A lithium-sulfur battery with a low cost, a long cycle life, safety, and high gravimetric energy density may be a viable option for overcoming the charge-storage limitations of lithium-ion batteries. This research describes how to increase the cycle life and performance of lithium-sulfur batteries by using highly conductive and lightweight cathode materials composed of poly(1,5-diaminoanthraquinone) (PDAAQ) and non-stoichiometric magnesium oxide nanoparticles (MgO). The cell with the MgO/PDAAQ/S cathode has a discharge capacity of 1239 mA h g^–1 after 200 cycles. The discharge capacity is maintained at 1020 mA h g^–1 after 500 cycles. When considering non-stoichiometric MgO, which is oxygen-rich, the adsorption energy of Li becomes highly negative (−4.648 eV/Li atom), making the structure active for adsorption of lithium polysulfide chains. The novel combination of a MgO/PDAAQ/S cathode has a significant potential for the fabrication of high gravimetric energy density Li–S batteries (570 W h kg^–1 per cell) over 200 cycles.