The Synthesis And Electrochemical Performance Of High-voltage Electrolyte For Supercapacitors

Supercapacitors are widely used in military industry,aerospace,rail rail transportation and other fields due to their high power characteristics,long cycle life and wide application temperature range.However,compared with lithium ion batteries,supercapacitors have low energy density,which limits their application as a single energy storage device.Therefore,supercapacitors need to greatly increase the energy density of their unit while maintaining high power and long cycle life.Improving specific capacitance and working voltage are the main ways to increase the energy density of supercapacitors.From the perspective of the electrolyte,this paper designs and synthesizes a series of new high-performance electrolyte salts for supercapacitors based on the introduction of functional anions in small ion size cations.The electrochemical properties of the electrolyte such as working voltage,energy density,power density and cycling performance were evaluated.Moreover,the relationship between electrolyte ion size,electrolyte concentration and energy contribution of anode and cathode was also discussed.The spiro-?1,1'?-bipyrrolidinium bisfluorosulfonylimide?SBP-FSI?salt was successfully prepared,the synthesis procedure was simple,and no organic solvent was involved in the synthesis process,which met the requirements of green synthetic chemistry.SBP-FSI was dissolved in PC solvent to form 1 mol/L SBP-FSI/PC electrolyte.Through the characterization of physicochemical properties and electrochemical stability,it was found that 1 mol/L SBP-FSI/PC showed higher electrical conductivity,lower viscosity,and a broader electrochemical window than commercial electrolyte.However,the aluminum corrosion test found that SBP-FSI has a significant corrosive effect on the aluminum foil current collector,and the experiment found that the low temperature could significantly inhibit the corrosion of the electrolyte to the aluminum foil.The supercapacitor based on 1 mol/L SBP-FSI/PC showed excellent electrochemical performance at-40?.Moreover,the new electrolyte can withstand a high operating voltage of 3.2 V at-40?,which greatly enhances the energy density and power density of the supercapacitor.The maximum energy density and power density can reach 42.67 Wh/kg and 5951 W/kg,respectively.The SBP-FSI/AN electrolyte with a high concentration of 4 mol/L exhibits similar physicochemical properties with ionic liquids,such as high thermal stability,non-volatilization and non-combustibility,but does not inherit the poor ionic mobility of ionic liquids.The conductivity of SBP-FSI/AN with 4 mol/L is as high as 39.8 m S/cm.The negative electrochemical window of SBP-FSI/AN electrolyte increased with the increase of concentration,while the positive electrochemical window remained unchanged.The positive and negative electrochemical windows of the 4 mol/L SBP-FSI/AN tend to be symmetrical,thus facilitating the full utilization of the entire electrochemical window for supercapacitor devices.After testing the three-electrode and two-electrode systems,the maximum safe working voltage of the supercapacitor assembled with 4 mol/L SBP-FSI/AN electrolyte is 3.4 V,which greatly improves the energy density of the supercapacitor.Maximum energy density and power can reach53.83 Wh/kg and 15.1 k W/kg,respectively,which is well balanced in the demand of high power density and high energy density.The spiro-?1,1'?-bipyrrolidinium difluoro?oxalato?borate?SBP-DFOB?salt was successfully synthesized and a suitable route for industrial production was explored.The physical and chemical properties of the new electrolyte salt were systematically characterized.It was found that the thermal stability of the new electrolyte salt was good,the initial decomposition temperature was 295.7?and the melting point was low,close to room temperature.SBP-DFOB was dissolved in PC solvent,1mol/L SBP-DFOB/PC exhibited higher conductivity than commercial electrolyte and 1mol/L SBP-BF₄/PC.The simulation combined experiment also proved that 1mol/L SBP-DFOB/PC possess the optimal electrochemical stability.1mol/L SBP-DFOB/PC was assembled into a symmetrical supercapacitor to test its electrochemical performance and withstand voltage characteristics,and the 1mol/L TEA-BF₄/PC and 1mol/L SBP-BF₄/PC were compared.Through cyclic voltammetry,charge and discharge,rate and long cycle tests at different voltages,we found that the maximum specific capacitance of SBP⁺based electrolyte was significantly higher than that of commercial electrolyte,indicating that electrolyte with smaller cationic size could achieve higher capacity.In addition,the maximum specific capacitance of 1 mol/L SBP-DFOB/PC and 1mol/L SBP-BF₄/PC electrolyte is almost the same,reaching 120 F/g,indicating that the replacement of DFOB^-anion hardly affects the electrode capacity.Finally,the 1mol/L SBP-DFOB/PC electrolyte proved to have the best voltage resistance among the three electrolytes.Based on the 1mol/L SBP-DFOB/PC,the supercapacitor can reach 3.5 V,the maximum energy density can reach 50.06 Wh/kg,and the maximum power density is 28.54 k W/kg,greatly improving the energy density and power density of the supercapacitor.Three kinds of DFOB^--based ionic liquids with different cationic sizes(Py₁₃DFOB,Py₁₄DFOB and Py₁₅DFOB)were successfully synthesized.Their melting points were 3?,-5?and-74?,respectively.With the increase of cationic size,the melting points decreased successively.Based on the electrochemical test results of three electrodes and two electrodes,the 1mol/L Py₁₃DFOB/PC can withstand the working voltage of 3V and the specific capacity can reach 130 F/g.Due to the increase of specific capacitance and voltage,the energy density and power density are further improved,and the maximum energy density and power can reach 39.5 Wh/kg and 14.52 k W/kg respectively.By changing the chain length(Py₁₃DFOB,Py₁₄DFOB and Py₁₅DFOB),we explore the effect of ion size on the positive and negative potential changes.It was found that,at a concentration of 1 mol/L,the ion size had little effect on the change trend of the positive and negative electrodes,and the potential changes of the positive electrode and the negative electrode at different voltages were completely symmetrical.While the cation size changes the overall capacity,the larger the ion size,the smaller the capacity.Exploring the influence of concentration(Py₁₃DFOB/PC)on the change of positive and negative potential,we found that there is a significant difference in the trend of positive and negative potential change in the solution system with higher concentration,especially in the pure ionic liquid system,the difference of voltage change in positive and negative electrode is the most dramatic.This may be due to the difference in the force exerted on the anions and cations in the dilute solution and the concentrated solution system?the force includes the repulsion between the same ion and the attraction between the opposite ions?.The difference of the force of anion and cations in concentrated solution causes the difference of ion diffusion rate,which leads to the uneven storage distribution of positive and negative electrode capacity per unit time,thus the positive and negative electrode potentials vary unevenly.