Lithium-sulfur batteries (LSBs) hold immense promise for next-generation energy storage due to high theoretical energy density and abundant electrode materials. However, their practical implementation is intrinsically hindered by sulfur’s insulating nature, polysulfide shuttling, and slow electrochemical reactions. The recently developed organic two-dimensional (2D) aromatic material C5N, with its exceptional charge transport, and stability, presents a promising cathode host for LSBs, extending its functionality beyond optoelectronics. Density functional theory calculations demonstrate that C5N achieves a dual benefit: it suppresses polysulfide shuttling and improves electrochemical reactions. C5N inhibits the shuttle effect by preferentially binding to lithium polysulfides (Li2Sn, 4 ≤ n ≤ 8) more strongly than electrolytes, facilitated by chemisorption through ionic Li-N bonds. Additionally, residual lithium in the cathode during cycling positively contributes to suppressing polysulfide shuttling. The electron-rich nitrogen sites in C5N serve as active adsorption sites, and the optimal N/C ratio results in moderately strong binding energies for Li2Sn (0.55–1.90 eV), preventing excessive binding as observed in C2N and C3N4. This enables C5N to offer the lowest energy barriers for polysulfide conversions in sulfur reduction reaction and the dissociation of the final discharge product Li2S among 2D carbon-nitrides, thereby accelerating electrochemical reactions.