Construction Of Carbon-based Catalysts For Lithium-oxygen Batteries

In the field of modern electrochemical energy storage,lithium-ion batteries(LIBs)are one of the most promising developments,which can power a large majority of mobile electronic devices.However,they have been unable to meet the emerging needs of society due to energy density constraints.Therefore,researchers around the world are committed to finding a new energy storage device with high energy density and efficiency.Lithiumoxygen batteries(LOBs)have attracted considerable attention in the past few years due to their theoretical energy density(～3500 Wh kg-1),which exceeds the state-of-the-art lithiumion batteries.However,due to their high overpotential and poor cycling stability,LOBs are not commonly used for commercial purposes.To improve the cycle stability and reduce the overpotential,research has been carried out in the direction of cathode catalyst,electrolyte,and lithium anode,among which the slow and irreversible three-phase reaction of the cathode seriously hinders the research progress of this system.It will be crucial for the study of lithium-oxygen batteries to develop efficient cathode catalysts that can accelerate the reversible formation/decomposition of Li2O2.The most efficient cathode catalysts for LOBs have the following three features:high electrical conductivity;abundant porous structure;and high catalytic performance.In this thesis,high conductivity carbon-based materials with a porous structure are used as the substrate for loading transition metals and their compounds,targetting to promote the electrochemical performance of LOBs and solve the serious polarization during the cycle process.The thesis work is summarized as follows:1.Ruthenium(Ru)nanoparticles loaded in zinc-based metal-organic framework(ZIF-8)-derived microporous carbon were designed according to the three elements required for LOBs cathode catalysts and applied as the cathode catalyst,which demonstrates that the pore structure and catalytic activity are significant to the improvementof LOBs performance.The cathode achieves a low overpotential of about 0.75 V and a high discharge capacity of about 17632 mAh g-1 in a lithium-oxygen battery and can cycle for more than 550 cycles at the current density of 1000 mA g-1.Using comprehensive in-situ and ex-situ characterizations,it is demonstrated that the excellent electrochemical performance of LOBs is attributed to the synergistic effect of the catalyst structure with high specific surface area and abundant active sites,which effectively suppress side reactions,promote O2 diffusion and transfer of electrons.2.The precursor ammonium tungstate was loaded in CNT-wrapped polyacrylonitrile by electrospinning,and selenized by chemical vapor deposition to obtain flexible and freestanding selenide tungsten(WSe2)-coated carbon nanofibers.The composite material avoids the side reaction caused by the cathode containing the binder,and the coating of the tungsten selenide with catalytic activity also effectively suppresses the side reaction caused by the carbon substrate.Its ultra-high discharge capacity of up to 57.17 mAh cm-2 and stable rate performance are attributed to the unique three-dimensional structures of WSe2-embedded carbon nanofibers.3.Taking advantage of the intrinsic semiconductor properties of WSe2,a light field was introduced into the lithium-oxygen battery system,and photogenerated electron holes were used to promote the decomposition of the discharge product lithium peroxide(Li2O2).The research results show that the electrochemical performance of the battery has been further improved and the corrosion of the lithium anode can be effectively alleviated under illumination.Using scanning electron microscopy,X-ray diffractometer,and differential electrochemical mass spectrometry,it was proved that easily decomposed thin-film Li2O2 could be generated under illumination,revealing the reason for its improved electrochemical performance.Meanwhile,a transparent pouch cell was fabricated using polyethylene terephthalate(PET)film,demonstrating a flexible rechargeable lithium-oxygen battery with excellent discharge capacity and mechanical deformability.The work provides favorable assistance for practical applications of lithiumoxygen batteries in flexible electronics and wearable devices.