Study On The Cathode Catalyst And Nonflammable Electrolyte In Nonaqueous Li-Air Battery

Non-aqueous lithium-air batteries have been considered as one of the most promising energy storage systems due to their extremely high energy density.The successful application of Li-air batteries could efficiently reduce greenhouse gas emissions and the consumption of non-renewable fossil fuels.However,the development of Li-air battery is still in its infancy stage and many key scientific and technical problems that restrict its practical use have not been well resolved.At present,the key problems in lithium-air batteries are poor rate performance,high polarization,instability of electrolytes and air electrodes,and high reactivity of lithium electrodes.Based on the above issues,we have conducted research work on the synthesis of highly efficient catalysts and the development of stable electrolytes.In this thesis,the main contents are mainly focused on:1.Firstly,the thesis outlines the classification and working principle of lithium-air batteries and the advantages and disadvantages of lithium-air batteries.It also summarizes the research progress of cathode materials,anode materials,and electrolytes for non-aqueous lithium-air batteries,and provides the existing challenges and perspectives for lithium-air batteries.2.The Co₂CrO₄ nanospheres?CCO?with a porous core-shell structure were synthesized by a simple hydrothermal method.The structural advantages of CCO promote the rapid diffusion of Li⁺and O₂,providing sufficient space for the deposition of discharge products.The surface states of CCO also favor the ORR/OER reaction.The overpotential of the lithium-air battery using the CCO catalyst was significantly reduced,and the rate and cycling performance were also obviously improved.More importantly,CCO has a strong adsorption on oxygen reduction intermediates,which can control the morphology of discharge products,and directly affect the performance of the battery.This strong adsorption was confirmed by DFT calculations.Based on the experimental results and theoretical calculations,for the first time,we proposed the catalytic mechanism for the formation and decomposition of discharge products under the catalyst control,and gave a reasonable explanation for the formation and decomposition of flower-like Li₂O₂ on CCO.This study gives insight into the understanding the catalytic mechanism of the lithium-air batteries,designing the catalyst structure,and modifying the surface state of the catalyst.3.As the solid catalysts are fixed on the electrode,issues pertaining to surface passivation and pore clogging of the cathode cannot be solved,especially in deep discharge.One strategy can deal with this issue by introducing a soluble catalyst?V?acac?₃?into the lithium air battery.During the charging process,the oxidized form of V?acac?₃ can promote the decomposition of Li₂O₂,reduce the charging voltage,avoid serious side reactions at high charging voltage,and improve rate performance.More importantly,the V?acac?₃ catalyst can promote the formation of Li₂O₂ by a two-electron transfer process,avoid the formation of highly reactive LiO₂ by one electron reaction,which can effectively reduce the occurrence of side reactions in the discharge process.V?acac?₃ is a rare soluble catalyst that promotes both the charge process and discharge process.This work provides new ideas for the development of multifunctional soluble catalyst for the lithium-air batteries in the future.4.The electrolyte plays a very important role in the lithium air battery.An efficient electrolyte is the guarantee for the stable operation and high-energy output of the battery.1,3-dimethylpropylene urea?DMPU?as a high Gutmann donor number solvent were introduced into the electrolyte.The DMPU electrolyte can facilitate the solution phase reaction mechanism,which promotes the promation of toroid Li₂O₂ and improve the battery capacity and rate performance.Equally,in a lithium air battery using DMPU as an electrolyte,the polarization of the battery is reduced,and the cycle performance is improved.Electrochemical studies indicate that the initial stage of the oxygen reduction process in the DMPU electrolyte is a two-step reaction mechanism.In addition,the DMPU electrolyte decomposes when the charging voltage is greater than 3.9 V,and its oxidation product will passivate the electrode and interfere with the assessment of battery performance.5.The design and synthesis of a stable and non-flammable electrolyte has a significant role in promoting the practical use of lithium-air batteries.Therefore,we first improved the oxidation stability by formulating a high concentration of G4 electrolyte?HCG4?.Then,1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether?TTE?,a low polarity,high electrochemical stability solvent was introduced into the HCG4 electrolyte.We confirmed that the complex cation structure?Li⁺-G4?is retained that avoiding the decomposition of the G4solvent during ORR and OER process,which is effective to improve the cycle stability.The Li-O₂ cell using the HCG4/TTE electrolyte exhibits outstanding cyclability.Moreover,the HCG4/TTE electrolyte helping to form a robust protective film on the Li anode,which can significantly restrain the parasitic reactions at the Li anode/electrolyte interface and improve the cycling performance of Li-O₂ cells.Equally,the HCG4/TTE electrolyte has good wettability and is nonflammable,which is beneficial for improving the power performances and safety in cell operations of Li-O₂ batteries.The electrolyte has positive effects for the development of high stability and high safety lithium-air batteries,and promotes the practical use of lithium-air batteries.