Improved lithium and sodium ion storage properties of WS2 anode with three-layer shell structure

Al2O3-Induced Phase Conversion Regulation of WS2 Anode Enhances the Lithium Storage Reversibility

WS2 is an attractive anode in alkali metal ion batteries (AMIBs) due to its 2D-layered structure and high theoretical capacity. However, the shuttle effect of sulfur and the spontaneous growth of W nanoparticles are key issues that limit the alkali-ion accommodation ability. Now, it is still a great challenge to achieve in situ control of the microstructure evolution paths in enclosed batteries for extending the cycling reversibility/lifespan. Herein, the phase conversion paths of both film- and powder-type WS2 anodes are investigated in lithium-ion batteries. It is found that the reversible conversion mechanism is beneficial for alleviating the shuttle effect through strong W-LixSy bonding. Also, once the size of the phase-converted W/WS2 redox pair exceeds ∼10 nm inside the anode layer, the Li+ storage ability will severely decay due to uncontrollable W precipitation. To maintain high reversibility, amorphous Al2O3 is introduced upon pristine WS2. After initializing the battery test, the particle size of the W/WS2 redox pair is in situ modulated within the range of ∼3-5 nm because of the refinement effect of gradually pulverized Al2O3. Thus, the decay suppression effect lasting over 750-1400 cycles is obtained with enhanced W ↔ WS2 conversion efficiency and good capacity retention. This is expected to promote the optimization of Mo-group sulfides/selenides/tellurides toward AMIBs.

Delicate Co-Control of Shell Structure and Sulfur Vacancies in Interlayer-Expanded Tungsten Disulfide Hollow Sphere for Fast and Stable Sodium Storage

Hollow multishelled structure (HoMS) is a promising multi-functional platform for energy storage, owing to its unique temporal-spatial ordering property and buffering function. Accurate co-control of its multiscale structures may bring fascinating properties and new opportunities, which is highly desired yet rarely achieved due to the challenging synthesis. Herein, a sequential sulfidation and etching approach is developed to achieve the delicate co-control over both molecular- and nano-/micro-scale structure of WS_{2- x} HoMS. Typically, sextuple-shelled WS_{2- x} HoMS with abundant sulfur vacancies and expanded-interlayer spacing is obtained from triple-shelled WO₃ HoMS. By further coating with nitrogen-doped carbon, WS_{2- x} HoMS maintains a reversible capacity of 241.7 mAh g^-1 at 5 A g^-1 after 1000 cycles for sodium storage, which is superior to the previously reported results. Mechanism analyses reveal that HoMS provides good electrode-electrolyte contact and plentiful sodium storage sites as well as an effective buffer of the stress/strain during cycling; sulfur vacancy and expanded interlayer of WS_{2- x} enhance ion diffusion kinetics; carbon coating improves the electron conductivity and benefits the structural stability. This finding offers prospects for realizing practical fast-charging, high-energy, and long-cycling sodium storage.

Improved Electrochemical Performance of NTs-WS2@C Nanocomposites for Lithium-Ion and Sodium-Ion Batteries

Even though WS₂ nanotubes (NTs-WS₂) have great potential as anode materials for lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs) thanks to their unusual layered structure, their conductivity and cycling stability are far from satisfactory. To tackle these issues, carbon-coated WS₂ (NTs-WS₂@C) nanocomposites were prepared through a facile synthesis method that involved precipitating a carbon precursor (20% sucrose) on WS₂ nanotubes, followed by annealing treatment under an argon environment. Thanks to the presence of highly conductive and mechanically robust carbon on the outer surface, NTs-WS₂@C nanocomposites show improved electrochemical performance compared with bare NTs-WS₂. After 60 cycles at 80 mA g^-1 current density, the cells display high capacities of 305 mAh g^-1 in LIBs and 152 mAh g^-1 in SIBs, respectively. As the current density increases to 600 mA g^-1, it provides specific capacities of 209 and 115 mAh g^-1, correspondingly. The enhanced electrochemical performance in LIBs and SIBs is primarily attributed to the synergistic effects of the tubular architecture of WS₂, carbon network and stable nanocomposite structure, which can effectively constrain volume variation during the metal ions intercalation/deintercalation processes.

