Preparation Of Vanadium Oxide Nanoarrays For Lithium-ion Hybrid Capacitors

Lithium-ion hybrid capacitors are hybrid energy storage devices that have attracted much attention because of their characteristics of high energy and power density.The two electrodes are respectively the battery electrode that can reversibly intercalate/de-intercalate lithium ions and the capacitive electrode that stores charges via ion adsorption/desorption.Solid-state lithium-ion hybrid capacitors with better safety and flexibility will be the key research and development directions in the future.Among various electrode materials,the rich composition and phase structure of vanadium oxide make it an excellent lithium ion storage capacity as a lithium ion battery electrode.However,vanadium oxide is often used as anode electrode in lithium-ion hybrid capacitors.The crystal structure will be severely damaged with huge volume expansion during cycling due to the conversion reaction mechanism,resulting in poor rate performance and cycling stability.In addition,the capacity(500–1000 m Ah g^-1)of vanadium oxide anode is much higher than that of the capacitive cathode,which causes great difficulty in realizing the charge balance between anode and cathode in lithium ion hybrid capacitors.In this thesis,we will report the design of non-solid-state and solid-state lithium-ion hybrid capacitors with V₂O₅ and VO_x as cathodes.The main contents are as follows:(1)Binder-free V₂O₅ porous nanodisk thin film is synthesized through a VO₂template oxidation strategy.This electrode structure can not only provide a direct electron transport channel,but also enhance the contact interface between electrode materials and electrolytes,providing more active sites for charge storage and effectively shortening the migration path of electrons/ions during charging and discharging,which in turn improves electrochemical performance.The electrochemical performance of V₂O₅ porous nanodisk thin film electrode was measured,the capacity is as high as?300 m Ah g^-1.With the current density increasing 64 times from 25 to1600 m A g^-1,a capacity of 104 m Ah g^-1 can still be maintained,indicative of excellent rate performance.Different from the previous hybrid capacitor configuration using V₂O₅ as the anode and the capacitive electrode as the cathode,we uses V₂O₅ as the battery-type cathode to build a lithium-ion hybrid capacitor with a pre-lithiated activated carbon capacitive anode.In our case,the benefits of the intrinsic layered structure of V₂O₅ can be utilized,which leads to high power density(1396 W kg^-1,71.9 m W cm^-3)and high energy density(146 Wh kg^-1,13 m Wh cm^-3).Furthermore,the device still works well when bended or even folded.(2)The electrochemical performance of VO_x nanodisk electrode was improved by polyvinylidene fluoride-acetylene black(PVDF-ACET)coating.PVDF-ACET forms a protective layer with porous structure on the surface of the active material,which connects each nanodisk to improve the structural stability of VO_x.The porosity enables the electrolyte to enter the nanodisk structure easily,which is beneficial to rapid ion diffusion.In addition,acetylene black has good electrical conductivity,which makes up for the defect of poor electrical conductivity of active materials.As a result,the capacity of the as-prepared VO_x@PVDF-ACET electrode can reach 352 m Ah g^-1 at25 m A g^-1.With the current density increasing to 1600 m A g^-1,the capacity of142 m Ah g^-1 can still be maintained,which is?3.8 times higher than that of VO_x,showing excellent rate performance.The electrochemical performance of VO_x@PVDF-ACET electrode as the cathode of solid-state Li-ion hybrid capacitor was tested with PVDF-HFP-based solid polymer electrolyte.It can provide a maximum discharge capacity of?350 m Ah g^-1.After 100 cycles,?60%of the initial capacity was retained,and the coulomb efficiency was as high as 95-100%.