Wenkui Zhang

Strong Sulfur Binding with Conducting Magnéli-Phase TinO2n–1 Nanomaterials for Improving Lithium–Sulfur Batteries

Lithium-sulfur batteries show fascinating potential for advanced energy storage systems due to their high specific capacity, low-cost, and environmental benignity. However, the shuttle effect and the uncontrollable deposition of lithium sulfide species result in poor cycling performance and low Coulombic efficiency. Despite the recent success in trapping soluble polysulfides via porous matrix and chemical binding, the important mechanism of such controllable deposition of sulfur species has not been well understood. Herein, we discovered that conductive Magnéli phase Ti4O7 is highly effective matrix to bind with sulfur species. Compared with the TiO2-S, the Ti4O7-S cathodes exhibit higher reversible capacity and improved cycling performance. It delivers high specific capacities at various C-rates (1342, 1044, and 623 mAh g(-1) at 0.02, 0.1, and 0.5 C, respectively) and remarkable capacity retention of 99% (100 cycles at 0.1 C). The superior properties of Ti4O7-S are attributed to the strong adsorption of sulfur species on the low-coordinated Ti sites of Ti4O7 as revealed by density functional theory calculations and confirmed through experimental characterizations. Our study demonstrates the importance of surface coordination environment for strongly influencing the S-species binding. These findings can be also applicable to numerous other metal oxide materials.

Green and Facile Fabrication of Hollow Porous MnO/C Microspheres from Microalgaes for Lithium-Ion Batteries

Hollow porous micro/nanostructures with high surface area and shell permeability have attracted tremendous attention. Particularly, the synthesis and structural tailoring of diverse hollow porous materials is regarded as a crucial step toward the realization of high-performance electrode materials, which has several advantages including a large contact area with electrolyte, a superior structural stability, and a short transport path for Li(+) ions. Meanwhile, owing to the inexpensive, abundant, environmentally benign, and renewable biological resources provided by nature, great efforts have been devoted to understand and practice the biotemplating technology, which has been considered as an effective strategy to achieve morphology-controllable materials with structural specialty, complexity, and related unique properties. Herein, we are inspired by the natural microalgae with its special features (easy availability, biological activity, and carbon sources) to develop a green and facile biotemplating method to fabricate monodisperse MnO/C microspheres for lithium-ion batteries. Due to the unique hollow porous structure in which MnO nanoparticles were tightly embedded into a porous carbon matrix and form a penetrative shell, MnO/C microspheres exhibited high reversible specific capacity of 700 mAh g(-1) at 0.1 A g(-1), excellent cycling stability with 94% capacity retention, and enhanced rate performance of 230 mAh g(-1) at 3 A g(-1). This green, sustainable, and economical strategy will extend the scope of biotemplating synthesis for exploring other functional materials in various structure-dependent applications such as catalysis, gas sensing, and energy storage.

Biotemplated fabrication of hierarchically porous NiO/C composite from lotus pollen grains for lithium-ion batteries

In this work, hierarchically porous NiO/C microspheres were successfully synthesized via a facile biotemplating method using natural porous lotus pollen grains as both the carbon source and the template. The as-prepared hierarchically porous NiO/C microspheres exhibited a large specific surface area and multiple pore size distribution, which could effectively increase the electrochemical reaction area and allow better penetration of the electrolyte. The Raman results also confirmed that the pollen grains have been well carbonized, which could provide good electronic conductivity. The specific capacities of the porous NiO/C microspheres after every 10 cycles at 0.1, 0.5, 1, and 3 A g−1 are about 698, 608, 454 and 352 mAh g−1. As an anode material in a Li ion half-cell, these unique hybrid hierarchically porous NiO/C microspheres exhibited fascinating electrochemical performance.

Nanocrystal-Constructed Mesoporous Single-Crystalline Co3O4 Nanobelts with Superior Rate Capability for Advanced Lithium-Ion Batteries

In this paper, one-dimensional (1D) mesoporous single-crystalline Co₃O₄ nanobelts are synthesized by a facile hydrothermal method followed by calcination treatment. The as-prepared nanobelts have unique mesoporous structures, which are constructed by many interconnected nanocrystals with sizes of about 20-30 nm. And typical size of the nanobelts is in the range of 100-300 nm in width and up to several micrometers in length. The BET surface area of Co₃O₄ nanobelts is determined to be about 36.5 m² g⁻¹ with dominant pore diameter of 29.2 nm. Because of the 1D structure, mesoporous morphologies and scrupulous nanoarchitectures, the Co₃O₄ nanobelts show excellent electrochemical performances such as high storage capacity and superior rate capability. The specific capacity of Co₃O₄ nanobelts could remains over 614 mA h g⁻¹ at a current density of 1 A g⁻¹ after 60 cycles. Even at a high current density of 3 A g⁻¹, these Co₃O₄ nanobelts still could deliver a remarkable discharge capacity of 605 mA h g⁻¹ with good cycling stability.

