Lithium-ion batteries (LIBs) have revolutionized many industrial sectors including portable consumer electronics, transportation, and grid-scale energy storage. The continuous advancement of battery technology is critical to reduce our carbon footprint and to meet the ever-growing demand for energy. Utilizing Li metal as the anode material can result in significant improvement in the energy density of the batteries compared to the traditional graphite anode based LIBs. However, several challenges associated with stability, safety, and long-term performance need to be solved for successful commercialization of Li metal batteries. One of the key challenges is high reactivity of Li metal resulting in rapid decomposition of the electrolyte molecules at the anode surface. The decomposed products lead to the formation of a complex solid electrolyte interphase (SEI) layer during battery cycling consisting of a mixture of inorganic and organic components. The SEI layer at the graphite anode in a functional LIB is usually stable over several charge/discharge cycles, and prevents continuous degradation of the electrolyte while facilitating the diffusion of Li ions. On the other hand, the SEI layer at the surface of a Li metal anode is typically unstable leading to performance degradation and capacity loss of the battery. The experimental characterization of SEI layer in batteries is challenging. Therefore, a detailed molecular-level understanding of the mechanism of SEI formation at Li metal anode surface is crucial for designing functional interphases and for the successful commercialization of Li metal batteries. Numerous experimental efforts have been dedicated to understanding the composition and morphology of the SEI layer in LIBs and Li-metal batteries over the last 20 years. These experiments have primarily been focused on ex-situ analysis of the SEI layer composition using various characterization techniques such as XPS and NMR. These experiments have been dedicated to investigating the effect of electrolyte composition and structure on the product distribution and morphology of SEI layers. Experiments have also been performed to study the effect of additives on the decomposition of the electrolyte and the subsequent composition of the SEI layer, where recent results have demonstrated the beneficial effect of adding vinylene carbonate (VC) on the robustness of the SEI layer formed on the Li metal surface. It has also been shown that the final SEI morphology can change significantly with the addition of additive molecules in electrolyte formulations.