ИСТИНА

The lithium-air batteries attract a lot of attention worldwide as a promising power source for electric vehicle propulsion as it could provide extremely high specific energy in comparison with other battery systems developed to date. The main features of lithium-air system include metallic lithium as an anode, and air cathode where oxygen reduction during battery discharge and its evolution at recharge occurs. Conventional cathode material for lithium-air cell comprises oxygen reduction catalyst (transition metal oxide or complex) mixed with electronically conductive carbon. In this case, active reaction sites would be boundaries between conductive carbon particles, absorbed oxygen species on the catalyst surface, and lithium-ion-conductive electrolyte. Thus, the discharge process could be limited by the electronic, ionic or oxygen transport in the positive electrode. The main discharge product is believed to be the lithium peroxide, that was confirmed using spectroscopic techniques. However, the electrode reaction pathways are not yet understood so the search for effective electrode materials is embarrassed and performed by trial- and-error route. High pressure XPS and Raman spectroscopy studies under electrochemical conditions provides unique tool to probe unusual chemistry at true operando conditions. The aim of the experiment is to get an insight to the cathode direct and reverse reaction pathways and to identify discharge products formed at the surface using high-pressure XPS and Raman scattering. Among different cathode materials we are focused on a reduced graphene oxide (RGO) as a convenient model electrode: it provides high performance for Li-air batteries, could be easily deposited on the substrate and has good electrical conductivity. In this work simple cell design for carrying out spectroscopic and electrochemical measurements was used: metallic lithium foil (anode) was brought into a contact with lithium-ion-conductive NASICON-type glass-ceramic membrane (solid electrolyte), graphene layer (cathode) was deposited on the opposite side of the membrane. Such electrode design provide developed 3-phase interface where Li+, e- and O2 can react thus making it possible to explore the reactions occurring in real air cathodes with high-pressure XPS and Raman techniques. Electrochemical and spectroscopic measurements were carried out in vacuum, argon and oxygen athmosphere in galvanostatic regime. Using operando XPS at high oxygen pressure (0.1 mbar) we have confirmed that the discharge product stoichiometry Li:O = 1:1. At the same time the appearance of lithium peroxide bands in Raman spectra makes us believe that discharge product is Li2O2. We demonstrated that RGO could effectively promote the discharge reaction as a catalyst. It was demonstrated that the discharge process involves the formation of intermediate compounds including carbonyl, carboxyl and other oxygen containing groups at the cathode surface.