The Study Of Stability Of Graphene At High Potential Limit

Hybrid Li-ion capacitors is a new generation of electrochemical energy storage device,which energy density is approximately twice orthrice than conventionalsupercapacitors,and also could achieve high power density and long cycle life.In order to improve energy density of hybrid Li-ion capacitors,and satisfy the demand of energy density(>20 Wh/L)of devices in the future,this dissertation explored method and mechanism of graphene materials at high stable working potential as cathode of hybrid Li-ion capacitors,andprovide theoretic guidance for choosing and preparation cathode of high voltage hybrid Li-ion capacitors.The electrochemical behaviors of graphene materials under high voltage Li-ion electrolyte were studied.The results show that the electrochemical performance at high potential limit(3-5v(vs.Li/Li?))are dependent on the structure and chemistry property of graphene materials.After oxidation potential of the electrolyte on different work electrode were compared,it found that oxidation reaction of the electrolyte could be catalyzed by surface of graphene materials,and oxidation potential is lower on graphene materials.The study on the electrochemical performance of graphene materials with different specific surface area indicates that the electrolyte oxidation reaction,catalyzed by surface of graphene materials,is influenced by the area ratio of mesoporous and macroporous and restricted by pore size of microporous;but through the pre-cycle process,the coulombic efficiency of graphene materials increase sharply,the solid electrolyte interface(SEI)-like film is formed on the surface of graphene materials,protect electrode and prohibit the reaction of electrolyte.The study on the electrochemical stability at high potential range of graphene materials with different pore structure indicates that graphene with small-meso-and micro-porous(pore size<6nm and average pore size is 2.6nm,volume of micropore divide volume of total pore is 38%)is better The study on the electrochemicalperformance of graphene materials with different oxygen content shows that reducing ratio of carbon and oxygen couldimprove stability at high potential.The reason of capacity decay of graphene materials at high potential range was analyzed by ex-situ SEM and XPS depth profiling of cycled electrode.It found that the more stable SEI-like film is formed on the graphene materials with abundant microporous and smaller mesoporous.After forming SEI-like film,graphene with small-meso-and micro-porous could work as stable cathode of high voltage hybrid Li-ion capacitors.