cathodes for high performance lithium-sulfur batteries.

Core-Shell Structured S@Co(OH)2 with a Carbon-Nanofiber Interlayer: A Conductive Cathode with Suppressed Shuttling Effect for High-Performance Lithium-Sulfur Batteries

Rechargeable lithium-sulfur batteries are potential candidates for storing electrochemical energy because of their extremely high energy density. However, their practical applications are prohibited by the sluggish charge transfer, the retarding Li ion diffusion, and the shuttle effect of lithium polysulfides. We report here a high-performance cathode material in which a S submicrosphere with a mass fraction of 80% was encapsulated within a permeable Co(OH)₂ nanoshell which functions as a physical barrier preventing the sulfur and polysulfides from leaking into the electrolyte and also contributes to the catalytic decomposition of polysulfides during the charge and discharge process. When an interlayer of carbon nanofibers is introduced between the S@Co(OH)₂ cathode and the separator, the performance of the Li-S batteries can be further significantly enhanced. Specifically, the S@Co(OH)₂ cathode possesses good cycling stability over 1000 cycles with an initial discharge capacity of 1100 mAh g^-1 at 2 C and a reversible capacity of 606 mAh g^-1. In particular, without the LiNO₃ additive, this S@Co(OH)₂ cathode also exhibits a Coulombic efficiency as high as 85%, just a little lower than that of commercial electrolyte with LiNO₃ additive. Relevant mechanistic studies revealed that such superior performances are attributed to the enhanced internal electrical and ionic conductivity and suppressed shuttling effect, owing to the presence of the Co(OH)₂ shell and the carbon-nanofiber interlayer. Theoretical simulations based on density functional theory were also carried out to figure out the interaction between the Co(OH)₂ nanosheets and the polysulfides. It revealed that the Co(OH)₂ nanoshell, rather than merely working as a physical barrier to trap the polysulfides, could also adsorb polysulfides and catalyze their decomposition during the cycling process, further helping to suppress the shuttling effect.

Synthesis of a Macroporous Conjugated Polymer Framework: Iron Doping for Highly Stable, Highly Efficient Lithium-Sulfur Batteries

Porous conjugated polymers offer enormous potential for energy storage because of the combined features of pores and extended π-conjugated structures. However, the drawbacks such as low pore volumes and insolubilities of micro- and mesoporous conjugated polymers restrict the loading of electroactive materials and thus energy storage performance. Herein, we report the synthesis of iron-doped macroporous conjugated polymers for hosting sulfur as the cathode of high-performance lithium-sulfur (Li-S) batteries. The macroporous conjugated polymers are synthesized via in situ growth of poly(3-hexylthiophene) (P3HT) from reduced graphene oxide (RGO) sheets, followed by gelation of the composite (RGO- g-P3HT) in p-xylene and freeze-drying. The network structures of the macroporous materials can be readily tuned by controlling the chain length of P3HT grafted to RGO sheets. The large pore volumes of the macroporous RGO- g-P3HT materials (ca. 34 cm³ g^-1) make them excellent frameworks for hosting sulfur as cathodes of Li-S batteries. Furthermore, incorporation of Fe into the macroporous RGO- g-P3HT cathode results in reduced polarization, enhanced specific capacity (1,288, 1,103, and 907 mA h g^-1 at 0.05, 0.1, and 0.2 C, respectively), and improved cycling stability (765 mA h g^-1 after 100 cycles at 0.2 C). Density functional theory calculations and in situ characterizations suggest that incorporation of Fe enhances the interactions between lithium polysulfides and the P3HT framework.

ZIF-67@Co-LDH yolk–shell spheres with micro-/meso-porous structures as vehicles for drug delivery

The encapsulated R6G molecules in ZIF-67@Co-LDH yolk–shell heterostructures are released with high loading capacity and long delivery time.

