A high concentration lithium salt in electrolytes has recently attracted attention to improve the performance of rechargeable lithium–oxygen (Li–O₂) batteries. In particular, lithium nitrate (LiNO₃) was noted as a promising salt for Li–O₂ batteries because of its catalytic effect during the charge process. Nevertheless, the effect of LiNO₃ concentration on the capability and electrochemical response of the cathode in Li–O₂ batteries is not entirely understood. Here, the correlation between the performance of the batteries and the concentration of LiNO₃ in electrolytes is studied, including the formation of Li₂O₂ and the decomposition of electrolytes in the discharge and charge processes. We show that a high concentration of LiNO₃ leads to a lower overpotential due to the catalytic effect of the oxygen evolution reaction (OER) in the charging process, which suppresses the decomposition of electrolytes. Using the XANES and SEM observation, we also found that the growth of Li₂O₂ during the discharge process is related to the concentration of LiNO₃. This result indicated that highly concentrated LiNO₃ salt in an electrolyte not only boosts the cell performance of Li–O₂ batteries but also possibly the safety and performance of the Li–O₂ cell.