| Electricity is the most important form of energy,and thus electrical energy storage devices have become a vital part of energy area.However,traditional electrochemical energy storage devices such as lithium-ion batteries(LIBs)and electric double layer capacitors(EDLCs)cannot meet the growing demand.It is urgent to develop an energy storage device with both high power density and high energy density.Unlike LIBs and EDLCs in which anode and cathode utilize the same type of reaction for energy storage,lithium-ion capacitors(LICs)composed by a cathode based on electric double-layer and an anode based on redox reactions deliver both high energy density and high power density.Due to two different charge-storage mechanisms occurred in one cell,kinetics mismatch between anode and cathode is the main issue for LICs,and therefore anode materials with fast kinetics are needed to be developed to improve the electrochemical performance of LICs.Pseudocapacitive materials are considered as the ideal anode candidates for LICs due to their fast kinetics.However,most of pseudocapacitive materials are subject to their relatively high redox potential,thus resulting in an undesirable decrease in energy density of LICs.As a battery anode material,silicon has the advantages of high theoretical capacity(3579 mAh g-1)and suitable lithium de-/lithiation potential(0.4 V(vs.Li/Li+)).However,the huge volume change during de-/lithiation process and sluggish kinetics hinder the application of silicon in LICs.Based on these problems,following research work was carried out:(1)To address the problem of high de-/lithiation potentials of traditional pseudocapacitive materials,a new anode material,lithium-rich disordered rock salt vanadium oxide(DRSLi3V2O5)with low de-/lithiation potentials(-0.6 V(vs.Li/Li+)),was synthesized by an electrochemical method.The electrochemical kinetics of DRX-Li3V2O5 was studied,which demonstrated that DRX-Li3V2O5 is an intrinsic intercalation pseudocapacitive material.The pseudocapacitive behaviors of DRX-Li3V2O5 is mainly arises upon a percolating network that offers three-dimensional lithium-ion transport pathways confirmed by the Monte Carlo simulations.DRX-Li3V2O5 delivers the high specific capacities of 217.9 and 84.3 mAh g-1 at current densities of 1 and 20 A g-1,respectively.The capacity retention rate was achieved at 86.5%after 2,000 cycles at the current density of 10 A g-1.A lithium-ion capacitor is further assembled by combining this pseudocapacitive DRX-Li3V2O5 anode with a capacitive activated carbon cathode,which yields a cell voltage of 4.0 V and high energy density of 126.8 Wh kg-1(at the power density of 184.0 W kg-1).Even after 30,000 constant current charge/discharge cycles at a current density of 2 A g-1,70%of its capacity was preserved.(2)To address the problems of volume change and sluggish kinetics of Si anode,the design concept of nano-scale and structure design were introduced to improve the rate performance and cycle stability of Si.A silicon-carbon composite Si@C with core-shell and pore structure was successfully synthesized by a facile coating approach using metal-organic framework(MOF)material ZIF-8 as the carbon precursor.The structural characterization analysis indicated that amorphous carbon shell has a microporous structure with size range from 0.5-2 nm,which is uniformly coated on Si nanoparticles.Si@C delivers the specific capacities of 1911.0 and 354.8 mAh g-1 at current densities of 1 and 10 A g-1,respectively.Si@C still delivers a specific capacity 917 mAh g-1 after 100 cycles at the current density of 1 A g-1.A lithium-ion capacitor is further assembled by combing this pseudocapacitive Si@C anode with a capacitive activated carbon cathode,yields the high energy density of 195.62 Wh kg-1.Even after 1000 constant current charge/discharge cycles at a current density of 2 A g-1,this LIC still preserved 89%of initial capacity. |
PDF Full Text Request