Reasonable Design And Mechanism Study Of High Efficient Catalysts In Lithium-Oxygen Batteries

Lithium-oxygen（Li-O₂）battery is an energy storage and conversion device that uses lithium metal as the negative electrode and oxygen in the air as the active material.In principle,its theoretical energy density is comparable to that of petroleum,which has great potential to achieve the long-range of electric vehicles.Therefore,it has received extensive attention from researchers.However,many scientific problems impede its large-scale commercial applications.Generally,discharge products Li₂O₂are electronically insulating and insoluble.Upon discharge,it easily passivates the oxygen cathode.As a result,the discharge capacity of batteries is far below the theoretical value.Besides,high overpotentials are frequently required for the oxygen evolution reaction（OER）,which leads to not only low energy efficiency but also degradation of battery components.To overcome these obstacles,various electrocatalysts have been extensively applied.Although the electrochemical performance of batteries has been improved to a certain extent,the internal catalytic mechanism has not been clarified,which hinders the design and development of efficient catalysts.Therefore,to point out specific research directions for the development of catalysts for Li-O₂ batteries,this dissertation systematically studies the effects of different catalysts on the electrochemical performance based on the reaction mechanism of Li-O₂ batteries.Besides,the first-principles calculation also is employed to clarify the specific catalytic mechanism and internal influencing factors of different catalysts.More details are as follows:1 The VK1 and DBBQ are employed to deeply explore the critical factors influencing the utilization efficiency of redox mediator（RM）for oxygen reduction reaction（ORR）.Combining experiments and first-principles calculations,it is demonstrated that the oxygen affinity of RM and the reaction free energy of intermediate Li（RM）O₂ disproportionating into Li₂O₂ have a greater impact on its utilization efficiency.Due to the strong oxygen affinity of VK1,more Li₂O₂ can be formed in the solution,which guarantees effective charge transport on the electrode surface.Besides,the low reaction free energy of Li（VK1）O₂ disproportionating into Li₂O₂ significantly accelerates the ORR process.Benefited from these,the utilization efficiency of VK1 is obviously higher than that of DBBQ,thereby significantly enhancing the discharge performance of batteries.2 The amorphous catalyst and its crystalline counterpart are fabricated by facile synthesis.Results show that decreasing the crystallinity of catalyst can improve its catalytic activity.Combining experiments and theoretical calculations,it is found that the amorphous drastically strengthen the adsorption of LiO₂,which can be attributed to the unique disordered structure and abundant defects.As a result,the particle size,morphology,crystallization,and distribution of Li₂O₂ are purposefully modulated.Benefited from the enhanced electronic/ionic conductivity of amorphous Li₂O₂ and the low impedance of the Li₂O₂/cathode interface,the electrochemical performance of the battery is greatly improved.3 Co₉S₈@carbon porous nanocages（Co₉S₈@CPNs）derived from a metal-organic framework are applied as an electrocatalyst in Li-O₂ battery.Results show that these Co₉S₈@CPNs are bifunctional,as both an efficient ORR and OER catalyst,which improve the energy efficiency and cycling stability of Li-O₂ batteries.Theoretical calculations reveal that the promising catalytic properties observed here originate from the high catalytic activity of Co₉S₈.Specifically,the Li-O bond in LiO₂is easier to break under the catalysis of Co₉S₈,thereby promoting the reaction kinetics of Li-O₂ batteries.4 The charging behavior of different types of charge RM in Li-O₂ batteries is investigated.Research shows single electron RM（SE-RM）usually exhibits a more stable charging curve at a lower potential than that of multiple electron RM（ME-RM）.Besides,the RM molecular with smaller steric effects exhibits higher catalytic activity thus a lower charging overpotential.Density functional theory（DFT）reveals RM²⁺/RM⁺has more effective electron transfer for the decomposition of Li₂O₂and thus dominating the charge reaction.Furthermore,smaller steric hindrance effects make the oxidizing center more reactive.These findings offer a guidance direction for subsequent explorations and optimization of high-performance RMs,which might further facilitate the development of Li-O₂ batteries.