Construction And Performances Study Of Ionic Liquid Based Supercapacitors

As a new type of clean energy storage device,supercapacitor has the characteristics of fast charge/discharge capability,high power density and ultra-long life.In most instances,it is used as the supplement or even the substitute of battery.However,comparing with batteries,the energy densities of supercapacitors is too low to meet the requirements of smart grid,rail transit and consumer electronics in the near future.Therefore,improving the energy density has become the research focus o f supercapacitors.As an important component of supercapacitor,the properties of electrolyte determine the device performance to a great extent.The current electrolytes of supercapacitor can be divided into aqueous electrolytes,organic electrolytes and ionic liquid electrolytes.Among these electrolyt es,the electrochemical stability of the ionic liquid is extremely high,which means wider electrochemical stability potential window,and that ensures higher energy density.In this thesis,we assembled supercapacitor by improving the properties of ionic liquid,so as to achieve the coexistence of high power density and energy density.Besides,the negative effect of trace water in humid ionic liquid on potential window is studied,and the mechanism of its action is clarified.Moreover,the ionic liquid an d electrode material with stable electrochemistry property was find out,and the supercapacitor with ultra-wide potential window was successfully assembled by further electrochemical passivation.More details are as follows:（1）Ionic liquids are considered to be the ideal electrolyte for high-energy-densit y supercapacitors due to their excellent electrochemical stability,but the high viscosit y and low ionic conductivity have limited their application in high-power output.In this work,we added small amounts of acetonitrile to optimize the viscosity and conductivity of EmimBF₄ while maintaining the excellent stable electrochemistry o f ionic liquid.The potential window of the device assembled with the optimized electrolyte can reach 3.2 V,which is much higher than that of the commercial ionic liquid electrolyte,but the rate capability is similar to that,leading high energy densit y at the high power output.More important,small addition of flammable ACN does not affect the flammability of the HCI electro lyte,which greatly guarantees its security in practical application.（2）Most ionic liquids are easily to absorb water from the surrounding environment,resulting in poor electrochemical performance.Thus,the storage conditions of ionic liquids are extremely severe and high cost,leading to the difficulty in industrialization.In this work,we added Li⁺into the ionic liquid EmimTFSI exposed to the open environment（which has reached water absorption saturation）.Through the solvation of Li⁺and water molecules,the activity of water has been eliminated,and the electrochemical stability window of the system was restored to the best without water.In addition,the effect of water on positive and negative potential was studied,and the mechanism of water action was clarified by Gromacs.（3）Electrochemical stability window has determined the energy density of a supercapacitor to a large extent.In this work,ionic liquid and electrode materials with high electrochemical stability were selected,and supercapa citor of 4.5 V was obtained.After that,the device was held at higher voltage for a period of time to passivate the electrode surface.After passivation,the device can reach 4.9 V with the energy density increase of 26%.In addition,the potential window of the device can be widened to 4.8v by negative pre-lithium,and the energy density can be increased by 20%.