N-substituted caprolactam (CL) derivatives are examined as novel solid electrolyte interphase (SEI)-forming additives for improving the cycle performance of LiCoO2/graphite lithium-ion batteries (LIBs). Our electrochemical experiments and density function calculations reveal that N-substitution of acetyl, methyl, and vinyl functional groups to ɛ-CL greatly affects the oxidation/reduction reactions of the CL additives. The cyclability of LiCoO2/graphite cells are in the following order: acetyl–CL > ɛ-CL > methyl-CL > no additive > vinyl-CL. The cycle performance of acetyl–CL is comparable to that of VC, currently the most widely used SEI-forming additive in LIBs. The excellent performance of acetyl–CL is ascribed to the electron withdrawing nature of the acetyl group, which enhances the anodic stability and facilitates the reduction reaction, leading to superior SEI formation at the graphite anode.