Synthesis Of Modified Carbon Based Materials And Their Battery Performance

With the rapid development of mobile electrical equipment and clean energy,the lithium ion battery with high capacity,high rate and long life has become a hot spot in energy storage research.The traditional graphite anode is limited by its low theoretical capacity(372 mAh g^-1)and serious attenuation at high current.However,carbon anode has the natural advantages of high conductivity,wide sources,low price,environmently friendly and high stability when comparing with other types of anode.Therefore,many researches focus on obtaining new carbon-based anodes with high specific capacity and high rate by changing the structure or component.In this paper,sevel factors affecting the lithium storage performance of carbon materials,including microstructure,hetero-atom doping and compound,were modified to obtain high performance carbon anodes.Through material characterization and electrochemical testing,the material structure and electrochemical performance were studied.The specific studies are summarized as follows:?1?Soft carbon?SC?with low degree of crystallization was synthesized by solvent-thermal method at a low temperature.Then the SC was calcined at higher temperatures to obtain different graphitized carbons.The results of HRTEM and XRD show that the lattice arrangement of soft carbon in the graphitization process goes through three stages:disorderly arrangement,relatively ordered with wide interlayer,and highly ordered with narrow interlayer.A high specific capacity(about 576 mAh g^-1)can be obtained by the soft carbon with disordered lattice arrangement.Soft carbon with relatively ordered lattice shows a higher rate performance due to the wider interlayer,showing a specific capacity of 169 mAh g^-1 at 20C with no obvious attenuation within1,000 cycles.The wide interlayer greatly reduces the lattice change after lithiation,improves the transport kinetics of lithium ions,and effectively alleviates the capacity attenuation.?2?By controlling the carbon source and iodine source,the pure bulk carbon microspheres?BC?and iodine doped bulk carbon microspheres?IBC?were prepared.The results of SEM characterization show that the carbon materials exhibit 4-8?m in diameter with spherical structure.The properties of these carbon materials when used as anode of lithium ion and potassium ion battery were tested.BC electrodes for lithium ion battery anode can reach a reversible capacity of 501 mAh g^-1;when used as potassium ion battery anode,the reversible capacity can reach 285 mAh g^-1.The IBC electrodes exhibit the reversible specific capacities of 760 mAh g^-1 and 377.9 mAh g^-11 when used as lithium ion and potassium ion battery anode,respectively.Iodine doping brings obvious pseudocapacitive behavior,which improves specific capacity and rate performance.Finally,the DFT results further prove that iodine doping can greatly improve the conductivity of electrode materials and change the adsorption energy toward ions.?3?Nitrogen doped carbon microspheres?NCM?and iodine,nitrogen co-doped carbon microspheres?INCM?with high material density were prepared by solvent-thermal method.The two carbon materials have sphere morphology and smooth surface.The corresponding EDS and XPS also suggest that the doping of nitrogen and iodine on the carbon microspheres are very uniform.When used as anode of lithium ion battery,the INCM electrode reach a high volumetric capacity up to 1401mAh cm^-3 at a small current of 0.05mA cm^-2,and the reversible volume capacity of 504 mAh cm^-3 can still be retained at a large current of 5mA cm^-2.It was found that the iodine nitrogen co-doped carbon microspheres can reach a better performance than those nitrogen-doped carbon microspheres or iodine-doped carbon microspheres.Iodine modification of commercial carbon microspheres further verify the effect of iodine on electrochemical performance on high density carbon electrode.?4?Graphene/cobalt boride composites were prepared by simple process of reduction at room temperature.The cobalt boride particles are dispersed uniformly on the graphene.The HRTEM and XRD results showed that the cobalt boride has an amorphous property.The optimal ratio of graphene/cobalt boron composite exhibits a specific capacity of 888mAh g^-1 at 50mA g^-1,and a specific capacity of 547 mAh g^-1 at 2000mA g^-1.After crystallization at different temperatures,the amorphous phase has the best performance because amorphous phase has no grain boundary with low diffusion barrier,which is favor to the transport and insertion/deinsertion of lithium ions.Graphene not only provides a fixation effect on borides,but also provides good electrical conductivity,promoting a larger pseudocapacitive effect.The lithium ion capacitor was assembled by using the amorphous composite as anode and activated carbon as cathode,reaching the maximum energy density of 139 Wh kg^-1 and the maximum power density of 40 000W kg^-1.