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. Two-dimensional (2D) materials, with unique chemical and electronic properties, have attracted great attention as one of the most promising electrode materials for rechargeable batteries to satisfy the ever-increasing demands of higher power and energy density, superior rate performance, and long cycling life. Hundreds of 2D materials and their derivatives, such as graphene, h-BN, g-C3N4, phosphorene, MoS2, and MXenes, have been fabricated and utilized as electrode materials. Creating new geometric defects within 2D nanosheets and constructing three-dimensional hierarchical materials made of such sheets have been proved to be an effective strategy to further improve the electrochemical performances. This is a newly emerged discipline combining the mathematical concept, i.e., geometry, materials design engineering, and electrochemistry, which can be referred as a novel ���geometry-driven��� energy storage. In this review, geometric sites of 2D materials are categorized into point-like (vacancy, protrusion, and heteroatom doping), line-like (edges, wrinkles, and lattice), and plane-like (interlay spacing) geometric sites based on the defect morphologies. The electronic structure of geometric sites, the binding energy, and diffusion barriers between geometric sites and lithium/sodium ions are systematically investigated. Hierarchical structures (crumpled particles, nanoflowers, and van der Waals heterostructures) obtained from 2D materials are also illustrated. Utilization of in situ transmission electron microscopy (TEM) techniques is shown to be very useful to shed new light onto electrochemical mechanisms affiliated with different geometric sites in 2D materials. Lastly, the perspectives of the regarded geometry concept for a material design, theoretical calculations for performance prediction, and in situ TEM techniques for uncovering electrochemical mechanisms are summarized and some possible research directions proposed to smartly utilize 2D materials in the fields of energy storage. Two-dimensional (2D) materials have been effectively utilized as electrodes for energy-storage devices to satisfy the ever-increasing demands of higher power and energy density, superior rate performance, and long cycling life. Creating new geometric defects within 2D nanosheets (such as point-like, line-like, and plane-like sites) and constructing 3D hierarchical materials from such 2D materials have been proved to be effective strategies to further improve the electrochemical performance.