Overcoming the grand energy and climate challenges of the 21st century will require advanced energy storage solutions. Growing demands for vehicle electrification and grid-level energy storage are quickly surpassing the capabilities of conventional lithium-ion batteries. Lithium metal batteries offer a much higher theoretical energy density, however, their practical applications have been limited by prohibitively short cycling lifetimes. The solid electrolyte interphase (SEI) is an interfacial structure that forms via decomposition of the electrolyte on reactive electrode surfaces, and it determines the (electro)chemical stability of the electrode. Although the concept of the SEI was introduced over 45 years ago, a rigorous understanding of how its structure and properties affects the stability and cycling lifetime of the lithium anode is still lacking. Another key open problem is understanding how the structure of the electrolyte can be manipulated to affect the structure of the SEI and elicit desired interfacial properties. A major challenge to reaching both of these ends has been achieving an adequate characterization of this nanometer-sized, dynamic, and environmentally sensitive interfacial layer. To tackle this ambitious problem we have employed a multi-modal and multi-length scale strategy that combines operando electrochemical methods, to track the heterogeneous surface layer properties under battery relevant dynamic operating conditions, with ex-situ chemical and structural characterization techniques that offer nanometer to atomic-scale spatial resolution. In this work, we first introduce the feedback mode of scanning electrochemical microscopy (SECM) as a powerful tool to track the potentiodynamic formation of the SEI on copper substrate electrodes. This methodology allows us to identify distinct stages in the SEI formation process, such as a buildup of incipient surface layers that was found to occur prior to effective surface passivation. Throughout this work, two complementary anions, LiTFSI and LiFSI, have been compared to gain insights into the role of electrolyte structure in determining the SEI structure and properties. The electrochemical reactions associated with the formation of the SEI were elucidated by identifying volatile reaction products using differential electrochemical mass spectrometry (DEMS). A systematic study on the effect of trace water in the electrolyte found that it plays a key role in catalyzing the decomposition of the anion and promoting the formation of the SEI. These results provide a mechanistic understanding for the observed improvement in lithium cycling performance in water contaminated electrolytes. Finally, we study the evolution of the SEI directly on lithium metal electrodes during lithium plating. The LiFSI-derived SEI was found to be far more robust to the surface stresses generated during plating, as compared to the LiTFSI derived SEI. These differences in SEI stability were correlated to corrosion of lithium deposits using cryogenic transmission electron microscopy (TEM) imaging, which helps to explain the improved cycling behavior of lithium anodes in LiFSI-based electrolytes. Atomic-scale TEM imaging of the interface provided a link between the structure of the SEI and its stability during lithium cycling, which helps to advance the design of interfaces for improving LMBs.