Weyl semimetal orthorhombic Td-WTe2as an electrode material for sodium- and potassium-ion batteries

Theoretical study on the effect of torsional deformation on WTe2 as a cathode material for calcium ion batteries

CONTEXT

In this study, the electronic structure and diffusion barrier of the Ca-adsorbed WTe2 system under different torsion angles were calculated based on the first-principles method. The results show that the structure and properties of the pure WTe2 system and Ca-adsorbed WTe2 system are affected by torsional deformation. When the torsion angle is 6°, the intrinsic WTe2 changes from a direct band gap to an indirect band gap. The torsional deformation does not change the metallicity of the Ca-adsorbed WTe2 system. The Ca-d state electrons contribute to the electron density of WTe2. The calculation of the diffusion barrier shows that the torsion deformation promotes the diffusion of calcium on the surface of WTe2. This study provides a theoretical basis for the regulation of electrode materials.

METHODS

In this study, Materials Studio 8.0 software was used to construct the WTe2 model and Ca-adsorbed WTe2 model, and CASTEP module was used for first-principles calculation.

Boosting cobalt ditelluride quantum-rods anode materials for excellent potassium-ion storage via hierarchical physicochemical encapsulation

The exploration of anode materials that can store large-sized K-ion to solve the poor kinetics and large volume expansion issues has become the key scientific bottlenecks hindering the development of potassium-ion batteries (PIBs). Herein, ultrafine CoTe₂ quantum rods physiochemically encapsulated by graphene and nitrogen-doped carbon (CoTe₂@rGO@NC) are regarded as anode electrodes for PIBs. Dual physicochemical confinement and quantum size effect not only enhance electrochemical kinetics but also restrain large lattice stress during repeated K-ion insertion/extraction process. Superior electronic conductivity, K-ion adsorption, and diffusion ability can be acquired for CoTe₂@rGO@NC, confirmed through first-principles calculations and kinetics study. K-ion insertion/extraction proceeds via a typical conversion mechanism relying on Co as the redox site, where the robust chemical bond of COCo plays an important role in maintaining the electrode stability. Accordingly, CoTe₂@rGO@NC contributes a high initial capacity of 237.6 mAh·g^-1 at 200 mA·g^-1, a long lifetime over 500 cycles with low-capacity decay of 0.10% per cycle. This research will lay the materials science foundation for the construction of quantum-rod electrodes.

Comprehensively Understanding the Role of Anion Vacancies on K-Ion Storage: A Case Study of Se-Vacancy-Engineered VSe2

Anion vacancy engineering (AVE) is widely used to improve the Li-ion and Na-ion storage of conversion-type anode materials. However, AVE is still an emerging strategy in K-ion batteries, which are promising for large-scale energy storage. In addition, the role of anion vacancies on ion storage is far from clear, despite several proposed explanations. Herein, by employing VSe₂ as a model conversion-type anode material, Se vacancies are intentionally introduced (labeled as P-VSe_2-x ) to investigate their effect on K⁺ storage. The P-VSe_2-x shows excellent cyclability in half cells (143 mA h g^-1 at 3.0 A g^-1 after 1000 cycles) and high energy density in coin-type full cells (206.8 Wh kg^-1 ). By applying various electrochemical techniques, the effects of Se vacancies on the redox potentials of K-ion insertion/extraction and the K-ion diffusion in electrodes upon cycling are uncovered. In addition, the structural evolution of Se vacancies during potassiation/de-potassiation using various operando and ex characterizations is revealed. Moreover, it is demonstrated that Se vacancies can facilitate the breaking of VSe bonds upon the P-VSe_2-x conversion using theoretical calculations. This work comprehensively explains the role of anion vacancies in ion storage for developing high-performance conversion-type anode materials.

Unlock the Potassium Storage Behavior of Single-Phased Tungsten Selenide Nanorods via Large Cation Insertion

Metal chalcogenide anodes with a layered structure have been regarded as potential K-based electrochemical energy storage devices with high energy density for large-scale energy storage applications. However, their development is impeded by the slow K-ion transport kinetics and poor structural stability. In this work, the energy-storage behavior is investigated first and decisively associated them with the capacity-degradation of the promising layer-structured WSe₂ from an integrated chemical and physical point of view. Then, a single-phased WSe₂ with pre-intercalated high K content (SP-K_x WSe₂ ) is designed to overcome the capacity-degradation issue fundamentally. Theoretical calculations clarify the beneficial effect of K-ions inside the interlayer of WSe₂ on boosting its electrochemical performance, including increasing the electronic conductivity, promoting the K-ion diffusivity, and improving the structural stability. The novel design enables the K-ions pre-intercalated WSe₂ anode material to exhibit a high reversible specific capacity of 211 mAh g^-1 at 5 A g^-1 and superior cycling stability (89.3% capacity retention after 5000 cycles at 1 A g^-1 ). Especially, the K-ion hybrid capacitor, assembled from the anode of SP-K_x WSe₂ and the cathode of porous activated carbon, delivers superior energy-density up to 175 Wh kg^-1 , high power-density as well as exceptional cycling stability.