Lithium-Oxygen Batteries and Related Systems: Potential, Status, and Future.

The goal of limiting global warming to 1.5 °C requires a drastic reduction in CO2 emissions across many sectors of the world economy. Batteries are vital to this endeavor, whether used in electric vehicles, to store renewable electricity, or in aviation. Present lithium-ion technologies are preparing the public for this inevitable change, but their maximum theoretical specific capacity presents a limitation. Their high cost is another concern for commercial viability. Metal-air batteries have the highest theoretical energy density of all possible secondary battery technologies and could yield step changes in energy storage, if their practical difficulties could be overcome. The scope of this review is to provide an objective, comprehensive, and authoritative assessment of the intensive work invested in nonaqueous rechargeable metal-air batteries over the past few years, which identified the key problems and guides directions to solve them. We focus primarily on the challenges and outlook for Li-O2 cells but include Na-O2, K-O2, and Mg-O2 cells for comparison. Our review highlights the interdisciplinary nature of this field that involves a combination of materials chemistry, electrochemistry, computation, microscopy, spectroscopy, and surface science. The mechanisms of O2 reduction and evolution are considered in the light of recent findings, along with developments in positive and negative electrodes, electrolytes, electrocatalysis on surfaces and in solution, and the degradative effect of singlet oxygen, which is typically formed in Li-O2 cells.

2,4-Dimethoxy-2,4-dimethylpentan-3-one: An Aprotic Solvent Designed for Stability in Li-O2 Cells.

In this study, we present a new aprotic solvent, 2,4-dimethoxy-2,4-dimethylpentan-3-one (DMDMP), which is designed to resist nucleophilic attack and hydrogen abstraction by reduced oxygen species. Li-O2 cells using DMDMP solutions were successfully cycled. By various analytical measurements, we showed that even after prolonged cycling only a negligible amount of DMDMP was degraded. We suggest that the observed capacity fading of the Li-O2 DMDMP-based cells was due to instability of the lithium anode during cycling. The stability toward oxygen species makes DMDMP an excellent solvent candidate for many kinds of electrochemical systems which involve oxygen reduction and assorted evaluation reactions.

1,1-diamino-2,2-dinitroethylenes are always zwitterions.

The nitration of tetraiodoethylene (7) yields 1,1-diiodo-2,2-dinitroethylene (8). The latter reacts with alkylamines 9 or alkyldiamines 11 to give the corresponding acyclic 1,1-diamino-2,2-dinitroethylenes 10 or their cyclic analogs 12, respectively. On the basis of liquid and solid-state (13)C and (15)N NMR data, x-ray analysis and ab initio calculations, we suggest that the title compounds are always zwitterionic and that the C(A)-C(N) bond is not a true double bond.

The effect of intercalants on the host liposome.

When phospholipids are vigorously dispersed in water, liposomes are formed. In the present study, we have explored the effect of intercalant concentration on various properties of unilamellar liposomes. Liposomes were sonically intercalated with vitamin E acetate (VitEAc) and hypericin (Hy) until no difference in light transmission was observed, which reflects the formation of liposomes of minimal diameter. Our studies indicate that the intercalant structure and concentration have an influence on the liposome diameter, which could be directly measured by cryogenic transmittance electronic microscopy. Thus, intercalated VitEAc substantially decreased the diameter of unilamellar dimyristoylphosphatidylcholine liposomes, whereas Hy did not. In addition, we followed peak intensities in the absorbance and fluorescence spectra of Hy as a function of intercalant concentration in the liposomal solution. Initially, the fluorescence intensity increased linearly with concentration; however, the curve then arched asymptotically, followed by a decrease in fluorescence at yet higher concentrations. Because the Hy monomer is the only species that emits fluorescence, we believe that the decrease of fluorescence intensity is the result of Hy aggregation.

Preparation of Tyrian purple (6,6'-dibromoindigo): past and present.

Over the past century, various synthetic approaches have been suggested to the most famous dye of antiquity, Tyrian purple (6,6'-dibromoindigo). These synthetic routes have been exhaustively surveyed and critically evaluated from the perspective of convenience, cost, safety and yield.

