A Model Study Of The Synergistic Effect Of Solvents And Electrode Materials In Lithium-oxygen Batteries

In recent years,rechargeable lithium-oxygen（Li-O₂）batteries have been widely studied because of their ultra-high theoretical energy density.However,the practical application of Li-O₂batteries is still restricted by the low practical capacity,large charge-discharge polarization,and poor cycle performance.In the research of battery materials,the physiochemical properties of electrolyte solvents are the critical factors that influence the performance of Li-O₂batteries.In the solvent effect research reported in the literature,the gold disk electrode is usually used as the oxygen electrode model,and the research results may be different from the reaction in the actual electrode.In addition,the additives in the composite electrode interfere with the charge-discharge mechanism study of Li-O₂batteries.Furthermore,most studies of solvent effect in literature ignore the synergistic effects of electrode materials.The interaction between discharge products,solvents,and electrode materials should be considered in the multiphase reaction system of practical Li-O₂batteries.Considering these issues in previous studies,this thesis uses nanoarray electrodes as a research model to study the effect of solvent donor number（DN）on the discharge capacity of Li-O₂batteries.The carbon cloth-supported nanoarrays not only mimic the carbon-supported catalyst but also eliminate the interference of binders,thus clearly illustrating the evolution of charge and discharge products by open architectures.In addition,a variety of metal oxide nanoarray electrodes were prepared in this dissertation,and the synergistic effect of solvent and electrode material on the kinetics of the charge-discharge reaction was discussed.The research in this thesis provides a new model method for the optimization of electrode materials and electrolyte solvents for Li-O₂batteries.The main research contents and conclusions of this thesis are as follows:（1）Mn O₂nanosheets（Mn O₂@CC）were prepared on carbon cloth by a simple electrodeposition method.The influences of solvents donor number（DN）on Mn O₂@CC electrode discharge capacities were studied with tetraethylene glycol dimethyl ether（G4）,dimethyl sulfoxide（DMSO）and DMSO-G4 binary solvents.Results show that Mn O₂@CC in DMSO solvent with a high DN（30）delivers the maximum discharge capacity of 2.54 m Ah cm^-2and a toroid-like discharge product,which indicates the"solvation"mechanism in the oxygen reduction process.However,Mn O₂@CC delivers the lowest discharge capacity of only 0.73 m Ah cm^-2in G4 solvent with a lower DN（17）.The discharge product is film type,which indicates that the redox process follows the"surface"mechanism.In DMSO-G4binary solvent,the discharge capacity of Mn O₂@CC increases with the increase of DMSO ratio,and the discharge products show a gradual transition from thin film to small particles,indicating that there is a combined mechanism of"solvation"process and"surface"process.All the results demonstrate that the nanoarray electrode is an ideal model to study the solvent effect.The model directly confirms the two discharge mechanisms of oxygen electrodes in aprotic electrolytes and provides a new idea for the optimization of electrolyte solvents.（2）The synergistic effect of electrolyte solvent and electrode material on the discharge reaction kinetics was studied by comparing the charge-discharge behaviors of Mn O₂@CC,Ni O@CC,Co₃O₄@CC and CNT@CP in G4,DMSO,and DMSO-G4binary solvents.Results show that Mn O₂@CC and Ni O@CC which have strong interactions with the discharge products fully demonstrate the DN effect of the solvent.However,CNT@CP and Co₃O₄@CC which have weak interactions with discharge products do not demonstrate the DN effect of the solvent and the discharge capacities of CNT@CP and Co₃O₄@CC are independent of solvent DN.In addition,this thesis calculates the interactions between electrode materials and discharge products by density functional theory（DFT）.The calculation results confirm the weak adsorption of discharge product on CNT@CP and Co₃O₄@CC and the strong adsorption of discharge product on Mn O₂@CC and Ni O@CC.In short,the discharge mechanism of Li-O₂batteries is determined by the adsorption energy of the electrode material and the solvation ability of the solvent.When the adsorption energy of the electrode material to the intermediate is stronger than the solvation ability of the solvent,the discharge tends to the surface mechanism,and vice versa towards a solution mechanism.This thesis reveals the synergistic and competitive relationship between electrode materials and electrolyte solvents for Li O₂adsorption,which has important guiding significance for the optimization of oxygen electrode materials and electrolyte solvents.