| The anode material of the secondary lithium-oxygen battery(Li-O2 battery)is lithium metal.The active material O2 for cathodic reactions comes from the air and it does not need to be stored in the electrode.Its theoretical specific energy is 11430 Wh.kg-1,comparable to that of petroleum.It is an electrochemical system with the highest known energy density,and has been hailed as the "ultimate battery." The cathode in the Li-O2 battery is a oxygen diffusion electrode.Generally,the porous cathode is made of cheap and light-weighted carbon or carbon-supported catalysts.The cathode is the main place for the electrochemical reactions of O2 and is responsible for the O2 diffusion and mass transfer,electron conduction,ion transmission,and deposition and storage of discharge products,thus becoming the key influencing factors and core components that determine the performance of Li-O2 batteries.The thesis focuses on the O2 diffusion electrode of Li-O2 battery,studies the composition,structure and performance of Fe/N@C catalyst materials,investigates the structure design of porous oxygen diffusion electrode in aprotic and solid-state systems,and systematically analyzes the correlation between the inconsistency of the cathode reaction and the battery performance for large-sized pouch-type Li-O2 batteries.And it can have important theoretical and practical significance for the development of Li-O2 batteries.The main research content and results of the thesis are as follows:(1)In the fixed ratio of ferrous acetate tetra-hydrate(FeAc2·4H2O)and 1,10-phenanthroline Fe/N precursors,carbon black(Super P)was mixed in them,and these four groups of Fe/N@C catalyst materials were prepared by a one-step pyrolysis method,and their composition,structure and performance were mainly characterized by XRD,Raman,TEM,XPS,TGA,LSV,CV,etc.Results show that when the C/N molar ratio is 70,50,and 30,the total doping concentration of Fe/N heteroatoms increases with the Fe/N precursor's adding ratio,and the content of pyridine-N in the material gradually increases;however,when the C/N molar ratio is 10,the total doping content of N decreases instead.Although the total content of Fe continues to increase,it is mainly accompanied by the agglomeration and precipitation of Fe3O4 particles in the material,resulting in the drastically dropping of the Fe-Nx proportion in the bonding components.And the content of pyridine-N has also dropped drastically.The electrochemical performance test results further show that the catalytic materials with a C/N molar ratio of 70,50,and 30 have successively enhanced battery cycle performance,and the reduction peak potential of oxygen increases successively.When the C/N molar ratio of the materials is 30,it has the best 50 stable cycles and the maximum reduction peak potential(2.75V vs.Li+/Li),and its half-wave potential is 28 mV higher than the sample with a C/N molar ratio of 10,and also 237 mV higher than the pristine Super P,showing the best catalytic performance.The analysis suggests that the active reaction site density in Fe/N@C catalyst materials can be directly controlled by the Fe/N proportioning ratio;at the optimal Fe/N ratio(Fe/N@C-30),a higher content of pyridine N and a larger proportion of Fe-Nx groups can be generated in the Super P matrix,which is beneficial to the material to exhibit higher electrocatalytic activity and enhance the cycle stability of the battery.(2)Different O2 diffusion electrode structures were optimized and designed and their influence on battery performance was analyzed.In the aprotic system,it was found that when the ratio of the cathode thickness and the loading of Super P carbon black was adjusted to 40 ?m/mg,it can promote the effective utilization of internal pore structures in the high-loading O2 diffusion electrode,and can realize the uniform distribution of the discharge products in the electrode pores with a good triple-phase reaction interface,and the battery cycle performance was also better;when the cathode was prepared using Fe/N@C-30 material under the coating thickness and the load capacity ratio of 40 ?m/mg,the battery discharge capacity can be as high as 5811.5 mAh/g,producing toroid-shaped lithium peroxide(Li2O2)products,further indicating the suitability of the designed oxygen/catalyst/electrolyte triple-phase interface structure for the high-loading electrode.In addition,two kinds of solid/solid composite oxygen diffusion electrodes with an integrated'"dense+porous”structure and a "flexible film type" structure were designed using LAGP solid-state electrolyte and carbon-containing catalyst materials.The two porous electrodes using as-prepared catalyst showed lower polarizations and better cyclabilities for oxygen redox reactions.In addition,the battery using Fe/N@CNT-LAGP electrode with a flexible structure can be cycled stably for 55 loops under the fixed capacity of 1000 mAh/gcatalyst,while the battery using Fe/N@CNT-RuO2-LAGP flexible electrode can achive 102 loops by adding the RuO2 catalyst.The gas/solid/solid triple-phase reaction interface formed in the designed O2 diffusion electrode based on the solid-state electrolyte LAGP can facilitate the diffusion and mass transfer of O2,and the electrode interface was stable,eventually improving the cycle stability of the cathodes.(3)The large-sized pouch-type Li-O2 batteries(13 cm x 13 cm)using aprotic electrolyte were designed and prepared,and the cyclability of such large-sized,high-energy-density(exceeding 600 Wh/Kg)pouch-type Li-O2 batteries was usually less than two loops.To find out the inherent reasons for their poorer cycle performance compared to coin-type cells,the cathodes after disassembly were conducted with subregional and post-mortem analysis.Results showed that obvious differences in the product morphology,structure and surface compositions existed in different regions of the cathode after discharge and the charge.In areas with reasonable electrolyte distribution and better oxygen diffusion conditions,more Li2O2 was generated after discharge,while in the lean-electrolyte area,especially in the oxygen-poor area,the content of Li2O2 output is less.The unavoidable local chemism inconsistency in the large-sized cathodes of the pouch-type batteries can aggravate the formation of LiAc·2H2O on the cathode surface during the charging process,especially in areas with less Li2O2 formation and higher proportion of unutilized carbon surface after the previous discharge.Further analysis found that the electrochemical activity of Li Ac·2H2O is very poor,and the potential for its electrochemical oxidation and decomposition is too high,thus making it a major factor contributing to the poor rechargeability of the large-sized pouch-type Li-O2 batteries. |
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