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.

The average increase in the rate of the energy density of secondary batteries has been about 3% in the past 60 years. Obviously, a great breakthrough is needed in order to increase the energy density from the current 210 Wh kg⁻¹ of Li-ion batteries to the ambitious target of 500–700 Wh kg⁻¹ to satisfy application in electrical vehicles. A thermodynamic calculation on the theoretical energy densities of 1172 systems is performed and energy storage mechanisms are discussed, aiming to determine the theoretical and practical limits of storing chemical energy and to screen possible systems. Among all calculated systems, the Li/F₂ battery processes the highest energy density and the Li/O₂ battery ranks as the second highest, theoretically about ten times higher than current Li-ion batteries. In this paper, energy densities of Li-ion batteries and a comparison of Li, Na, Mg, Al, Zn-based batteries, Li-storage capacities of the electrode materials and conversion reactions for energy storage, in addition to resource and environmental concerns, are analyzed.

A novel rechargeablebattery is reported. It comprises aconductive organic polymer electrolyte membrane sandwiched by a thin Li metal foil anode, and a thin carbon composite electrode on which oxygen, the electroactive cathode material, accessed from the environment, is reduced during discharge to generate electric power. It features an all solid state design in which electrode and electrolyte layers are laminated to form a 200 to 300 μm thick battery cell. The overall cell reaction during discharge appears to be. It has an open‐circuit voltage of about 3 V, and a load voltage that spans between 2 and 2.8 V depending upon the load resistance. The cell can be recharged with good coulombic efficiency using a cobalt phthalocyanine catalyzed carbon electrode.

Rechargeable lithium batteries represent one of the most important developments in energy storage for 100 years, with the potential to address the key problem of global warming. However, their ability to store energy is limited by the quantity of lithium that may be removed from and reinserted into the positive intercalation electrode, Li_xCoO₂, 0.5<x<1 (corresponding to 140 mA·h g^-1 of charge storage). Abandoning the intercalation electrode and allowing Li to react directly with O₂ from the air at a porous electrode increases the theoretical charge storage by a remarkable 5−10 times! Here we demonstrate two essential prerequisites for the successful operation of a rechargeable Li/O₂ battery; that the Li₂O₂ formed on discharging such an O₂ electrode is decomposed to Li and O₂ on charging (shown here by in situ mass spectrometry), with or without a catalyst, and that charge/discharge cycling is sustainable for many cycles.

Li–air batteries are potentially viable ultrahigh energy density chemical power sources, which could potentially offer specific energies up to ∼3000 Wh kg⁻¹ being rechargeable. The modern state of art and the challenges in the field of Li–air batteries are considered. Although their implementation holds the greatest promise in a number of applications ranging from portable electronics to electric vehicles, there are also impressive challenges in development of cathode materials and electrolyte systems of these batteries.

The Li–air battery has recently emerged as a potentially transformational energy storage technology for both transportation and stationary energy storage applications because of its very high specific energy; however, its practical application is currently limited by the poor power capability (low current density), poor cyclability, and low energy efficiency. All of these are largely determined by interfacial reactions on oxygen electrocatalysts in the air electrode. In this article, we review the fundamental understanding of oxygen electrocatalysis in nonaqueous electrolytes and the status and challenges of oxygen electrocatalysts and provide a perspective on new electrocatalysts' design and development.

A Li-air battery could potentially provide three to five times higher energy density/specific energy than conventional batteries and, thus, enable the driving range of an electric vehicle to be comparable to gasoline vehicles. However, making Li-air batteries rechargeable presents significant challenges, mostly related to the materials. Here, the key factors that influence the rechargeability of Li-air batteries are discussed with a focus on nonaqueous systems. The status and materials challenges for nonaqueous rechargeable Li-air batteries are reviewed. These include electrolytes, cathode (electrocatalysts), lithium metal anodes, and oxygen-selective membranes (oxygen supply from air). A perspective for the future of rechargeable Li-air batteries is provided.

