SEI or Solid Electrolyte Interfaces are becoming one of the hottest areas of battery design fundamentals. Once limited to mostly lithium ion anode studies, today it is being viewed as the solution to improved efficiency, higher voltage compatibilities, reduced corrosion, improved cycle life, increased operating temperature windows, improved shelf life, impedance control, a means to control reaction product solubility, selective ionic diffusion control, a means to expanded electrolyte options, and the list goes on. Once often over looked in aqueous battery systems, the transition to mercury free design stressed the importance of stabilizing the surface of zinc from corrosion and hydrogen generation, while balancing impedance contributions associated with the passivation SEI. Whether referred to as particle encapsulation, electrolyte additives, or corrosion and passivation, there is no denying the significance of SEI films and their contribution and role in electrochemical optimization. The lithium metal battery would not be possible without the formation of a stable SEI passivation layer on its surface. Advances in material science and nano-chemistry, as well as coatings and encapsulation technologies will make this a hot topic of study and enable continued optimization of existing and future product designs.