In order to prevent polysulfide dissolution into liquid electrolytes and to promote the Li/S redox reaction (16Li 1 S8 $ Li2Sn $ Li2S), nanosized Mg0.6Ni0.4O, which has the catalytic effect of chemical bond dissociating and is expected to have an adsorbing effect due to the effect of retaining liquid electrolyte of MgO in a Li/iron sulfide secondary battery,16 was prepared by the sol-gel method as an electrochemically inactive additive for an elemental sulfur cathode for Li/S rechargeable batteries. The Li/S battery using an elemental sulfur cathode with a nanosized Mg0.6Ni0.4O added showed the improvement of not only the discharge capacity but also cycle durability ~maximum discharge capacity: 1185 mAh/g sulfur, C50 /C1 5 85%). The rate capability of the sulfur cathode was also increased with the addition of the nanosized Mg0.6Ni0.4O. From the results, it is confirmed that the nanosized Mg0.6Ni0.4O had the polysulfide adsorbing effect and the catalytic effect of promoting Li/S redox reaction. Furthermore, it is found that the nanosized Mg0.6Ni0.4O also increased the porosity of the sulfur cathode.