A simple, safe and efficient synthesis of Tyrian purple (6,6'-dibromoindigo).

6,6'-Dibromoindigo is a major component of the historic pigment Tyrian purple, arguably the most famous dye of antiquity. Over the last century, chemists have been interested in developing practical syntheses of the compound We describe herein a new, reasonably simple and efficient synthesis of Tyrian purple which opens the way to the production of large quantities of the dye with minimal hazards and at low cost.

Locating intercalants within lipid bilayers using fluorescence quenching by bromophospholipids and iodophospholipids.

In previous work, we have been able to determine the depth of intercalated molecules within the lipid bilayer using the solvent polarity sensitivity of three spectroscopic techniques: the 13C NMR chemical shift (δ); the fluorescence emission wavelength (λem), and the ESR ß-H splitting constants (aß-H). In the present paper, we use the quenching by a heavy atom (Br or I), situated at a known location along a phospholipid chain, as a probe of the location of a fluorescent moiety. We have synthesized various phospholipids with bromine (or iodine) atoms substituted at various locations along the lipid chain. The latter halolipids were intercalated in turn with various fluorophores into DMPC liposomes, biomembranes and erythrocyte ghosts. The most effective fluorescence quenching occurs when the heavy atom location corresponds to that of the fluorophore. The results show that generally speaking the fluorophore intercalates the same depth independent of which lipid bilayer is used. KBr (or KI) is the most effective quencher when the fluorophore resides in or at the aqueous phase. Presumably because of iodine's larger radius and spin coupling constant, the iodine analogs are far less discriminating in the depth range it quenches.

Determining radical penetration of lipid bilayers with new lipophilic spin traps.

Predicting the susceptibility of lipid moieties to radical attack requires a determination of the depth of radical penetration into a lipid membrane. We thus synthesized three homologous series of lipophilic spin traps--DMPO analogs 2-alkanoyl-2-methyl-1-pyrroline N-oxides (11) and PBN derivatives 4-alkoxyphenyl N-tert-butylnitrones (18) and 4-alkoxyphenyl N-admantylnitrones (20). The intercalation depth of these spin traps within the liposomal bilayer was determined via the previously reported NMR technique, which correlates the chemical shift and the micropolarity (measured in ET(30) units) experienced by the pivotal nitronyl carbon. Hydroxyl and alpha-hydroxyalkyl radicals were generated in the extraliposomal aqueous phase and the lowest depth at which a radical could be spin trapped was determined. The ESR data indicate that these radicals can exit the aqueous phase, penetrate the lipid bilayer past the head groups (ET(30)=63 kcal/mol) and the glycerol ester (ET(30)=52 kcal/mol), and pass down to an ET(30) polarity of at least 44 kcal/mol. The latter depth presumably corresponds to the upper portion of the lipid slab. It is likely, if not probable, that having come this far they can abstract the allylic/diallylic hydrogens resident in the midslab at ET(30) values of >31 kcal/mol.

Aggregate formation in the intercalation of long-chain fatty acid esters into liposomes.

Various hydrophobic benzenediacetic esters, the corresponding benzenedipropionic esters, and branched alkyl esters were intercalated into DMPC liposomes, where the molar ratio (n/n) of ester:DMPC was 1:5. In the case of the very long-chain derivatives, double carbonyl peaks were observed in the 13C NMR spectrum. This doubling phenomenon was observed only for the carbonyl peaks, whose chemical shift is most sensitive to solvent polarity, and disappeared when the ester:DMPC molar ratio drops below 1:15. This doubling reflects the presence of two populations in these samples: one group includes those molecules which are intercalated within the liposome and feel the polarity corresponding to the liposomal microenvironment; the other consists of aggregates of these long-chain derivatives located in the extra-liposomal aqueous phase.

NMR-based molecular ruler for determining the depth of intercalants within the lipid bilayer Part II. The preparation of a molecular ruler.

