The Exploration On Transition Metal Carbides/Nitrides As Cathode Catalysts Of Lithium-Oxygen Batteries

Aprotic Lithium-oxygen batteries are considered as promising next-generation energy storage devices to replace Li-ion battery due to the ultrahigh theoretical energy density(3500 Wh kg-1).However,there is a large gap between current Lithium-oxygen batteries and the requirement of practical application,which is largely due to the sluggish kinetics of oxygen reduction reaction and oxygen evolution reaction kinetics.Especially under high current density,it tends to cause high overpotential,leading to the accelerated degradation of electrolyte and electrode materials,which causes the declined energy efficiency and curtate cycle life.Therefore,the development of efficient cathode catalysts plays a key role in promoting the performance improvement and future commercial application of Lithium-oxygen batteries.Transition metal carbides and nitrides have similar structures and desireable properties,such as the advantages of noble metal-like catalytic activity,low cost,high electronic conductivity and chemical stability.Therefore,we chose transition metal carbide/nitride as the research object,and focus on the cycling properties of related materials at relatively high current density(≥500 mA g-1).(1)Mo2C/CNT composites were synthesized by two-step method.Mo2C nanoparticles were highly dispersed on the surface of carbon nanotubes,which can effectively avoid the agglomeration of Mo2C particles.At the same time,the one-dimensional carbon nanotubes serve as the fast channel for electron transport and effectively promot the material transport during charge and discharge process.It helps to realize the long cycle life of Mo2C/CNT composite as cathode catalyst of lithium oxygen battery at a current density of 500 mA g-1.(2)A new salt-assisted synthesis method was developed for the first time to fabricate Mo2N nanofibers,and the unique roles of NaCl in the synthesis process was explained,which provided a new idea for the synthesis of one-dimensional nitrides.At the same time,Mo2N nanofibers show very good catalytic activity as cathode catalyst of Lithium-oxygen battery,which can achieve a large specific capacity of 5703 mAh g-1 and cycle life of more than 90 cycles at a current density of 500 mA g-1.This is the first study on the application of Mo2N based materials for Lithium-oxygen batteries,which lays a foundation for the future research and application of nitride materials in Lithium-oxygen batteries.(3)A series of slightly oxidized MXene materials(mo-Nb2CTx,mo-Ti3C2Tx and mo-V2CTx)were successfully prepared by one-step hydrothermal method and used as efficient catalysts for Lithium-oxygen batteries.The presence of metal oxides expands the interlayer spacing of MXene and provides more space for nucleation of Li2O2.Moreover,in situ formed metal oxides optimize the surface composition and improve the intrinsic activity of MXche.The Lithium-oxygen battery based on V2CTx cathode achieves a high specific capacity of 22752 mAh g-1 at 100 mA g-1.In particular,DFT calculation further confirmed that mo-V2CTx possess a moderate adsorption strength for LiO2 at the atomic level,and thus has an ultra-low ORR/OER overpotential,which effectively inhibits the occurrence of side reactions.Therefore,mo-V2CTx cathode shows excellent rate performance and can complete 90,89 and 70 cycles at high current density of 1000,2000 and 3000 mA g-1,respectively.