Preparation And Electrochemical Performance Of Carbon-based Materials Using Layered Double Hydroxides As The Template For Lithium Ion Batteries

Nanocarbons materials have been attracted much attentions for their distinct physical and chemical properties.Layered double hydroxides(LDHs)as a 2D advanced functional material become the research focus in the field of energy storage and conversion.Not only LDHs can be used as the template or catalyst to prepare novel nanocarbons,but also the electro-active LDHs and nanocarbons can be compounded to fabricate the hierarchical composites.These strategies can inherit the advantages of either component material,resulting in the formation of the multifunctional materials with unexpected properties.The carbon materials prepared by LDHs template have great potential in the field of energy storage and conversion.These carbon materials usually have rich porous structure and proper pores distribution.When used for lithium ion batteries(LIBs),the porous structure of the carbon materials is beneficial to the contact of active material and electrolyte,thus improving the lithium storage performance.The electro-active LDHs are also popular in the field of energy storage and conversion.In this thesis,LDHs are used as the template to prepare novel carbon-based materials with excellent energy storage performance.The changes of the morphology and composition of LDHs have a significant impact on the morphology,structure and energy storage performance of the prepared carbon-based materials.We also reveal the design principle,preparation method and lithium-storage mechanism of the carbon-based materials with excellent electrochemical performance.Firstly,novel hierarchical porous carbons(NHPCs)with carbon nanosheets have been prepared by using delaminated MgAl-LDHs as template and sucrose as carbon source.The delaminated MgAl-LDHs can create slit-shaped pores in the carbon matrix,in the meantime,the sucrose can be adsorbed on the LDHs layers and convert to carbon nanosheets in the subsequent carbonization process.By controlling the mass ratio of template/sucrose from 5:1 to 1:1,NHPCs with different specific surface area,mesopores content and the amount of carbon nanosheets can be fabricated.NHPC-5.0 electrode exhibits the specific reversible capacity of 1151.9 mA h g-1 under the current density of 50 mA g-1 due to existence of abundant carbon nanosheets and slit-mesopores.The porous structure can provide fast transport channels for the conductive ions and more active sites for lithium storage,thus improve the conductivity and the electrochemical performance.The existence of carbon nanosheets is beneficial to the electrolyte/electrode contact and enhances the wettability of electrolyte.Secondly,hierarchical porous carbon microspheres(HPCMs)have been fabricated successfully,using MgAl-layered double oxides(LDOs)microspheres as template and sucrose as carbon source.The MgAl-LDOs microspheres template is prepared by the self-assembly of MgAl-LDHs precursor under one-step calcination process.The proper mass ratio of sucrose/template is beneficial to the formation of HPCM with good spherical shape and well-developed porous structure.When used as the anode materials,HPCM-1 have the specific capacities of 1140.5?650.3 and 347.9 mA h g-1 under the current densities of 0.05?0.2 and 1 A g-1,respectively.The spherical shape and abundant hierarchical porous structure can not only shorten the ion diffusion distance but also provide more reactive sites for lithium-insertion/desertion,which beneficial to the improvement of specific capacities and rate performance.Thirdly,graphene nanosheets(GNS)are introduced to wrap the hydrothermal-prepared CoNi-LDHs microspheres and the GNS-wrapped CoNi-LDHs microspheres(LDH/Gs)were obtained.The mass ratio of graphene and LDHs has an important influence on the lithium storage performance of LDH/Gs.The composite electrode with the mass ratio of 2.5:1(CoNi-LDHs/graphene)shows high reversible specific capacity of 1428.0 mA h g-1 at 0.05 A g-1,excellent rate performance(1099.9,670.8 and 328.1 mA h g-1 at 0.2,2 and 10 A g-1,respectively)and cyclic stability(75%retention at 10 A g-1 after 10000 cycles).The composite has the advantages of high chemical reactivity of CoNi-LDHs and good electrical conductivity of GNS.During the electrochemical charge and discharge process,GNS can efficiently suppress the volume expansion of LDHs and enhances the cyclic stability of the electrode.Finally,by controlling the catalytic temperature(Tc),the N-doped 3D carbonaceous scaffolds with different morphologies can be prepared by the in situ catalytic growth approach,using CoNi-LDHs as the catalyst and pyridine as the carbon and nitrogen source.At 600?,pyridine deposits on the CoNi-LDHs microsphere skeleton to form N-doped spherical graphene network(NSGN).When Tc is 800?,the carbon nanoonions(CNO)catalytically grow on the graphene nanosheets.When Tc increases to 1000?,the catalytic components in CoNi-LDHs effectively promote the growth of CNO to CNT and assist the synthesis of CNT decorated NSGN(CNT-NSGN).The interconnected structure,higher nitrogen content(-8%)and better crystallinity of CNT-NSGN effectively improve the conductivity and the electrochemical performance.