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.

The growth and electrical characteristics of vertically oriented graphene nanosheets grown by radio frequency plasma-enhanced chemical vapor deposition from C2H2 feedstock on nickel substrates and used as electrodes in symmetric electric double layer capacitors (EDLC) are presented. The nanosheets exhibited 2.7 times faster growth rate and much greater specific capacitance for a given growth time than CH4 synthesized films. Raman spectra showed that the intensity ratio of the D band to G band versus temperature initially decreased to a minimum value of 0.45 at a growth temperature of 750 °C, but increased rapidly with further temperature increase (1.15 at 850 °C). The AC specific capacitance at 120 Hz of these EDLC devices increased in a linear fashion with growth temperature, up to 265 μF/cm(2) (2 μm high film, 850 °C with 10 min growth). These devices exhibited ultrafast frequency response: the frequency response at -45° phase angle reached over 20 kHz. Consistent with the increase in D band to G band ratio, the morphology of the films became less vertical, less crystalline, and disordered at substrate temperatures of 800 °C and above. This deterioration in morphology resulted in an increase in graphene surface area and defect density, which, in turn, contributed to the increased capacitance, as well as a slight decrease in frequency response. The low equivalent series resistance varied from 0.07 to 0.08 Ω and was attributed to the significant carbon incorporation into the Ni substrate.

In-plane defects have been introduced into graphene nanosheets by treatment with hydrochloric acid. Acid treatment induces bond cleavage in the CC network via electrophilic attack. These resultant vacancy sites will then undergo further reactions with the surrounding ambient to produce CO and C-H bonds. A σ * resonance at 287 eV in the carbon K-edge x-ray absorption spectra is observed with acid treatment and is assigned to CO states. Theoretical modelling of a di-vacancy in a graphene bilayer reproduces all essential features of this resonance and in addition predicts a metallic conductivity of states around this vacancy. The possibility of engineering the properties of graphene via the routes explored here is an important step towards establishing strategies for building devices based on this material.