The electrochemical reaction of a lithium-oxygen cell using a tetraethylene glycol dimethyl ether-lithium triflate, TEGDME-LiCF(3)SO(3) electrolyte, is investigated by a detailed transmission electron microscopy analysis. The results confirm the reversibility of the process by showing the formation-dissolution of lithium peroxide, Li(2)O(2), upon repeating cell charge and discharge cycles.