Laser Induced Fabrication Of Graphene Based Planar High Voltage Supercapacitor And Its Electrochemical Properties

With the development of wearable and integrated electronic devices towards miniaturization and high energy/power density,portable flexible power facilities have become an urgent energy storage/supply device.In recent years,porous laser induced graphene（LIG）supercapacitors（SCs）,as a flexible energy storage device with high efficiency,low cost and large-scale manufacturing,prepared by laser processing polyimide（PI）film have great application prospects.However,the existing LIG supercapacitors generally have significant problems,such as low potential window,lack of compact series structure design,insufficient areal voltage and specific energy/power density.The key to solve the abovementioned problems is to explore the forming mechanism of laser-induced graphene and laser processing parameters to manufacture high-quality graphene,and design/manufacture supercapacitors with new high-voltage structures.In view of this,this paper proposes the method of laser processing PI film to generate porous graphene electrode to prepare planar high voltage supercapacitors（HVSCs）.A comprehensive and in-depth study is carried out on the laser micro nano manufacturing mechanism of graphene electrode materials,the influence of laser processing parameters on the performance of LIG electrode,interdigital electrode modeling and simulation analysis,and the structural design of high voltage supercapacitors,the main contents of this paper are as follows:（1）Laser micro nano fabrication mechanism of graphene electrode materialsTo explore the forming mechanism of porous graphene prepared by laser processing PI film,firstly,the thermal decomposition temperature of PI polymer layer was analyzed by thermogravimetric analysis.Secondly,the microstructure of LIG is characterized in pulse mode,and the evolution of surface material morphology and pore distribution in the spot area under different laser powers and diverse radiation durations are studied.Furthermore,Raman spectroscopy was used to detect the changes of peak intensity ratio on I_D/I_G and I_2D/I_G under different laser powers,and further analyze the disorder defect degree,steady-state sp²-C content and surface crystal size of LIG.Then,EDS/XPS is used to explore the element composition of the radiation region under different laser powers,the fracture and reorganization of C,O and N atomic bond energy.Finally,refining the thermal decomposition and transformation process of porous graphene materials,and the local reaction mechanisms of thermoforming zone,thermal melting zone and heat affected zone were summarized.（2）Modeling and Simulation of PI heat transfer and the influence of laser processing parameters on the manufacture of graphene electrode materialsThe heat transfer model of PI is established and COMSOL simulation of laser processing is carried out to explore the temperature field distribution on the surface of PI film and the reaction layer thickness in the direction of heat transfer.The micro morphology,pore distribution and surface roughness of LIG under different laser powers,scanning speeds,laser focal lengths and scanning spacings are investigated by using the control variable method.At the same time,the water contact angle and EDS element content of graphene electrode materials under different scanning spacings are analyzed.Based on the above research,the optimized laser processing parameters are used to prepare high-performance LIG.The materials are characterized by Raman spectrum detection,XPS elemental analysis and SEM characterization.In terms of performance,the surface 3D morphology contour scanning,dynamic water contact and conductivity are tested.In this paper,it is found that the optimal laser processing parameters are laser power 9.9%,scanning speed of 100mm/s,laser focal length of 0mm and scanning spacing of 100μm.the Raman peak intensity ratio of I_D/I_G corresponding to high-performance LIG is 0.3,the XPS detection percentages of C and O elements are 94.49%and 5.51%,respectively,the transient water contact angle is 13.3°（full penetration after 20s）,and the conductivity is 105.54S/m.（3）Mathematical modeling and simulation analysis of planar interdigital graphene based supercapacitorsThe mathematical models for capacitance and equivalent series impedance of planar interdigital supercapacitors are established,the local physical field and equivalent circuit diagram of cross section electrode are analyzed.The plane interdigital electrode capacitance is transformed into parallel plate electrode capacitance by conformal transformation method,and the resistance of gel electrolyte is solved by Ohm’s law and Maxwell equation,the resistance of different numbers of interdigital electrodes is solved by converting into series/parallel structure in terms of equivalent circuit.The functional expressions of interdigital capacitance and equivalent series impedance related to interdigital number N,length L_e,width W_e and spacing D_e are constructed.Based on the above mathematical model,the capacitance,equivalent series impedance and specific energy/power density of interdigital capacitor are simulated and fitted by MATLAB,and the variation trend and difference percentage between simulation values and experimental values are compared and analyzed.The results show that a single interdigital supercapacitor with dimension parameters of N=10,L_e=10mm,W_e=1.5mm and D_e=0.35mm can obtain high areal capacitance（2.43m F/cm²）,areal energy density（225μWh/cm²）and areal power density（3.65m W/cm²）.（4）Fabrication and electrochemical properties of planar high voltage supercapacitorsIn order to improve the high voltage and compactness of supercapacitors,the planar micro high voltage supercapacitors are fabricated by laser processing in twice.Among them,the laser etching lines with a width of only 300μm,used as series electrical connection channels,are made of conductive carbon materials irradiated from gel electrolyte layers.This method is capable of realizing the compact package of series high voltage supercapacitors（3mm×21.15mm）composed of square LIG electrodes.The electrochemical performance test shows that its working voltage reaches up to 10V.In the existing reported lig supercapacitors,it has the smallest electrode design area of 0.54cm² and the highest areal voltage of 18.52V/cm².To improve the high voltage and compactness of interdigital capacitors,a three-dimensional packaging method is proposed to prepare planar interdigital high voltage supercapacitors.Flexible and high conductivity ultra-thin phosphorus copper sheet with thickness of 40μm is used as the substrate and series electrical connection medium,and multiple interdigital supercapacitors are prepared by laser processing PI film on the upper and lower surfaces of phosphorus copper sheet.The series conduction of interdigital supercapacitors is realized through the phosphor copper sheet in the middle layer,and the compact series interdigital high voltage supercapacitors（overall thickness of 0.48mm）with three-dimensional packaging can be realized.The experimental results show that the tensile strength of interdigital high voltage supercapacitor is as high as 18.23Mpa,the working potential window is as high as10V,the capacitance retention under arbitrary bending or torsion is maintained at 97.78%,and it has an ultra-high areal energy density of 37.68μWh/cm² and areal power density of3.38m W/cm²,highlighting its application prospect in the field of flexible electronics and wearable devices.