| Lithium ion batteries?LIBs?have been widely used as the power sources of portable electronic equipments and electric vehicles due to their advantages of high specific energy,high working voltage,long cycle life,good safety perfor mance,small self-discharge rate,and friendly environment,etc.Anode material is one of the key materials of LIBs,and graphite materials are the most commercial anode materials of LIBs at present,such as natural graphite and artificial graphite,etc.H owever,the practical application of these graphite materials as anodes in LIBs for high-energy electric vehicles is severely hindered by their low specific capacities.Microcrystalline graphite?MG?is a kind of resourceful and cheap natural graphite with moderate degree of graphitization?95%?and consists of many microcrystals?less than 1?m in size?with different orientations.Natural MG holds the great potential as promising anode materials for LIBs in view of low price of raw material,good electrolyte compatibility,high rate performance and remarkable cycle stability.Nevertheless,the relatively low reversible capacity of MG largely limited its application process in LIBs as their anode materials.In order to improve the lithium storage capacity of MG,this thesis focuses on the preparation of two kinds of modified MG-based anode materials:pitch-based carbon coated mildly expanded MG and FeCl3-intercalated MG intercalation compounds.The correlation between the ingredients,preparation parameters and structures of MG-based materials and their electrochemical performance of LIBs as anodes will be systematically studied.First,the preparation of pitch-based carbon coated mildly expanded MG and its lithium storage properties.This part firstly prepared the graphite intercalation compounds?GICs?with a intercalationg treatment in the condition of natural MG as raw material,inorganic acid as intercalating agent and organic acid as oxidan t agent.The obtained GICs were subsequently heat-treated to decompose the intercalated component and the producted gas will enlarge the interlayer spacing of graphite to obtain the mildly expanded MG.At last,the mildly expanded MG was uniformly coated by a carbon layer from the pitch source.The experimental results show that the optimal performance of this material can be achieved in the condition of nitric acid as oxidant agent,formic acid as intercalating agent,40?of oxidation temperature and40 min of oxidation time.As a result,the prepared mildly expanded MG exhibits a initial charge capacity of 353.5 mAh g-1(30.4 mAh g-1 more than that of pristine MG),and a initial Coulombic efficiency of 76.17%.After coated by pitch-based carbon,it was found that the optimal carbon coating content was 6 wt%and this material delivered a initial charging specific capacity is up to 374.5 mAh g-1,the initial coulomb efficiency is 94.31%,and the charging specific capacity is 329.8 mAh g-1after 100 charging and discharging cycles,the cycle retention rate is up to 100%.Second,the synthesis and electrochemical study of FeCl 3-intercalated microcrystalline graphite intercalation compounds?GICs?.FeCl 3-intercalated GIC was synthesized by a molten salt method with the natural MG as host,and the influence of its microstructure on the electrochemical properties of lithium storage were studied.As a result,the FeCl3-GIC anode showed a high gravietric capacity of about 905.4 mAh g-1 and high volumetric capacity of 859 mAh cm-3 with a high tap density of 0.95g cm-3.This anode material achieves the integration of isotropic orientation structure,amorphous carbon ingredients between graphite grains,and crumpled graphite layers,thus displays excellent structure integrity with long cycle stability during 100 cycles.In comparison,such GICs from natural flake graphite?NG-GIC?shows a low initial charge specific capacity of only 752 mAh g-1,and unstable cycle performance with a fast capacity fade during repeated discharge/charge processes.The capacities of NG-GIC are even lower than that of the graphite host after 30 cycles. |
PDF Full Text Request