Highly mesoporous carbon foams synthesized by a facile, cost-effective and template-free Pechini method for advanced lithium–sulfur batteries

Highly mesoporous carbon foam (MCF) with a high specific surface area has been successfully synthesized via a facile, cost-effective and template-free Pechini method. The as-prepared MCF exhibits a high specific surface area of 1478.55 m2 g−1 and a commendable pore size distribution for impregnating sulfur. After sulfur loaded in MCF, the relationship between pore size distribution of mesoporous carbon foam/sulfur nanocomposite (MCF/S) and the content of loaded sulfur is investigated in detail, which impacts on subtle variation of lithium storage performance. MCF/S (57.22 wt%) delivers an initial discharge of 1285 mA h g−1 and retains 878 mA h g−1 after 50 cycles. Compared with pristine sulfur, MCF/S cathodes display enhanced electrochemical performances, which can be attributed to the cross-linked hierarchical structure of MCF conductive matrix. Based on the advantages of the template-free Pechini method such as low cost, relative simplicity and atomic-scaled mixing, the MCF with hierarchical porous structure can be generalized to other practical applications including electrochemical double-layer capacitors, adsorption, separation, catalyst supports, etc. In addition, we believe that this modified Pechini method is general and can be extended to the fabrication of other types of mesoporous carbon by changing metal salts and organic reagents.

Pillared Structure Design of MXene with Ultralarge Interlayer Spacing for High-Performance Lithium-Ion Capacitors

Two-dimensional transition-metal carbide materials (termed MXene) have attracted huge attention in the field of electrochemical energy storage due to their excellent electrical conductivity, high volumetric capacity, etc. Herein, with inspiration from the interesting structure of pillared interlayered clays, we attempt to fabricate pillared Ti3C2 MXene (CTAB-Sn(IV)@Ti3C2) via a facile liquid-phase cetyltrimethylammonium bromide (CTAB) prepillaring and Sn4+ pillaring method. The interlayer spacing of Ti3C2 MXene can be controlled according to the size of the intercalated prepillaring agent (cationic surfactant) and can reach 2.708 nm with 177% increase compared with the original spacing of 0.977 nm, which is currently the maximum value according to our knowledge. Because of the pillar effect, the assembled LIC exhibits a superior energy density of 239.50 Wh kg-1 based on the weight of CTAB-Sn(IV)@Ti3C2 even under higher power density of 10.8 kW kg-1. When CTAB-Sn(IV)@Ti3C2 anode couples with commercial AC cathode, LIC reveals higher energy density and power density compared with conventional MXene materials.

B4C‐Nanowires/Carbon‐Microfiber Hybrid Structures and Composites from Cotton T‐shirts

Nanocomposites, where nanoscale inclusions are embedded in a matrix material, have attracted increasing research attention in recent years. Here, we limit our focus to polymer nanocomposites. Nano-reinforcements (for instance, nanoparticles, nanowires, and nanotubes) have largely disappointed us in manufacturing lightweight, high-strength, and high-toughness polymer composites because of their agglomeration in the matrix—one of the major drawbacks limiting their reinforcing effect. Dispersion of nano-reinforcements is not trivial and often requires expensive facilities and/or processing procedures. It has proven difficult to disperse nano-reinforcements in the matrix via conventional chemical, mechanical, and physical methods. To date, a perfect dispersion of nanoreinforcements in a matrix material has not been achieved. Despite considerable efforts, the mechanical properties of polymer nanocomposites are still far below their theoretically predicted potential. On the other hand, nano-reinforcements and their composites produced through the currently available methods are usually too expensive for use in large-scale industrial applications. To solve these two critical problems, we turned our attention to investigate new concepts for nano-reinforcement dispersion and new synthesis techniques that enable low-cost, high-throughput manufacturing of nanocomposites. Boron carbide (B4C), a lightweight refractory semiconductor, is the third hardest material known to man at room temperature and becomes the hardest above 1100 8C. It has many unique properties such as low density (2.5 g cm ), a small thermal extension coefficient (5.73 10 6 K ), a high melting point (>2400 8C), high resistance to chemical attacks, high thermal stability, a high Seebeck coefficient, and a large neutron absorption cross-section. The combination of these superior properties gives rise to numerous applications, especially under extreme conditions, for example, lightweight body armor, aircraft armor, abrasive wear-resistant materials, solid-state neutron detectors, and, potentially, power generation in deep-space flight applications. Recently, 1D B4C nanostructures, including nanowires and nanorods, have attracted significant attention. To synthesize B4C nanowires, the carbon source is essential. Among most of the available synthesis methods for 1D B4C nanostructures, artificial materials such as carbon black, carbon nanotubes, and activate carbon are selected as carbon source. A stepwise synthetic procedure was usually adopted: metal-catalyst nanoparticles were first synthesized and subsequently added to the precursors as the catalyst. Cotton has been used by mankind for at least 7000 years. It is the oldest and most commonly used nanoporous material, which is constructed from polysaccharide chains arranged into amorphous and crystalline regions. Today, the world production of cotton is approximately 20 million tons per annum, mainly for clothing, paper, and medical uses. In this work, a commercial cotton T-shirt was used as both the template and the carbon source to synthesize large quantities of radially aligned B4C nanowires on carbon microfibers. The as-synthesized B4C-nanowire/carbon-microfiber hybrid structures were then simply dipped in an epoxy resin bath to achieve a high dispersion of B4C nanowires in the matrix and highthroughput manufacturing of laminated polymer nanocomposites. A digital camera image of the 100% cotton T-shirt is shown in Figure 1a. A striking and extremely useful feature of cotton is its ability of absorbing large quantities of liquids, particularly water. Figure 1b is a representative digital camera image of a piece of the cotton T-shirt after the absorption of a solution with Ni(NO3)2 6H2O and amorphous boron powders. Figure 1c shows 6 pieces of final textiles. A typical scanning electron microscopy (SEM) image of carbon microfibers covered with B4C nanowires is shown in Figure 1d. The B4C nanowires, with a diameter ranging from 80 to 200 nm and a length greater than 4mm, grew radially on the entire length of the carbon microfiber (Fig. 1e). The SEM image (Fig. 1f) and transmission electron microscopy (TEM) image (Fig. 1g) together reveal a catalyst particle on the tip of each nanowire. Most of the catalyst particles were spherical and had a diameter distribution of 90 to 250 nm. TEM image (Fig. 1g) also shows that the diameter of the catalyst is larger than that of the corresponding B4C nanowire. Energy dispersive spectroscopy (EDS) reveals that the catalyst particle on the nanowire tip is nickel boride. This suggests that a catalyst is essential for growing such B4C nanowires. Our experimental results proved that most of the catalyst particles can be easily removed by acid treatment (HNO3þHF or HNO3). The B4C

