Construction of 1T-MoSe2 /TiC@C Branch-Core Arrays as Advanced Anodes for Enhanced Sodium Ion Storage

A dual heterostructure enables the stabilization of 1T-rich MoSe2 for enhanced storage of sodium ions

Electron injection effectively induces the formation of a 1T-rich phase to address the low conductivity of MoSe2. Nevertheless, overcoming the inherent metastability of the 1T phase (particularly during the conversion reactions that entail the decomposition-reconstruction of MoSe2 and volume expansion) remains a challenge. Guided by DFT results, we designed a composite with bimetal selenides-based heterostructures anchored on reduced graphene oxide (rGO) nanosheets (G-Cu2Se@MoSe2) to obtain stabilized 1T-rich MoSe2 and enhanced ion transfer. The construction of 1T-rich MoSe2 and built-in electric fields (BiEF) through electron transfer at the heterointerfaces were realized. Moreover, the rGO-metal selenides heterostructures with in situ-formed interfacial bonds could facilitate the reconstruction of the 1T-rich MoSe2-involved heterostructure and interfacial BiEF. Such a dual heterostructure endowed G-Cu2Se@MoSe2 with an excellent rate capability with a capacity of 288 mA h g-1 at 50 A g-1 and superior cycling stability with a capacity retention ratio of 89.6% (291 mA h g-1) after 15 000 cycles at 10 A g-1. Insights into the functional mechanism and structural evolution of the 1T MoSe2-involved dual heterostructure from this work may provide guidelines for the development of MoSe2 and phase-engineering strategies for other polymorphistic materials.

Molecular Intercalation Enables Phase Transition of MoSe2 for Durable Na-Ion Storage

1T-MoSe2 is recognized as a promising anode material for sodium-ion batteries, thanks to its excellent electrical conductivity and large interlayer distance. However, its inherent thermodynamic instability often presents unparalleled challenges in phase control and stabilization. Here, a molecular intercalation strategy is developed to synthesize thermally stable 1T-rich MoSe2, covalently bonded to an intercalated carbon layer (1TR/2H-MoSe2@C). Density functional theory calculations uncover that the introduced ethylene glycol molecules not only serve as electron donors, inducing a reorganization of Mo 4d orbitals, but also as sacrificial guest materials that generate a conductive carbon layer. Furthermore, the C─Se/C─O─Mo bonds encourage strong interfacial electronic coupling, and the carbon layer prevents the restacking of MoSe2, regulating the maximum 1T phase to an impressive 80.3%. Consequently, the 1TR/2H-MoSe2@C exhibits an extraordinary rate capacity of 326 mAh g-1 at 5 A g-1 and maintains a long-term cycle stability up to 1500 cycles, with a capacity of 365 mAh g-1 at 2 A g-1. Additionally, the full cell delivers an appealing energy output of 194 Wh kg-1 at 208 W kg-1, with a capacity retention of 87.3% over 200 cycles. These findings contribute valuable insights toward the development of innovative transition metal dichalcogenides for next-generation energy storage technologies.

Multiscale Structural Design of 2D Nanomaterials-based Flexible Electrodes for Wearable Energy Storage Applications

2D nanomaterials play a critical role in realizing high-performance flexible electrodes for wearable energy storge devices, owing to their merits of large surface area, high conductivity and high strength. The electrode is a complex system and the performance is determined by multiple and interrelated factors including the intrinsic properties of materials and the structures at different scales from macroscale to atomic scale. Multiscale design strategies have been developed to engineer the structures to exploit full potential and mitigate drawbacks of 2D materials. Analyzing the design strategies and understanding the working mechanisms are essential to facilitate the integration and harvest the synergistic effects. This review summarizes the multiscale design strategies from macroscale down to micro/nano-scale structures and atomic-scale structures for developing 2D nanomaterials-based flexible electrodes. It starts with brief introduction of 2D nanomaterials, followed by analysis of structural design strategies at different scales focusing on the elucidation of structure-property relationship, and ends with the presentation of challenges and future prospects. This review highlights the importance of integrating multiscale design strategies. Finding from this review may deepen the understanding of electrode performance and provide valuable guidelines for designing 2D nanomaterials-based flexible electrodes.

Preparation of MoP-based quasi-two-dimensional belt-like composite fibers and its performance modulation for sodium/potassium ion storage

Beyond round nanofibers, electrospinning method is able to fabricate thin fibers with various cross-sectional shapes. In this work, using phosphomolybdic acid to trigger the polymerization reaction of pyrrole with addition of tin salt for the filler of carbon fibers, we prepared quasi-two-dimensional thick belt-like fibers through single spinneret electrospinning. The side-by-side welding nanofiber bundles with dog-bone-shaped cross-section improved the connection between fibers, endowing the fiber films with some flexibility and enabling it to be used as freestanding electrode. Employed as anode for sodium/potassium ion storage, the carbon fiber encapsulated MoP/SnO2 material exhibited promising electrochemical performance. Controlling the collection process of electrospinning, the electrode mass loading can be increased to 10 mg cm-2. Combined with surface selenizing modification, the performance of as-prepared MoP/SnO2/MoSe2 was further improved, which exhibited areal capacity about 1.5 mAh cm-2 as anode for sodium/potassium ion storage with mass loading of 8-9 mg cm-2.