UV absorber co-intercalated layered double hydroxides as efficient hybrid UV-shielding materials for polypropylene

UV absorber co-intercalated layered double hydroxides can efficiently shield UV light and greatly enhance the anti-photoaging performance of polypropylene.

A novel route for the generation of Co/CoZn/CoNi layered double hydroxides at ambient temperature

Cobalt-based LDHs with high electrocatalytic OER performance are selectively produced by simply adjusting the amount of reagents in methanol.

Strong Surface-Bound Sulfur in Carbon Nanotube Bridged Hierarchical Mo2 C-Based MXene Nanosheets for Lithium-Sulfur Batteries

In this work, hydroxyl-functionalized Mo₂ C-based MXene nanosheets are synthesized by facilely removing the Sn layer of Mo₂ SnC. The hydroxyl-functionalized surface of Mo₂ C suppresses the shuttle effect of lithium polysulfides (LiPSs) through strong interaction between Mo atoms on the MXenes surface and LiPSs. Carbon nanotubes (CNTs) are further introduced into Mo₂ C phase to enlarge the specific surface area of the composite, improve its electronic conductivity, and alleviate the volume change during discharging/charging. The strong surface-bound sulfur in the hierarchical Mo₂ C-CNTs host can lead to a superior electrochemical performance in lithium-sulfur batteries. A large reversible capacity of ≈925 mAh g^-1 is observed after 250 cycles at a current density of 0.1 C (1 C = 1675 mAh g^-1 ) with good rate capability. Notably, the electrodes with high loading amounts of sulfur can also deliver good electrochemical performances, i.e., initial reversible capacities of ≈1314 mAh g^-1 (2.4 mAh cm^-2 ), ≈1068 mAh g^-1 (3.7 mAh cm^-2 ), and ≈959 mAh g^-1 (5.3 mAh cm^-2 ) at various areal loading amounts of sulfur (1.8, 3.5, and 5.6 mg cm^-2 ) are also observed, respectively.

Pretreating anaerobic fermentation liquid with calcium addition to improve short chain fatty acids extraction via in situ synthesis of layered double hydroxides

In situ synthesis of layered double hydroxides (LDHs) was proved to be an effective way to extract short chain fatty acids (SCFAs) from anaerobic fermentation liquid (AFL) as carbon source for biodenitrification, but the SCFAs content in SCFAs-LDH was unsatisfactory because of the existence of much carbonate in AFL. Pretreatment of AFL with calcium addition was investigated to remove carbonate and improve SCFAs extraction via LDHs synthesis. Results of batch tests showed that, the carbonate removal efficiency was as high as 76.6% when the calcium addition was 0.06 mol/L at pH 12. When using the optimal SCFAs/Al³⁺ ratio of 3.0, the total SCFAs content in SCFAs-LDH with pretreatment was improved to 46.5 mg COD/g LDH, which was 4.5 times of the control (10.4 mg COD/g LDH). These results suggest that adding calcium to AFL was an effective way to eliminate the negative effect of carbonates on SCFAs-LDH synthesis.

Metal–organic framework derived yolk–shell NiS2/carbon spheres for lithium–sulfur batteries with enhanced polysulfide redox kinetics

A yolk–shell NiS₂/C–S cathode exhibits enhanced electrochemical performances in lithium–sulfur batteries due to the improved redox kinetics of polysulfide confined within the yolk–shell structure.

