ИСТИНА

In the modern era of portable electronic devices, electric vehicles and other energy-intensive industries, increasing attention is paid to the problems of electrochemical energy storage. Metal-air and especially lithium-air rechargeable batteries are among the most promising electrochemical power sources because of high theoretical energy density. The oxygen reduction reaction, which occurs in the cathodes during the discharge of the aprotic metal-air batteries, leads to the formation of superoxide anions O2 ⁻that can survive in some solvents for a certain time . In lithium-oxygen cells, O2⁻ cannot form stable associates with Li⁺ to form LiO2 as it does not exist as a bulk phase at room temperature . Instead, all the intermediates have to transform into lithium peroxide (Li2O2) , which always demonstrates a very complex morphology. We report the study of Li2O2crystals growth upon the discharge of aprotic lithium-oxygen cells. To perform all the experiments, we utilized porous gold electrodes with an enhanced surface area and high stability with respect to all redox processes and interaction with peroxide and superoxide species. It allowed us to observe changes in the morphology of lithium peroxide discharge products at different discharge current densities. By a simple experiment purely based on the chemical generation of lithium peroxide in the ion exchange reaction between KO2 and lithium salt we show that the morphology of lithium peroxide precipitated after the chemical reaction is quite similar to that of lithium peroxide produced in Li–O2 cells. This finding suggests that lithium peroxide particles can be formed right upon the formation of superoxide anions without the influence of the surface of the electrode. We show that lithium peroxide plate-like crystals are likely to be formed in the liquid electrolyte phase rather than directly on the electrode surface. Li2O2 particles aggregate to produce finally submicron crystal clusters with different morphologies that are deposited on electrode surface. This layer, however, remains porous, which allows a further mass transport between the electrode and the electrolyte. These deposits can then lose their electric contact with the electrode and thus additionally limit the rechargeablity of the Li–O2cell. On the other hand, the special morphology of Li2O2provides a larger surface compared to well-facetted crystals or uniform films that might allow a faster recharge if a proper liquid phase redox shuttle can be found.