Electrical energy storage holds the key to wider adoption of renewable energy. Electrical double layer capacitors (or supercapacitors) and Li-ion batteries are two complementary technologies for storing electricity, both involving a working solid/electrolyte interface. Despite their commercialization, some fundamental questions remain unanswered. Supercapacitors store energy as the charge on the surface of a porous carbon material. Here the dependence of the capacitance on the pore size has been a hot topic in the experimental efforts. Using a classical density functional theory, we show that the specific capacitance in fact oscillates with the pore size due to the interference of electrical double layers. This novel and interesting behavior from prediction now calls for experimental confirmation. Li-ion batteries work by shuttling Li ions back and forth between cathode and anode through the electrolyte. The function of Li-ion batteries is enabled by the formation of a solid-electrolyte interphase at the graphite anode. Using first principles molecular dynamics, we examine the solvation of Li ions in the carbonate electrolyte and the initial chemical reactions between the electrolyte and the lithiated graphite. We find that the LiF species plays an important role at the interface.