Self-Supported FeCo2S4 Nanotube Arrays as Binder-Free Cathodes for Lithium-Sulfur Batteries

Inhibiting the shuttle effect, buffering the volume expansion, and improving the utilization of sulfur have been the three strategic points for developing a high-performance lithium-sulfur (Li-S) battery. Driven by this background, a flexible sulfur host material composed of FeCo₂S₄ nanotube arrays grown on the surface of carbon cloth is designed for a binder-free cathode of the Li-S battery through two-step hydrothermal method. Among the rest, the interconnected carbon fiber skeleton of the composite electrode ensures the basic electrical conductivity, whereas the FeCo₂S₄ nanotube arrays not only boost the electron and electrolyte transfer but also inhibit the dissolution of polysulfides because of their strong chemical adsorption. Meanwhile, the hollow structures of these arrays can provide a large inner space to accommodate the volume expansion of sulfur. More significantly, the developed composite electrode also reveals a catalytic action for accelerating the reaction kinetic of the Li-S battery. As a result, the FeCo₂S₄/CC@S electrode delivers a high discharge capacity of 1384 mA h g^-1 at the current density of 0.1 C and simultaneously exhibits a stable Coulombic efficiency of about 98%.

A MOF-derived coral-like NiSe@NC nanohybrid: an efficient electrocatalyst for the hydrogen evolution reaction at all pH values

A coral-like NiSe@NC nanohybrid as an effective electrocatalyst for the hydrogen evolution reaction (HER) at all pH values, constructed via the in situ selenation of a Ni-MOFs precursor, is reported. The electrocatalyst shows overpotentials of 123 mV, 250 mV and 300 mV in 0.5 M H2SO4, 1.0 M KOH and 1.0 M PBS, respectively, to afford a current density of 10 mA cm-2. Meanwhile, NiSe@NC also exhibits a small Tafel slope and superior long-term stability over a wide pH range. The excellent electrocatalytic performance should be ascribed to the unique coral-like structure with a large BET specific surface area (125.4 m2 g-1) and mesoporous features, as well as synergistic effects between NiSe nanocrystals and highly conductive N-doped porous carbon.

Insight of Enhanced Redox Chemistry for Porous MoO2 Carbon-Derived Framework as Polysulfide Reservoir in Lithium-Sulfur Batteries

As a promising energy-storage system, lithium-sulfur batteries (LSBs) with a high energy density suffer from the polysulfide shuttle effect and sluggish reaction kinetics, which have been studied for a few decades. Incorporation of polar metal oxides is an efficient addition for LSBs to suppress the dissolution of soluble polysulfides, increase the utilization of sulfur, and improve cycling stability. Herein, a model (MoO₂/C-NCs) based on a porous octahedral carbon framework decorated with MoO₂ nanoparticles (MoO₂ NPs) as a sulfur host is proposed. Adsorption experiments of lithium polysulfides (LiPSs) to MoO₂/C-NCs and cyclic voltammetry analysis showed that the MoO₂ NPs facilitate interfacial charge transfer and provide numerous active sites for the electrochemical redox reactions of LiPSs. Density functional theory calculations further reveal that LiPSs are diffused and strongly adsorbed on the surface of MoO₂ NPs because of the powerful van der Waals forces via Mo-S and Li-O bonds, which helps achieve a stable long-term cycling performance. As a result, the fabricated LSBs display a high initial specific capacity of 1317 mA h g^-1 at 0.2C and a promising capacity of 602 mA h g^-1 and a capacity retention of 65.6% at 1C when proceeding to 500 cycles.

Enhanced Adsorptions to Polysulfides on Graphene-Supported BN Nanosheets with Excellent Li-S Battery Performance in a Wide Temperature Range

For Li-S batteries, the catalysis for S redox reaction is indispensable. A lot of multifunctional sulfur electrode support materials with have been investigated widely. However, most of these studies were carried out at room temperature, and the interaction between different components in the matrix is not often paid enough attention. Here, we report a graphene supported BN nanosheet composite in which the synergistic effect between BN and graphene greatly enhanced the adsorption for polysulfides, thus leading to excellent performance in a wide temperature range. When used as a host material of sulfur, it can make the Li-S battery apply to a wide range of temperatures, from -40 to 70 °C, delivering a high utilization of sulfur, an excellent rate capability, and outstanding cycling life. The capacity can stabilized at 888 mAh g^-1 at 2 C after 300 cycles with a capacity attenuation of <0.04% per cycle at 70 °C, and the battery can deliver a capacity above 650 mAh g^-1 at -40 °C.

