Structure Modulation Of Ion Channels For TiO₂/Graphene Thick And Dense Electrodes For Improving Electrochemical Performance

Nowadays,along with the rapid increase of the electronics products,current commercial batteries and supercapacitors can hardly meet the growing demand for high energy density and fast charge-discharge.One of the crucial studies in lithium-ion batteries（LIBs）and sodium-ion batteries（SIBs）is to develop the anodes with high volumetric capacity and rate performance.TiO₂ is a promising anode material,which is promising to be used in commercial batteries because of its long cycle life,low cost,high safty and environment friendliness.However,the electrochemical performance of TiO₂ suffers from its low intrinsic diffusion coefficient and electrical conductivity.Constructing nanoarchitectures reasonably and hybridizing TiO₂ with carbon material have been proposed to improve the electrochemical performance of TiO₂,but the areal mass loadings of reported TiO₂/C nanomaterial electrodes still remain to be increased.Moreover,the nanoarchitectures and hybridization with carbon material can severely decrease electrode density.In this thesis,the thick and dense TiO₂/graphene electrodes have been prepared.In the thick and dense electrodes,the structures with vertically aligned TiO₂/graphene nanosheets and microchannel arrays were constructed separately to simultaneously achieve high areal capacities,high volumetric capacities and satisfied rate performance.To attain high areal capacity,a spongy three-dimensional porous thick electrode（G@TiO₂/G）composed of TiO₂/graphene upper layer and graphene under layer was constructed by a filtration method.In the electrode,the interconnected macropores between TiO₂/G nanosheets offer fast ion transport channels.The stacked graphene sheets not only provide robust framework but also favor fast electron transport.Moreover,the ultrasmall TiO₂ nanoparticles offer the short diffusion paths and fast pseudocapacitive storage capability.These characteristics make the G@TiO₂/G electrode attain high areal capacities and long cycle performance.As an anode for LIBs,the G@TiO₂/G electrode with an areal mass loading of 25.5 mg cm^-2 achived a high areal capacity of 5.63 m Ah cm^-2 at 4.27 m A cm^-2.When the G@TiO₂/G electrode was assembled with Li₃V₂（PO₄）₃,the energy density of the battery is about112 Wh kg^-1 calculated with the total mass of anode and cathode electrodes.As an anode for SIBs,the G@TiO₂/G electrode with an areal mass loading of 10.5 mg cm^-2delivers a higher areal capacity of 2.27 m Ah cm^-2 at 0.53 m A cm^-2.In addition,the G@TiO₂/G electrode has potential to be used in flexible sodium-ion storage.This thesis proposes a wet pressing method to densify the G@TiO₂/G electrode and construct directional ion transport channels.Compared with traditional dry pressing,the wet pressing is more effective to increase the density of the electrode.Based on the directional structure in thick and dense G@TiO₂/G electrode,an approach including the diamond wire saw cutting process was employed to change the structural orientation of TiO₂/graphene nanosheets to obtain a thick and dense vertically aligned TiO₂/graphene nanosheet electrode（VATiO₂-G）.The vertical alignment not only offers less tortuous ion transport paths to facilitate ion diffusion kinetics,but also enhances the electronic conductivity.As an anode for LIBs,the VATiO₂-G electrode shows high volumetric and high areal capacities of 243 m Ah cm^-3 and 3.16 m Ah cm^-2 at 2.8 m A cm^-2,respectively.Femtosecond laser drilling technology was adopted to construct microchannel arrays with a mean hole diameter of 9μm in the thick and dense horizontally aligned TiO₂/graphene nanosheets electrode（d G@TiO₂/G）.If the hole spacing is 40μm,the microchannel array only brings 4.0%decrease in electrode density,and the relative density of the drilled electrode still reaches 64.3%.The microchannel array can effectively facilitate ion transport kinetics in d G@TiO₂/G electrode.Ion can directly reach the bottom part of the electrode,and the ion transport paths are further shorted.As a result,the microchannel array greatly improves the gravimetric capacity,volumetric capacity,rate performance and cycling performance of the d G@TiO₂/G electrode.The raise rates are enhanced with the decrease of the hole spacing.The microchannel array with a hole spacing of 40μm improves the volumetric capacity of the d G@TiO₂/G electrode from 172 to 266 m Ah cm^-3 at 0.5C,from 15.8 to 97m Ah cm^-3 at 10C,and attains high areal capacities of 2.26 m Ah cm^-2 at 0.5C,respectively.Furthermore,such microchannel array also improves the volumetric capacity and rate performance of a dense graphene electrode.This result indicates that the microchannel array is generally effective for a variety of dense 2D nanomaterial electrodes to improve their volumetric performance.