| At present,supercapacitors and lithium-ion batteries are the mainstream electrochemical energy storage devices,and both of them have their own special characteristics.Supercapacitors have the advantage of fast charging and discharging speed,high power density and long cycle life,but the energy density is very low.Lithium-ion batteries have a very high energy density,but the cycle life is short.With the increasing demand in the fields of portable electronic devices,electric vehicles and so on,it is extremely urgent to develop high-performance lithium-ion batteries and supercapacitors.For both of the electrochemical energy storage devices,the key issue is the electrode material.Therefore,it is necessary to develop a new type of electrode material,design a reasonable electrode composition and structure,and study its energy storage mechanism.The main research contents are as follows:1.MnO2 was deposited on a nickel foam substrate by using the electrodeposition method,and then the conductive polymer PEDOT-PSS was in situ polymerized on its surface,which was designed to format the composite materials.The impact of wrapped conductive polymer layer on the electrochemical properties has been studied with different polymerization time.The results of the electrochemical tests show that the P-Mn2-2 composite with electropolymerization for 10s has the highest specific capacity(346.5 F/g),which is 1.9 times higher than MnO2 electrode(179.1 F/g).The P-MnO2-2 composite still has a specific capacity of 223.5 F/g at the high current density of 6 A/g.Finally,a flexible symmetrical supercapacitor has been assembled using the KOH gel-solid electrolyte.2.In this work,the rate performances of the layer-structured LiNi1/3Co1/3Mn1/3O2(NCM)cathodes are enhanced by adding both highly conductive mesoporous graphene and carbon black(SG)as conductive additives.The electrochemical properties of NCM cathodes using various conductive additives have been comparatively studied.The results suggest that the NCM electrode with SG conductive additive has the best rate performance,with a discharge capacity of 78 mAh/g at the current density of 2000 mA/g in the voltage range of 2.5-4.1 V versus lithium electrode,which is～1.7 times higher than that of NCM electrode with SK additive.The electrochemical impedance spectra(EIS)results reveal that the NCM electrode with SG addition has the lowest charge transfer resistance.Meanwhile,the structural evolution of NCM material with SG conductive additive during the first charge process have been studied via an in situ synchrotron X-ray diffraction technique(XRD).The XRD patterns show that a thorough phase transition from hexagonal H1 to hexagonal H2 occurs at the voltage of 4.3 V,and the diffraction peaks of(0 1 8)/(1 1 0)split gradually,representing the extraction of lithium from the ordered layer-structured NCM materials.3.A bifunctional electrode with the capacitor-type activated carbon(AC)and battery-type NCM has been fabricated by using internal parallel connection mechanisim.Compared with NCM electrode and AC capacitor electrode,the bifunctional electrode can not only maintain the high capacity of the battery material,but also possess the property of AC material with high rate performance.The discharge capacity of the bifunctional electrode was 57.1 mAh/g at 2000 mA/g current density,which was much higher than NCM electrode and activated carbon electrode.Further,we assembled a multilayer pouch cell with the bifunctional electrode and the pre-lithiated soft carbon anode(SC).Based on the mass of the entire device,the pouch cell has the highest energy density of 52.8 Wh/kg,and the highest power density of 9.3 kW/kg.After 2000 cycles at 10C rate,the capacity retention of the pouch cell is 84%.It can be seen that the application of bifunctional electrodes in new high-performance energy storage devices is very promising. |
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