Facile fabrication of WS2 nanocrystals confined in chlorella-derived N, P co-doped bio-carbon for sodium-ion batteries with ultra-long lifespan

Sodium-ion batteries (SIBs) have been regarded as a promising substitute for lithium-ion batteries but there are still formidable challenges in developing an anode material with adequate lifespan and outstanding rate performance. Transition metal dichalcogenides (TMDs) are promising anode materials for SIBs due to their high theoretical capacities. However, their severe volume expansions and low electronic conductivity impede their practical developments. In addition, the synthesis of energy storage materials from waste biomass has aroused extensive attention. Herein, we synthesize WS₂ nanocrystals embedded in N and P co-doped biochar via a facile bio-sorption followed by sulphurization, employing waste chlorella as the adsorbent and bio-reactor. The WS₂ nanocrystals are beneficial for storing more sodium ions and expediting the transportation of sodium ions, thus improving the capacity and reaction kinetics. Chlorella acts as a reactor and not only inhibits the stacking of WS₂ nanocrystals during the synthesis process but also alleviates the mechanical pressure of composite during the charge/discharge process. As a result, the WS₂/NPC-2 electrode delivers a high specific capacity (436 mA h g^-1 at 0.1 A g^-1) and superior rate performance of 311 mA h g^-1 at 3 A g^-1 for SIBs. It also exhibits excellent stability even up to 6000 cycles at 5 A g^-1, which is one of the optimal long-cycle properties reported for WS₂-based materials to date.

Atomic layer deposition of tungsten sulfide using a new metal-organic precursor and H2S: thin film catalyst for water splitting

Transition metal dichalcogenides (TMDs) are extensively researched in the past few years due to their two-dimensional layered structure similar to graphite. This group of materials offers tunable optoelectronic properties depending on the number of layers and therefore have a wide range of applications. Tungsten disulfide (WS₂) is one of such TMDs that has been studied relatively less compared to MoS₂. Herein, WS _x thin films are grown on several types of substrates by atomic layer deposition (ALD) using a new metal-organic precursor [tris(hexyne) tungsten monocarbonyl, W(CO)(CH₃CH₂C≡CCH₂CH₃)₃] and H₂S molecules at a relatively low temperature of 300 °C. The typical self-limiting film growth by varying both, precursor and reactant, is obtained with a relatively high growth per cycle value of ∼0.13 nm. Perfect growth linearity with negligible incubation period is also evident in this ALD process. While the as-grown films are amorphous with considerable S-deficiency, they can be crystallized as h-WS₂ film by post-annealing in the H₂S atmosphere above 700 °C as observed from x-ray diffractometry analysis. Several other analyses like Raman and x-ray photoelectron spectroscopy, transmission electron microscopy, UV-vis. spectroscopy are performed to find out the physical, optical, and microstructural properties of as-grown and annealed films. The post-annealing in H₂S helps to promote the S content in the film significantly as confirmed by the Rutherford backscattering spectrometry. Extremely thin (∼4.5 nm), as-grown WS _x films with excellent conformality (∼100% step coverage) are achieved on the dual trench substrate (minimum width: 15 nm, aspect ratio: 6.3). Finally, the thin films of WS _x (as-grown and 600/700 °C annealed) on W/Si and carbon cloth substrate are investigated for electrochemical hydrogen evolution reaction (HER). The as-grown WS _x shows poor performance towards HER and is attributed to the S-deficiency, amorphous character, and oxygen contamination of the WS _x film. Annealing the WS _x film at 700 °C results in the formation of a crystalline layered WS₂ phase, which significantly improves the HER performance of the electrode. The study reveals the importance of sulfur content and crystallinity on the HER performance of W-based sulfides.