Graphitized Porous Carbon And Stereotaxically Constructed Graphene Based-composites Application In Lithium-ion Batteries

Lithium-ion batteries(LIBs)are of great interest in modern society due to their excellent performance characteristics,which includes:their low self-discharge,high voltage,high energy density,safety,pollutant-free nature,long cycle life,and absence of memory effect.Graphite is currently the most widely used LIB anode material due to its low Li insertion potential,stable chemical properties and abundant reserves.However,graphite is limited to 372 mAh g-1 in theoretical specific capacity,requires relatively long charge periods,and can form Li dendrites at fast charge leading to short circuits,which therefore limits its practical applications.At present,two alternatives kinds of carbon anode materials are of great research interest as LIB anode material.One is a hard carbon material.Its theoretical capacity is much higher than that of graphite,but due to it being an amorphous carbon material,its conductivity is far lower than that of graphite.Much research has focused on tuning the structural morphology of hard carbon materials to improve their rate performance and cycle stability.The other one is graphene.Its conductivity is very high and its theoretical capacity is twice that of graphite,but due to the van der Waals forces,the graphene sheets easily agglomerate,which greatly reduces their performance.However,in recent years a new type of graphene material has been developed known as three-dimensional graphene.Three-dimensional graphene retains the beneficial characteristics of graphene,but crucially its 3D self-supporting network structure prevents graphene sheet agglomeration.However,because of its self-supporting stereotaxically constructed,it can have low tap density and therefore an undesirably low volumetric capacity.One solution to this problem is to compound three-dimensional graphene with high volumetric capacity anode materials to form graphene-based composites with improved electrochemical performance.In this thesis,nickel ion exchanged resin was used as a carbon source to synthesize various porous carbon with different degrees of graphitization and stereotaxically constructed graphene loaded NiO nanobelts composite anode materials.The layout of the work in this thesis is summarized below:(1)In the first chapter,the synthesis of low-temperature graphitized porous carbon material from a Ni-ion exchanged resin is described.Hydrazine hydrate is used to reduce Ni-ions to Ni metal nanoparticles at room temperature,which allows low temperature graphitized of carbon,while addition of KOH produces high porosity in the carbon.The use of hydrazine hydrate lowers graphitization temperature by at least 200 ?.The as-prepared highly porous carbon(HPC)material has a 3D interconnected framework structure,while the degree of graphitization and the specific surface area could be tuned by adjusting heating temperature.(2)In the second chapter,HPC is evaluated as a LIB anode material.HPC synthesized at 500 ? has "house of cards" graphene arrangement and has the best electrochemical performance of the materials produced at different graphitization temperatures.At a current density of 0.1 A g-1,HPC delivers a reversible specific capacity of 1170 mAh g-1 and at 5 A g-1 can maintain 380 mAh g-1,even after 3000 cycles.Overall,the rate performance,cycling stability and specific capacity of HPC is far superior to commercial graphite and other porous carbon materials.(3)In the third chapter,a novel method to grow NiO nanobelts from stereotaxically constructed graphene is introduced to make an effective composite LIB anode material.The method uses the residual Ni metal in the stereotaxically constructed graphene as a seeding agent for the nanobelts,which are grown during a hydrothermal step followed by oxidative heating.The nanobelts are highly porous and well anchored to the stereotaxically constructed graphene,which results in a composite material that has high specific capacity,good rate capability and excellent stability.After 360 cycles at 2 A g-1 current density,a high capacity of 445 mAh g-1 is maintained.Although the mass content of NiO in the 3D graphene is only 26.8%,it causes a 41.6 and 75.7%increase in specific and volumetric capacity respectively.The developed synthesis method not only provides an efficient new means to produce high performance NiO-3DG anode material,but has the potential to extend to other transition metal oxide/carbon composites for LIB anodes and other applications.