Study On Modified Mesocarbon Microbeads As Anode Material For Sodium-Ion Batteries

Mesocarbon microbeads?MCMB?has been applied as negative materials of lithium-ion batteries.Compared with lithium-ion batteries,sodium-ion batteries?SIB?have a rich source and low cost,SIB have great application prospects.However,it cannot be used as negative materials of sodium-ion battery,because sodium ions radius larger than lithium ions.Recently,some researches find that carbonized MCMBs under low temperature can applied in sodium-ion battery,carbonized process expand interlayer distance,which is more easy for sodium-ion extraction and insertion in carbonized MCMBs.This research studies a surface modified MCMBs and N-doped MCMBs,it can increase surface pore size and suface defects,the material shows more high capacities and better cycle life.In this research,mixing NaOH with MCMB,and carbonized at 650?.The NaOH-etched MCMB exhibit surface fissures and more mesopores,the diameter of mesopores is 5 nm,thier interlayer spaces expand from 342 pm?carbonized MCMBs?to 356 pm?NaOH-etched MCMB?,wider interlayer spaces benefits sodium-ion extraction/insertion and decrease resistence.Compared carbonized MCMBs and NaOH-etched MCMBs,the etched materials as anodes for SIBs display improved rate capability and better cyclability.The NaOH-etched MCMBs discharge/charge curves at a low current density 15 mA g^–1,it delivers initial discharge/charge capacity of 441/249 mAh g^–1,the coulombic efficiency is 56.5%,this result mainly attributed to sodium loss during the formation of solid electrolyte interphase?SEI?layers.After 60^thh cycle,the charge capacity remain 217 mAh g^–1,the second cycle coulombic efficiency is 97%.The capacities rapidly fell down with increasing current density,when current density at 25 mA g^–1,the NaOH-etched MCMBs anode delivers charge/discharge capacities of 210/369 mA g^-1,at 800 mA g^-11 current density,the charge capacity is 69 mAh g^–1.Nevertheless,the NaOH-etched MCMBs anode still shows improved rate capacity.The N-doped MCMBs materials have not high nitrogen content?¹.7%?,as result of ammonia water volatility.The nitrogen and carbon can combine result which appear some surface defects,and nitrogen electronegativity stronger than carbon,the result that nitrogen can obtain electronic from carbon,this character benefits to accelerate sodium-ion extraction and insertion.When current density at15mA g^-1,the N-doped MCMBs anode delivers initial charge/discharge capacities of173/359 mAh g^–1,and coulombic efficiency is 48.4%.At the 10^thh cycled,the coulombic efficiency increase to 98%.The result shows that reversible capacity restore faster and the capacities slowly fell down.In this work,the NaOH-etched MCMBs and the N-doped MCMBs improve capacities and cyclability.The NaOH-etched MCMBs appear surface fissures that can enhance the sodium transfer in the etched MCMBs during discharge and charge and increase sodium-ion capacity.The N-doped MCMBs produce surface defects which accelerate sodium-ion transfer and improve electrochemical performance.