It is crucial to comprehend the effect of the solid electrolyte interphase (SEI) on battery performance to develop stable Li metal batteries. Nonetheless, the exact nanostructure and working mechanisms of the SEI remain obscure. Here, we have investigated the relationship between electrolyte components and the structural configuration of interfacial layers using an optimized cryogenic transmission electron microscopy (Cryo-TEM) analysis and theoretical calculation. We revealed a unique dual-layered inorganic-rich nanostructure, in contrast to the widely known simple specific component-rich SEI layers. The origin of stable Li cycling is closely related to the Li-ion diffusion mechanism via diverse crystalline grains and numerous grain boundaries in the fine crystalline-rich SEI layer. The results can elucidate a particular issue pertaining to the chemical structure of SEI layers that can induce uniform Li diffusion and rapid Li-ion conduction on Li metal anodes, developing stable Li metal batteries.