Studies On Preparation Of Ionic Liquid-Based Electrolytes And Supercapacitor Performance

Electrochemical double layer capacitors（EDLCs）have attracted much attention due to their fast charge-discharge kinetics and long cycle life,but its large-scale application is greatly limited by the low energy density.Ionic liquids（ILs）can be used in EDLCs to improve the working voltage and energy density of capacitors and improve the safety performance with the advantages of wide temperature range,non-volatile,non-flammability and good electrochemical stability.Based on this,a novel double ILs voltage-resistant liquid electrolyte and IL-based flexible quasi-solid electrolyte system were designed and constructed from the perspective of optimizing the electrolyte.The structure-activity relationship of the electrolyte and its influence on the electrochemical performance of the capacitor were studied.The application value and prospect of the two electrolytes in EDLCs were discussed.The main contents and results are as follows:（1）1-ethyl-3-methylimidazolium bistrifluoromethanesulfonyl imide salt（EMITFSI）and N-methylbutylpyrrole bistrifluoromethanesulfonyl imide salt(PYR₁₄TFSI)were used to blend and modulate the double ILs liquid electrolyte.The stable potential window of the electrolyte was optimized by adjusting the ratio of ILs.When the content of PYR₁₄TFSI was 5 wt%,the ionic conductivity and viscosity of the electrolyte were moderate,and the stable potential was up to-4～4 V.The capacitance retention rate of EDLCs after 5000 cycles in the voltage window of 0～3 V reached 94.1%,and the calculated maximum energy density was up to 72 Wh/kg.（2）Construction of ionic liquid based quasi-solid electrolyte.A simple and stable quasi-solid electrolyte was prepared based on ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate（EMIBF₄）and polymer polyvinylidene fluoride hexafluoropropylene（PVDF-HFP）.Its ionic conductivity（7.2 ms/cm）and toughness reached a reasonable quasi-solid electrolyte was prepared based on ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate（EMIBF₄）and polymer polyvinylidene fluoride hexafluoropropylene（PVDF-HFP）.Its ionic conductivity（7.2 ms/cm）and toughness reached a reasonable balance.At the upper voltage of 3 V,after 10000 cycles,the capacity retention rate and energy density of EDLCs assembled by PE3 electrolyte can reach～100%and 32.6 Wh/kg.（3）PVDF-HFP and EMIBF₄ in the quasi-solid electrolyte were pre-doped into activated carbon electrode to optimize ion transport path and enhance interfacial compatibility.The use of moderate content of PVDF-HFP（25 wt%）in the electrode can promote IL absorption,while excessive PVDF-HFP will cause the destruction of electrode structure.After the electrode was infiltrated by EMIBF₄,the equivalent series internal resistance decreased and the cycle stability was improved.