Al<sub>2</sub>O<sub>3</sub>‑Induced Phase Conversion\nRegulation of WS<sub>2</sub> Anode Enhances the Lithium Storage Reversibility

WS<sub>2</sub> is an attractive anode in alkali metal ion batteries\n(AMIBs) due to its 2D-layered structure and high theoretical capacity.\nHowever, the shuttle effect of sulfur and the spontaneous growth of\nW nanoparticles are key issues that limit the alkali-ion accommodation\nability. Now, it is still a great challenge to achieve in situ control\nof the microstructure evolution paths in enclosed batteries for extending\nthe cycling reversibility/lifespan. Herein, the phase conversion paths\nof both film- and powder-type WS<sub>2</sub> anodes are investigated\nin lithium-ion batteries. It is found that the reversible conversion\nmechanism is beneficial for alleviating the shuttle effect through\nstrong W–Li<sub><i>x</i></sub>S<sub><i>y</i></sub> bonding. Also, once the size of the phase-converted W/WS<sub>2</sub> redox pair exceeds ∼10 nm inside the anode layer,\nthe Li<sup>+</sup> storage ability will severely decay due to uncontrollable\nW precipitation. To maintain high reversibility, amorphous Al<sub>2</sub>O<sub>3</sub> is introduced upon pristine WS<sub>2</sub>.\nAfter initializing the battery test, the particle size of the W/WS<sub>2</sub> redox pair is in situ modulated within the range of ∼3–5\nnm because of the refinement effect of gradually pulverized Al<sub>2</sub>O<sub>3</sub>. Thus, the decay suppression effect lasting\nover 750–1400 cycles is obtained with enhanced W ↔ WS<sub>2</sub> conversion efficiency and good capacity retention. This is\nexpected to promote the optimization of Mo-group sulfides/selenides/tellurides\ntoward AMIBs.