Synthesis And Electrochemical Performance Of Ni/Co-based Metal Oxides/Carbon Nanofibers Anode Materials For Lithium-ion Batterics

Lithium ion batteries（LIBs）with high energy density,long cyclic life,and environmental compatibility have been intensively investigated due to the growing demanding from portable electronics,electric vehicles and other energy storage system.Commercialized graphite is commonly utilized as anode materials due to its low cost and highly chemical stability.However,its low theoretical capacity and slow lithium diffusion kinetics limit the application in high energy LIBs.Transition metal oxides have high theoretical capacity but poor conductivity and significant volume effects.In order to improve their electrochemical performance,nickel/cobalt-based metal oxides/carbon nanofiber anode materials were prepared and their electrochemical properties were studied.The present work can be summarized as follows:（1）In this work,hierarchical NiO hexagonal nanoplates have been synthesized by a simple hydrothermal process with carbon nanofibers distributing in the solution.The first cycle presents a very high discharge capacity of 1357.2 mAh·g-1 and a charge capacity of 1002.6 mAh-g-1 with an initial Coulombic efficiency of 73.87%.The H-NiO nanoplates exhibits an excellent cycling stability,with a high reversible capacity of 1111.6 mAh·g-1 at a current density of 100 mA·g-1after 45 cycles.The lithium diffusion coefficient of the H-NiO is calculated eo be 7.517×10-12 cm2·s-1 with fast Li+diffusion during the charge and discharge processes.The stable hexagonal structure can relieve the stress of the volume change and then enhance its electrochemical performance.（2）A novel heterostructure of NiO/Ni3S2 nanoflakes have been synthesized and composited with carbon nanofibers（CNF）membrane.NiO/Ni3S2 nanoflakes are homogeneously dispersed in CNF network,herein,NiO/Ni3S2 like leaf and CNF like branch.Carbon nanofibers network efficiently prevents the pulverization and buffers the volume changes of NiO/Ni3S2,meanwhile,NiO/Ni3S2 nanoflakes through the conductive channels of carbon nanofibers own improved Li+diffusion ability and structural stability.The capacity of NiO/Ni3S2/CNF reaches to 519.2 mA·g-1 after 200 cycles at the current density of 0.5 A-g₁ while NiO/Ni3S2 fades to 71 mAh·g-1 after 40 cycles.Owing to the synergetic structure,the resultant binder-free electrode NiO/Ni3S2/carbon nanofibers shows an excellent reversible lithium storage capability.（3）In this work,hierarchical CNF/CoO composite films have been synthesized through a reflux method in which cobalt compounds can grow on the substrate of CNFs under low temperature of 120℃ and easy for large-scaled output.The crystal morphologies of CoO nanoflakes are facilely controlled by adjusting the weight ratio of starting cobalt salt.The newly hierarchical structure of nanoparticle-nanoflake-micron particle plays a key role in achieving extraordinary electrochemical performance.CNF with good conductivity and stable structure provides fast electronic conductive pathway in the composite electrode,which is beneficial for the high rate performance and cyclic performance.CoO nanoflakes with the porous structure enhance the ionic conductivity,and a proper weight ratio finally enlarge the specific capacity of CNF/CoO composite electrode.By combining the advantages of CNF and CoO,the composite film of CNF/CoO-4 with a proper amount of CoO achieves an excellent electrochemical performance in LIBs and SIBs.CNF/CoO-4 sample presents capacity of 530 mAh·g-1 after 100 cycles at 0.2 in LIBs,and delivers 193 mAh·g-1 after 50 cycles at 0.05 A·g-1 in SIBs.Furthermore,in CNF/CoO//LiCoO2/Al coin-type full cell,CNF/CoO-4 delivers a high reversible discharge capacity of 430 mAh·g-1 after 55 cycles at 0.1 A·g-1.The reflux method can be used to synthesize other CNF supported metal oxides with various crystal morphologies for highly reversible energy storage.