Controllable Synthesis Of Carbon Nanotube Array Based Composites And Their Energy Storage Mechanism For Lithium Oxygen Battery

In recent years,lithium-oxygen batteries（LOBs）have attracted great attention in the field of high energy storage devices due to their high theoretical energy density(3623 Wh kg^-1),low cost,high efficiency,and non-toxicity merits.However,the issues including the premature failure of the catalytic activity of cathode,interfacial instability and dendrites of lithium metal anode,lead to the low practical energy density,low Coulombic efficiency and short cycle life,which limit the practical application of LOBs.Carbon nanotubes and graphene possess excellent electrical conductivity and mechanical properties.Through structural design and controllable preparation,they are expected to be used as LOBs electrode materials to promote electron transport of discharge products and obtain stable electrochemical performance.In this study,carbon nanotube array（CNTA）was in-situ synthesized by hot filament-assisted chemical vapor deposition（HFCVD）method using graphene as a substrate.Structural design and surface modification were carried out to construct a cathode with high catalytic activity and a lithium anode without dendrites,respectively.The growth mechanism of high-density,highly-ordered carbon nanotube arrays/graphene composites was systematically studied.The efficient oxygen reduction/oxygen evolution reaction（ORR/OER）mechanism of O₂ cathode was also elucidated.In addition,the formation mechanism of dendrite-free metal lithium anode was proposed.Finally,a CNTA-based lithium-oxygen full batteries was assembled.The main research contents and results are as follows:（1）Controllable preparation of ultrafine catalysts.Fe₃O₄/Al O_x catalysts were prepared by refluxing method using iron acetylacetonate and aluminum acetylacetonate as precursors.The effects of amounts of precursors and refluxing temperature on catalyst size were investigated.Ultrafine Fe₃O₄/Al O_x catalysts with an average particle size of 2.5 nm are obtained,which can be used for the growth of high-density CNTA and ensure the yield of single-walled carbon nanotubes.（2）In-situ growth of high-density,highly-ordered CNTA on graphene.CNTA was grown on graphene substrate by HFCVD with Fe₃O₄/Al O_x catalysts.The key growth parameters affecting CNTA growth,including tungsten filament power,growth time,H₂O flow,and catalysts size,were systematically investigated.The results show that the power of tungsten filament is the main influence factor for obtaining high-density and high-ordered CNTA.When the power of the tungsten filament was adjusted to 65 W,the average particle size of the Fe nanoparticals is 9.8nm and its areal density is 3.52×10¹¹ cm^-2（nearly 5 times higher than that of the sample without tungsten filament treatment）.The W atomic clusters deposited form high-power tungsten filament were extremely able to suppress the aggregation and growing up of Fe catalyst particles,thus ensuring the growth of high-density and highly-ordered CNTA.（3）Catalytic mechanism of self-supporting N,S co-doped carbon nanotubes arrays/three-dimensional graphene（NS-CNTA/3DG）cathode.NS-CNTA/3DG composites were prepared by N,S co-doping treatment of carbon nanotube arrays/three-dimensional graphene（CNTA/3DG）.The electrochemical performance and ORR/OER mechanism of self-supporting NS-CNTA/3DG cathode for LOBs was systematically investigated.The results show that N and S co-doped carbon sites induce the formation of amorphous Li₂O₂ films on large-area CNTA substrates during the ORR process;accelerate the dissociation of Li⁺during the OER process,and synergize with the highly conductive CNTA to promote reversible decomposition of amorphous Li₂O₂.The NS-CNTA/3DG cathode exhibited a high first discharge capacity of 23778 m Ah g^-1 and a high first Coulombic efficiency of 87.8%at a current density of 200 m A g^-1.（4）Preparation of self-supporting alumina-carbon nanotube array/3D graphene（Al₂O₃-CNTA/3DG）composite.Al₂O₃ nanoparticles were deposited on the surface of CNTA/3DG by atomic layer deposition method.The electrochemical performance and electrolyte interface stability of Al₂O₃-CNTA/3DG Li hosts were systematically investigated.The results show that the Al₂O₃-CNTA/3DG composite has high stregthen,highly ordered structure and large specific surface area.Al₂O₃ nanoparticles can not only improve the structural stability of CNTA and induce uniform Li deposition,but also stabilize the solid-state electrolyte interface films,inhibiting the growth of lithium dendrites.When Al₂O₃-CNTA/3DG was used as the host material for Li deposition,its mass specific capacity could reach 3722 m Ah g^-1 and achieved a high Coulombic efficiency over 97%at 140 cycles(at a current density of 4 m A cm^-2and an areal capacity of 8 m Ah cm^-2).（5）Assembly and electrochemical performance of CNTA-based lithium-oxygen full battery.Al₂O₃-CNTA/3DG-Li anode and CNTA/3DG cathode were assembled into a lithium-oxygen full battery,and its electrochemical performance and storage mechanism were investigated.The results show that the Al₂O₃-CNTA/3DG-Li anode can effectively inhibit the formation of lithium dendrites and the cracking of the SEI film,and ensure fast Li⁺transport during cycling.The CNTA/3DG cathode can provide large amounts of deposition sites for Li₂O₂ as well as rapid charge transport,promoting the reversible Li₂O₂ formation/decomposition.The reversible electrochemical reaction enables the CNTA-based lithium-oxygen full battery to achieve good cycling stability.As a result,it exhibited a long life-span up to 160cycles at a current density of 100 m A g^-1 and a limited capacity of 500 m Ah g^-1.In this study,LOBs with high storage capacity,high coulombic efficiency and long cycle life are obtained by improving the catalytic activity of the cathode and improving the stability of the anode,which can provide relevant theoretical basis and experimental reference for the design and preparation of LOBs based on carbon-based electrode materials.