Formation of Si Hollow Structures as Promising Anode Materials through Reduction of Silica in AlCl3-NaCl Molten Salt

Hollow nanostructures are attractive for energy storage and conversion, drug delivery, and catalysis applications. Although these hollow nanostructures of compounds can be generated through the processes involving the well-established Kirkendall effect or ion exchange method, a similar process for the synthesis of the pure-substance one ( e. g., Si) remains elusive. Inspired by the above two methods, we introduce a continuous ultrathin carbon layer on the silica nano/microstructures (Stöber spheres, diatom frustules, sphere in sphere) as the stable reaction interface. With the layer as the diffusion mediator of the reactants, silica structures are successfully reduced into their porous silicon hollow counterparts with metal Al powder in AlCl₃-NaCl molten salt. The structures are composed of silicon nanocrystallites with sizes of 15-25 nm. The formation mechanism can be explained as an etching-reduction/nucleation-growth process. When used as the anode material, the silicon hollow structure from diatom frustules delivers specific capacities of 2179, 1988, 1798, 1505, 1240, and 974 mA h g^-1 at 0.5, 1, 2, 4, 6, and 8 A g^-1, respectively. After being prelithiated, it retains 80% of the initial capacity after 1100 cycles at 8 A g^-1. This work provides a general way to synthesize versatile silicon hollow structures for high-performance lithium ion batteries due to the existence of ample silica reactants and can be extended to the synthesis of hollow structures of other materials.

Microwave-assisted rapid preparation of hollow carbon nanospheres@TiN nanoparticles for lithium-sulfur batteries

Highly conductive titanium nitride (TiN) has a strong anchoring ability for lithium polysulfides (LiPSs). However, the complexity and high cost of fabrication limit their practical applications. Herein, a typical structure of hollow carbon nanospheres@TiN nanoparticles (HCNs@TiN) was designed and successfully synthesized via a microwave reduction method with the advantages of economy and efficiency. With unique structural and outstanding functional behavior, HCN@TiN-S hybrid electrodes display not only a high initial discharge capacity of 1097.8 mA h g^-1 at 0.1C, but also excellent rate performance and cycling stability. After 200 cycles, a reversible capacity of 812.6 mA h g^-1 is still retained, corresponding to 74% capacity retention of the original capacity and 0.13% decay rate per cycle, which are much better than those of HCNs-S electrodes.

Novel Non-Carbon Sulfur Hosts Based on Strong Chemisorption for Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries are considered as promising candidates for energy storage systems owing to their high theoretical capacity and high energy density. The application of Li-S batteries is hindered by several obstacles, however, including the shuttle effect, poor electrical conductivity, and the severe volume expansion of sulfur. The traditional method is to integrate sulfur with carbon materials. But the interaction between polysulfide intermediates and carbon is only weak physical adsorption, which easily leads to the escape of species from the framework (shuttle effect) of the material causing capacity loss. Recently, however, there has been a trend for the introduction of novel non-carbon materials as sulfur hosts based on the strong chemisorption. This review highlights recent research progress on novel non-carbon sulfur hosts based on strong chemisorption, in Li-S batteries. In comparison with carbon-based sulfur hosts, most non-carbon sulfur hosts have been demonstrated to be polar host materials that could efficiently adsorb polysulfide via strong chemisorption, mitigating their dissolution. The intrinsic mechanism associated with the role of non-carbon-based host materials in improving the performance of Li-S batteries is discussed.