We have previously shown how the location of an intercalant within the lipid bilayer can be qualitatively determined by using the excellent correlation that exists between the 13C NMR chemical shift of a polarizable carbon (e.g., the carbonyl or nitronyl carbon) and the polarity (using the Dimroth-Reichardt's ET(30) parameter) of the microenvironment in which that carbon resides. In a companion paper, we have determined criteria for reporter molecules that will assist us in converting this qualitative polarity data into quantitative Angstrom values. In the present paper, we report on our initial success in quantitatively mapping of the DMPC bilayer by linking two or more vertical points within a bilayer by both distance (in Angstroms) and ET(30) polarity. The results correlated well with the values obtained using the "parallax method" of Erwin London.

NMR-based molecular ruler for determining the depth of intercalants within the lipid bilayer Part I. Discovering the guidelines.

The development of "molecular rulers" would allow one to quantitatively locate intercalants within the liposomal bilayer. To this end, we have attempted to correlate the 13C NMR chemical shift of a polarizable "reporter" carbon (e.g., carbonyl) of the intercalant-with the ET30 polarity it experiences, and with its Angstrom distance from the interface. This requires families of molecules with the same two "reporter carbons" separated by a fixed distance, residing at various depths/polarities within the bilayer. The families studied included 4,4-dialkylcyclohexa-2,5-dienones 1, benzenediacetic esters 15, benzenedipropionic esters 17, 4-alkoxybenzaldehydes 19 and methyl 4-alkoxybenzoates 22. These compounds possessed the following characteristics: (1) a planar backbone; (2) polar/hydrophilic "head" groups; (3) modular hydrophobic tails; (4) large changes in the 13C NMR chemical shift (Deltadelta) of the reporter atoms with solvent polarity. These studies revealed a fifth requirement, namely: (5) the reporter carbons must not be strongly conjugated, lest it reflect the charge build-up at another site within the conjugated system.

Shedding Light on the Oxygen Reduction Reaction Mechanism in Ether-Based Electrolyte Solutions: A Study Using Operando UV-Vis Spectroscopy.

Using UV-vis spectroscopy in conjunction with various electrochemical techniques, we have developed a new effective operando methodology for investigating the oxygen reduction reactions (ORRs) and their mechanisms in nonaqueous solutions. We can follow the in situ formation and presence of superoxide moieties during ORR as a function of solvent, cations, anions, and additives in the solution. Thus, using operando UV-vis spectroscopy, we found evidence for the formation of superoxide radical anions during oxygen reduction in LiTFSI/diglyme electrolyte solutions. Nitro blue tetrazolium (NBT) was used to indicate the presence of superoxide moieties based on its unique spectral response. Indeed, the spectral response of NBT containing solutions undergoing ORR could provide a direct indication for the level of association of the Li cations with the electrolyte anions.

The importance of solvent selection in Li-O2 cells.

We have examined the effect of glyme selection on the cycling behavior of Li-O2 cells. We conclude that diglyme is the optimal solvent for prolonged cycling. We have also focused on the effect of electrolyte solution instability as compared to other cell components.

Feasibility of Full (Li-Ion)-O2 Cells Comprised of Hard Carbon Anodes.

Aprotic Li-O2 battery is an exciting concept. The enormous theoretical energy density and cell assembly simplicity make this technology very appealing. Nevertheless, the instability of the cell components, such as cathode, anode, and electrolyte solution during cycling, does not allow this technology to be fully commercialized. One of the intrinsic challenges facing researchers is the use of lithium metal as an anode in Li-O2 cells. The high activity toward chemical moieties and lack of control of the dissolution/deposition processes of lithium metal makes this anode material unreliable. The safety issues accompanied by these processes intimidate battery manufacturers. The need for a reliable anode is crucial. In this work we have examined the replacement of metallic lithium anode in Li-O2 cells with lithiated hard carbon (HC) electrodes. HC anodes have many benefits that are suitable for oxygen reduction in the presence of solvated lithium cations. In contrast to lithium metal, the insertion of lithium cations into the carbon host is much more systematic and safe. In addition, with HC anodes we can use aprotic solvents such as glymes that are suitable for oxygen reduction applications. By contrast, lithium cations fail to intercalate reversibly into ordered carbon such as graphite and soft carbons using ethereal electrolyte solutions, due to detrimental co-intercalation of solvent molecules with Li ions into ordered carbon structures. The hard carbon electrodes were prelithiated prior to being used as anodes in the Li-O2 rechargeable battery systems. Full cells containing diglyme based solutions and a monolithic carbon cathode were measured by various electrochemical methods. To identify the products and surface films that were formed during cells operation, both the cathodes and anodes were examined ex situ by XRD, FTIR, and electron microscopy. The HC anodes were found to be a suitable material for (Li-ion)-O2 cell. Although there are still many challenges to tackle, this study offers a more practical direction for this promising battery technology and sets up a platform for further systematic optimization of its various components.