Construction of sheet–belt hybrid nanostructures from one-dimensional mesoporous TiO2(B) nanobelts and graphene sheets for advanced lithium-ion batteries

TiO2(B) is considered as a new kind of anode material, and an alternative to graphite, for high-power lithium ion batteries (LIBs) due to its characteristic pseudocapacitive energy storage mechanism. Herein, we firstly report the synthesis of one-dimensional (1D) mesoporous TiO2(B) nanobelts by hydrothermal treatment of commercial TiO2 (P25) powders in NaOH medium. The as-prepared TiO2(B) nanobelts, with typical sizes of 50–100 nm in width and several micrometers in length, have mesopore channels in the range of 10–30 nm. Moreover, we demonstrate the use of graphene as an excellent mini-current collector to in situ construct unique hybrid sheet–belt nanostructures (G–TiO2(B)) to optimize the performance. Such a 1D mesoporous TiO2(B) structure can provide numerous open channels for the electrolyte to access and facilitate the ultrafast diffusion of lithium ions. In addition, the introduced graphene layers will both be favorable for the fast electron transport in the electrode and make a great contribution to the specific capacity. As a consequence, this G–TiO2(B) hybrid can deliver an ultrahigh reversible capacity (over 430 mA h g−1 at a low current density of 0.15 A g−1), and present a superior rate capability (210 mA h g−1 at 3 A g−1).

Highly efficient electrolytic exfoliation of graphite into graphene sheets based on Li ions intercalation–expansion–microexplosion mechanism

In this communication, we report a facile and novel molten salt electrolysis method to prepare high-quality graphene sheets. Transmission electron microscopy (TEM) and atomic force microscopy (AFM) confirmed that the final products exfoliated from the electrolysis of graphite cathode in the molten LiOH medium are mainly graphene sheets (approx. 80 wt% conversion efficiency). Raman spectroscopy revealed that the as-formed graphene sheets have significantly low density of defects. Based on these observations, the exfoliation mechanism of graphite cathode into graphene sheets through lithium intercalation–expansion–microexplosion processes was proposed. The discovery of a molten salt electrolysis method presents us with the possibility for large scale and continuous production of graphene.

Bio-inspired fabrication of carbon nanotiles for high performance cathode of Li–S batteries

Learning from biological materials with complex, optimized and hierarchical morphologies and microstructures has become one of the hottest subjects in material design. Herein, we report a bio-inspired fabrication of fish scale-like carbon nanotiles, which were synthesized by a facile carbonization and grind procedure using kapok fibers (KFs) as green carbon source. Kapok fiber derived carbon nanotiles (KFCNTs) were used as the host of sulfur to construct KFCNTs/S cathodes for Li–S batteries. KFCNTs with scale-like microstructure are important for retarding the shuttling of soluble polysulfides, rendering S particles electrically conducting, and accommodating volume variation of S during the Li+ insertion/extraction. Owing to their unique microstructure, the resulting KFCNTs/S (93.2 wt%) electrodes exhibit a high and stable volumetric capacity of 504 mA h cm−3 (calculated from the whole electrode, the corresponding gravimetric capacity is 524 mA h g−1) with a superior capacity retention up to 95.4% after 90 cycles at 0.4 A g−1, representing a promising cathode material for rechargeable Li–S batteries. KFCNTs may also find potential applications in catalysis, electronics, sensors and separation technology. The bionic strategy outlined here can be generalized to other advanced electrode materials.