ИСТИНА

Lithium–air (Li–O2) batteries (LAB) have received extraordinary research attention recently owing to their potential to provide positive electrode gravimetric energies considerably higher than Li-ion positive electrodes [1]. Carbon would be ideally suited as a positive electrode material due to its low density, high specific surface area and high conductivity. However, LAB with carbon cathode has poor cycleability due to the number of problems, mainly oxygen transport limitations, passivation of the electrode with an electrically insulating product, and Li2CO3 side product formation at the cathode as a result of lithium superoxide reactions with the carbon material. Our group recently performed an experiment with solid-state lithium oxygen cell comprising carbon cathode based on reduced graphene oxide [2]. It allowed us to conclude that various defects and functional groups on graphite surface might be responsible for the formation of discharge products, as well as byproduct Li2CO3. However, to reveal whether carbon defects possess any electrocatalytic activity towards oxygen reduction reaction, model carbon electrode with controllable surface structure should be the subjects of investigation. The aim of the present experiment was to shed the light on the discharge product composition, morphology and growth dynamics, on the model carbon electrodes, namely single- and bi-layered epitaxial graphene. Graphenes were synthesized by CVD method on Cu foil and transferred onto ceramic Li+-conductive membrane that served as solid electrolyte. Li-O2 cell was assembled as reported previously [2] and stepwise discharge in constant current regime in 10-6 - 10-5 mbar O2 was performed inside the analytical chamber of the spectrometer, each step followed by photoemission spectroscopy and mapping of Li 1s, O 1s and C 1s core levels. We observed that discharge product in such conditions is a composition of Li2O2 and Li2O, and the fraction of Li2O grows with the charge passed. Product growth appeared to be lateral, starting from micrometer scale “islands” on bi-layered graphene electrode, and grid-like structure for single-layered graphene (Fig. 1). In case of bi-layered graphene Li2CO3 byproduct was observed at last discharge steps.