Layered CoS@NC in situ loaded onto Ti3C2Tx MXene as an efficient lithium-ion battery anode.

Due to its large capacity and relatively high conductivity, cobalt sulfide has been considered an excellent electrode material for lithium-ion batteries, but its extreme volume change during charging and discharging and lower conductivity than graphite limits its development. In this work, composite nanosheets of MXene and N-doped carbon-confined cobalt sulfide nanosheets (CoS@NC/MXene) were synthesized by growing the Co metal-organic framework of ZIF-67 onto MXene sheets, followed by sulfidation treatment. Different from normal ZIF-67 generally prepared in methanol, this work fabricates ZIF-67 in aqueous solution, which induces ZIF-67 to undergo some degree of hydrolysis and form more dispersed Co layered hydroxides mounted onto MXene. Also, the MXene incorporation imparts better water stability to ZIF-67(Co) and helps maintain its morphology during the sulfidation. CoS@NC/MXene has a conductive network supported by MXene and enhanced by NC, as well as a 3D hierarchical porous structure offered by the rational combination of its components. These favorable characteristics allow CoS@NC/MXene to deliver a capacity of 691 mA h g-1 at 200 mA g-1 in the 100th cycle and retain the specific capacity of 382 mA h g-1 at a higher current density of 8000 mA g-1.

Promoted OH- adsorption and electron-transfer kinetics by electrospinning mono-disperse NiCo2S4 nanocrystals within porous CNFs for solid asymmetric supercapacitors.

Bimetallic sulfide NiCo2S4 has been regarded as a potential supercapacitor electrode material with excellent electrochemical performance. However, the origin of its high specific capacity is little studied, and the design of a rational structure still remains a challenge to exert its intrinsic advantage. In this work, the advantage of NiCo2S4 over NiS and CoS is explained by density functional theory calculation from the aspects of energy band, density of electronic states and OH- adsorption energy. It is proved that the synergistic effect of Ni and Co in NiCo2S4 can reduce its OH- adsorption energy and provide more active electrons near the Fermi level, thus promoting electrochemical reaction kinetics in supercapacitors. Then, a simple electrospinning method is used to in-situ load mono-disperse NiCo2S4 nanocrystals within amorphous carbon nanofibers, obtaining a porous, lotus-leaf-stem-like one-dimensional nanocomposite of NiCo2S4/CNF. Ex-situ XPS characterization confirms that the proportion of metal ions involved in electrochemical reactions and the number of transferred electrons in NiCo2S4/CNF during the redox reaction are significantly higher than those in mono-metallic sulfides (NiS/CNF and CoS/CNF), verifying the calculation results. With its boosting reaction kinetics, the NiCo2S4/CNF gives the specific capacity of 757.97C g-1 at 1 A/g and the capacity retention of 95.15 % after 10,000 cycles at 5 A/g, both greater than NiS/CNF and CoS/CNF. The NiCo2S4/CNF, as the positive electrode, and activated carbon, as the negative electrode, are assembled into liquid-state and solid-state asymmetric supercapacitor (ASC) devices, and both show high power density (760.6 W kg-1 for liquid-state device and 1067.4 W kg-1 for solid-state device), high energy density (52.25 Wh kg-1 for liquid-state device and 48.54 Wh kg-1 for solid-state device) and great cycle stability. Moreover, the solid-state ASC device possesses excellent low temperature capacity and reversibility, further demonstrating the wide application potential of the NiCo2S4/CNF composite.

Few-layer MoS2 nanosheets vertically supported on Ti3C2-MXene sheets promoting lithium storage performance.

2H phase MoS2 with a two-dimensional nanostructure, high chemical stability and large theoretical capacity has been considered as a potential anode material for lithium-ion batteries. However, some practical problems hinder the direct use of 2H-MoS2 for lithium storage, such as its volume expansion effect that leads to capacity loss and its semiconductor properties that cannot provide sufficient conductivity. Herein, the surface of an MXene with abundant surface groups was modified with CTAB to promote its ability to adsorb MoO42- anions, and then 2H-MoS2 with a few layers was directly grown on the surface of MXene sheets vertically. Thanks to the conductive MXene sheets and the vertically-supported high-capacity MoS2 on them, the as-obtained composite MXene@MoS2 offers enhanced performance in specific capacity, long cycling stability and high rate capability. A reversible specific capacity of 1198 mA h g-1 was retained after 100 cycles at 200 mA g-1 and a specific capacity of 717 mA h g-1 was exhibited at 8000 mA g-1.

SnS nanosheets firmly bound in alkali-treated wrinkled MXene framework with enhanced lithium-ion storage.

An alkali-treated MXene-SnS hybrid was prepared through hydrothermal methods. The Alk-MXene microplates provide highway for electronic transport, and the 3D wrinkled morphology ensures sufficient channels for Li+ diffusion. The alkali treatment of MXene gives Alk-MXene@SnS enhanced binding strength, which allows the SnS nanosheets to remain firm binding with the Alk-MXene substrate during cycling and overcome capacity decay caused by large volume change. The synergy between the two components guarantees the hybrid excellent electrochemical properties by enabling high electronic/ionic conductivity and superior kinetic properties as evidenced by EIS, GITT tests and DFT calculation. As a result, the Alk-MXene@SnS retains specific capacities of 519 mAh/g after 100 cycles at 200 mA g-1, and 330 mAh/g at the high rate of 8000 mA g-1. In addition, a reversible capacity of 421 mAh/g can be provided after long term cycle test at 1000 mA g-1 for 800 cycles.

Polypyrrole-coated Boron-doped Nickel-Cobalt sulfide on electrospinning carbon nanofibers for high performance asymmetric supercapacitors.

Although nickel-cobalt bimetallic sulfides have been widely studied for supercapacitor electrodes, how to obtain high specific capacity and cycle stability is still an important challenge. Here, an efficient chemical redox method is used to adjust the crystal and electronic structure of cobalt-nickel sulfide (NCS) via B doping, combined with electrospinning technology and conductive polymer polypyrrole (PPy) coating to facilitate faraday redox reactions and obtain high energy density electrode materials. The resulting composite with boron-doped nickel-cobalt sulfide on electrospinned carbon nanofibers with polypyrrole-coating (PPy@B-NCS/CNF) has a high specific capacity (751.61C/g at 1 A/g) and good cycle stability (82.49 % retention after 4000 cycles at 5 A/g). With PPy@B-NCS/CNF as the positive electrode and activated carbon as the negative electrode, an asymmetric supercapacitor (ASC) is prepared. It has excellent electrochemical properties with a power density of 65.58 Wh kg-1 and an energy density of 819.72 W kg-1. The low-temperature performance test shows high reversibility, which provides the possibility for the development of low-temperature electrolytes. Finally, density functional theory (DFT) explains that B-doped NCS has better electrochemical properties from the energy band and state density.