In an attempt to ameliorate the poor cyclability of LiNi0.5Mn1.5O4 at elevated temperature, quercetin is applied as an additive. The irreversible oxidative behavior of quercetin is thoroughly investigated by electrochemical method. The improved cyclability of the quercetin-containing cell at high temperature implies that by forming robust and less-resistive SEI, quercetin is preferentially oxidized and passivates the LiNi0.5Mn1.5O4 electrode. EIS result coherently suggests that the quercetin-added electrolyte forms a more compact and Li-ion conducting interface. The surface sensitive XPS analysis confirms that the presence of quercetin restrains the formation of LiF, suppresses the reaction of PF5, and alleviates Mn dissolution. Meanwhile, ICP-MS analysis affirms the effectiveness of quercetin against Mn dissolution. The self-discharge experiment which exhibits the retained charged state of LiNi0.5Mn1.5O4 at high temperature, gives convincing evidence of the effect of quercetin. Intensive analyses confirm that quercetin can effectively prolong the cycle-life of LiNi0.5Mn1.5O4 at elevated temperature. We envision its potential and practical usage as an electrolyte additive for high-voltage cathode.