In this chapter, the authors provide insights into the roles and progresses of electrolyte and separator in the Lithium-Ion Supercapacitors (LISC). For LISCs, electrolytes must cater to the needs of both electrodes; hence, in principle, new battery chemistries would have incurred new electrolyte compositions. The role of electrolytes is to serve as the medium for the transfer of charge, which are in the form of ions, between anode and cathode, which are in close interaction with both electrodes. The electrolytes often include aqueous and nonaqueous electrolytes. The aqueous electrolytes show higher ionic concentration and higher ionic conductivity, which can satisfy the high power of LISC. Polymer electrolytes, serving as both the electrolyte and the separator, can show good compatibility between the electrolyte and the separator. With the objective of improving the electrochemical performance, authors dedicated strategies to stabilize electrolyte and separator during charge–discharge cycling.