Study On Electrochemical Properties Of Mesocarbon Microbeads For Lithium-ion Batteries And Sodium-ion Batteries

As a kind of typical soft carbon,the graphitized mesocarbon microbeads(MCMBs)have been applied in industrial production as anode materials in lithium ion batteries(LIBs).Compared with the MCMB after graphitization,the MCMBs carbonized under low temperature are expected to become the ideal anode materials for LIBs and sodium ion batteries(SIBs).Their interlayer distance is bigger,which is more conducive to the insertion and extraction of lithium/sodium ions in the process of charge-discharge.In this thesis,the MCMBs are carbonized at 700 oC and 1000 oC,and the products were named MCMB700 and MCMB1000,respectively.The anode performance and the energy storag mechanism of MCMB700 are tested as they are used as anode materials for SIBs.To further explore the energy storage mechanism of MCMB,the anode performance of MCMB1000 is investigated and compared as they are used in SIBs and LIBs.Further more,the sodium storage mechanism is discussed based on the lithium storage mechanism in lithium ion batteries.The MCMBs carbonized at 700 oC consist of graphite-like crystallites with an interlayer distance of ~357 pm,and with a crystal stack height of 1.61 nm.The MCMB700 as anode materias in SIBs show good electrochemical performance.They deliver an initial charge capacity of 232 mAh g-1 at a current density of 25 mA g-1,and keep a reversible capacity of 161 mAh g-1 after 60 cycles.The MCMB700 anodes exhibit good reversible sodium insertion/extraction behavior,which results from their internal crystallites and their unique external spherical surface layers.XRD and Raman analyses indicate that the MCMB700 anode stores energy by the electrochemical intercalation of sodium into the graphene sheets.Comoared with MCMB700,MCMB1000 have higher crystallinity and less defect structure,which is beneficial to the analysis of energy storag mechanisms in the aspect of microstructure.The anode performance of MCMB1000 is compared as it is used in SIBs and in LIBs.In LIBs and SIBs,the first charge capacity is 445/180 mA g-1 at 20 mA g-1,and drops to 318/180 mAh g-1 after 50 cycles,corresponding to the capacity retention of 71.5% and 68.3%.MCMB1000 anodes for SIBs show nice reversible capacities and good stability during cycling.The MCMB1000 anodes in SIBs show much less capacities than in LIBs,which mainly results from the radius of sodium ion is bigger than the radius of lithium ion.Then it?s more difficult for the insertion of sodium into carbon layers than lithium ion.The MCMB1000?s interlayer spacing available for the insertion/extraction of bigger sodium ions is smaller than that for lithium ions.That?s why the MCMB1000 anodes in SIBs show much less capacities than in LIBs...