Rational design of yolk-shell silicon dioxide@hollow carbon spheres as advanced Li-S cathode hosts

Hollow carbon spheres loaded with NiSe2 nanoplates as multifunctional SeS2 hosts for Li-SeS2 batteries

Selenium sulfide as a new alternative cathode material can effectively address the inferior electronic conductivity of sulfur, which is the main cause for poor electrochemical reactivity of conventional lithium-sulfur batteries (Li-S batteries). Therefore, in this work, hollow carbon spheres loaded with NiSe₂ nanoplates were prepared as SeS₂ hosts for Li-SeS₂ batteries. The unique micro-mesoporous hollow carbon spheres not only provide channels for the diffusion of SeS₂, but also afford spaces for alleviating the volume expansion of the active substance. Besides, the external polar NiSe₂ nanoplates increase active sites for capturing polysulfides or polyselenides during the charge/discharge process. Meanwhile, the excellent electronic conductivity of NiSe₂ can accelerate the catalytic reaction on the surface, thus reducing the loss of soluble intermediate products and finally suppressing the "shuttle effect". These extraordinary features of the as-proposed cathode offer many superiorities in electrochemical performances in terms of a high initial discharge capacity of 1139 mA h g^-1 at a current rate of 0.1C and an excellent cycling life of up to 1000 cycles at 1C.

Customized Structure Design and Functional Mechanism Analysis of Carbon Spheres for Advanced Lithium-Sulfur Batteries

Lithium-sulfur batteries (LSBs) are attracting much attention due to their high theoretical energy density and are considered to be the predominant competitors for next-generation energy storage systems. The practical commercial application of LSBs is mainly hindered by the severe "shuttle effect" of the lithium polysulfides (LiPSs) and the serious damage of lithium dendrites. Various carbon materials with different characteristics have played an important role in overcoming the above-mentioned problems. Carbon spheres (CSs) are extensively explored to enhance the performance of LSBs owing to their superior structures. The review presents the state-of-the-art advances of CSs for advanced high-energy LSBs, including their preparation strategies and applications in inhibiting the "shuttle effect" of the LiPSs and protecting lithium anodes. The unique restriction effect of CSs on LiPSs is explained from three working mechanisms: physical confinement, chemical interaction, and catalytic conversion. From the perspective of interfacial engineering and 3D structure designing, the protective effect of CSs on the lithium anode is also analyzed. Not only does this review article contain a summary of CSs in LSBs, but also future directions and prospects are discussed. The systematic discussions and suggested directions can enlighten thoughts in the reasonable design of CSs for LSBs in near future.

Hollow urchin-like Mn3O4 microspheres as an advanced sulfur host for enabling Li-S batteries with high gravimetric energy density

Lithium-sulfur (Li-S) batteries are considered to be promising candidates for next-generation storage systems. However, the practical applications are still hindered by the severe capacity decay, mainly caused by the large volume change, polysulfide shuttle and sluggish sulfur conversion kinetics. Herein, hollow urchin-like Mn₃O₄ (HU-Mn₃O₄) microspheres as sulfur hosts have been synthesized by the hydrothermal method and calcination treatment, aiming to prevent the polysulfide dissolution (benefiting from the strong polysulfide anchoring effect of Mn₃O₄) and alleviate the volume expansion of sulfur (benefiting from the special hollow structure). Meanwhile, the urchin-like thorny surface also facilitates the rapid ion/electron transfer and the abundant active sites for the fast sulfur redox kinetics. When used as the sulfur host in Li-S batteries, the S@HU-Mn₃O₄ cathode delivers a high initial capacity of 1137.4 mAh g^-1 with a slow capacity decay of 0.042% after 200 cycles at 0.2 C. Even under the conditions of lean electrolyte (E/S = 7 mL g^-1) and low N/P ratio (N/P = 2.1), the S@HU-Mn₃O₄ cathode still enables a stable cycling performance with a high gravimetric energy density (202 Wh kg cell^-1), demonstrating its great potential in the development of future practical Li-S battery materials.

Engineering hollow carbon spheres: directly from solid resin spheres to porous hollow carbon spheres via air induced linker cleaving

Hollow carbon spheres (HCSs) have broad application in many fields such as catalysis, adsorption and energy storage. Due to various restrictions on hard and soft templates, self-templating methods have received extensive attention. Generally, the conventional self-templating method includes two steps, including the hollowing and carbonization process. Herein, a facile novel one-step air induced linker cleaving (AILC) method was developed to synthesize HCSs using 3-aminophenol formaldehyde (APF) resin spheres as the carbon precursor. In this case, the cavitation and carbonization processes occur simultaneously. The as-prepared HCSs were characterized by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) and Raman spectroscopy. It was found that the cleavage of the ether bond groups (Ar-O-C) and the methylene (-CH₂) in the APF resulted in cavitation and carbonization. The degree of cavitation and carbonization can be adjusted by controlling the thermal treatment temperature and time in air. Furthermore, the sulfur cathode containing HCSs heated at 400 °C exhibited excellent electrochemical performance with an initial discharge capacity of 1006 mA h g^-1 at 0.2 C, and a low capacity decay rate of 0.097% per cycle over 500 cycles at 1 C. The novel one-step AILC strategy will pave a new avenue for the synthesis of hollow carbon spheres and their promising application in different areas.

