Interlayer Structure Design And Charge Storage Performance Of MXene-based Self-assembled Thin Film Electrodes

The 2D transition metal carbonitrides/nitrides(MXenes),based on their characteristic advantages of multilayered structure,rich surface redox chemistry,and metallic conductivity,have been identified as the most competitive intercalated charge storage electrode in micro energy storage devices.Nonetheless,during the self-assembly process of MXene thin films,the restacking of MXene nanosheets induced by strong van der Waals force limits the interlayer space,thus severely blocking the(de)intercalation of ions,especially for the preferable anions and cations(such as Cl-,Zn2+)with high charge densities and large ionic radii.This results in limited charge storage capacity and sluggish charge transfer kinetics,thus low areal capacitance,and the linearly inclined unstable voltage output of micro energy storage devices.Therefore,for addressing the above challenges,through the interlayer structure design of MXene based self-assembled thin film electrode,good matching of electrolyte and optimization of device configuration,further breakthroughs in area energy/power density of micro energy storage devices can be realized,and the rapid development of flexible wearable/implantable microelectronics can be promoted.The main research contents are as follows:(1)MXene/bacterial cellulose fiber(BC)hybrid films with effectively expanded interlayer spacing between re-stacked few-layered MXene sheets via homogeneous intercalation of BC nanospacer were designed and prepared.The electrochemical testing and molecular dynamics simulation demonstrated the consequently widened ions-transport-channels between few-layered MXene sheets can effectively reduce the diffusion barrier of Zn2+within the MXene/BC host electrodes compared to pure MXene film electrodes without BC nanospacer.Further in-situ Raman and ex-situ XPS spectroscopy characterizations suggest:the consequently accelerated Zn2+diffusion involved adsorption/desorption featured charge storage,and followed electrochemical reaction of Zn2+with oxygen terminal groups on the MXene flakes induced additional pseudocapacitance within the MXene/BC host electrodes simultaneously contribute to the acquired superior areal capacitance of the fabricated symmetrical aqueous Zn-ions MSCs(ZIMSCs)based on the MXene/BC host electrodes and Zn(CF3SO3)2/polyacrylamide hydrogel electrolyte.Meanwhile,combined with doubled voltage window(1.2 V)benefiting from the cocurrently suppressed kinetics of hydrogen/oxygen evolution within the MXene/BC host electrodes,a tremendously increased areal energy density(34 μWh cm-2)in comparison with that employing conventional H2SO4/polyacrylamide hydrogel electrolyte(0.6 V/9 μWh cm-2)is finally realized.The work demonstrates a simple and effective strategy to synchronously boost bivalent Zn2+diffusion and depress hydrogen/oxygen evolution within MXene host electrodes toward symmetric ZIMSCs of high areal energy density.(2)We also designed a novel Zn2+-Cl-dual-ions micro-redoxcapacitor(MRC)by combining the zinc atoms injection strategy and interlayer coupling engineering to scientifically and effectively utilize the precious interlayer space between 2D MXene sheets.The novel device structure is first present by coupling silver nanowires(AgNWs)between MXene interlayers(MXene/AgNWs)as cathode,simultaneously in situ inpouring zinc atoms between MXene/AgNWs interlayers as anode,and well-designed polyacrylamide/ZnCl2 hydrogel as solid-state electrolyte.Moreover,in situ inpouring zinc atoms between MXene/AgNWs interlayers enabling Zn/Zn2+redox conversion of MRC toward high areal energy density.In addition,the introduced AgNWs effectively alleviate the MXene sheets restacking to facilitate Zn2+,Cl-transfer kinetics,and synchronous Ag/AgCl redox conversion benefiting from a ultra-flat discharge plateau(～0.9 V).Thence,the integration of the zinc atoms injection strategy and interlayer coupling engineering enables the Zn2+-Cl-dual-ions micro-redoxcapacitor to achieve stable output of voltage and battery-level areal energy density of 117 μWh cm-2.(3)By coupling silver nano wires(AgNWs)between MXene interlayers with the help of bacterial cellulose(BC)as bio-dispersant toward MXene/AgNWs&BC hybrid cathode to pair with Zn anode,a novel Zn2+-Cl-dual-ions micro-redoxcapacitor(MRC)employing polyacrylamide/ZnCl2+NH4Cl hydrogel electrolyte is present.The introduced AgNWs nanopillars alleviate the MXene nanosheets restacking to facilitate Cl-transfer kinetics,and concurrently strengthen the charge storage capacity and output stability benefiting from a flat discharge plateau(～1 V)stemming from the extra phase transition behavior(Ag?AgCl).Thus,an appealing dual energy storage mechanism,featuring of(i)expedited Cl-diffusion involved de/intercalation and(ii)reversible solid-to-solid conversion of Ag/AgCl redox couple confined between MXene interlayers,is established and revealed by in-situ XRD/Raman analyses.Consequently,remarkably boosted areal energy density up to 227 μWhcm-2 along with significantly improved output stability and suppressed notorious self-discharge behavior,are achieved in the resultant MRC.This work provides a brand-new strategy for designing innovative MXene-based MRC featuring a hybrid charge storage mechanism of ions-intercalation and phase conversion to simultaneously realize high and steady energy output.