A fundamental understanding of electrolytes is critical for designing lithium-ion batteries with excellent performance and high safety. The traditional solvents in electrolytes of lithium-ion batteries are mainly ethylene carbonate and propylene carbonate. Despite their similar structures and chemical properties, ethylene carbonate-based electrolytes have been reported to enable the reversible reaction of graphite anodes, whereas propylene carbonate-based electrolytes cause the exfoliation of graphite. Herein, we have investigated the oxidation stability and the reductive decomposition of ethylene carbonate and propylene carbonate from electron-level quantum calculations. While small differences in their oxidation stability were presented, we found disparity in their reductive decomposition. The reductive product of lithium alkyl carbonate exhibits different geometrical and molecular orbitals, which was considered to influence the quality of the ethylene/propylene carbonate solvent-based solid–electrolyte interphase (SEI). This study presents the disparity of redox decomposition of ethylene carbonate–propylene carbonate in Li-ion batteries, which is expected to guide the design of new electrolyte systems, thereby enhancing the performance of Li-ion batteries.