Research On Supercapacitors Based On Redox-active Hydrogel Electrolytes With Wide Operating Voltage

The non-renewability and high pollution of fossil fuels limit the use of energy resources by human society.Supercapacitors?SCs?have become a kind of energy storage devices with huge development potential due to its outstanding advantages such as high power density,long cycle life,fast charge and discharge,safety and environmental protection.However,compared with lithium-ion batteries,the disadvantage of supercapacitors is that the energy density is relatively lower.Therefore,the current research direction in the field of supercapacitors mainly focuses on increasing the energy density while maintaining the power density.According to the energy density formula E = 0.5 CV²,in order to increasing the energy density,it is necessary to increase the specific capacitance and the operating voltage of SCs.The introduction of a redox substance into an electrolyte has become a research hotspot.The redox reactions of redox substance occur at the electrode-electrolyte interface increase specific capacitance.This method is safe and easy to operate.In comparison with liquid electrolytes including organic electrolytes and ionic liquid electrolytes which exhibit the disadvantages of easy leakage,high corrosion and difficult packaging,hydrogel polymer electrolytes?HPEs?have greater application propect due to their advantages such as high ionic conductivity,easy encapsulation,no corrosion and safety.However,the decomposition voltage of water is about 1.23 V.It was found that the enhancement of working voltage of aqueous SCs could be achieved through the strong solvation effect of electrolyte ions or the formation of “water-in-salt”?WiS?electrolytes.In this paper,on one hand,the specific capacitance of SCs was increased by adding redox-active anions Br^-or I^-into HPEs,and on the other hand,the working voltage was broadened by introducing sulfates with strong solvation effect or high-concentration lithium bistrifluoromethanesulfonimide?LiTFSI?into HPEs.This article explored SCs from the following aspects:1.Ionic liquid N-butyl-N-methylpyrrolidine bromide(Pyr₁₄Br)was selected as the redox additive and plasticizer,and the wide working voltage of SCs was achieved by the strong solvation of sodium sulfate in neutral aqueous medium.A redox-active HPE which consisted of polyvinyl alcohol?PVA?,sodium sulfate?Na₂SO₄?and Pyr₁₄Br was prepared.and was assembled with two activated carbon electrodes to form a supercapacitor.The prepared PVA-Na₂SO₄-Pyr₁₄Br HPE has an elongation at break of 300% and an ionic conductivity of 27.1 mS cm^-1.The assembled symmetric supercapacitor can deliver a working voltage of 2.0 V.At a current density of 0.5 A g^-1,the specific capacitance of electrode reaches 266.6 F g^-1and the energy density reaches 33.0 Wh kg^-1(power density 459.2 W kg^-1).After 8000 galvanostatic charge/discharge tests,the capacitance retention rate reaches 81%,showing good cycle stability.2.A "water in salt"?WiS?electrolyte was prepared by taking advantage of the high solubility of lithium bistrifluoromethanesulfonimide?LiTFSI?.In this system,based on 5 M LiTFSI,PVA-LiTFSI matrix was prepared by sol-gel method,and then PVA-LiTFSI-KBr HPE with the maximum ionic conductivity of 43.3 mS cm^-1was prepared by doping a small amount of potassium bromide?KBr?into the above matrix.The PVA-LiTFSI-KBr HPE assembled with two activated carbon electrodes into a supercapacitor,which possessesthe operating voltage of 2.0 V.At a current density of 0.5 A g^-1,the specific capacitance of electrode reaches 267.2 F g^-1and the energy density is 35.4 Wh kg^-1(the power density is 533.7 W kg^-1).Moreover,after 5000 galvanostatic charge/discharge cycles,the capacitance retention rate was 81%,indicating good cycle stability.3.In order to further increase the working voltage,10 M LiTFSI solution was prepared,and then the PVA-LiTFSI matrix was synthesized by a sol-gel method.On the other hand,carbon nanotubes?CNTs?were dispered into ionic liquid 1-butyl-3-methylimidazole iodide?BMIMI?.The PVA-LiTFSI-BMIMI-CNTs HPE was then prepared by the addition of CNTs/BMIMI dispersion into the maxtrix,which can improve the ionic conductivity and produce redox reactions.The PVA-LiTFSI-BMIMI-CNTs HPE was assembled with two activated carbon electrodes into a supercapacitor.The ionic conductivity of optimized HPE is 47.5 mS cm^-1.The working voltage of the constructed SC can reach 2.1 V.At a current density of 0.5 A g^-1,the specific capacitance of electrode reaches 292.2 F g^-1and the energy density is 40.6 Wh kg^-1(the power density is 355.84 W kg^-1).Furthermore,at the end of 5000 galvanostatic charge/discharge cycles,the capacitor retention rate is 90%,demonstrating outstanding cycle life.