A lithium–oxygen battery based on lithium superoxide

Lithium–oxygen batteries allow oxygen to be reduced at the battery’s cathode when a current is drawn; in present-day batteries, this results in formation of Li2O2, but it is now shown that another high energy density material, namely LiO2, with better electronic conduction can be used instead as the discharge product, if the electrode is decorated with iridium nanoparticles. Nonaqueous lithium–air batteries have a much superior theoretical gravimetric energy density compared to conventional lithium ion batteries, and thus have the potential for making long-range electric vehicles a reality. Batteries based on sodium and potassium superoxides have recently been reported, but thermodynamically unstable lithium superoxide (LiO2), with its potential high energy density, has proved more problematic. This paper demonstrates that crystalline LiO2 can be stabilized in a Li–O2 battery by using a suitable cathode material — reduced graphene oxide decorated with iridium nanoparticles. A battery based on this new lithium–oxygen chemistry was demonstrated through 40 cycles before failure, achieving high efficiency and good capacity. Batteries based on sodium superoxide and on potassium superoxide have recently been reported1,2,3. However, there have been no reports of a battery based on lithium superoxide (LiO2), despite much research4,5,6,7,8 into the lithium–oxygen (Li–O2) battery because of its potential high energy density. Several studies9,10,11,12,13,14,15,16 of Li–O2 batteries have found evidence of LiO2 being formed as one component of the discharge product along with lithium peroxide (Li2O2). In addition, theoretical calculations have indicated that some forms of LiO2 may have a long lifetime17. These studies also suggest that it might be possible to form LiO2 alone for use in a battery. However, solid LiO2 has been difficult to synthesize in pure form18 because it is thermodynamically unstable with respect to disproportionation, giving Li2O2 (refs 19, 20). Here we show that crystalline LiO2 can be stabilized in a Li–O2 battery by using a suitable graphene-based cathode. Various characterization techniques reveal no evidence for the presence of Li2O2. A novel templating growth mechanism involving the use of iridium nanoparticles on the cathode surface may be responsible for the growth of crystalline LiO2. Our results demonstrate that the LiO2 formed in the Li–O2 battery is stable enough for the battery to be repeatedly charged and discharged with a very low charge potential (about 3.2 volts). We anticipate that this discovery will lead to methods of synthesizing and stabilizing LiO2, which could open the way to high-energy-density batteries based on LiO2 as well as to other possible uses of this compound, such as oxygen storage.