| Because of the ultrahigh theoretical energy density(3505 Wh kg-1)that is about ten times higher than that of conventional lithium-ion batteries,lithium-oxygen battery has attracted extensive attention from researchers.What is more,it has even been considered as the so-called "ultimate" electrochemical power source.However,it is also very complex,involving a series of key problems and challenges,such as multiphase reaction kinetics,oxygen cathode passivation,product/electrode interface,side reactions,lithium metal anode dendrites and corrosion.These problems directly or indirectly affect the electrochemical performance of lithium-oxygen battery,make it not only has a large discharge/charge overpotential(or low energy efficiency),but also has poor cycling stability,rate capability and reversibility.In this thesis,from the perspective of "catalysis" and "structure",the electrochemical performance of the lithium-oxygen battery was greatly improved by the reasonable design and optimization of the catalytic site and structure of the oxygen catalyst on the cathode side and the structure of the solid-electrolyte interphase(SEI)on the lithium metal anode side.The main research contents and results are shown as follows:1.Design and study of solid-phase catalyst with open structure.By combining the hydrothermal reaction and sulfuration,we have successfully designed and synthesized a sisal-like Co9S8 material which is assembled by numerous acicular nanorods.This material has very good O2 affinity,which can adsorb O2 and induce O2 to react on the surface of its acicular nanorods to generate Li2O2,and make Li2O2 grow around the acicular nanorods uniformly,thus forming a better Li2O2/catalyst contact interface,similar to the flower-bud-bunch structure.The special flower-bud-bunch like Li2O2/catalyst interface can not only improve the catalytic efficiency of the catalyst during the charging process and promote the complete decomposition of Li2O2,but also effectively improve the oxygen cathode passivation issue.In addition,its special open structure is also conducive to the capture and release of O2,enabling a high-efficient and rapid electrode reaction.When using it as a solid-phase oxygen catalyst for a lithium-oxygen battery,we obtained higher reversible specific capacity and lower discharge/charge overpotential.This study provides a new idea on how to design and optimize gas electrode.2.Design and study of solution-phase catalyst with "single site" activation.Based on the hard-soft acid-base theory,we have successfully designed and synthesized an original ruthenium bipyridine complex(RuPC)with soft acid site.Since this material can be dissolved in the electrolyte,it can form a better solid/liquid contact interface with the product,thereby effectively improving the product/catalyst interface issue.Secondly,a RuPC(LiO2-3DMSO)complex can be formed through the interaction between the soft acid site and the O2-intermediate with soft alkalinity,so as to induce O2-/LiO2 to dissolve in the electrolyte,and then promote the solution-mediated mechanism,thereby improving the oxygen cathode passivation issue.When charging,the interaction between Li2O2 and O2-/LiO2 intermediate can also promote the decomposition of Li2O2 in the way of one-electron delithiation,which provides a more reversible and kinetically favorable reaction pathway than the traditional direct twoelectron decomposition pathway.In addition,since there are no free highly reactive O2-/LiO2 species during the whole discharge and charge processes,and only the relatively stable and less reactive RuPC(LiO2-3DMSO)complex will exist,it can significantly suppress the superoxide-related side reactions.When using it as a solutionphase catalyst for a lithium-oxygen battery,it can not only significantly reduce the discharge/charge overpotential,but also improve the discharge capacity,reversibility and cycle life of the lithium-oxygen battery.3.Study on solution-phase catalyst with "dual sites" activation.On the basis of the research on "single site" RuPC,a solution-phase catalyst with both Lewis acidic and basic sites—iodosylbenzene(PhIO)was developed.The I3+=O2-of this material can be provided as Lewis acidic and basic dual sites to coordinate with the terminal O atom and Li atom of LiO2 species,respectively,to form a LiO2-3PhIO complex,which then rapidly disproportionates into the Li2O2-4PhIO complex,and finally dissociates into the Li2O2 product.Therefore,similar to RuPC in research 2,it can also promote the solution-mediated mechanism during discharge process,thereby improving the oxygen cathode passivation issue,and then enhancing the discharge capacity of the lithium-oxygen battery;and it can also promote the one-electron delithiation mechanism during charge process,thus reducing the charge overpotential of the lithium-oxygen battery;at the same time,it can also suppress the superoxide-related side reactions during discharge and charge processes,thereby improving the reversibility and cycle life of the lithium-oxygen battery.4.An indirect lithium-oxygen battery based on oxygen-exchange mechanism.Combining the aforementioned research results that the reaction mechanism can be modulating by capturing the LiO2 intermediate,so as to improve the electrochemical performance of the lithium-oxygen battery,a new idea of constructing lithium-oxygen batteries by capturing and activating oxygen was creatively proposed for the first time.Then,a novel metallacycle([Os]O2)with the function of capturing and activating the O2 reactant was successfully designed and synthesized,meanwhile,an indirect lithium-oxygen battery has also been fabricated.During discharge,the "activated oxygen" of this material is easier and more preferentially to react to form Li2O2 products and[Os]-i I intermediate than O2 in the ambient atmosphere,then the[Os]-?intermediate can recapture and activate a new O2 molecule to recover to[Os]O2 species for the next reaction cycle.This process does not produce any free superoxide species,so the superoxide-related side reactions can be avoided.In addition,because the whole process is carried out in the electrolyte,the oxygen cathode passivation issue can also be improved and then a higher discharge capacity can be obtained.As for the charge process,the material can be firstly oxidized and release O2 to form[Os]2+-? species,and then the[Os]2+-? species can directly oxidized the Li2O2 product to return itself to its origin state([Os]O2)for the next reaction cycle via a chemical reaction;this charge process bypasses the slow electrochemical oxidation decomposition process of Li2O2,and the charge voltage becomes determined by its own oxidation potential,so that the overpotential of the charge process can be significantly reduced for its low oxidation potential.The concept of "indirect lithium-oxygen battery based on oxygen-exchange mechanism" proposed here provides new inspiration and idea for the design and construction of new lithium-oxygen battery systems in the future.5.Design and study of SEI protective film with oxygen-blocking function.By combining electrochemical polishing technology and the reduction chemistry of LiNO3,a molecularly smooth LiNO3-derived SEI(N-SEI)film with a unique multilayer structure has been successfully designed and constructed on lithium metal surface.This N-SEI film can be divided into three layers,in which an organic-mixed middle layer mainly composed of ROLi is sandwiched by two inorganic-rich layers in the outer(mainly Li2SxOy,LiCF3 and Li3N)and inner(mainly LiF and Li2S)regions,respectively.Moreover,in this N-SEI film,the LiNO3-derived soluble NO2-species are encapsulated in the inner region of the SEI film,thus,the negative effects caused by the dissolution of NO2-species can be effectively avoided.In addition,the outer layer of the N-SEI film has the ability to block O2,which can serve as an effective barrier to prevent O2 from corroding the lithium metal anode.Based on the above advantages,when the lithium metal with N-SEI film is used as the anode material for a lithium-oxygen battery,the cycle life of the lithium-oxygen battery can be improved significantly. |
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