The Design and Synthesis of Hollow Micro-/Nanostructures: Present and Future Trends

Hollow micro-/nanostructures have attracted tremendous interest owing to their intriguing structure-induced physicochemical properties and great potential for widespread applications. With the development of modern synthetic methodology and analytical instruments, a rapid structural/compositional evolution of hollow structures from simple to complex has occurred in recent decades. Here, an updated overview of research progress made in the synthesis of hollow structures is provided. After an introduction of definition and classification, achievements in synthetic approaches for these delicate hollow architectures are presented in detail. According to formation mechanisms, these strategies can be categorized into four different types, including hard-templating, soft-templating, self-templated, and template-free methods. In particular, the rationales and emerging innovations in conventional templating syntheses are in focus. The development of burgeoning self-templating strategies based on controlled etching, outward diffusion, and heterogeneous contraction is also summarized. In addition, a brief overview of template-free methods and recent advances on combined mechanisms is provided. Notably, the strengths and weaknesses of each category are discussed in detail. In conclusion, a perspective on future trends in the research of hollow micro-/nanostructures is given.

Advances in Cathode Materials for High-Performance Lithium-Sulfur Batteries

Lithium-sulfur batteries (LSBs) represent a promising energy storage technology, and they show potential for next-generation high-energy systems due to their high specific capacity, abundant constitutive resources, non-toxicity, low cost, and environment friendliness. Unlike their ubiquitous lithium-ion battery counterparts, the application of LSBs is challenged by several obstacles, including short cycling life, limited sulfur loading, and severe shuttling effect of polysulfides. To make LSBs a viable technology, it is very important to design and synthesize outstanding cathode materials with novel structures and properties. In this review, we summarize recent progress in designs, preparations, structures, and properties of cathode materials for LSBs, emphasizing binary, ternary, and quaternary sulfur-based composite materials. We especially highlight the utilization of carbons to construct sulfur-based composite materials in this exciting field. An extensive discussion of the emerging challenges and possible future research directions for cathode materials for LSBs is provided.

Multi-Heteroatom-Doped Hollow Carbon Attached on Graphene Using LiFePO4 Nanoparticles as Hard Templates for High-Performance Lithium-Sulfur Batteries

P, O, and N heteroatom-doped hollow carbon on graphene (PONHC/G) from nanosized LiFePO₄ (LFP) as a hard template is shown to be a very efficient sulfur host for lithium-sulfur (Li-S) batteries. The PONHC/G made from LFP nanoparticles as hard materials provides sufficient voids with various pore sizes for sulfur storage, and doping of the carbon structures with various heteroatoms minimized dissolution/diffusion of the polysulfides. The obtained PONHC/G can store sulfur and mitigate diffusion of the dissolved polysulfide owing to the well-organized host structure and the strong chemical affinity for polysulfides because of the polarization effect of the heteroatom dopants. As a cathode, S@PONHC/G shows excellent cycle stability and rate capability, as confirmed by polysulfide adsorption analysis. Therefore, PONHC/G may show high potential as a sulfur scaffold in the commercialization of Li-S batteries through additional modification and optimization of these host materials.

Dual-template engineering of triple-layered nanoarray electrode of metal chalcogenides sandwiched with hydrogen-substituted graphdiyne

Hybrid nanostructures integrating electroactive materials with functional species, such as metal-organic frameworks, covalent organic frameworks, graphdiyne etc., are of significance for both fundamental research and energy conversion/storage applications. Here, hierarchical triple-layered nanotube arrays, which consist of hydrogen-substituted graphdiyne frameworks seamlessly sandwiched between an outer layer of nickel–cobalt co-doped molybdenum disulfide nanosheets and an inner layer of mixed cobalt sulfide and nickel sulfide (Co₉S₈/Ni₃S₂), are directly fabricated on conductive carbon paper. The elaborate triple-layered structure emerges as a useful hybrid electrode for energy conversion and storage, in which the organic hydrogen-substituted graphdiyne middle layer, with an extended π-conjugated system between the electroactive nanomaterials, provides built-in electron and ion channels that are crucial for performance enhancement. This dual-template synthetic method, which makes use of microporous organic networks to confine a self-template, is shown to be versatile and thus provides a promising platform for advanced nanostructure-engineering of hierarchical multi-layered nanostructures towards a wide range of electrochemical applications.

