Influence Of Nano-Additives On The Fomation And Electrochemical Properties Of Mesocarbon Microbeads

Mesocarbon microbeads（MCMBs）are a new kind of carbon materials and have a wide range of applications in many fields such as lithium-ion batteries due to their special spherical shape,high chemical stability and high thermal/electrical conductivities.However,in the common preparation of polycondensation process using coal tar pitch as the precursor,the spherical shape is difficult to control which can be easily coalesced,and the obtained MCMBs usually possess wide and uneven particle size distribution.Therefore,the method for preparing MCMBs by adding nucleating agent has gained extensive researches and developments.The objective of this study is to explore two kinds of new preparation approaches for MCMBs.One is adding a new nucleating agent,polyhedral oligomeric silsesquioxane（POSS）,to coal tar pitch to promote the uniform nucleation and growth of MCMBs during the thermal polycondensation process.The effects of POSS addition,condensation temperature and maintaining time on the formation,growth,coalescence,and yield of MCMBs were investigated.The optimal preparation parameters were obtained.The other is that a new fullerene C₆₀induced MCMBs formation strategy was firstly proposed,in that C₆₀is used as a structural inducer to enhance the orientational assembly of coalesced mesophase pitch into MCMBs.The effects of C₆₀addition,temperature,and viscosity of pitch on the formation of MCMBs were investigated.The mechanism of microsphere assembly was also deduced.The morphologies and structures of MCMBs prepared by the above two methods were characterized by various measurements such as polarized optical microscope,XRD,SEM,and Raman.The electrochemical performances of the carbonized and graphitized MCMBs as anode materials for lithium-ion batteries were investigated.These results will provide some basic data for MCMBs further practical applications.The results indicated that increasing the reaction temperature and prolonging the holding time are both beneficial to promote the formation of MCMBs in coal tar pitch.The addition of 0.5wt%of octaphenylsilsesquioxane（OPS）to the refined coal tar pitch at a constant temperature of 420°C for 3hours enhances the formation of MCMBs with a larger number and uniform particle size.The yield of 29.3%was attained under such circumstance.When the addition amount reaches 1 wt%,MCMBs are easily oalescence into a bulk mesophase pitch.The mesocarbon microbeads prepared by adding different proportions of POSS show disordered carbon structures.The MCMBs prepared by 0.5%POSS addition were carbonized at 1000℃and then were assembled into lithium-ion batteries for electrochemical characterization.The results shown that MCMBs possess the specific capacity of 422.5m Ah/g after 100 cycles at a current density of 50 m A/g,and the capacity retention of 96.9%.The graphitized MCMBs at 2800℃show higher Li-storage performance compared with that of without OPS addition.C₆₀addition to naphalene-based mesophase pitch can induce the bulk mesophase orientation and help to assemble into MCMBs.When C₆₀was added with small amounts（less than 5 wt%）,it is difficult to induce a large domain of mesophase pitch,but the continuous liquid crystal phase gradually tends to become discontinuous state.When the addition of C₆₀reached 5 wt%,its spherical morphology induces the rearrangement of the coalesced mesophase liquid crystals.MCMBs with the particle sizes of 10-30μm can be prepared under the effect of surface tension.When the addition of fullerene C₆₀reached 10 wt%,a large amount of C₆₀makes the induction effect too dispersed,which is difficult to make aromatic molecules concentrated,resulting in the formation of sea-island mesophase pitch rather than MCMBs.The MCMBs prepared by C₆₀inducing were carbonized and then assembled into a lithium-ion batteries for electrochemical testing.It was found that MCMBs exhibit the specific capacity of 425.6 m Ah/g after 50 cycles at a current density of 100 m A/g.In addition,after 100 cycles at a current density of 1 A/g,the specific capacity still remains 148.2m Ah/g.