Construction Of Transition Metal Oxides/Graphene Composites Electrodes And Applications In The Lithium Ion Capacitor

With the rapid development of global economy,diminishing supply of fossil fuels and increasing environmental pollution,the construction of efficient,green and sustainable chemical power supplies have attracted much attention.Many reserchers have devoted to developing high performance lithium ion capacitors（LIC）due to their higher power density and longer cycling life than lithium ion battery（LIB）and higher energy density than electric double-layer capacitor（EDLC）.LIC is expected to bridge the gap between the LIB and EDLC and will become the ultimate power source for hybrid electric vehicles（HEV）and electric vehicles（EV）in the near future.The electrochemical performance of LIC are determined by the properties of electrode materials,especially for anode materials.Thus,the fabrication of high-performanced negative materials plays an importance role to the construction of high performance LICs.The main contents and results are as follows:Firstly,pomegranate shaped Fe₃O₄/RGO nanohybrids were fabricated by a facile solvothermal method.As observed from SEM and TEM,Fe₃O₄ particles are uniformly and highly hybridized with graphene nanosheets,and have an average size of 250³00 nm.Moreover,a high specific surface area of 130 m2/g for the nanohybrids can be obtained.And then,as anode material for LIB,the electrode exhibits superior cycling stability and enhanced reversible capacity compared with pure Fe₃O₄ NPs and the mixture of（Fe₃O₄+RGO）: A high intial specific capacity of 1266 m Ah/g can be attained and the electrode could still remain 992 m Ah/g after 100 cycles at 150 m A/g.Furthermore,The cells also manifested outstanding rate capability（384 m Ah/g at 5 C）.Finally,a LIC was assembled based on activated carbon（AC）positive,Fe₃O₄/RGO negative,and Li PF6-based organic electrolyte.the highest energy density of 65 Wh/kg can be achieved at 1450 W/kg.Even at the highest power density of 9.3 k W/kg,a high energy density of 26.5 Wh/kg can be retained.The LIC can still exhibit a superior cycling stability（At 1 A/g,a capacitance retention of 73.3% can be obtained after 1000 cycles）.The relationship between microstructure and performance was exploited to obtain the key factors that are helpful for design and construction of hybridized materials with tunable performance.Secondly,a high-performance Li+ insertion pseudocapacitive material of T-Nb2O5/RGO nanohybrids was prepared by hydrothermal method and heat-treatment processes.As observed from SEM and TEM,T-Nb2O5 nanowires are highly hybridized with graphene nanosheets,forming tight and cross-linked nanohybrids.The hybridized anode material exhibits enhanced reversible capacity and superior cycling stability.The initial capacity of the nanohybrids could reach to 227 m Ah/g and remained 165 m Ah/g after 100 cycles at 100 m A/g.Even at a high rate density of 10 C,a capacity of 86 m Ah/g could be obtained,which is much higher than the pure T-Nb2O5 electrode.Furhrmore,LICs based on T-Nb2O5/RGO negative material,AC positive material in Li PF6/EC+DEC+DMC organic electrolyte and PVDF-HFP-Li TFSI/EMIMBF4 ionic liquids gel electrolyte has been constructed,respectively.The maxium of energy density and power density of 45 Wh/kg and 9.1 k W/kg for the LIC in the organic electrolyte can be achieved.At the same time,the electrochemical performance of PVDF-HFP-Li TFSI/EMIMBF4 ionic liquid gel electrolyte based quasi-solid LIC at different temperatures was also determined.We can concluded that the quasi-solid state LIC at 60 oC could deliver the best electrochemical performance.A highest energy density of 70 Wh/kg at 1 k W/kg can be achieved.Even at a largest power density of 14.7 k W/kg,an energy density of 24 Wh/kg can be still obtained.