Study On The Regulation Of Superoxide Intermediates Activity And The Enhancement Of The Cathodic Oxygen Reduction Activity In Aprotic Li-O₂ Batteries

With the rapid development and extensive use of vehicles,energy crisis and environmental problems are becoming the major problems in today's society.Energy conservation and emission reduction,and the development of environmental and efficient electric vehicles have become a major national strategic demand.Currently,electric vehicles have the disadvantage of short range,which is mainly limited by the actual energy density of commercial lithium-ion batteries.At present,the energy density of commercial lithium-ion battery is only 200-300 Wh/kg,and the corresponding electric vehicle mileage is about 200-300 km.Therefore,the development of high specific energy batteries,such as lithium oxygen batteries with a theoretical energy density of about 3500 Wh/kg,will become an effective means to solve the problem of the short endurance of electric vehicles.However,the practical application of lithium-oxygen battery still faces many challenges,including low energy conversion efficiency and poor cycle stability.There are many reasons for the above problems,including:?1?low reaction rate at the oxygen cathode/electrolyte interface,resulting in high charge discharge overpotential and low energy efficiency;?2?a large number of active intermediates,such as singlet oxygen and LiO₂,are produced by the reaction at the oxygen cathode/electrolyte interface,and too many byproducts are deposited on the oxygen electrode,seriously degrading the electrode circulation stability.These problems directly lead to the low capacity,poor cycle stability and rate performance,which become the technical bottleneck restricting the development of lithium-oxygen battery.Therefore,it is an effective way to reduce the negative side reaction and improve the capacity,cycle stability and rate performance of lithiumoxygen battery by improving the cathode reaction rate,and simultaneously stabilizing and rapidly transferring the active intermediates.In this study,anthraquinone derivatives were introduced into the organic electrolyte as the capture agent of the LiO₂ intermediate.By changing the type of anthraquinone molecular substituent to adjuste its electron supply and absorption ability,the stability of the intermediate LiO₂ produced by anthraquinone on the electrode surface in the cathodic oxygen reduction process was regulated.The change rules of kinetic rate and stability of cathodic oxygen reduction reaction with the stable state of LiO₂ intermediate were clarified.Finally,the rate and stability of cathodic oxygen reduction were improved.The research contents include two parts:?1?Controlling the molecular structure of anthraquinone capture agent aims to realize its regulation of the molecular adsorption energy of LiO₂.By changing the substituents of anthraquinone?such as tert butyl and fluoro group?,the ability of supplying and absorbing electrons of anthraquinone is regulated,so as to realize the regulation of its adsorption ability of LiO₂.Compared with anthraquinone,2-tert-butyl anthraquinone?2-TBAQ?with electron donor group has weaker adsorption on LiO₂ and lower cathodic oxygen reduction reaction rate,while 1,4-difluoroanthaquinone?1,4-DFAQ?with electron donor group has stronger adsorption on LiO₂,which can significantly improve the cathodic surface oxygen reduction reaction rate and stability.Combined with the experimental and theoretical research,the influence of adsorption capacity of anthraquinone and LiO₂ on the cycle stability,rate performance and capacity of the battery was clarified.The discharge capacity of the lithium oxygen battery assembled with carbon nanofiber network and lithium sheet as cathode and anode is 13.5 m Ah/cm²,3 times higher than before,and the cycle stability is 4 times higher than that of the lithium oxygen battery assembled with anthraquinone molecule,2 times and 2.5 times higher than that of the lithium oxygen battery assembled with 10 m M 1,4-DFAQ in electrolyte.It is also important that the capacity of the battery reaches 80% of that of 100 u A/cm² at 500 u A/cm²,which is much higher than that of the lithium oxygen battery without molecular additives or anthraquinone molecular additives.More importantly,compared with the best liquid-phase catalyst DBBQ,the capacity,cycle stability and rate performance of the lithium-oxygen battery are increased by about 2.5 times,2.5 times and 40%,respectively.It was found that the formation of by-product Li2CO3 could be inhibited by the addition of 1,4-DFAQ,a strong adsorption intermediate,during the discharge process.Furthermore,the theoretical calculation and experimental process showed that 1,4-DFAQ could realize the stabilization and transfer of the intermediate LiO₂ from the electrode surface to the electrolyte,avoid the electrolyte reaction caused by the active intermediate LiO₂,and deeply analyze the basic reason for the improvement of battery performance.?2?Regulating the concentration of anthraquinone capture agent aims to realize the complete capture of LiO₂ on the electrode surface,which further improves the performance of lithium oxygen battery.In order to fully capture LiO₂,we tried to increase the concentration of quinones.The results showed that when the concentration of 2-TBAQ and 1,4-DFAQ were increased to 100 and 30 m M,respectively,the performance of lithium-oxygen battery is the best.The capacity?7.5?13 m Ah/cm²?and cycle stability of 2-TBAQ at 100 m M are increased about 2 times and 2 times,respectively,The capacity?13.5?16.5 m Ah/cm²?and cycle stability of 1,4-DFAQ with 30 m M concentration were 1.5 and 1.5 times higher than those of 10 m M.The capacity and cycle stability of 30 m M 1,4-DFAQ additive were better than that of 100 m M 2-TBAQ,which further proved that 1,4-DFAQ molecules showed strong ability to adsorb LiO₂,which realized the stable and efficient transfer of LiO₂.