Chemical activation of hollow carbon nanospheres induced self-assembly of metallic 1T phase MoS2 ultrathin nanosheets for electrochemical lithium storage

Enhanced Interfacial Magnetization is Responsible for the Negative Capacity Fading of Cobalt Ditelluride Anodes for Lithium Storage

In lithium-ion batteries (LIBs), the stabilized capacities of transition metal compound anodes usually exhibit higher values than their theoretical values due to the interfacial charge storage, the formation of reversible electrolyte-derived surface layer, or interfacial magnetization. But the effectively utilizing the mechanisms to achieve novel anodes is rarely explored. Herein, a novel nanosized cobalt ditelluride (CoTe₂ ) anodes with ultra-high capacity and long term stability is reported. Electrochemical tests show that the lithium storage capacity of the best sample reaches 1194.7 mA h g^-1 after 150 cycles at 0.12 A g^-1 , which increases by 57.8% compared to that after 20 cycles. In addition, the sample offers capacities of 546.6 and 492.1 mA h g^-1 at 0.6 and 1.8 A g^-1 , respectively. During cycles, CoTe₂ particles (average size 20 nm) are gradually pulverized into the smaller nanoparticles (<3 nm), making the magnetization more fully due to the larger contact area of Co/Li₂ Te interface, yielding an increased capacity. The negative capacity fading is observed, and verified by ex situ structural characterizations and in situ electrochemical measurements. The proposed strategy can be further extended to obtain other high-performance ferromagnetic metal based electrodes for energy storage applications.

Unveiling the relationship between the multilayer structure of metallic MoS2 and the cycling performance for lithium ion batteries

Molybdenum disulfide (MoS₂) with a layered structure is a desirable substitute for the graphite anode in lithium ion storage. Compared with the semiconducting phase (2H-MoS₂), the metallic polymorph (1T-MoS₂) usually shows much better cycling stability. Nevertheless, the origin of this remarkable cycling stability is still ambiguous, hindering further development of MoS₂-based anodes. Herein, we assembled multilayered 1T-MoS₂ nanosheets directly on Ti foil to investigate the Li⁺ storage mechanism. Based on experimental observation and computational simulation, we found that the cycling stability correlates with the layer number of MoS₂. Multilayered 1T-MoS₂ can accommodate inserted Li⁺ in a ternary compound Li-Mo-S through a reversible reaction, which is favorable for retaining a substantial number of MoS₂ nanodomains upon Li intercalation. These residual MoS₂ nanodomains can serve as an anchor to adhere Li_xS species, thereby suppressing the "shuttle effect" of polysulfides and enhancing cycling stability. This work sheds light on the development of high-performance anodes based on metallic MoS₂ for LIBs.

Covalent Pinning of Highly Dispersed Ultrathin Metallic-Phase Molybdenum Disulfide Nanosheets on the Inner Surface of Mesoporous Carbon Spheres for Durable and Rapid Sodium Storage

Two-dimensional (2D) transition-metal dichalcogenide materials show potential for use in alkali metal ion batteries owing to their remarkable physical and chemical properties. Nevertheless, the electrochemical energy storage performance is still impaired by the tendency of aggregation, volume, and morphological change during the conversion reaction and poor intrinsic conductivity. Until now, ultrathin molybdenum disulfide nanosheets with a metallic-phase structure on the inner surface of mesoporous hollow carbon spheres (M-MoS₂@HCS) have rarely been investigated as an anode for sodium-ion batteries. In this work, a novel M-MoS₂@HCS anode was designed and synthesized by employing a template-assisted solvothermal reaction. Structural and chemical analyses indicate that the M-MoS₂ nanosheets with a larger interlayer spacing compared to their semiconductor counterpart grow on the inner surface of HCS via covalent interactions. When used as the anode materials for Na⁺ storage, the M-MoS₂@HCS anode presents durable and rapid sodium storage properties. The developed electrode shows a reversible capacity of 291.2 mAh g^-1 at a high current density of 5 A g^-1. After 100 cycles at 0.1 A g^-1, the reversible capacity is 401.3 mAh g^-1 with a capacity retention rate of 79%. After 2500 cycles at 1.0 A g^-1, the electrode still delivers a reversible capacity of 320.1 mAh g^-1 with a capacity retention rate of 75%. The excellent sodium storage capability of the MoS₂@HCS electrode is explained by the special structural design, which reveals great potential to accelerate the practical applications of transition-metal dichalcogenide electrodes for sodium storage.

Atomic-scale evidence for highly selective electrocatalytic N-N coupling on metallic MoS2

Molybdenum sulfide (MoS₂) is the most studied two-dimensional (2D) material bar graphene. Current research on crystal-phase engineering focuses almost exclusively on the improvement of catalytic activity. However, the potential advantages of phase engineering toward regulation of selectivity control during multistep catalytic processes remain unexplored. Here, we report atomic-scale evidence on how metallic MoS₂ shows significantly higher selectivity compared to the semiconducting phase during multielectron reduction of nitrite to nitrous oxide. Namely, a reaction intermediate specific to metallic MoS₂ increases the selectivity by decoupling the proton and electron transfer steps. This has previously been shown to be a universal mechanism to enhance selectivity, and therefore, our work opens directions of the application of 2D materials toward selective electrocatalysis.

Molybdenum sulfide (MoS₂) is the most widely studied transition-metal dichalcogenide (TMDs) and phase engineering can markedly improve its electrocatalytic activity. However, the selectivity toward desired products remains poorly explored, limiting its application in complex chemical reactions. Here we report how phase engineering of MoS₂ significantly improves the selectivity for nitrite reduction to nitrous oxide, a critical process in biological denitrification, using continuous-wave and pulsed electron paramagnetic resonance spectroscopy. We reveal that metallic 1T-MoS₂ has a protonation site with a pK_a of ∼5.5, where the proton is located ∼3.26 Å from redox-active Mo site. This protonation site is unique to 1T-MoS₂ and induces sequential proton−electron transfer which inhibits ammonium formation while promoting nitrous oxide production, as confirmed by the pH-dependent selectivity and deuterium kinetic isotope effect. This is atomic-scale evidence of phase-dependent selectivity on MoS₂, expanding the application of TMDs to selective electrocatalysis.