The Preparation Of Three-dimensionalgraphene/Carbon Composites And Their Application In Lithium-air Batteries

Along with the carbon neutrality and the carbon emission,the global transportation equipment industry is facing a new development situation,which makes the energy transformation imminent.Lithium air batteries（LABs）have captured world-wide attention as a promising next-generation power source,due to its sky-high theoretical energy density(3 500 Wh kg^-1).However,it is at early stage of research because there are still many scientific and technical challenges for developing lithium air battery,such as poor electrode catalytic activity,the oxygen channel easily blocked by Li₂O₂products,design strategy of the whole battery,the relationship between the material structure and the performance of the whole battery,etc.In this paper,the several kinds of catalytic active carbon were prepared by using different biomass carbon sources,and their catalytic activity were studied.In addition,the structure design,catalytic performance and mechanism of biomass carbon/three diamond（3D）graphene composites were investigated,and the whole battery design and performance research were also carried out.Firstly,two kinds of biomass carbon materials were prepared with yeast and oak as precursors.By using carbon paper as the supporter,the physical properties and electrochemical properties of biomass carbon/carbon paper electrodes were studied to find out the suitable preparation process.The results indicated that the capacity of yeast biomass carbon electrode prepared by hydrothermal and spraying method could reach 21 000 m Ah g^-1,but the preparation process is complicated and the biomass carbon distributed uniformity on carbon paper supporter.The capacity of the yeast biomass carbon on carbon paper obtained through hydrothermal synthesis and secondary carbonization was 13 000 m Ah g^-1.The above preparation process was simple,and the biomass carbon was evenly distributed in the carbon paper fiber mesh structure.With the end capacity of 800 m Ah g^-1,the cycle life of the yeast biomass carbon could reach 130 cycles and show good comprehensive performance.The study showed that oak biomass carbon had better performance with the heat treatment condition at 800℃,which the higher capacity of 11 600m Ah g^-1after 400 cycles could be reached,but the platform voltage was lower than biomass carbon.Thus,it is established that the yeast biomass carbon prepared by one-step hydrothermal synthesis and secondary carbonization exhibited better oxygen reduction.Considering that the density of graphene is less than 1/30 of that of the carbon paper,bend arbitrarily,and the 3D structure could be used as a reaction products store,a new technology route was designed by using 3D graphene instead of carbon paper.Based on the nickle foam supporter,3D graphene was prepared by chemical vapor deposition（CVD）.The analysis of the scanning electron microscope（SEM）showed that the graphene has a 3D connected large pore structure,a pore diameter in the range of 100 to 300μm,good microscopic integrity,and no obvious crushing defects,and the lamina is very thin,indicating that the structure of graphene is intact.3D graphene-carbon nanotube composite positive electrode was prepared through CVD method by using 3D graphene as the electrode supporter.Through the micromorphology characterization,the introduction of carbon nanotubes greatly increased the real surface area of the electrode,and the sufficient electrochemical reaction active sites resulting to the electrode discharge capacity increase sharply.At the current density of 0.05 m A cm^-2(113 m A g^-1),the composite electrode capacity was 20 300 m Ah g^-1,and it could remain stable after 150 charge and discharge cycles,indicating that the composite electrode has excellent discharged performance and good stability.Meanwhile,the grapheme/biomass carbon composite electrode was prepared by using 3D graphene as the supporter.According to results of the Raman spectroscopy,constituent elements and electrochemical kinetic analysis,there were abundant miscellaneous elements existed in the optimized graphene yeast carbon and graphene-oak carbon electrode,which increase the active points and show excellent discharge performance.At the working condition of 200 m A g^-1,the discharged capacity of the composite electrodes prepared by hydrothermal and spraying method or the one-step method,and the oak biomass carbon composite electrode were 16 700 m Ah g^-1,15 600 m Ah g^-1,and 13 000 m Ah g^-1,respectively.With the current density increasing,the discharged capacity of graphene-yeast carbon composite electrode was higher than that by one-step synthesis,which exhibited that graphene-yeast carbon composite obtained a better rate capability.Through the density functional theory calculation,the adsorption energy of O element after the biomass carbon composite three-dimensional graphene was reduced and make it easier for O element to participate in the reaction,which theoretically explained the influence mechanism of biomass carbon.A lightweight and flexible LABs was designed with the three-dimensional graphene/yeast biomass carbon as the positive electrode,the lithium metal as the cathode,the PP as the diaphragm,and the 1 mol/L Li TFSI/TEGDME as the electrolyte.The batteries were folded to 90°and still could work normally,with a high specific capacity up to 1100 Wh kg^-1.Based on the engineering application,the LABs of capacity could reach to Ampere hour（～Ah）level was designed for safety test and environmental test,including short circuit,acupuncture,extrusion,the value of 24.08 g vibration,1 800 g impact,30 g acceleration and so on.After the above tests,the LABs worked normally and well,which means the flexible LABs could meet the requirements for special occasions.