Development of lithium-oxygen (Li-O2) batteries is hindered by large overpotentials, low energy efficiency and unsatisfactory cycling life, which are predominately caused by the oxygen evolution reaction (OER) involving sluggish oxidation of lithium peroxide (Li2O2). Herein, a highly-efficient soluble redox mediator (RM) of N,N’-Diphenyl-p-phenylenediamine (DPPD) is introduced to catalyze the oxygen evolution reaction for Li-O2 batteries. During charging process, DPPD is electrochemically oxidized into DPPD+, which can chemically decompose Li2O2 on the cathode with a much-reduced overpotential. The in-situ differential electrochemical mass spectrometry confirms that the charging process is dominated by Li2O2 decomposition. With the assistance of DPPD, Li-O2 cells can achieve lower charging potential, improved cycling stability, and prolonged lifespan (294 cycles) which is three times longer than that of tetrathiafulvalene counterpart. This work presents a credible and viable opportunity for long lifespan Li-O2 batteries.