Design Of Mesocarbon Microbeads-based Anode Materials And Their Lithium Storage Properties

Mesocarbon microbeads?Mesocarbon microbeads,MCMB?stand out from many lithium-ion battery anode materials because of their unique lamellar structure,excellent electrical conductivity,and high bulk density.And,MCMB has received extensive attention and research from scholars,which has now been commercialized.However,MCMB also have inherent defects,such as lower theoretical specific capacity and poor rate performance,etc.These defects also limit their further applications in power batteries.In order to expand the application range of MCMB,it is urgent to modify MCMB,and the construction of MCMB-base composites is an effective method to improve the electrochemical performance of MCMB.In this paper,medium temperature coal tar pitch was used as a raw material,and a new type of MCMB-based anode material with controllable composition and structure was prepared by using thermal condensation method,mainly including MCMB and Fe_1-xS/MCMB.The MCMB were prepared by direct thermal condensation method using medium temperature coal tar pitch as raw material,and the influence of reaction conditions on the growth process of MCMB was investigated.Among them,the increase of the reaction temperature and the extension of the holding time are beneficial to the formation and growth of MCMB,and increase the particle size and yield of MCMB.The reaction pressure changes the viscosity of the system mainly by controlling the escape of the light components,thus affecting the formation process of MCMB.The polarizing microscope analysis showed that the optimum process parameter combination for the preparation of MCMB was 420?-4 h-1Mpa.Then,the effects of quinoline insoluble?QI?on the morphology and lithium storage performance of MCMB were investigated under the optimum conditions.The results show that QI promotes the formation of MCMB,but also increases the surface roughness and specific surface area of MCMB.Therefore,MCMB with rough surface will increase the occurrence of electrochemical side reactions and consumes more Li⁺in the process of intercalation and delithiation,its coulomb efficiency in the first cycle is only 68.16%,and the reversible specific capacity is maintained at about 294 mAh g^-1after 30 cycles.Fe_1-xS/MCMB was successfully prepared by a facile one-step in situ thermal condensation method with refined coal tar pitch as carbon source.Fe_1-xS/MCMB is a hierarchical structure with MCMB with Fe_1-xS crystals attached on the surface as the core and amorphous carbon as the shell.And the effects of Fe_1-xS crystals content on the structure and electrochemical performance of the composite was investigated.The results show that Fe_1-x-x S crystals are beneficial to the formation of Fe_1-xS/MCMB and induce the formation of a more orderly carbon layer structure.Fe_1-xS crystals adhered to the surface of MCMB will also inhibit the coalescence of carbon microspheres and produce Fe_1-xS/MCMB with a more uniform particle size.The results of electrochemical characterization showed that15%-Fe_1-xS/MCMB composite exhibited excellent lithium storage performance.The first charging and discharging specific capacity of 15%-Fe_1-x-x S/MCMB is 481.5/614.3 mAh g^-1.After 120 cycles at 200 mA g^-1,the charge-discharge specific capacity of 15%-Fe_1-xS/MCMB is as high as 531.7/536.9 mAh g^-1,which is much higher than the reversible capacity of MCMB(226.6 mAh g^-1).Further research shows that the excellent electrochemical performance of 15%-Fe_1-xS/MCMB benefits from the good synergistic effect between Fe_1-xS crystals and carbon materials.To sum up,a series of MCMB-base anode materials were prepared by thermal condensation method from medium-temperature coal tar pitch,and their structure and electrochemical properties were characterized.On this basis,the controlled synthesis of MCMB was realized by investigating the reaction conditions,and MCMB with uniform particle size,smooth surface and good sphericity was obtained.And it was found that MCMB has excellent cycle stability,but its specific capacity is relatively low.In view of the above problems,a new type of core-shell structure Fe_1-x-x S/MCMB composite anode material was successfully prepared in this study.The synergistic effect of Fe_1-x-x S crystal and MCMB gives the composite excellent electrochemical properties,such as higher specific capacity,good cycling stability and rate performance.