In the search for improved energy storage, rechargeable metal-oxygen batteries are very attractive owing to their reliance on molecular oxygen, which forms oxides on discharge that decompose reversibly on charge. Much focus has been directed at aprotic Li-O2 cells, but the aprotic Na-O2 system is of equal interest because of its better reversibility. We report here on the critical role and mechanism of phase-transfer catalysis in Na-O2 batteries. We find that it is solely responsible for the growth and dissolution of micrometre-sized cubic NaO2 crystals and for the reversible cell capacity. In the absence of phase-transfer catalysis, quasi-amorphous NaO2 films are formed and cells exhibit negligible capacity. Electrochemical investigations provide a measure of the transportation of superoxide from the carbon electrode to the electrolyte phase by the phase transfer catalyst. This leads to a new understanding of the mechanism of Na-O2 batteries that, significantly, extends to Li-O2 cells and explains their different behaviour.

mass of both reactants and a favorable 3.0 V potential. [3][4][5][6] The theoretical specifi c energy of 3505 Wh kg -1 , and energy density of 3435 Wh L -1 (based on Li 2 O 2 ), is very attractive compared to contemporary Li-ion couples that are calculated to have theoretical values near 400 Wh kg -1 and 1400 Wh L -1 . Practical values for Li-air batteries are estimated near 1000 Wh kg -1 , [ 7 ] which compare favorably to current Li-ion commercial cells (210 Wh kg -1 ), but realizing this specifi c energy over continuous operation requires surmounting many challenges in the underlying chemistry.The most prominent of these is creating an electrolyte for the non-aqueous cell which is inert to nucleophilic attack on discharge and charge of the battery, is stable to metallic lithium and solvates Li salts. Although the search for more stable systems has resulted in many investigations of different solvent/ salt combinations, it is generally agreed that there is presently no electrolyte that fi ts these requirements. Dimethylacetamide (DMA) [ 8 ] and dimethylformamide (DMF) [ 9 ] have recently been shown to be quasi-stable in combination with the LiTFSI salt. Nonetheless, both solvents react to form Li-X salts on cycling (X = formate, acetate and carbonate). [ 10 ] These decomposition products, particularly Li 2 CO 3, precipitate on the cathode where they increase impedance and create high cell polarization on charge owing to their high oxidation potentials. [11][12][13] Similar problems are created by the carbon support typically used for the gas diffusion membrane cathode which has been shown to react with the peroxide discharge product and produce Li 2 CO 3 interfacial impedance layers. [ 10 ] Two promising solutions to this dilemma have been presented by Peng et al., [ 14 ] who employed dimethyl sulfoxide (DMSO) as a solvent in combination with a nanoporous gold foil as a gas diffusion membrane, or Thotiyl et al., [ 15 ] utilizing TiC as a stable cathode material. Decomposition of DMSO leads to soluble products such as dimethyl sulfone and lithium sulfate, [16][17][18] which do not passivate the cathode surface to the same extent as do the carbonates. The high reactivity of DMSO with the lithium metal anode, and the eventual precipitation of the decomposition products renders this a fi rst-step solution. The higher rate capability of the noncarbonaceous cathode materials is also highly benefi cial because it favors formation of quasi-amorphous Li 2 O 2 thin-fi lms, which exhibit lower charging potentials. [ 19,20 ] TiC exhibits limited cycling behavior at fast rates even with tetraglyme (TEGDME).A new lithium-ether-derived chelate ionic liquid is synthesized to serve as an electrolyte for the Li-O 2 battery that is stable to metallic lithium, and whose ethereal framework is much more inherently stable to superoxide-initiated hydrogen abstraction than the simple glyme, dimethoxyethane (DME). Reactions of chemically generated superoxide with this electrolyte show that virtually no decomposition products such as lith...

Advanced large-scale electrochemical energy storage requires cost-effective battery systems with high energy densities. Aprotic sodium-oxygen (Na-O2) batteries offer advantages, being comprised of low-cost elements and possessing much lower charge overpotential and higher reversibility compared to their lithium-oxygen battery cousins. Although such differences have been explained by solution-mediated superoxide transport, the underlying nature of this mechanism is not fully understood. Water has been suggested to solubilize superoxide via formation of hydroperoxyl (HO2), but direct evidence of these HO2 radical species in cells has proven elusive. Here, we use ESR spectroscopy at 210 K to identify and quantify soluble HO2 radicals in the electrolyte-cold-trapped in situ to prolong their lifetime-in a Na-O2 cell. These investigations are coupled to parallel SEM studies that image crystalline sodium superoxide (NaO2) on the carbon cathode. The superoxide radicals were spin-trapped via reaction with 5,5-dimethyl-pyrroline N-oxide at different electrochemical stages, allowing monitoring of their production and consumption during cycling. Our results conclusively demonstrate that transport of superoxide from cathode to electrolyte leads to the nucleation and growth of NaO2, which follows classical mechanisms based on the variation of superoxide content in the electrolyte and its correlation with the crystallization of cubic NaO2. The changes in superoxide content upon charge show that charge proceeds through the reverse solution process. Furthermore, we identify the carbon-centered/oxygen-centered alkyl radicals arising from attack of these solubilized HO2 species on the diglyme solvent. This is the first direct evidence of such species, which are likely responsible for electrolyte degradation.

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