Double Layer Structure Of Electrode/Ionic Liquid-lithium Salt Mixtures Studied By Differential Capacitance Measurements

Ionic liquids (ILs), which are completely composed of anions and cations, areliquid salts below100℃. Due to their excellent properties, such as low vapor pressure,good thermal stability, wide electrochemical window as well as favorable ionicconductivity, ionic liquids are widely used as new solvent systems in the field ofbatteries, electro-deposition, electro-synthesis, supercapacitors and so on. As theelectrolyte of secondary lithium-ion batteries, ionic liquids have the potential toenhance the thermal stability and the safety of the whole system. However, most ofthe ionic liquids have lower conductivity compared to traditional eletrolyte solvent,while mixing with lithium salt is an effective method to enhance the conductivity. Forthe application of electrolytes in lithium-ion batteries, it is necessary to introducelithium salt in ionic liquids. Therefore, mechanism of lithium ion arrangement atelectrolyte/electrode interface, where electrochemical behaviors such as electrontransfer are usually taken place, is practically important. Thus, the study of interfacialproperties of electrode/mixture containing lithium salt and ionic liquid is an importantfoundation to understand and improve their performance in lithium-ion batteries.In this study, interfacial structure of double layer between solid electrodes andseveral mixtures of imidazolium-based ionic liquids and lithium salt is investgated byelectrochemical methods including cyclic voltammetry (CV) and electrochemicalimpedance spectroscopy (EIS).Firstly, differential capacitance and cyclic voltammograms measurements havebeen carried out for gold electrode in mixture of1-butyl-3-methyl imidazoliumtrifluoromethanesulfonate ([Bmim][OTf]) and lithium trifluoromethanesulfonate(Li[OTf])compared with the pure ionic liquid. Variation of differential capacitancecurves is obtained by changing the direction of potential scan andtemperature.Hysteresis has been observed from differential capacitance curves both inpure ionic liquid and the mixture. PZC is defined according toKornyshev’s theory.Besides, schematic illustrationof the interfacial electric double layer is discussed onthe basis of differential capacitance curves. Secondly, the effect of anion size is studied by changing the lithium salt.Mixtures containing LiCl， LiBF4and Li[TFSI] in [Bmim][OTf] have beenpreparedrespectively. Electrochemical window is determined by CV. EIS is carriedout within the electric double layer region. Variation of differential capacitancecurvesis obtained by changing the direction of potential scan and temperature. Results of thethree mixtures are dicussed by comparing with [Bmim][OTf]+Li[OTf] mixture. Theanion size has an effect on differential capacitance,fature of differential capacitancecurves,PZC and structure of double layer.Thirdly, the effect of cation size is studied by introducing ionic liquids withdifferent alkyl group：1-ethyl-3-methyl imidazolium trifluoromethanesulfonate([Emim][OTf])and1-octyl-3-methyl imidazolium trifluoromethanesulfonate([Omim][OTf]). The electrochemical window and electric double layer region of[Emim][OTf]+Li[OTf] and [Omim][OTf]+Li[OTf]are determined by CV, then EIS isperformed in the electric double layer region. The PZC, influnce of potential-scaningdircetion and temperature, as well as the hysteresis effect is studied on the basis ofdifferential capacitance curves and compared with [Bmim][OTf]+Li[OTf].Finally, Pt and GC are used as working electrode to investigate the effect ofelectrode material on the doubly layer structure. Compared with the interface ofAu/[Bmim][OTf]+Li[OTf], Pt/[Bmim][OTf]+Li[OTf] andGC/[Bmim][OTf]+Li[OTf]present different featurs in their differential capacitancecurves. Influnce of potential-scaning dircetion and temperature, hysteresis effect, andstucture of electric double layer is discussed.Systematically studies of the interficial double layer structure betweem solidelectrode and ionic liquid-lithium salt mixtuers not only deepen the understanding oftheirfundamentalelectrochemistry knowledge, but also provide important informationfor their application in various electrochemical devices.