Lithium/air batteries, based on their high theoretical specific energy, are an extremely attractive technology for electrical energy storage that could make long-range electric vehicles widely affordable. However, the impact of this technology has so far fallen short of its potential due to several daunting challenges. In nonaqueous Li/air cells, reversible chemistry with a high current efficiency over several cycles has not yet been established, and the deposition of an electrically resistive discharge product appears to limit the capacity. Aqueous cells require water-stable lithium-protection membranes that tend to be thick, heavy, and highly resistive. Both types of cell suffer from poor oxygen redox kinetics at the positive electrode and deleterious volume and morphology changes at the negative electrode. Closed Li/air systems that include oxygen storage are much larger and heavier than open systems, but so far oxygen- and OH⁻-selective membranes are not effective in preventing contamination of cells. In this review we discuss the most critical challenges to developing robust, high-energy Li/air batteries and suggest future research directions to understand and overcome these challenges. We predict that Li/air batteries will primarily remain a research topic for the next several years. However, if the fundamental challenges can be met, the Li/air battery has the potential to significantly surpass the energy storage capability of today's Li-ion batteries.

With the increasing importance of electrified transport, the need for high energy density storage is also increasing. Possible candidates include Li-O₂ batteries, which are the subject of rapidly increasing focus worldwide despite being in their infancy of understanding. This excitement owes to the high energy density of Li-O₂ (up to 2-3 kWh kg⁻¹), theoretically much higher compared to that of other rechargeable systems, and the open “semi-fuel” cell battery configuration that uses oxygen as the positive electrode material. To bring Li-O₂ batteries closer to reality as viable energy storage devices, and to attain suitable power delivery, understanding of the underlying chemistry is essential. Several concepts have been proposed in the last year to account for the function and target future design of Li-O₂ batteries and these are reviewed. An overview is given of the efforts to understand oxygen reduction/evolution and capacity limitations in these systems, and of electrode and electrolyte materials that are suitable for non-aqueous and hybrid (nonaqueous/aqueous) cells.

A lot of attention has been paid to Li–O₂ batteries in recent years, due to the huge potential specific energy and energy density, and they are extensively studied around the world. Much advance has been achieved, however, the fundamental understanding is still insufficient and challenges remain. Here, we provide a specific perspective on the development of non-aqueous Li–O₂ batteries excluding those with aqueous, ionic liquid, hybrid, and solid-state electrolytes, because non-aqueous Li–O₂ batteries possess a relatively simple configuration and the research on non-aqueous Li–O₂ batteries is the most active of all Li–O₂ batteries. The discussion will be focused on non-aqueous electrolytes, cathode catalysts, and anodes, and corresponding perspectives are provided at the end.

The nonaqueous rechargeable lithium–O₂ battery containing an alkyl carbonate electrolyte discharges by formation of C₃H₆(OCO₂Li)₂, Li₂CO₃, HCO₂Li, CH₃CO₂Li, CO₂, and H₂O at the cathode, due to electrolyte decomposition. Charging involves oxidation of C₃H₆(OCO₂Li)₂, Li₂CO₃, HCO₂Li, CH₃CO₂Li accompanied by CO₂ and H₂O evolution. Mechanisms are proposed for the reactions on discharge and charge. The different pathways for discharge and charge are consistent with the widely observed voltage gap in Li–O₂ cells. Oxidation of C₃H₆(OCO₂Li)₂ involves terminal carbonate groups leaving behind the OC₃H₆O moiety that reacts to form a thick gel on the Li anode. Li₂CO₃, HCO₂Li, CH₃CO₂Li, and C₃H₆(OCO₂Li)₂ accumulate in the cathode on cycling correlating with capacity fading and cell failure. The latter is compounded by continuous consumption of the electrolyte on each discharge.

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.

A rechargeable Li–air cell utilizing an electrolyte composed of a solution ofin tetraethylene glycol dimethyl ether,(TEGDME), and an uncatalyzed porous carbon electrode, investigated to elucidate the baseline Li–air battery chemistry, is reported. From the x-ray diffraction patterns of the discharged carbon electrodes, the discharge product of the cell was identified to beduring normal discharge to. Discharging the cell toor below producesas well. The cell can be recharged without a catalyst in the carbon cathode, albeit at low depths of discharge. The high resistance of the discharged carbon cathode is a major impediment to recharging cells displaying a high specific capacity. The cell capacity decreases with continued cycling, which was found to be due to the poor cycling efficiency of the Li anode and the high resistance of the discharge products, which slowly accumulate in the porous electrode.