Harnessing Plasma-Assisted Doping Engineering to Stabilize Metallic Phase MoSe2 for Fast and Durable Sodium-Ion Storage

Metallic-phase selenide molybdenum (1T-MoSe₂ ) has become a rising star for sodium storage in comparison with its semiconductor phase (2H-MoSe₂ ) owing to the intrinsic metallic electronic conductivity and unimpeded Na⁺ diffusion structure. However, the thermodynamically unstable nature of 1T phase renders it an unprecedented challenge to realize its phase control and stabilization. Herein, a plasma-assisted P-doping-triggered phase-transition engineering is proposed to synthesize stabilized P-doped 1T phase MoSe₂ nanoflower composites (P-1T-MoSe₂ NFs). Mechanism analysis reveals significantly decreased phase-transition energy barriers of the plasma-induced Se-vacancy-rich MoSe₂ from 2H to 1T owing to its low crystallinity and reduced structure stability. The vacancy-rich structure promotes highly concentrated P doping, which manipulates the electronic structure of the MoSe₂ and urges its phase transition, acquiring a high transition efficiency of 91% accompanied with ultrahigh phase stability. As a result, the P-1T-MoSe₂ NFs deliver an exceptional high reversible capacity of 510.8 mAh g^-1 at 50 mA g^-1 with no capacity fading over 1000 cycles at 5000 mA g^-1 for sodium storage. The underlying mechanism of this phase-transition engineering verified by profound analysis provides informative guide for designing advanced materials for next-generation energy-storage systems.

Metallic Transition Metal Dichalcogenides of Group VIB: Preparation, Stabilization, and Energy Applications

Layered transition metal dichalcogenides (TMDs) of group VIB have been widely used in the realms of energy storage and conversions. Along with the existence of semiconducting states, their metallic phases have recently attracted numerous attentions owing to their fascinating physical and chemical properties. Many efforts have been devoted to obtain metallic TMDs with high purity and yield. Nevertheless, such metallic phase is thermodynamically metastable and tends to convert into semiconducting phase, which necessitates the exploration over effective strategies to ensure the stability. In this review, typical fabrication routes are introduced and those critical factors during preparation are elaborately discussed. Moreover, the stabilized strategies are summarized with concrete examples highlighting the key mechanisms toward efficient stabilization. Finally, emerging energy applications are overviewed. This review presents comprehensive research status of metallic group VIB TMDs, aiming to facilitate further scientific investigations and promote future practical applications in the fields of energy storage and conversion.

A stable and ultrafast K ion storage anode based on phase-engineered MoSe2

Potassium-ion batteries (PIBs) are attracting increasing attention due to the abundance of K resources, but the sluggish kinetics and inferior cycling stability of anodes still hinder their application. Herein, we present a hybrid 1T/2H phase MoSe₂ anode, which shows noticeable pseudocapacitive response and fast kinetics for K storage. Correspondingly, superior electrochemical performances including a high reversible capacity of 440 mA h g^-1 after 100 cycles at 0.1 A g^-1 and superb rate capacity of 211 mA h g^-1 at 20.0 A g^-1 are achieved. We believe this work may shed light on the phase engineering of transition metal compounds for rapid charging PIBs.

Anchoring SnS2 on TiC/C Backbone to Promote Sodium Ion Storage by Phosphate Ion Doping

Tin disulfide (SnS₂ ) shows promising properties toward sodium ion storage with high capacity, but its cycle life and high rate capability are still undermined as a result of poor reaction kinetics and unstable structure. In this work, phosphate ion (PO₄ ^3- )-doped SnS₂ (P-SnS₂ ) nanoflake arrays on conductive TiC/C backbone are reported to form high-quality P-SnS₂ @TiC/C arrays via a hydrothermal-chemical vapor deposition method. By virtue of the synergistic effect between PO₄ ^3- doping and conductive network of TiC/C arrays, enhanced electronic conductivity and enlarged interlayer spacing are realized in the designed P-SnS₂ @TiC/C arrays. Moreover, the introduced PO₄ ^3- can result in favorable intercalation/deintercalation of Na⁺ and accelerate electrochemical reaction kinetics. Notably, lower bandgap and enhanced electronic conductivity owing to the introduction of PO₄ ^3- are demonstrated by density function theory calculations and UV-visible absorption spectra. In view of these positive factors above, the P-SnS₂ @TiC/C electrode delivers a high capacity of 1293.5 mAh g^-1 at 0.1 A g^-1 and exhibits good rate capability (476.7 mAh g^-1 at 5 A g^-1 ), much better than the SnS₂ @TiC/C counterpart. This work may trigger new enthusiasm on construction of advanced metal sulfide electrodes for application in rechargeable alkali ion batteries.