Zhangquan Peng

A reversible and higher-rate Li-O2 battery.

Improving Lithium Batteries Lithium-oxygen batteries have similar volumetric energy densities to lithium-ion batteries, but, because the oxygen part of the battery can be extracted from the air, they have a significant advantage in their gravimetric energy densities. One of the fundamental problems plaguing the nonaqueous Li-O2 system is that the Li2O2 that forms on discharge must be completely reversed on charging, but for most systems, a range of side products form instead of Li2O2. Peng et al. (p. 563, published online 19 July) show that by using dimethyl sulfoxide as the electrolyte, and a porous gold cathode, they can get reversible production and removal of Li2O2 during discharge and charge cycles. Furthermore, the electrolyte-electrode system operates with much faster kinetics than carbon electrodes. A viable lithium-oxygen battery is demonstrated using dimethylsulfoxide electrolyte and a porous gold cathode. The rechargeable nonaqueous lithium-air (Li-O2) battery is receiving a great deal of interest because, theoretically, its specific energy far exceeds the best that can be achieved with lithium-ion cells. Operation of the rechargeable Li-O2 battery depends critically on repeated and highly reversible formation/decomposition of lithium peroxide (Li2O2) at the cathode upon cycling. Here, we show that this process is possible with the use of a dimethyl sulfoxide electrolyte and a porous gold electrode (95% capacity retention from cycles 1 to 100), whereas previously only partial Li2O2 formation/decomposition and limited cycling could occur. Furthermore, we present data indicating that the kinetics of Li2O2 oxidation on charge is approximately 10 times faster than on carbon electrodes.

A stable cathode for the aprotic Li–O2 battery

Rechargeable lithium-air (O2) batteries are receiving intense interest because their high theoretical specific energy exceeds that of lithium-ion batteries. If the Li-O2 battery is ever to succeed, highly reversible formation/decomposition of Li2O2 must take place at the cathode on cycling. However, carbon, used ubiquitously as the basis of the cathode, decomposes during Li2O2 oxidation on charge and actively promotes electrolyte decomposition on cycling. Replacing carbon with a nanoporous gold cathode, when in contact with a dimethyl sulphoxide-based electrolyte, does seem to demonstrate better stability. However, nanoporous gold is not a suitable cathode; its high mass destroys the key advantage of Li-O2 over Li ion (specific energy), it is too expensive and too difficult to fabricate. Identifying a suitable cathode material for the Li-O2 cell is one of the greatest challenges at present. Here we show that a TiC-based cathode reduces greatly side reactions (arising from the electrolyte and electrode degradation) compared with carbon and exhibits better reversible formation/decomposition of Li2O2 even than nanoporous gold (>98% capacity retention after 100 cycles, compared with 95% for nanoporous gold); it is also four times lighter, of lower cost and easier to fabricate. The stability may originate from the presence of TiO2 (along with some TiOC) on the surface of TiC. In contrast to carbon or nanoporous gold, TiC seems to represent a more viable, stable, cathode for aprotic Li-O2 cells.

Oxygen reactions in a non-aqueous Li+ electrolyte.

Oxygen (O2) reduction is one of the most studied reactions in chemistry.1 Widely investigated in aqueous media, O2 reduction in non-aqueous solvents, such as CH3CN, has been studied for several decades.2–7 Today, O2 reduction in non-aqueous Li+ electrolytes is receiving considerable attention because it is the reaction on which operation of the Li–air (O2) battery depends.8–29 The Li–O2 battery is generating a great deal of interest because theoretically its high energy density could transform energy storage.8, 9 As a result, it is crucial to understand the O2 reaction mechanisms in non-aqueous Li+ electrolytes. Important progress has been made using electrochemical measurements including recently by Laoire et al.29 No less than five different mechanisms for O2 reduction in Li+ electrolytes have been proposed over the last 40 years based on electrochemical measurements alone.25–29 The value of using spectroelectrochemical methods is that they can identify directly the species involved in the reaction. Here we present in situ spectroscopic data that provide direct evidence that LiO2 is indeed an intermediate on O2 reduction, which then disproportionates to the final product Li2O2. Spectroscopic studies of Li2O2 oxidation demonstrate that LiO2 is not an intermediate on oxidation, that is, oxidation does not follow the reverse pathway to reduction.