The lithium-air system captured worldwide attention in 2009 as a possible battery for electric vehicle propulsion applications. If successfully developed, this battery could provide an energy source for electric vehicles rivaling that of gasoline in terms of usable energy density. However, there are numerous scientific and technical challenges that must be overcome if this alluring promise is to turn into reality. The fundamental battery chemistry during discharge is thought to be the electrochemical oxidation of lithium metal at the anode and reduction of oxygen from air at the cathode. With aprotic electrolytes, as used in Li-ion batteries, there is some evidence that the process can be reversed by applying an external potential, i.e., that such a battery can be electrically recharged. This paper summarizes the authors' view of the promise and challenges facing development of practical Li-air batteries and the current understanding of its chemistry. However, it must be appreciated that this perspective represents only a snapshot in a very rapidly evolving picture.

Among the many important challenges facing the development of Li-air batteries, understanding the electrolyte's role in producing the appropriate reversible electrochemistry (i.e., 2Li(+) + O2 + 2e(-) ↔ Li2O2) is critical. Quantitative differential electrochemical mass spectrometry (DEMS), coupled with isotopic labeling of oxygen gas, was used to study Li-O2 electrochemistry in various solvents, including carbonates (typical Li ion battery solvents) and dimethoxyethane (DME). In conjunction with the gas-phase DEMS analysis, electrodeposits formed during discharge on Li-O2 cell cathodes were characterized using ex situ analytical techniques, such as X-ray diffraction and Raman spectroscopy. Carbonate-based solvents were found to irreversibly decompose upon cell discharge. DME-based cells, however, produced mainly lithium peroxide on discharge. Upon cell charge, the lithium peroxide both decomposed to evolve oxygen and oxidized DME at high potentials. Our results lead to two conclusions; (1) coulometry has to be coupled with quantitative gas consumption and evolution data to properly characterize the rechargeability of Li-air batteries, and (2) chemical and electrochemical electrolyte stability in the presence of lithium peroxide and its intermediates is essential to produce a truly reversible Li-O2 electrochemistry.

New photoacoustic cell design for studying aqueous solutions and gels Rev. Sci. Instrum. 82, 084903 (2011) Electric response of a cell of hydrogel: Role of the electrodes Appl. Phys. Lett. 98, 064101 (2011) Effect of the gate electrode on the response of organic electrochemical transistors APL: Org. Electron. Photonics 3, 205 (2010) Effect of the gate electrode on the response of organic electrochemical transistors Appl. Phys. Lett. 97, 123304 (2010) Note: Fixture for characterizing electrochemical devices in-operando in traditional vacuum systems Rev. Sci. Instrum. 81, 086104 (2010) Additional information on J. Chem. Phys. Non-aqueous Li-air or Li-O 2 cells show considerable promise as a very high energy density battery couple. Such cells, however, show sudden death at capacities far below their theoretical capacity and this, among other problems, limits their practicality. In this paper, we show that this sudden death arises from limited charge transport through the growing Li 2 O 2 film to the Li 2 O 2 -electrolyte interface, and this limitation defines a critical film thickness, above which it is not possible to support electrochemistry at the Li 2 O 2 -electrolyte interface. We report both electrochemical experiments using a reversible internal redox couple and a first principles metal-insulator-metal charge transport model to probe the electrical conductivity through Li 2 O 2 films produced during Li-O 2 discharge. Both experiment and theory show a "sudden death" in charge transport when film thickness is ∼5 to 10 nm. The theoretical model shows that this occurs when the tunneling current through the film can no longer support the electrochemical current. Thus, engineering charge transport through Li 2 O 2 is a serious challenge if Li-O 2 batteries are ever to reach their potential.

Quantitative differential electrochemical mass spectrometry and cyclic voltammetry have been combined to probe possible mechanisms and the kinetic overpotentials, responsible for discharge and charge in a Li−O 2 battery, using C as the cathode and an electrolyte based on dimethoxyethane as the solvent. Previous spectroscopy experiments (X-ray diffraction, μRaman, IR, XPS) have shown that Li 2 O 2 is the principle product formed during Li−O 2 discharge using this electrolyte/cathode combination. At all discharge potentials and charge potentials <4.0 V, the observed electrochemistry is ∼2e − /O 2 consumed or produced, also implying that Li 2 O 2 is the dominant thermodynamically stable species formed and consumed in the electrochemistry. No evidence exists at any potential for formation of stable LiO 2 (1e − /O 2 ) or Li 2 O (4e − /O 2 ) during discharge. At charging potentials >4.0 V, the electrochemistry requires significantly more than 2e − /O 2 , and we take this as evidence for electrolyte decomposition. We find that sequential concerted (Li + + e − ) ion transfers to/from adsorbed O 2 * and LiO 2 * to produce/consume Li 2 O 2 is the mechanism that is most compatible with these experiments. The kinetic overpotentials are extremely low relative to aqueous O 2 reduction and evolution, and this implies that in principle a discharge−charge Li−O 2 cycle is possible with high voltaic efficiency (∼85%) if electrolyte and cathode stability issues are resolved.

The Li/oxygen battery may achieve a high practical specific energy as its theoretical specific energy is 11,400 Wh/kg Li assuming Li 2 O 2 is the product. To help understand the physics of the Li/oxygen battery we present the first physics-based model that incorporates the major thermodynamic, transport, and kinetic processes. We obtain a good match between porous-electrode experiments and simulations by using an empirical fit to the resistance of the discharge products ͑which include carbonates and oxides when using carbonate solvents͒ as a function of thickness that is obtained from flat-electrode experiments. The experiments and model indicate that the discharge products are electronically resistive, limiting their thickness to tens of nanometers and their volume fraction in one of our discharged porous electrodes to a few percent. Flat-electrode experiments, where pore clogging is impossible, show passivation similar to porous-electrode experiments and allow us to conclude that electrical passivation is the dominant capacity-limiting mechanism in our cells. Although in carbonate solvents Li 2 O 2 is not the dominant discharge product, we argue that the implications of this model, ͑i.e., electrical passivation by the discharge products limits the capacity͒ also apply if Li 2 O 2 is the discharge product, as it is an intrinsic electronic insulator.