The O₂electrode in Li–O₂cells was shown to exhibit gravimetric energy densities (considering the total weight of oxygenelectrode in the discharged state) four times that of LiCoO₂ with comparable gravimetric power. The discharge rate capability of Au-catalyzed Vulcan carbon and pure Vulcan carbon (VC) as the O₂electrode was studied in the range of 100 to 2000 mA g_carbon⁻¹. The discharge voltage and capacity of the Li−O₂ cells were shown to decrease with increasing rates. Unlike propylene carbonate based electrolytes, the rate capability of Li−O₂ cells tested with1,2-dimethoxyethane was found not to be limited by oxygen transport in the electrolyte. X-Ray diffraction (XRD) showed lithium peroxide as the discharge product and no evidence of Li₂CO₃ and LiOH was found. It is hypothesized that higher discharge voltages of cells with Au/C than VC at low rates could have originated from higher oxygenreduction activity of Au/C. At high rates, higher discharge voltages with Au/C than VC could be attributed to faster lithium transport in nonstoichiometric and defective lithium peroxide formed upon discharge, which is supported byXRD and X-ray absorption near edge structure O and Li K edge data.

The addition of potassium superoxide (KO₂) into the electrolyte for Li-air battery was proved to be an effective screening method to examine the stability of the solvents against superoxide anion radicals (O₂^•−). It was also confirmed that the coupled compound between Li⁺ and O₂^•−, namely LiO₂^•, was highly reactive for many solvents to decompose themselves. Specific ionic liquid and dimethylsulfoxide, selected solvents by the KO₂ screening test, actually performed stable operation of Li-air battery.

Raman, infrared, and X-ray photoelectron spectroscopies were used to characterize the thick coating of reaction products on carbon and MnO₂-coated carbon cathodes produced during discharge of Li–air cells. The results show that neither Li₂O₂ nor Li₂O are major components of the insoluble discharge products; instead, the products are composed primarily of fluorine, lithium, and carbon, with surprisingly little oxygen. The complex reaction chemistry also appears to involve the formation of ethers or alkoxide products at the expense of the carbonate solvent molecules (ethylene carbonate and dimethyl carbonate). The irreversible discharge reaction is likely electrochemically promoted with Li⁺, anions, and dissolved oxygen. Exactly how the molecular O₂ participates in the reaction is unclear and requires further study. The addition of a conformal coating of MnO₂ on the carbon lowers the cell’s operating voltage but does not alter the overall discharge chemistry.

Adimethyl sulfoxide (DMSO) based electrolyte is first proposed for rechargeable lithium–O₂ (Li–O₂) batteries. Superior battery performances, including high discharge capacity and low charge potential, are successfully obtained.

Study of formation and decomposition of Li₂O₂ during operations of Li–O₂ cells is essential for understanding the reaction mechanism and finding solutions to improve the cell performance. Using vertically aligned carbon nanotubes (VACNTs) directly grown on stainless steel meshes as the cathodes in the Li–O₂ cells with dimethoxyethane (DME) electrolytes, nucleation, growth, and decomposition processes of the Li₂O₂ in the first cycle are clearly visualized. Through cycles with the controlled discharge and charge capacities, the abacus-ball-shaped Li₂O₂ and the rust-like carbonates simultaneously formed around the VACNTs are further identified. It is indicated that the increasing coverage of carbonates on the cathode surface suppresses the formation of Li₂O₂, which maintains the shape of abacus ball. When the VACNT surfaces are predominantly covered by the carbonates, the cells tend to terminate.

