Computational tools for studying the atomic and molecular structures of complex systems are very useful. This work uses the first principle method to investigate the structural and optoelectronic properties of a polyethylene oxide (PEO)-based solid polymer electrolyte including lithium salt (LiFSI) as an ion-conducting species. DFT has been used for the analysis of electrolyte conducting properties. The electrical band gap of the polymer electrolyte (PEO)8-LiFSI system has been quantitatively examined by density of states (DOS) studies. From electronical analysis, it is verified that (PEO)8 exhibits insulator properties with a wide band gap of 6.2797 eV, whereas the bandgap of (PEO)8-LiFSI slightly drops to 6.0028 eV. These findings suggest that the current polymer electrolyte system (PEO)8-LiFSI could be a viable option for the electrolyte in next-generation energy storage devices.