Batteries based on sodium superoxide and on potassium superoxide have recently been reported. However, there have been no reports of a battery based on lithium superoxide (LiO2), despite much research into the lithium-oxygen (Li-O2) battery because of its potential high energy density. Several studies 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 lifetime. 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 form 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.

In this paper we report on a kinetics study of the discharge process and its relationship to the charge overpotential in a Li-O2 cell for large surface area cathode material. The kinetics study reveals evidence for a first-order disproportionation reaction during discharge from an oxygen-rich Li2O2 component with superoxide-like character to a Li2O2 component. The oxygen-rich superoxide-like component has a much smaller potential during charge (3.2-3.5 V) than the Li2O2 component (∼4.2 V). The formation of the superoxide-like component is likely due to the porosity of the activated carbon used in the Li-O2 cell cathode that provides a good environment for growth during discharge. The discharge product containing these two components is characterized by toroids, which are assemblies of nanoparticles. The morphologic growth and decomposition process of the toroids during the reversible discharge/charge process was observed by scanning electron microscopy and is consistent with the presence of the two components in the discharge product. The results of this study provide new insight into how growth conditions control the nature of discharge product, which can be used to achieve improved performance in Li-O2 cell.

We report on the use of a petroleum coke-based activated carbon (AC) with very high surface area for a Li-O(2) battery cathode without the use of any additional metal catalysts. Electrochemical measurement in a tetra(ethylene) glycol dimethyl ether-lithium triflate (TEGDME-LiCF(3)SO(3)) electrolyte results in two voltage plateaus during charging at 3.2-3.5 and 4.2-4.3 V versus Li(+)/Li. Herein we present evidence from Raman and magnetic measurements that the lower plateau corresponds to a form of lithium peroxide with superoxide-like properties characterized by a low temperature magnetic phase transition and a high O-O stretching frequency (1125 cm(-1)). The magnetic phase transition and the high O-O stretching frequency disappear when charged to above 3.7 V. Theoretical calculations indicate that a surface superoxide structure on lithium peroxide clusters and some lithium peroxide surfaces have an unpaired electron and a high O-O stretching frequency that help explain the observations. These results provide evidence that the form of the lithium peroxide discharge product is important to obtaining a low charge overpotential, and thus improving the round-trip efficiency between discharge and charge.

Raman spectroscopy is used to characterize the composition of toroids formed in an aprotic Li-O2 cell based on an activated carbon cathode. The trends in the Raman data as a function of discharge current density and charging cutoff voltage provide evidence that the toroids are made up of outer LiO2-like and inner Li2O2 regions, consistent with a disproportionation reaction occurring in the solid phase. The LiO2-like component is found to be associated with a new Raman peak identified in the carbon stretching region at ∼1505 cm(-1), which appears only when the LiO2 peak at 1123 cm(-1) is present. The new peak is assigned to distortion of the graphitic ring stretching due to coupling with the LiO2-like component based on density functional calculations. These new results on the LiO2-like component from Raman spectroscopy provide evidence that a late stage disproportionation mechanism can occur during discharge and add new understanding to the complexities of possible processes occurring in Li-O2 batteries.

The successful development of Li-air batteries would significantly increase the possibility of extending the range of electric vehicles. There is much evidence that typical organic carbonate based electrolytes used in lithium ion batteries form lithium carbonates from reaction with oxygen reduction products during discharge in lithium-air cells so more stable electrolytes need to be found. This combined experimental and computational study of an electrolyte based on a tri(ethylene glycol)-substituted trimethylsilane () provides evidence that the ethers are more stable toward oxygen reduction discharge species. X-ray photoelectron spectroscopy (XPS) and FTIR experiments show that only lithium oxides and no carbonates are formed when electrolyte is used. In contrast XPS shows that propylene carbonate (PC) in the same cell configuration decomposes to form lithium carbonates during discharge. Density functional calculations of probable decomposition reaction pathways involving solvated oxygen reduction species confirm that oligoether substituted silanes, as well as other ethers, are more stable to the oxygen reduction products than propylene carbonate. These results indicate that the choice of electrolyte plays a key role in the performance of Li-air batteries.