This study, by using the ortho, meta, and para forms of phenylenedimaleimide (o-, m-, and p-phenylenedimaleimide), presents an analysis of electrolyte-additive positional isomer effects that are significant to the formation of the solid electrolyte interface (SEI), and thus the resulting battery's performance. The analysis is presented in terms of the SEI's electrochemical properties, growth mechanism, free volume, ionic transfer properties, and its charge–discharge performance. This study shows that the reduction reaction mechanism for each of the positional isomers is the same; however, the isomers when used individually provide different degrees of reduction reaction and varying SEI physical infrastructures. O-phenylenedimaleimide can inhibit SEI formation because its three-dimensional barrier is greater than that of either m- or p-phenylenedimaleimide. This three-dimensional barrier gives rise to a loose SEI structure that reduces ionic motion resulting in discontinuous SEI formation and a decrease in ionic diffusion velocity. Battery performance testing showed that p-phenylenedimaleimide has the greatest discharge capacity, together with significant reversibility, because of dense and uniform structure of its SEI. Our results indicate that by selecting an appropriately functionalized isomer functional groups on SEI formation and thus improve battery performance. The maleimide-based additives provide valuable information on SEI formation mechanisms for future electrolytes.