ИСТИНА

The development of lithium-air batteries today faces a number of fundamental problems restricting the system rechargeability. Nevertheless, its promise of achieving of 1 kWh/kg at cell according to some estimations [1] drives an active interest to the lithium-oxygen electrochemical system. The possibility of achieving such specific energy, however, is also still under question as a number of processes can limit the battery capacity. One of the most important is electrode passivation by both insulating discharge and side products [1, 2]. Deposition of the discharge product mainly composed of lithium peroxide is a multistep process. One of the intermediate processes is the disproportionation of electrochemically generated superoxide to lithium peroxide. The rate of this reaction significantly impacts the morphology of the deposited particles and, according to literature, is governed by a solvent donor number [3]. Although, a few works devoted to study the Li2O2 precipitation on cathode and in most of them carbon black is used as cathode material, but we found that Li2O2 morphology is completely different inside and on the surface of carbon black agglomerates [4] that complicates results interpretation. Here, we study solvent influence on the Li2O2 morphology formed during the galvanostatic discharge at various current densities using a model cell with a carbon fiber cathode where discharge product grows more or less uniformly in the whole electrode volume. After discharge and cell disassembly we performed SEM study of the cathodes. For our experiments, we chose solvents that are relatively stable both to oxidation by oxygen reduction reaction products and to reduction by metallic lithium. We found that Li2O2 particles (spheres or toroids) are formed only in the electrolytes based on dimethylsulfoxide and tetramethylene sulfone while films are formed in electrolytes based on dimethoxyethane, γ-butyrolactone and tetraethylene glycol dimethyl ether. It was established that there is no correlation between both Li2O2 morphology and discharge capacity and a solvent donor number that contradicts with Li2O2 formation mechanism suggested in literature [3].