Biocompatible Mesoporous Hollow Carbon Nanocapsules for High Performance Supercapacitors

A facile and general method for the controllable synthesis of N-doped hollow mesoporous carbon nanocapsules (NHCNCs) with four different geometries has been developed. The spheres (NHCNC-1), low-concaves (NHCNC-2), semi-concaves (NHCNC-3) and wrinkles (NHCNC-4) shaped samples were prepared and systematically investigated to understand the structural effects of hollow particles on their supercapacitor performances. Compared with the other three different shaped samples (NHCNC-1, NHCNC-2, and NHCNC-4), the as-synthesized semi-concave structured NHCNC-3 demonstrated excellent performance with high gravimetric capacitance of 326 F g⁻¹ (419 F cm⁻³) and ultra-stable cycling stability (96.6% after 5000 cycles). The outstanding performances achieved are attributed to the unique semi-concave structure, high specific surface area (1400 m² g⁻¹), hierarchical porosity, high packing density (1.41 g cm⁻³) and high nitrogen (N) content (up to 3.73%) of the new materials. These carbon nanocapsules with tailorable structures and properties enable them as outstanding carriers and platforms for various emerging applications, such as nanoscale chemical reactors, catalysis, batteries, solar energy harvest, gas storage and so on. In addition, these novel carbons have negligible cytotoxicity and high biocompatibility for human cells, promising a wide range of bio applications, such as biomaterials, drug delivery, biomedicine, biotherapy and bioelectronic devices.

Encapsulation of Few-Layer MoS2 in the Pores of Mesoporous Carbon Hollow Spheres for Lithium-Sulfur Batteries

Integrating a highly conductive carbon host and polar inorganic compounds has been widely reported to improve the electrochemical performances for promising low-cost lithium sulfur batteries. Herein, a MoS₂/mesoporous carbon hollow sphere (MoS₂/MCHS) structure has been proposed as an efficient sulfur cathode via a simple wet impregnation method and gas phase vulcanization method. Multi-fold structural merits have been demonstrated for the MoS₂/MCHS structures. On one hand, the mesoporous carbon hollow sphere (MCHS) matrix, with abundant pore structures and high specific surface areas, could load a large amount of sulfur, improve the electronical conductivity of sulfur electrodes, and suppress the volume changes during the repeated sulfur conversion processes. On the other hand, ultrathin multi-layer MoS₂ nanosheets are revealed to be uniformly distributed in the mesoporous carbon hollow spheres, enhancing the physical adsorption and chemical entrapment functionalities towards the soluble polysulfide species. Having benefited from these structural advantages, the sulfur-impregnated MoS₂/MCHS cathode presents remarkably improved electrochemical performances in terms of lower voltage polarization, higher reversible capacity (1094.3 mAh g⁻¹), higher rate capability (590.2 mAh g⁻¹ at 2 C), and better cycling stability (556 mAh g⁻¹ after 400 cycles at 2 C) compared to the sulfur-impregnated MCHS cathode. This work offers a novel delicate design strategy for functional materials to achieve high performance lithium sulfur batteries.

Hierarchical porous Fe/N doped carbon nanofibers as host materials for high sulfur loading Li-S batteries

Improving the sulfur utilization and cycling stability especially under high sulfur loading (>2 mg cm-2) is challenging due to the poor conductivity of sulfur and high soluble nature of polysulfides. Herein, we develop novel self-supporting carbon nanofibers with hierarchical porous structures and Fe/N absorption/nucleation centers (Fe/N-HPCNF) as high performance sulfur hosts via a facile co-spinning method. The highly interior porous carbon fiber structure provides good electrolyte infiltration, stable conductive networks and sufficient surfaces for fast Li+/electron transport and sulfur redox while maintaining high sulfur area loading. In addition, the abundant Fe/N heteroatoms evenly dispersed in the fiber strongly restrain polysulfide diffusion through a chemisorption effect and meanwhile regulate homogeneous sulfur nucleation, enhancing the stability of cathodes. Consequently, S@Fe/N-HPCNF cathodes realize a high initial specific capacity of 1273 mA h g-1 (areal capacity: 4.5 mA h cm-2) and long cycle life over 500 cycles with a high sulfur loading of 3.5 mg cm-2. A stable capacity of 6.6 mA h cm-2 is achieved even under 9 mg cm-2 sulfur. What's more, a pouch cell prototype with an ultrahigh sulfur area density (54 mg cm-2) was assembled and successfully lighted 10 yellow light-emitting diodes (LED), demonstrating the convenient scale-up of our S@Fe/N-HPCNF cathodes.

