The development of sodium-ion batteries for large-scale applications requires the synthesis of electrode materials with high capacity, high initial Coulombic efficiency (ICE), high rate performance, long cycle life, and low cost. A rational design of freestanding anode materials is reported for sodium-ion batteries, consisting of molybdenum disulfide (MoS2) nanosheets aligned vertically on carbon paper derived from paper towel. The hierarchical structure enables sufficient electrode/electrolyte interaction and fast electron transportation. Meanwhile, the unique architecture can minimize the excessive interface between carbon and electrolyte, enabling high ICE. The as-prepared MoS2@carbon paper composites as freestanding electrodes for sodium-ion batteries can liberate the traditional electrode manufacturing procedure, thereby reducing the cost of sodium-ion batteries. The freestanding MoS2@carbon paper electrode exhibits a high reversible capacity, high ICE, good cycling performance, and excellent rate capability. By exploiting in situ Raman spectroscopy, the reversibility of the phase transition from 2H-MoS2 to 1T-MoS2 is observed during the sodium-ion intercalation/deintercalation process. This work is expected to inspire the development of advanced electrode materials for high-performance sodium-ion batteries.

MoS2 Nanosheets Vertically Aligned on Carbon Paper: A Freestanding Electrode for Highly Reversible Sodium Ion Batteries

2D/2D heterostructures can combine the collective advantages of each 2D material and even show improved properties from synergistic effects. 2D Transition metal carbide Ti3C2 MXene and 2D 1T-MoS2 have emerged as attractive prototypes in electrochemistry due to their rich properties. Construction of these two 2D materials, as well as investigation about synergistic effects, is absent due to the instability of 1T-MoS2. Here, 3D interconnected networks of 1T-MoS2/Ti3C2 MXene heterostructure are constructed by magneto-hydrothermal synthesis, and the electrochemical storage mechanisms are investigated. Improved extra capacitance is observed due to enlarged ion storage space from a synergistically interplayed effect in 3D interconnected networks. Outstanding rate performance is realized because of ultrafast electron transport originating from Ti3C2 MXene. This work provides an archetype to realize excellent electrochemical properties in 2D/2D heterostructures.

2D/2D 1T-MoS2/Ti3C2 MXene Heterostructure with Excellent Supercapacitor Performance

Molybdenum disulfide (MoS2) is a promising nonprecious catalyst for the hydrogen evolution reaction (HER) that has been extensively studied due to its excellent performance, but the lack of understanding of the factors that impact its catalytic activity hinders further design and enhancement of MoS2-based electrocatalysts. Here, by using novel porous (holey) metallic 1T phase MoS2 nanosheets synthesized by a liquid-ammonia-assisted lithiation route, we systematically investigated the contributions of crystal structure (phase), edges, and sulfur vacancies (S-vacancies) to the catalytic activity toward HER from five representative MoS2 nanosheet samples, including 2H and 1T phase, porous 2H and 1T phase, and sulfur-compensated porous 2H phase. Superior HER catalytic activity was achieved in the porous 1T phase MoS2 nanosheets that have even more edges and S-vacancies than conventional 1T phase MoS2. A comparative study revealed that the phase serves as the key role in determining the HER performance, as 1T phase MoS2 always outperforms the corresponding 2H phase MoS2 samples, and that both edges and S-vacancies also contribute significantly to the catalytic activity in porous MoS2 samples. Then, using combined defect characterization techniques of electron spin resonance spectroscopy and positron annihilation lifetime spectroscopy to quantify the S-vacancies, the contributions of each factor were individually elucidated. This study presents new insights and opens up new avenues for designing electrocatalysts based on MoS2 or other layered materials with enhanced HER performance.

(Invited) Contributions of Phase, Sulfur Vacancies, and Edges to the Hydrogen Evolution Reaction Catalytic Activity of Porous Molybdenum Disulfide Nanosheets

To promote the development of supercapacitors and their applications in modern electronics, it is crucial to explore novel supercapacitor electrode materials. As a representative member of the rising 2D MXenes, Ti3C2Tx MXene has shown tremendous potential for supercapacitor electrodes owing to its unique physicochemical properties. Here, the most recent advances in Ti3C2Tx-based supercapacitor electrodes are comprehensively reviewed, with an emphasis on the vital role that Ti3C2Tx MXene plays in the remarkable electrochemical performance and related mechanisms. The fabrication methods, electrode structures, working mechanisms, electrochemical performance and related influencing factors, mechanical properties and applications, as well as the associated advantages/disadvantages of Ti3C2Tx-based supercapacitor electrodes are thoroughly and exhaustively summarized and discussed. Based on the recent progress, the existing challenges along with the corresponding possible solutions, and the future prospects of Ti3C2Tx-based materials for supercapacitors are also outlined and discussed.

Ti3C2Tx MXene for electrode materials of supercapacitors

Flexible Transparent Supercapacitors: Materials and Devices

1T MoS2 is metallic octahedral type molybdenum disulfide (MoS2). Compared to the most stable and widely existing triangle prismatic (2H) phase of MoS2, 1T MoS2 possesses distinct electrochemical and electronic properties, such as a high conductivity, abundant active sites both at the edges and on the basal plane for electrochemical catalysis, and enlarged interlayer distance. Due to these merits, the last few years have witnessed a growing amount of interest in both synthesis and application of this unique material. So far, a series of methods have been developed to obtain 1T rich or 1T containing MoS2 and the applications of 1T MoS2 have covered a wide range of areas from electrochemical reactions, photoelectrocatalysis and photodetectors to photothermal agents, energy storage devices, and biosensors. In this article, to obtain a hint of the key reasons for the formation of 1T structured MoS2 and instruct the exploration of more desirable new approaches, the means for fabricating 1T MoS2 that have been reported are carefully classified and compared with a focus on the detailed procedures and the underlying mechanisms. Meanwhile, a full image of the explored applications with 1T MoS2 is provided with an emphasis on the relationship between the properties of 1T MoS2 and the performance and a hope to inspire the development of other potential applications for 1T MoS2. Additionally, the stabilization issue of 1T MoS2 is discussed with a review of the current efforts. The content will be not only helpful for researchers currently working in the related areas but also instructive for the ones new to this field.

Shangli Shi

Papers

Synthesis, stabilization and applications of 2-dimensional 1T metallic MoS2

The special route toward conversion of methane to methanol on a fluffy metal-free carbon nitride photocatalyst in the presence of H2O2

Photocatalytic conversion of ethane: Status and perspective