Multi-shelled nanomaterials offer interesting electrochemical properties, but have been limited in composition. Here the authors use dual templating to integrate electroactive metal chalcogenide layers with hydrogen-substituted graphdiyne, achieving electrocatalytic activity for hydrogen evolution.

Hollow N-Doped Carbon Polyhedron Containing CoNi Alloy Nanoparticles Embedded within Few-Layer N-Doped Graphene as High-Performance Electromagnetic Wave Absorbing Material

Magnetic metal nanostructures have exhibited good electromagnetic wave (EMW) absorption properties. However, the surface of the nanostructures is easily oxidized upon exposure to air, leading to the bad stability of the EMW absorption properties. We use metal-organic framework structure as a template to fabricate hollow N-doped carbon polyhedron containing CoNi alloy nanoparticles embedded within N-doped graphene (CoNi@NG-NCPs). The atomic ratio of Co/Ni can be tuned from 1:0.54 to 1:0.91 in the hollow CoNi@NG-NCPs. Experimental results demonstrate that the EMW absorption properties of the CoNi@NG-NCPs can be improved through the Ni introduction and increased with an increase of the Ni content. Typically, the minimal reflection loss of the optimal CoNi@NG-NCP can reach -24.03 dB and the effective absorption bandwidth (reflection loss below -10 dB) is as large as 4.32 GHz at the thickness of 2.5 mm. Furthermore, our CoNi@NG-NCPs exhibit favorably comparable or superior EMW absorption properties to other magnetic absorbers. In addition, because the CoNi alloy nanoparticles are coated with N-doped graphene layers, their surface oxidation behavior can be efficiently limited. The mechanism of the enhanced EMW absorption property is relevant to the enhanced dielectric loss and better impedance matching characteristic caused by the Ni incorporation.

A multi-shelled CoP nanosphere modified separator for highly efficient Li-S batteries

Lithium-sulfur batteries are considered to be one of the most promising energy-storage systems because of their high theoretical energy density, as well as low cost, nontoxicity and natural abundance of sulfur. However, their poor cycling stability mostly originates from the shuttling of polysulfides which hinders their future practical applications. Here, multi-shelled CoP nanospheres are designed as a coated separator material for Li-S batteries for the first time. Conductive CoP can efficiently anchor polysulfides not only owing to its polar character but also its partial natural surface oxidation feature as evidenced by XPS results, which further activates Co sites for chemically trapping polysulfides via strong Co-S bonding. Furthermore, the unique multi-shelled structure can capture polysulfides to alleviate the "shuttle effect". Consequently, the battery using a CoP coated separator exhibits outstanding cycling stability with a capacity degradation of 0.078% per cycle over 500 cycles at a current density of 1 C and excellent rate performance (725 mA h g-1 at 5 C). It is also worth noting that a high areal capacity of 3.2 mA h cm-2 can be achieved even with a sulfur loading of 3.24 mg cm-2. Our approach demonstrates the convenient fabrication and application potential for a multi-shelled CoP nanosphere modified separator for highly efficient Li-S batteries.

Structural Design of Lithium–Sulfur Batteries: From Fundamental Research to Practical Application

Abstract

Lithium–sulfur (Li–S) batteries have been considered as one of the most promising energy storage devices that have the potential to deliver energy densities that supersede that of state-of-the-art lithium ion batteries. Due to their high theoretical energy density and cost-effectiveness, Li–S batteries have received great attention and have made great progress in the last few years. However, the insurmountable gap between fundamental research and practical application is still a major stumbling block that has hindered the commercialization of Li–S batteries. This review provides insight from an engineering point of view to discuss the reasonable structural design and parameters for the application of Li–S batteries. Firstly, a systematic analysis of various parameters (sulfur loading, electrolyte/sulfur (E/S) ratio, discharge capacity, discharge voltage, Li excess percentage, sulfur content, etc.) that influence the gravimetric energy density, volumetric energy density and cost is investigated. Through comparing and analyzing the statistical information collected from recent Li–S publications to find the shortcomings of Li–S technology, we supply potential strategies aimed at addressing the major issues that are still needed to be overcome. Finally, potential future directions and prospects in the engineering of Li–S batteries are discussed.