Cycle performance of Li–O₂ cells withN-methyl-N-propylpiperidinium bis(trifluoromethansulfony)imide (PP13TFSI)–LiClO₄ electrolytes and vertically aligned carbon nanotube (VACNT) cathodes has been investigated. It is found that both the current density and the depth of discharge have great influence on the capacity utilization, voltage polarization and cycle life. Abacus-ball-shaped Li₂O₂ particles produced at discharge are identified, and their complete decomposition at charge is confirmed. Meanwhile, formation of Li carbonates around the VACNTs is detected after the first cycle. The carbonates accumulate and gradually become predominant over Li₂O₂ upon cycling, leading to decrease of the discharge capacity and increase of the cell resistance. These results indicate that reduction of the carbonates formed around the VACNTs is of critical importance for improving cycle performance of the cells investigated here.

Li–O₂ batteries, wherein solid Li₂O₂ is formed at the porous air cathode during discharge, are candidates for high gravimetric energy (3212 Wh/kg_Li2O2) storage for electric vehicles. Understanding and controlling the nucleation and morphological evolution of Li₂O₂ particles upon discharge is key to achieving high volumetric energy densities. Scanning and transmission electron microscopy were used to characterize the discharge product formed in Li–O₂ batteries on electrodes composed of carpets of aligned carbon nanotubes. At low discharge rates, Li₂O₂ particles form first as stacked thin plates,10 nm in thickness, which spontaneously splay so that secondary nucleation of new plates eventually leads to the development of a particle with a toroidal shape. Li₂O₂ crystallites have large (001) crystal faces consistent with the theoretical Wulff shape and appear to grow by a layer-by-layer mechanism. In contrast, at high discharge rates, copious nucleation of equiaxed Li₂O₂ particles precedes growth of discs and toroids.

The thermodynamic stability and electronic structure of 40 surfaces of lithium peroxide (Li₂O₂) and lithium oxide (Li₂O) were characterized using first-principles calculations. As these compounds constitute potential discharge products in Li–oxygen batteries, their surface properties are expected to play a key role in understanding electrochemical behavior in these systems. Stable surfaces were identified by comparing 23 distinct Li₂O₂ surfaces and 17 unique Li₂O surfaces; crystallite areal fractions were determined through application of the Wulff construction. Accounting for the oxygen overbinding error in density functional theory results in the identification of several new Li₂O₂ oxygen-rich {0001} and {11̅00} terminations that are more stable than those previously reported. Although oxygen-rich facets predominate in Li₂O₂, in Li₂O stoichiometric surfaces are preferred, consistent with prior studies. Surprisingly, surface-state analyses reveal that the stable surfaces of Li₂O₂ are half-metallic, despite the fact that Li₂O₂ is a bulk insulator. Surface oxygens in these facets are ferromagnetic with magnetic moments ranging from 0.2 to 0.5 μ_B. In contrast, the stable surfaces of Li₂O are insulating and nonmagnetic. The distinct surface properties of these compounds may explain observations of electrochemical reversibility for systems in which Li₂O₂ is the discharge product and the irreversibility of systems that discharge to Li₂O. Moreover, the presence of conductive surface pathways in Li₂O₂ could offset capacity limitations expected to arise from limited electron transport through the bulk.

The main discharge products formed at the cathode of nonaqueous Li–air batteries are known to be Li₂O₂ and residual Li₂CO₃. Recent experiments indicate that the charge transport through these materials is the main limiting factor for the battery performance. It has been also shown that the performance of the battery decreases drastically when the amount of Li₂CO₃ at the cathode increases with respect to Li₂O₂. In this work, we study the formation and transport of hole and electron polarons in Li₂O₂ and Li₂CO₃ using density functional theory (DFT) within the PBE+U approximation. For both materials, we find that the formation of polarons (both hole and electron) is stabilized with respect to the delocalized states for all physically relevant values of U. We find a much higher mobility for hole polarons than for the electron polarons, and we show that the poor charge transport in Li₂CO₃ compared to Li₂O₂ can be understood through a polaronic model for the conduction. Furthermore, the hole polaronic model in Li₂O₂ provides a possible explanation for the experimentally observed preferential growth direction of the films. Our results also suggest that doping is unlikely to be a viable route for improving the transport properties of Li₂O₂ or Li₂CO₃.

