Dendrite growth during electrodeposition is a critical challenge faced by high-energy metallic electrodes (e.g. Li, Zn) in rechargeable batteries. Under typical battery operation conditions, mossy structure forms on Li and Zn anode surface, which may be described as colonies of intertwined and branched filaments with diameters of ~ 100 nm. Compared to the well-known diffusion-limited dendritic growth, which occurs at much higher current density, the mossy growth process is vaguely described as “kinetically limited” in literature and its mechanism remains poorly understood. Recent study shows that the detachment of mossy filaments from substrate during electrostripping, which forms “dead lithium”, is a major cause of the battery capacity fading. A better understanding of the structure evolution of mossy deposits during cycling is therefore of considerable importance for developing successful strategies to improve the cycling life of metallic anodes.In this study, we performed operando tomography of mossy zinc structure growth on Cu substrate during electrodeposition with nanoscale resolution at the National Synchrotron Light Source II. Zinc electroplating was carried out at a constant overpotential of 50mv in an in-situ electrodeposition cell made of a capillary tube with the working (Cu wire) and counter (Zn wire) electrodes inserted from opposite ends. Tomography datasets were collected every 2 minutes with an effective voxel size of 43 nm. The 3D structure of zinc deposits was reconstructed from the datasets using open-source code TomoPy.Three different nucleation sites were observed, and the Zn mossy structure was divided into three groups according to these sites for a more detailed analysis. The calculated average porosity of the three mosses was ~35% and this was supported by ex-situ analysis. These three groups were separated into shells from their cores, and the most significant growth was occurring at the outermost shell. Further, the porosity and the volume of the inner shell and the core reached a steady state after a certain amount of time of electrodeposition. This result showed the penetration of the electrolyte to the inner parts of the mossy Zn was limited. All three mossy structure showed the same growth pattern without a preferred growth direction; however, there was an apparent difference of volume growth rate between them, indicating a possible diffusion limitation or uneven electric field distribution, which was in contrast with the previous studies. From ex-situ focus-ion beam scanning electron microscopy examination, it was shown that the moss structure was comprised of individual filaments, which repeatedly branched and entangled to form a mossy Zn. Figure 1