Sulfurized polyacrylonitrile (SPAN) has emerged as an excellent cathode material for lithium–sulfur batteries (LiSBs), and it addresses the shuttle effect through a solid‒solid reaction. However, the actual sulfur loadings in SPAN often remain below 40 wt%. Due to the susceptibility of polysulfides-to-nucleophilic reactions with electrolytes, achieving physical encapsulation of elemental sulfur is a challenging task. In this study, a free-standing cathode material with a high sulfur/selenium (S/Se) loading of 55 wt% was fabricated by introducing SeSx into the unique lotus root-like pores of porous SeSxPAN nanofiber membranes by electrospinning and a two-step heat treatment. Insoluble compounds were formed due to nucleophilic interactions between lithium polyselenosulfides (LiSeSx) and the electrolyte, which potently blocked the existing lotus root-like pores and facilitated the creation of a thin cathode–electrolyte interphase on the fiber surface. This dual functionality of LiSeSx safeguarded the active material embedded within the porous structure. The SeS15PAN cathode exhibited remarkable cycling stability with almost no degradation after 200 cycles at 0.2 C, along with a high discharge capacity of 580 mAh/g. This approach presents a solution for addressing the insufficient sulfur content in SPAN.