Synthesis And Investigation Of Multi-doped Carbon-based Catalysts As Cathode Materials For Li-O₂ Batteries

Lithium air battery is one of the most promising electrochemical energy storage technologies with high energy density and less environmental pollution.The high theoretical energy density(?3.500 Wh kg-1)is more than ten times of lithium ion battery(?300 Wh kg-1).However,the development of practical lithium air batteries needs to overcome many challenges,including the better understanding of the electrochemical reaction mechanism,reasonable design of electrode materials and structure,and application in an open environment.Among them,the selection of the air cathode materials and the design of their channel structure have great influence on the energy density of batteries.Therefore,the exploration of cathode materials with high activity and good stability is crucial to the further development and practical application of lithium air batteries.Following the requirements of cathode catalysts for Li-O2 battery,the three-dimensional graphene based catalysts and multi-ligand metal MOF derived carbon nanotube carbon catalysts were designed and prepared in this thesis.The prepared catalysts showed excellent catalytic performances by doping,compositing and modifying the catalysts.Moreover,various physical characterization techniques were also used to characterize the structure of the materials,and the mechanism of the battery catalytic reaction was further explored.The main work and achievements are as follows:(1)A new type of three-dimensional(3D)nitrogen-doped graphene decorated with high specific surface area were prepared from graphene oxide(GO)by cross-linking and reduction techniques.Then,highly dispersed Ru nanoclusters was prepared using a hydrothermal synthesis method followed by a reduction process.The results show that the three-dimensional crosslinking structure of graphene in the catalyst is not only conducive to the highly dispersed Ru to form clusters,but also can provide enough space to accommodate the discharge products.It is found that increasing active N content in graphene and Ru nanoclusters are beneficial to enhance the electrochemical activity of the catalyst.Further,the battery can deliver a specific discharge capacity high up 23922 mAh g-1 at the discharge current density of 100 mA g-1,and the overcharge potential is only 0.89 V.It also demonstrates a long-term stability for 200 cycles with limited capacity of 1000 mAh g-1.(2)In the bases of successful synthesis of RuNC/3DNG catalyst,a high-performance catalyst,in which the highly dispersed Ru nanocluster are anchored on single Co atom and N-doped 3D graphenes,was prepared through introducing a small amount of transition metal cobalt.Compared with our previous work,the Ru loading is reduced to 8.82 wt.%.It is revealed by STEM and XAFS analysis that the Co is existed as single atoms in the graphene based catalyst,and cooperated with N to form a Co-N moiety,enhancing the performance of the catalyst,furthermore,the existence of doped Co atoms seems change the electron distribution of Ru.The battery with RuNC/COSA-3DNG as cathodel exhibited excellent and significantly improved rate performance and cycle stability,compared with the battery with RuNC/3DNG cathode.When the current density increased as high as 1000 mAh g-1,the battery with RuNC/COSA-3DNG as cathode still delivered a large discharge capacity of 12362 mAh g-1 with lower charge overpotential(0.85 V).Moreover,it can be operated for 300 cycles without obvious attenuation under the limited capacity of 1000 mAh g-1.It is revealed by XPS and SEM analysis that the doping of single Co atoms significantly changes the electronic state of Ru,resulting in an electron deficiency of Ru,which affects the formation and decomposition of discharge product LiO2,and finally improved performance of the battery.(3)The double-layer metal hydroxides were obtained by etching ZIF-67 with nickel nitrate as the metal precursor.Then,the film material was prepared by filtratting the mixture of precursor and graphene oxide,followed by vacuum freeze-drying and pyrolysis at high temperature,and was directly used as a self-supported cathode for Li-O2 battery.The self-supported cathode with a relatively regular porous can avoid the negative effects caused by binder and carbon paper,which may promote the transfer of materials and electrons,and the uniform deposition of discharge products.The vertical carbon nanotubes formed on the surface of graphene are beneficial to the dispersion of catalytic active sites,and reduce the agglomeration of discharge products,making them more easily decomposed in the process of charging to achieve a long-term stability(496 cycles).(4)A metal-organic framework complex formed by 2,2'-biimidazole,1,3,5-homophthallic acid and Co2+was first synthesized via hydrothermal method.The multi-atom(N,O and Co)doped carbon nanotubes were cross-linked with melamine during pyrolysis to obtain a three-dimensional network structure.The catalyst was further coated on carbon paper to build the cathode of Li-O2 battery.The battery with Co(L1/L2)-CNT cathode delivered a high discharge capacity of 21842 mAh g-1,and can achieve 300 cycles with limited capacity of 1000 mAh g-1.It is found that the porous structure of carbon nanotubes can provide enough space for the deposition of discharge products,which makes the battery have a high discharge specific capacity.The synergistic effect of co-doped N and O enhances the ORR/OER activity of the catalysts,which can decompose the discharge products at a lower charging voltage,thus improving the cycle stability of the battery.