Mechanistic Role of Li⁺ Dissociation Level in Aprotic Li-O2 Battery.

The kinetics and thermodynamics of oxygen reduction reactions (ORR) in aprotic Li electrolyte were shown to be highly dependent on the surrounding chemical environment and electrochemical conditions. Numerous reports have demonstrated the importance of high donor number (DN) solvents for enhanced ORR, and attributed this phenomenon to the stabilizing interactions between the reduced oxygen species and the solvent molecules. We focus herein on the often overlooked effect of the Li salt used in the electrolyte solution. We show that the level of dissociation of the salt used plays a significant role in the ORR, even as important as the effect of the solvent DN. We clearly show that the salt used dictates the kinetics and thermodynamic of the ORR, and also enables control of the reduced Li2O2 morphology. By optimizing the salt composition, we have managed to demonstrate a superior ORR behavior in diglyme solutions, even when compared to the high DN DMSO solutions. Our work paves the way for optimization of various solvents with reasonable anodic and cathodic stabilities, which have so far been overlooked due to their relatively low DN.

Catalytic Behavior of Lithium Nitrate in Li-O2 Cells.

The development of a successful Li-O2 battery depends to a large extent on the discovery of electrolyte solutions that remain chemically stable through the reduction and oxidation reactions that occur during cell operations. The influence of the electrolyte anions on the behavior of Li-O2 cells was thought to be negligible. However, it has recently been suggested that specific anions can have a dramatic effect on the chemistry of a Li-O2 cell. In the present paper, we describe how LiNO3 in polyether solvents can improve both oxygen reduction (ORR) and oxygen evolution (OER) reactions. In particular, the nitrate anion can enhance the ORR by enabling a mechanism that involves solubilized species like superoxide radicals, which allows for the formation of submicronic Li2O2 particles. Such phenomena were also observed in Li-O2 cells with high donor number solvents, such as dimethyl sulfoxide dimethylformamide (DMF) and dimethylacetamide (DMA). Nevertheless, their instability toward oxygen reduction, lithium metals, and high oxidation potentials renders them less suitable than polyether solvents. In turn, using catalysts like LiI to reduce the OER overpotential might enhance parasitic reactions. We show herein that LiNO3 can serve as an electrolyte and useful redox mediator. NO2(-) ions are formed by the reduction of nitrate ions on the anode. Their oxidation forms NO2, which readily oxidizes to Li2O2. The latter process moves the OER overpotentials down into a potential window suitable for polyether solvent-based cells. Advanced analytical tools, including in situ electrochemical quartz microbalance (EQCM) and ESR plus XPS, HR-SEM, and impedance spectroscopy, were used for the studies reported herein.

Superoxide organic chemistry within the liposomal bilayer, part II: a correlation between location and chemistry.

Coumarin ester derivatives 1, substituted at C-4 and/or C-12 with alkyl chains, were synthesized and intercalated within DMPC liposomal bilayers. By correlating the 13C chemical shift with medium polarity [E(T)(30)], the relative location of these substrates within the liposomal bilayer was determined. The length of the alkyl chain substituents clearly influences the lipophilicity of the substrates and their location and orientation within the liposome: Superoxide readily saponifies the C-12 esteric linkage of 1, when this reaction site lies in a polar region of the liposome (E(T)(30) > 45 kcal/mol), to give the corresponding 7-hydroxy coumarin derivatives 2. However, when C-12 lies deeper and is hence less available to O(2)(*-), the lactonic carbon C-2, which lies in a shallower region (E(T)(30) = 43-49), is the preferred site for superoxide-mediated cleavage. When coumarin 1 is disubstituted with long chains at both C-12 and C-4, these derivatives lie deep within the bilayer and react only slowly with O(2)(*-). These results indicate there is indeed a correlation between location within the bilayer and substrate reactivity. Contrary to the suggestion of Dix and Aikens (Chem. Res. Toxicol.6:2-18; 1993) superoxide can penetrate deep within the liposomal bilayer. Nevertheless, its concentration drops precipitously (to approximately 16% of what it is near the interface) below E(T) values of 38, thereby precluding substantial reaction with many highly lipophilic substrates. This work also confirms the findings of others that reactions of small oxy-radicals occur within cellular membranes and appear to be of significant biological importance.

