Lithium nitrate (LiNO3) has been reported as a novel additive to improve the cycling performance of Li/S batteries because LiNO3 suppresses the polysulfide shuttling problem. However, several studies indicate that LiNO3 instead decreases the battery performance due to the formation of irreversible products at the cathode. In this study, we investigated the role of LiNO3 in irreversible product formation. To elucidate the effect of LiNO3, electrolytes with an excess concentration of LiNO3 were employed in the Li/S cell for discharge and cyclic voltammetry tests. In the discharge test, a distortion of the discharge profile near the end of the discharge was clearly observed for the Li/S battery using a 0.8?M LiNO3 electrolyte. The third plateau (distortion) representing the irreversible reaction was significantly more pronounced when a poor cathode that was not subjected to heat-treatment was used. In addition, cyclic voltammetry tests showed an extra cathodic peak at 1.5 V corresponding to the irreversible reaction. It was confirmed that the irreversible product can be partially recovered to high-order polysulfides and elemental S8 by applying a potential of over 2.9 V. Finally, to provide a more detailed explanation, we carried out a computational simulation of the irreversible reaction. The simulation indicated that the reaction was related to the formation of crystallized lithium sulfide, and the simulation results successfully reproduced the third plateau (distortion) in the discharge profile.