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Recent years have witnessed a surge of research in two-dimensional (2D) nanostructures for development of new rechargeable Li/Na-ion battery systems. Herein, via first-principles calculations we demonstrate strain-engineered Li/Na adsorption and storage in 2D MoS2 as anode material, aiming to enhance the operating performance of Li/Na-ion batteries. Our results show that tensile strain greatly increases the adsorption of Li/Na atoms on MoS2, and a modest strain of 6% increases Li (Na) adsorption energy by over 70%, which originates from the strain-induced upshift of Mo d states towards Fermi level that interact strongly with Li/Na s states, in analogy with the d-band model in metal catalyst. Significant narrowing of the n-doped semiconducting gap of MoS2 suggests the improved electric conductivity that may benefit charge carrier transport. By mapping out the potential energy surfaces, we show shallow energy barriers of ion diffusion with ~0.2 eV for Li and 0.1 eV for Na. Furthermore, the strain-steered competition between chemical bonding and coulomb repulsion results in high Li/Na storage capability and relatively low average operating voltage. We believe that the fundamental principle underlying the use of strain to enhance performance of renewable ion battery is applicable to other stretchable low-dimensional nanomaterials.

Recent years have seen a surge in the use of low-dimensional transition metal dichacolgenides, such as MoS 2 , as catalysts for the electrochemical hydrogen evolution reaction. In particular, sulfur vacancies in MoS 2 can activate the inert basal plane, but that requires an unrealistically high defect concentration (~9%) to achieve optimal activity. In this work, we demonstrate by firstprinciples calculations that assembling van der Waals heterostructures can enhance the catalytic activity of MoS 2 with low concentrations of sulfur vacancies. We integrate MoS 2 with various two-dimensional nanostructures, including graphene, h-BN, phosphorene, transition metal dichacolgenides, MXenes, and their derivatives, aiming to fine-tune the free energy of atomic hydrogen adsorption. Remarkably, an optimal free energy can be achieved for a low sulfur vacancy concentration of~2.5% in the MoS 2 /MXene-OH heterostructure, as well as high porosity and tunability. These results demonstrate the potential of combining two-dimensional van der Waals assembly with defect engineering for efficient hydrogen production.

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