Cine Substitution in 2-Oxabicyclo[4.2.0]octanones and Subsequent Unusual Rearrangements(1).

Reaction of 2-chlorooxabicyclo[4.2.0]octanone 5 with several nucleophiles was examined and found to differ significantly from those of carbon analog 1. MeO(-) and PhS(-) led either to products of cine substitution 9 or of ring opening to cyclobutenones 8. With most enolates cine substitution occured via C-alkylation of the intermediate oxidoallyl cation in spite of formation of a new C-C bond between two quaternary carbons; with nitroalkanes O-alkylation was preferred. With azide as a nucleophile, further transformations occurred, among them an oxy-promoted electrocyclic cyclobutane opening, with incorporation of a phenyl triazole unit and final formation of the unusual product 19a. Evidence for a mechanism explaining formation of 19a was obtained by isolation of intermediates. Thermolysis or photolysis of 8e or9b led via electrocyclic ring opening to a vinyl ketene which was trapped by MeOH, alkenes, dienes, or oxygen to produce polyfunctional unsaturated esters 29 and 30 or 8-membered ring lactone 31, fused cyclobutanones 33 and 34, pyranone 38, or gamma-lactone 39, respectively.

Active oxygen chemistry within the liposomal bilayer. Part IV: Locating 2',7'-dichlorofluorescein (DCF), 2',7'-dichlorodihydrofluorescein (DCFH) and 2',7'-dichlorodihydrofluorescein diacetate (DCFH-DA) in the lipid bilayer.

2',7'-Dichlorodihydrofluorescein diacetate (DCFH-DA) is commonly used to detect the generation of reactive oxygen intermediates and for assessing the overall oxidative stress in toxicological phenomenon. It has been suggested that DCFH-DA crosses the cell membrane, subsequently undergoing deacetylation by intracellular esterases. The resulting 2',7'-dichlorodihydrofluorescein (DCFH) is proposed to react with intracellular hydrogen peroxide or other oxidizing ROS to give the fluorescent 2',7'-dichlorofluorescein (DCF). Using an NMR chemical shift-polarity correlation, we have determined that DCFH-DA and DCFH are located well within the lipid bilayer and certainly not at the interface. These results, therefore, put into serious question the proposed ability of DCFH to come in contact with the aqueous phase and thereby interact with aqueous intracellular ROS and components. However, H2O2 and superoxide can cross or at least penetrate the lipid bilayer and react with certain lipophilic substrates. This may well describe the mode of reaction of these and other ROS with DCFH.

Active oxygen chemistry within the liposomal bilayer. Part III: Locating Vitamin E, ubiquinol and ubiquinone and their derivatives in the lipid bilayer.

We have previously shown that the location and orientation of compounds intercalated within the lipid bilayer can be qualitatively determined using an NMR chemical shift-polarity correlation. We describe herein the results of our application of this method to analogs of Vitamin E, ubiquinol and ubiquinone. The results indicate that tocopherol--and presumably the corresponding tocopheroxyl radical--reside adjacent to the interface, and can, therefore, abstract a hydrogen atom from ascorbic acid. On the other hand, the decaprenyl substituted ubiquinol and ubiquinone lie substantially deeper within the lipid membrane. Yet, contrary to the prevailing literature, their location is far from being the same. Ubiquinone-10 is situated above the long-chain fatty acid "slab". Ubiquinol-10 dwells well within the lipid slab, presumably out of "striking range" of Vitamin C. Nevertheless, ubiquinol can act as an antioxidant by reducing C- or O-centered lipid radicals or by recycling the lipid-resident tocopheroxyl radical.