We report a significant difference in the growth mechanism of Li₂O₂ in Li–O₂ batteries for toroidal and thin-film morphologies which is dependent on the current rate that governs the electrochemical pathway. Evidence from diffraction, electrochemical,FESEM and STEM measurements shows that slower current densities favor aggregation of lithium peroxide nanocrystallites nucleatedvia solution dismutase on the surface of the electrode; whereas fast rates deposit quasi-amorphous thin films. The latter provide a lower overpotential on charge due to their nature and close contact with the conductive electrode surface, albeit at the expense of lower discharge capacity.

In this Letter, we report the first in situ transmission electron microscopy observation of electrochemical oxidation of Li₂O₂, providing insights into the rate limiting processes that govern charge in Li–O₂ cells. In these studies, oxidation of electrochemically formed Li₂O₂ particles, supported on multiwall carbon nanotutubes (MWCNTs), was found to occur preferentially at the MWCNT/Li₂O₂ interface, suggesting that electron transport in Li₂O₂ ultimately limits the oxidation kinetics at high rates or overpotentials.

Carbon has been used widely as the basis of porous cathodes for nonaqueous Li–O₂ cells. However, the stability of carbon and the effect of carbon on electrolyte decomposition in such cells are complex and depend on the hydrophobicity/hydrophilicity of the carbon surface. Analyzing carbon cathodes, cycled in Li–O₂ cells between 2 and 4 V, using acid treatment and Fenton’s reagent, and combined with differential electrochemical mass spectrometry and FTIR, demonstrates the following: Carbon is relatively stable below 3.5 V (vs Li/Li⁺) on discharge or charge, especially so for hydrophobic carbon, but is unstable on charging above 3.5 V (in the presence of Li₂O₂), oxidatively decomposing to form Li₂CO₃. Direct chemical reaction with Li₂O₂ accounts for only a small proportion of the total carbon decomposition on cycling. Carbon promotes electrolyte decomposition during discharge and charge in a Li–O₂ cell, giving rise to Li₂CO₃ and Li carboxylates (DMSO and tetraglyme electrolytes). The Li₂CO₃ and Li carboxylates present at the end of discharge and those that form on charge result in polarization on the subsequent charge. Li₂CO₃ (derived from carbon and from the electrolyte) as well as the Li carboxylates (derived from the electrolyte) decompose and form on charging. Oxidation of Li₂CO₃ on charging to4 V is incomplete; Li₂CO₃ accumulates on cycling resulting in electrode passivation and capacity fading. Hydrophilic carbon is less stable and more catalytically active toward electrolyte decomposition than carbon with a hydrophobic surface. If the Li–O₂ cell could be charged at or below 3.5 V, then carbon may be relatively stable, however, its ability to promote electrolyte decomposition, presenting problems for its use in a practical Li–O₂ battery. The results emphasize that stable cycling of Li₂O₂ at the cathode in a Li–O₂ cell depends on the synergy between electrolyte and electrode; the stability of the electrode and the electrolyte cannot be considered in isolation.

In this Letter, the effect of CO₂ contamination on nonaqueous Li–O₂ battery rechargeability is explored. Although CO₂ contamination was found to increase the cell’s discharge capacity, it also spontaneously reacts with Li₂O₂ (the primary discharge product of a nonaqueous Li–O₂ battery) to form Li₂CO₃. CO₂ evolution from Li₂CO₃ during battery charging was found to occur only at very high potentials (>4 V) compared to O₂ evolution from Li₂O₂ (3–3.5 V), and as a result, the presence of CO₂ during discharge dramatically reduced the voltaic efficiency of the discharge–charge cycle. These results emphasize the importance of not only completely removing CO₂ from air fed to a Li-air battery, but also developing stable cathodes and electrolytes that will not decompose during battery operation to form carbonate deposits.

