Herein, free-standing supertubes, composed of a single layer of close-packed carbon-coated nanoparticles, are fabricated by a confined-epitaxial-assembly strategy. Benefiting from the tubular geometry, monolayer superlattice structure, and uniform and conformal carbon coating, such free-standing supertubes promise high electrochemical performance while simultaneously serving as a robust platform for reliably elucidating the structure-performance relationship of lithium-ion batteries (LIBs). As a model, Fe3O4 supertubes, when used as LIB anodes, can deliver a capacity of ∼800 mAh g−1 after 500 cycles at 5 A g−1, outperforming most Fe3O4-based materials reported previously. More importantly, the structural evolution of Fe3O4 supertubes is revealed at meso-/nano-/atomic scales simultaneously upon lithiation and delithiation, which correlates well with the battery's capacity reactivation, stabilization, and degradation behaviors during the course of 500 cycles. Complex nanostructures are expected to outperform their simple counterparts with enhanced electrochemical performance. Investigating the complex structural evolution of active materials with electrochemical performance is a crucial step toward the development of high-performance electrochemical devices. Herein, free-standing supertubes comprising a monolayer of carbon-coated nanoparticles enabling homogeneous electrochemical process have been fabricated. Such supertubes provide a fundamentally important platform capable of revealing long-term structural evolution on multiple length scales, the better understanding of which may provide insights into the device operation mechanisms and shed light on the rational design of high-performance electrode materials. Given the flexibility in tuning nanoparticle composition, it is anticipated that such supertubes may serve as a platform for elucidating the structure-performance relationship of many other energy-storage and energy-conversion devices. Well-defined supertubes, which integrate the merits of the tubular geometry, conformal carbon coating, and nanoparticle monolayer superlattice structure, are designed and fabricated by a confined-epitaxial-assembly method. Such self-assembled supertubes, when evaluated as electrode materials for lithium-ion batteries, exhibit superior electrochemical performance while also providing a fundamentally important platform capable of visually revealing a structure-performance relationship on multiple length scales over long-term cycling.