Reducing The Self-discharge Of Supercapacitors By Electrolyte Additives

With the rapid development of electronic products and electric vehicles,portable energy storage technology has attracted increasing attention.Supercapacitors have become indispensable electrochemical energy storage devices because they can be charged and discharged rapidly.However,the practical applications of supercapacitors are limited by their relatively low working voltages.Compared with batteries,the energy density of supercapacitors is low,and there is still a self-discharge situation that cannot be ignored.Self-discharge is generally caused by ohmic leakage,charge distribution and faradaic reactions,while faradaic reaction process can be divided into activation-controlled faradaic reaction(ACFR)or diffusion-controlled faradaic reaction(DCFR)process.In this work,we added liquid additives into the electrolyte of supercapacitors to improve the working voltage window and reduce selfdischarge.1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether(TTE),1,1,1,3,3,3-hexafluoroisopropyl methyl ether(HFPM),and bis(2,2,2-trifluoroethyl)ether(BTFE)were selected as the electrolyte additives.The use of ether additive for electrolytes provides a new idea for improving the working voltage window of supercapacitors and suppressing self-discharge.The work of this thesis consists of the following two parts:(1)TTE was added to the electrolyte containing acetonitrile and salt solution for supercapacitors.Electrochemical tests of the supercapacitors with and without TTE in the electrolyte were carried out.The working voltage of the supercapacitors containing TTE electrolyte was increased to3.6 V with a relatively low leakage current of 118 μA.After selfdischarging for 24 h,the open circuit voltage(OCV)of the supercapacitor remained above 1.48 V.Compared with the supercapacitors without TTE in the electrolyte,the OCV drop was reduced by 0.4 V.Based on the analysis of self-discharge mechanism of supercapacitors,it is found that the reason for the decrease of self-discharge rate is because the addition of TTE inhibited the decomposition of the electrolyte and reduced faradaic reactions controlled by activation.(2)With a high charging voltage of 3.6 V,HFPM and BTFE were compared with TTE as electrolyte additives.Electrochemical tests showed that the leakage current decreased following the order of BTFE > TTE >HFPM.Other performance tests,such as rate performance,cycle performance and impedance test indicated good performance of BTFE supercapacitors.This result suggests that the addition of BTFE can further improved the electrochemical performance of the supercapacitors while maintaining the high working voltage.