Two-dimensional nanostructures may effectively be utilized as electrode materials for lithium/sodium-ion batteries to enhance the energy and power densities and cycling stability, and to satisfy the increasing demands of electrical vehicles and power stations. This review introduces the ''geometry-driven'' concept to illuminate the mechanisms related to various geometric sites in 2D materials for improving their electrochemical performance. The geometric sites of 2D materials are categorized into point-like, line-like, and plane-like defects. The electronic structures of geometric sites are then discussed. Hierarchical materials constructed from 2D materials, such as 3D crumpled nanoparticles, nanoflowers, and heterostructures, are highlighted. A summary of applications of the in situ transmission electron microscopy (TEM) technique is presented toward understanding the mechanisms of geometric-site effects in 2D materials. Finally, some perspectives on the geometry concept for material designs, theoretical calculations for performance prediction, and modern in situ TEM techniques for uncovering electrochemical mechanisms are discussed.

Inspired by a folded protein, multistage structural MoS 2 is designed as an advanced anode material for lithium-ion batteries (LIBs). Density functional theory (DFT) calculations are initially performed, demonstrating that the ideal primary structure (PÀ MoS 2 ) has saw-tooth-like edges terminated by Mo atoms and the desired secondary structure (CÀ MoS 2 ) may form via crumpling. For the latter, more exposed (002) planes exist within the wrinkled parts, creating more active sites and promoting isotropic Li + insertion. Importantly, the rate capability and capacity of a MoS 2 anode are enhanced after such a PÀ MoS 2 to CÀ MoS 2 transition: a superb specific capacity of 1490 mAh/g for CÀ MoS 2 at 0.1 A/g (vs. 1083 mAh/g for PÀ MoS 2 ), an excellent cycling stability (858 mAh/g after 450 cycles at 0.5 A/ g), and an improved rate capability of 591 mAh/g at 1 A/g (vs. 465 mAh/g) are documented. The curving effects and mechanical properties of a single CÀ MoS 2 particle are further visualized by in situ TEM. Drastically enlarged spacing changes upon Liinsertion and high elasticity are confirmed, which lead to enhanced LIB performances and the excellent mechanical strength of CÀ MoS 2 . The present multistage design of a MoS 2 structure should pave the way toward high-energy MoS 2 anode materials for future LIBs.

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