Graphical Abstract

Three-dimensional sp2 carbon networks prepared by ultrahigh temperature treatment for ultrafast lithium-sulfur batteries

The current challenge in the development of high-performance lithium-sulfur (Li-S) batteries is to facilitate the redox kinetics of sulfur species as well as to suppress the shuttle effect of polysulfides, especially at high current rates. Herein, aiming the application of Li-S at high current rates, we coupled a sp2 carbon configuration consisting of 3D carbon nanotubes/graphene prepared by ultrahigh temperature treatment (2850 °C) with S (2850CNTs-Gra-S) for application in Li-S batteries. The 2850CNTs-Gra as the host material exhibits a nearly perfect sp2 hybridized structure because ultrahigh temperature treatment not only repairs the raw defects in CNTs and graphene, but it also forms new sp2 C-C bonds between them. The 3D sp2 carbon network ensures ultrafast ion/electron transfer and efficient heat dissipation to protect the integrity of the separator when the Li-S battery is running at an ultrahigh rate. Based on these unique advantages, the 2850CNTs-Gra-S cathode shows a high current rate performance. Critically, it still delivers a considerable specific capacity after 1500 cycles even at a current rate of 15C and exhibits an extremely low capacity degradation rate of 0.0087% per cycle.

Polyelectrolyte Binder for Sulfur Cathode To Improve the Cycle Performance and Discharge Property of Lithium-Sulfur Battery

To achieve the higher capacity and the better cycle performance of the lithium-sulfur (L-S) batteries, a copolymer electrolyte prepared via emulsifier-free emulsion polymerization was used as the binder for the sulfur cathode in this study. This polyelectrolyte binder has uniform dispersion and good Li⁺ conductivity in the cathode that can improve the kinetics of sulfur electrochemical reactions. As a result, the capacity and cycle performance of the battery are improved evidently when the cell is discharged to 1.8 V. Moreover, when the cell is discharged to 1.5 V, the difficult deposition of Li₂S₂ will take place easily at 1.75 V, and the difficult transformation from solid Li₂S₂ to solid Li₂S will progress smoothly and completely during the voltage range of 1.55-1.75 V, too. The capacity of this L-S battery discharged to 1.5 V is as much as 1700 mAh g^-1, which is very close to the theoretical value of sulfur cathode. The knowledge acquired in this study is valuable not only for the design of an efficient new polyelectrolyte binder for sulfur cathode but also the discovery that the discharge degree is the main fact that limits the capacity to reach its theoretical value.

NIR-Triggered Crystal Phase Transformation of NiTi-Layered Double Hydroxides Films for Localized Chemothermal Tumor Therapy

Construction of localized drug-eluting systems with synergistic chemothermal tumor-killing abilities is promising for biomedical implants directly contacting with tumor tissues. In this study, an intelligent and biocompatible drug-loading platform, based on a gold nanorods-modified butyrate-inserted NiTi-layered double hydroxides film (Au@LDH/B), is prepared on the surface of nitinol alloy. The prepared films function as drug-loading "sponges," which pump butyrate out under near-infrared (NIR) irradiation and resorb drugs in water when the NIR laser is shut off. The stimuli-responsive release of butyrate is verified to be related with the NIR-triggered crystal phase transformation of Au@LDH/B. In vitro and in vivo studies reveal that the prepared films possess excellent biosafety and high efficiency in synergistic thermochemo tumor therapy, showing a promising application in the construction of localized stimuli-responsive drug-delivery systems.