The non-aqueous Li–air (O₂) battery is receiving intense interest because its theoretical specific energy exceeds that of Li-ion batteries. Recharging the Li–O₂ battery depends on oxidizing solidlithium peroxide (Li₂O₂), which is formed on discharge within the porous cathode. However, transporting charge betweenLi₂O₂ particles and the solid electrode surface is at best very difficult and leads to voltage polarization on charging, even at modest rates. This is a significant problem facing the non-aqueous Li–O₂ battery. Here we show that incorporation of a redox mediator,tetrathiafulvalene (TTF), enables recharging at rates that are impossible for the cell in the absence of the mediator. On charging,TTF is oxidized toTTF⁺ at the cathode surface;TTF⁺ in turn oxidizes the solidLi₂O₂, which results in the regeneration ofTTF. The mediator acts as an electron–hole transfer agent that permits efficient oxidation of solidLi₂O₂. The cell with the mediator demonstrated 100 charge/discharge cycles.

Li/O₂ batteries are a breakthrough in battery technology for powering long-range electric vehicles. The viability of high efficiency, rechargeable Li/O₂ battery is demonstrated by the use of catalyst-free meso-macroporous carbon cathode and N-butyl-N-methyl pyrrolidinium bis(trifluoromethanesulfonyl)imide ionic liquid-based electrolyte. The carbon electrode, obtained by a simple, low-cost template method, features a high specific capacity of 2500 mAh g⁻¹ at 2.6 V vs. Li⁺/Li and, more importantly, a recharge potential lower than 3.8 V vs Li⁺/Li that prevents secondary reactions in the ionic liquid detrimental for battery rechargeability and makes it possible to reach a recharge efficiency of 90%.

Abstract

Nanocrystalline MnO thin film has been prepared by a pulsed laser deposition (PLD) method. The reversible lithium storage capacity of the MnO thin film electrodes at 0.125C is over 472 mAh g⁻¹ (3484 mAh cm⁻³) and can be retained more than 90% after 25 cycles. At a rate of 6C, 55% value of the capacity at 0.125C rate can be obtained for both charge and discharge. As-prepared MnO thin film electrodes show the lowest values of overpotential for both charge and discharge among transition metal oxides. All these performances make MnO a promising high capacity anode material for Li-ion batteries.

Thin films of manganese oxides have been deposited on Cu foils by radio-frequency (RF) sputtering under different conditions. Crystalline MnO films cannot be obtained but Mn³⁺/M⁴⁺ oxides are formed when the growth proceeds under the Ar atmosphere, due to the oxidation of MnO. While they can be obtained under the Ar:H₂ (95:5 vol%) reduction atmosphere at both room temperature and 500 °C. The latter films in thickness of ∼0.5 μm exhibit an initial coulombic efficiency of 75%, reversible capacities of 380 μAh cm⁻² μm⁻¹ (∼700 mAh g⁻¹) at 4 μA cm⁻² (∼0.05 C) after 100 cycles, and 230 μAh cm⁻² μm⁻¹ (∼428 mAh g⁻¹) at 20 C. These values demonstrate that the sputtering-grown MnO films here exhibit excellent cyclability and rate performance in comparison to the reported data of MnO anodes. Pure phase with low oxidation state and certain porosity could be favorable factors accounting for such improved electrochemical performance.

MnO thin films were grown on Cu-foil substrates at various temperatures by the pulsed laser deposition (PLD) technique. The elevated growth temperatures led to the improved storage capacity, initial Coulombic efficiency and rate performance of the thin film anodes. It also had an observable effect on the voltage polarization at the first discharge process, whereas negligible at the subsequent cycles. The origins for these behaviours were clarified. The superior electrochemical performance was attributed to the improved kinetic properties, which relate to the initial better crystallinity and more ordered microstructure of the films caused by the elevated deposition temperatures. The similar behaviour of voltage polarization in the cycles after the first discharge was ascribed to the similar nanocomposited structure induced by the initial conversion reaction. Moreover, the mechanisms for the low initial Coulombic efficiency and the large voltage polarization, which are the appealing and essential topics for the conversion reactions, were elucidated.

