The tendency of metals to form uncontrolled dendritic morphologies during electrodeposition hinders the development of safe and reliable metal batteries. Multiphase nanostructured electrolytes can suppress dendritic growth if the mechanical modulus of the electrolyte is high relative to that of the metal or if the conducting channels are confined to nanoscale dimensions. Direct visualization and analysis of electrodeposition within polymeric nanostructures elucidates the structure−property relationships and mechanisms underlying the suppression of dendrite growth. To overcome these challenges, we introduce here a generalizable platform by which we can directly characterize electrodeposited metal morphologies in well-defined, multiphase nanostructured electrolytes by top-down SEM imaging [1]. This approach requires no destructive sample preparation techniques between electrodeposition and imaging. The platform consists of coplanar electrodes coated by polymeric structures consisting of alternating ion conducting and nonconducting material which are precisely created by lithography and etch techniques. These fabricated polymeric structures are designed to carefully model deposition of metals in nanostructured polymeric separators and electrolytes. In this study we track the electrodeposition of silver ions (Ag⁺) dissolved in dry, aprotic poly(ethylene oxide) (PEO) polymer electrolyte confined within channels that are defined by a microfabricated nonconductive polystyrene structures. We find that Ag metal electrodeposition occurs only in the conductive domains with no significant deformation of the adjacent low-modulus nonconductive and confining domains. Moreover, we have shown that the Ag metal deposit morphology was strongly dependent on the dimensions of the conducting domains. When confinement width was similar in scale to the critical length scale for unstable deposition, the metal morphology was dense, unbranched filaments. Gradually increasing the confinement width led to branched dendritic deposition, although the side branch feature size was limited by the size of the confinement. These results demonstrate that in addition to mechanical and transport properties, the nanodimension of the structured electrolyte can also serve to stabilize dendritic morphology growth. This work highlights critical parameters for design of structured battery components such as solid-electrolytes, separators, and current collectors. [1] Sharon, D.; Bennington, P.; Patel, S.N.; Nealey, P.F.*, “Stabilizing Metal Electrodeposition by Limiting Spatial Dimensions in Nanostructured Electrolytes,” ACS Energy Lett. 2020, 5, 2889−2896. Figure 1