The lack of fundamental understanding of the oxygen reduction and oxygen evolution in nonaqueous electrolytes significantly hinders the development of rechargeable lithium-air batteries. Here we employ a solid-state Li_4+xTi₅O₁₂/LiPON/Li_xV₂O₅ cell and examine in situ the chemistry of Li-O₂ reaction products on Li_xV₂O₅ as a function of applied voltage under ultra high vacuum (UHV) and at 500 mtorr of oxygen pressure using ambient pressure X-ray photoelectron spectroscopy (APXPS). Under UHV, lithium intercalated into Li_xV₂O₅ while molecular oxygen was reduced to form lithium peroxide on Li_xV₂O₅ in the presence of oxygen upon discharge. Interestingly, the oxidation of Li₂O₂ began at much lower overpotentials (~240 mV) than the charge overpotentials of conventional Li-O₂ cells with aprotic electrolytes (~1000 mV). Our study provides the first evidence of reversible lithium peroxide formation and decomposition in situ on an oxide surface using a solid-state cell, and new insights into the reaction mechanism of Li-O₂ chemistry.

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