Rechargeable lithium batteries represent one of the most important developments in energy storage for 100 years, with the potential to address the key problem of global warming. However, their ability to store energy is limited by the quantity of lithium that may be removed from and reinserted into the positive intercalation electrode, Li_xCoO₂, 0.5<x<1 (corresponding to 140 mA·h g^-1 of charge storage). Abandoning the intercalation electrode and allowing Li to react directly with O₂ from the air at a porous electrode increases the theoretical charge storage by a remarkable 5−10 times! Here we demonstrate two essential prerequisites for the successful operation of a rechargeable Li/O₂ battery; that the Li₂O₂ formed on discharging such an O₂ electrode is decomposed to Li and O₂ on charging (shown here by in situ mass spectrometry), with or without a catalyst, and that charge/discharge cycling is sustainable for many cycles.

Abstract

A mesocellular carbon foam (MCF-C) was prepared by nanocasting technology using mesocellular foam (MCF) silica hard template. The obtained carbon sample exhibits bimodal mesopores with narrow pore size distribution, centered at 4.3 and 30.4 nm. The MCF-C was evaluated as positive electrode in lithium/oxygen battery. It showed a higher discharge capacity, about 40% increased capacity compared to several commercial carbon black. The enhanced performance is probably ascribed to their large pore volumes and ultra-large mesoporous structures, which allow more lithium oxide deposit during discharge process.

Mesoporous cobalt oxides (Co₃O₄) with different porosities were employed as the electrochemical catalysts of lithium–oxygen batteries. The mesoporous Co₃O₄ demonstrated significantly improved round-trip efficiency and specific capacity over the bulk catalyst. Moreover, electrochemical measurement results suggest that the highly open and continuous three-dimensional cubic mesoporous structural features of the catalyst are imperative for its much higher catalytic reactivity than the two-dimensional hexagonal mesoporous Co₃O₄. A capacity close to 2250 mA h g^− 1_carbon and a round-trip efficiency of 81.42% were obtained for the mesoporous Co₃O₄-KIT-6-40 catalyzed air electrode. It is suggested that the mesoporous metal oxide could be a promising catalyst of air electrode for the high performance lithium–oxygen battery.

Understanding the reaction mechanism of nonaqueous oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is key to increase the low round-trip efficiency and power capability of rechargeable Li-air batteries. Here we show that the ORR kinetics are much faster than OER kinetics and OER occurs in two distinct stages upon Li-air battery charging. The first OER stage occurs at low overpotentials (<400 mV) with a slopping voltage profile, whose kinetics are relatively insensitive to charge rates and catalysts. This OER stage could be attributed to the delithiation of the outer part of Li₂O₂ forming lithium-deficient Li_2–xO₂, which is chemically disproportionate to evolve O₂. The second stage takes place at high overpotentials (400–1200 mV), whose kinetics are sensitive to discharge/charge rates and catalysts, which can be attributed to the oxidation of bulk Li₂O₂ particles. Our study provides insights into bridging current two schools of thought on the OER mechanism.

Lithium–oxygen chemistry offers the highest energy density for a rechargeable system as a “lithium–air battery”. Most studies of lithium–air batteries have focused on demonstrating battery operations in pure oxygen conditions; such a battery should technically be described as a “lithium–dioxygen battery”. Consequently, the next step for the lithium–“air” battery is to understand how the reaction chemistry is affected by the constituents of ambient air. Among the components of air, CO₂ is of particular interest because of its high solubility in organic solvents and it can react actively with O₂^–•, which is the key intermediate species in Li–O₂ battery reactions. In this work, we investigated the reaction mechanisms in the Li–O₂/CO₂ cell under various electrolyte conditions using quantum mechanical simulations combined with experimental verification. Our most important finding is that the subtle balance among various reaction pathways influencing the potential energy surfaces can be modified by the electrolyte solvation effect. Thus, a low dielectric electrolyte tends to primarily form Li₂O₂, while a high dielectric electrolyte is effective in electrochemically activating CO₂, yielding only Li₂CO₃. Most surprisingly, we further discovered that a high dielectric medium such as DMSO can result in the reversible reaction of Li₂CO₃ over multiple cycles. We believe that the current mechanistic understanding of the chemistry of CO₂ in a Li–air cell and the interplay of CO₂ with electrolyte solvation will provide an important guideline for developing Li–air batteries. Furthermore, the possibility for a rechargeable Li–O₂/CO₂ battery based on Li₂CO₃ may have merits in enhancing cyclability by minimizing side reactions.