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Solid-electrolyte interphases (SEIs), oftentimes viewed as the most important yet least understood part of alkali ion/metal batteries, remain a key bottleneck for battery design. Despite extensive research in the past few decades, to date we have only begun to unravel the structure of SEIs, while its dynamic nucleation and growth mechanism is still elusive. Here we discuss the existing progress in characterizing SEIs in the battery community, and propose that SEI growth depends critically on the electrical double layer (EDL) structure, a factor that has been largely hidden or ignored to date. We will further discuss methods for simultaneously characterizing EDL and SEIs, with a particular focus on the emerging electrochemical 3D atomic force microscopy (EC-3D-AFM) and shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) techniques. In the end, we will propose strategies for predictive design of electrolytes to enable controlled EDL and SEI structures and achieve desired battery performance.