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Understanding the thermal stability of major reaction products, Li2O2 (space group P63/mmc) and Li2O (space group Fm3¯m) is critical to improve the safety characteristics of Li-air batteries. The changes in the crystal structure and surface chemistry of Li2O2 and Li2O were examined as a function of temperature via in situ X-ray diffraction (XRD) and in situ X-ray photoelectron spectroscopy (XPS). Significant decreases in the lattice parameters and the c/a ratio of Li2O2 were found at 280°C and higher. These structural changes can be attributed to the transformation of Li2O2 to Li2O2-δ, which is supported by density functional theory calculations. Upon further heating to 700°C, a lithium-deficient Li2-δO phase appeared at 300°C and gradually became stoichiometric upon further heating to ∼550°C. XPS measurements of Li2O2 revealed that Li2O appeared on the surface starting at 250°C, which is in agreement with the onset temperature of phase transformation as detected by XRD. In addition, the growth of Li2CO3 on the surface was found at 250°C, which can be attributed to chemical reactions between Li2O2/Li2O and carbon-containing species (e.g. hydrocarbons) present in the XPS chamber. This finding highlights the challenges of developing stable carbon-based oxygen electrode for Li-air batteries.

Battery performance is strongly correlated with electrode microstructural properties. Of the relevant properties, the tortuosity factor of the electrolyte transport paths through microstructure pores is important as it limits battery maximum charge/discharge rate, particularly for energy-dense thick electrodes. Tortuosity factor however, is difficult to precisely measure, and thus its estimation has been debated frequently in the literature. Herein, three independent approaches have been applied to quantify the tortuosity factor of lithium-ion battery electrodes. The first approach is a microstructure model based on three-dimensional geometries from X-ray computed tomography (CT) and stochastic reconstructions enhanced with computationally generated carbon/binder domain (CBD), as CT is often unable to resolve the CBD. The second approach uses a macro-homogeneous model to fit electrochemical data at several rates, providing a separate estimation of the tortuosity factor. The third approach experimentally measures tortuosity factor via symmetric cells employing a blocking electrolyte. Comparisons have been made across the three approaches for 14 graphite and nickel-manganese-cobalt oxide electrodes. Analysis suggests that if the tortuosity factor were characterized based on the active material skeleton only, the actual tortuosities would be 1.35-1.81 times higher for calendered electrodes. Correlations are provided for varying porosity, CBD phase interfacial arrangement and solid particle morphology.

Understanding the oxygen reduction reaction kinetics in the presence of Na ions and the formation mechanism of discharge product(s) is key to enhancing Na− O 2 battery performance. Here we show NaO 2 as the only discharge product from Na− O 2 cells with carbon nanotubes in 1,2-dimethoxyethane from X-ray diffraction and Raman spectroscopy. Sodium peroxide dihydrate was not detected in the discharged electrode with up to 6000 ppm of H 2 O added to the electrolyte, but it was detected with ambient air exposure. In addition, we show that the sizes and distributions of NaO 2 can be highly dependent on the discharge rate, and we discuss the formation mechanisms responsible for this rate dependence. Micron-sized (∼500 nm) and nanometer-scale (∼50 nm) cubes were found on the top and bottom of a carbon nanotube (CNT) carpet electrode and along CNT sidewalls at 10 mA/g, while only micron-scale cubes (∼2 μm) were found on the top and bottom of the CNT carpet at 1000 mA/g, respectively.R echargeable metal-air (oxygen) batteries are receiving intense interest as possible alternatives to lithium-ion batteries, in particular due to their potential to provide higher gravimetric energies.1−5 While much attention has been focused on aprotic Li−O 2 batteries since their introduction in 1996 by Abraham et al.,6 substantial challenges must be addressed before widespread commercial exploitation is possible. These include the instability of aprotic electrolytes 7−9 and oxygen electrodes, 10,11 which contribute to low round-trip efficiency, poor rate capability, and poor cycle life. Recently, a metal-air battery in which lithium has been replaced by sodium has received increasing attention. Unfortunately, the factors responsible for dissimilar discharge product chemistry and morphologies are still unclear, and no correlation has been found between the type of air electrode or electrolyte and the discharge product formed. Because the discharge product crystal structure, morphology (e.g., shape and thickness), and distribution are important parameters that influence the voltage profile on discharge and charge, the rate capability, the discharge capacity, and the reversibility in metalair batteries, 23 it is critical to gain insights into the oxygen reduction kinetics in the presence of Na ions and the nucleation and growth mechanisms of discharge products.In this study, we investigate discharged Na−O 2 cells with carbon nanotube (CNT) carpet cathodes to understand the effect of discharge kinetics on the formation of the discharge product. We analyzed the chemical composition of the discharge product using X-ray diffraction (XRD) and Raman spectroscopy and found that it consisted solely of NaO 2 . The formation of Na 2 O 2 ·H 2 O was not readily detected with the addition of water to the solvent (<6000 ppm); however, the evolution of Na 2 O 2 ·2H 2 O was detected when we intentionally exposed the sample to the ambient environment. We also observed rate-dependent effects on the size and distribution of the discharge product. ...

Electrochemical processes in lithium–oxygen (Li–O2 or Li–air) batteries are complex, with chemistry depending on cycling conditions, electrode materials and electrolytes. In non‐aqueous Li–O2 cells, reversible lithium peroxide (Li2O2) and irreversible parasitic products (i.e., LiOH, Li2CO3, Li2O) are common. Superoxide intermediates (O2−, LiO2) contribute to the formation of these species and are transiently stable in their own right. While characterization techniques like XRD, XPS and FTIR have been used to observe many Li–O2 species, these methods are poorly suited to superoxide detection. Raman spectroscopy, however, may uniquely identify superoxides from O−O vibrations. The ability to fingerprint Li–O2 products in situ or ex situ, even at very low concentrations, makes Raman an essential tool for the physicochemical characterization of these systems. This review contextualizes the application of Raman spectroscopy and advocates for its wider adoption in the study of Li–O2 batteries.