Recent Advances in Hollow Porous Carbon Materials for Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries are considered as one of the most potential next-generation rechargeable batteries due to their high theoretical energy density. However, some critical issues, such as low capacity, poor cycling stability, and safety concerns, must be solved before Li-S batteries can be used practically. During the past decade, tremendous efforts have been devoted to the design and synthesis of electrode materials. Benefiting from their tunable structural parameters, hollow porous carbon materials (HPCM) remarkably enhance the performances of both sulfur cathodes and lithium anodes, promoting the development of high-performance Li-S batteries. Here, together with the templated synthesis of HPCM, recent progresses of Li-S batteries based on HPCM are reviewed. Several important issues in Li-S batteries, including sulfur loading, polysulfide entrapping, and Li metal protection, are discussed, followed by a summary on recent research on HPCM-based sulfur cathodes, modified separators, and lithium anodes. After the discussion on emerging technical obstacles toward high-energy Li-S batteries, prospects for the future directions of HPCM research in the field of Li-S batteries are also proposed.

Tuning Confined Nanospace for Preparation of N-doped Hollow Carbon Spheres for High Performance Supercapacitors

The structural parameters and surface functionalities of hollow carbon spheres are critical for their electrochemical performance. Herein, preparation of N-doped-hollow carbon spheres (N-HCS) with tunable structural parameters and surface properties is reported by using a confined pyrolysis strategy. Polystyrene/polyaniline (PSPAN) was pyrolyzed in a silica shell, which provided a confined nanospace. PSPAN functioned as both the core and a source of carbon and nitrogen. The surface properties and structural parameters of the obtained N-HCS could be optimized by regulating the pore size of the silica shell. The silica shell coating also prevented agglomeration of the N-HCS, leading to a regular and well-distributed spherical morphology. The N-HCS synthesized in relatively porous conditions had thinner shell, larger mesopore size, higher surface area, and higher nitrogen content and showed excellent electrochemical performance as a supercapacitor with a specific capacitance of 436.5 F g^-1 at 0.5 A g^-1 .

Electrocatalysis of polysulfide conversion by conductive RuO2 nano dots for lithium-sulfur batteries

The practical use of the rechargeable lithium-sulfur (Li-S) battery is still restricted by poor cycle life and rate performance caused by the shuttle of soluble redox intermediates and low conductivity of S/Li2S. A comprehensive approach is to tune the multi-electron redox reactions and construct reversible chemical bonds with polysulfide intermediates. In this study, RuO2 nano dots (NDs) are proposed to anchor polysulfides, trigger the surface-mediated reduction of polysulfides and to facilitate the formation of Li2S2/Li2S through its catalytic effect for the first time. When serving as the sulfur host, the RuO2 NDs can retard the shuttle of polysulfides, accelerate the redox reaction of polysulfides, and therefore result in improved sulfur utilization and enhanced rate performance. The designed RuO2@NMCs/S ternary electrodes with high sulfur loading of 70 wt% could achieve a low decay rate of 0.07% per cycle for 500 cycles at a 0.5 C-rate. Realized by fast electrode kinetics, the reversible capacity of 634 mA h g-1 is attained at a high C-rate of 5 C. Overall, this strategy sheds new light on the oxide mediators for reversible modulation of electrochemical reactions in lithium-sulfur (Li-S) batteries.

Lithium Sulfonate/Carboxylate-Anchored Polyvinyl Alcohol Separators for Lithium Sulfur Batteries

A monolayer poly(vinyl alcohol) (PVA)-based separator with pendant sulfonate/carboxylate groups and compact morphology is synthesized to suppress the essential lithium polysulfide permeation in lithium sulfur batteries (LSBs). The Li⁺ transference number is significantly increased to 0.8, much higher than that of a commercial separator (0.43). The polysulfide retention is verified by idle test in a polysulfide-rich electrolyte under the internal electric field of the cell. The LSB with an additive-free electrolyte attains a Coulombic efficiency around 98% and delivered capacity of 804 mA h g^-1 at 2.5 A g^-1. After 500 cycles, it retains 901 mA h g^-1 at 1.5 A g^-1 with extra low fading rate of 0.016% per cycle. Overall, this monolayer PVA-based separator provides a facile and effective technique to assemble highly stable LSBs.

In situ sulfur loading in graphene-like nano-cell by template-free method for Li-S batteries

Carbon nanomaterials with 3D structures as sulfur hosts have been widely developed in lithium-sulfur batteries because of their high specific surface area, high conductivity and structural stability. However, sulfur, loaded by melting-diffusion method, is usually attached to the outside surface of carbon host, resulting in weak adsorption to expose polysulfide. Herein, we report a template-free method for synthesizing graphene-like nano-cell (GLC) with high in situ sulfur loading (S@GLC). The GLC is expected to provide physical adsorption by enclosed graphene cell architecture and chemical adsorption by pyridinic N-doping and oxygen functional group. With these merits, the S@GLC cathode owned high sulfur content (72%) and also, it exhibited a reversible specific capacity of 1253 mA h g^-1 at 0.2C, excellent rate performance, and long cycling stability (502 mA h g^-1 after 400 cycles at 1C).