Effect Of Room Temperture Ionic Liquid On Carbonate Electrolyte Structure And Performances

Ionic liquids,as salts or solvents,are composed of big cations and anions.In this paper,the compatibility of room temperature ionic liquids PP13*TFSI as solutes or solvents with electrodes has been systematically studied in the virtue of the duality of ionic liquids.Firstly,ionic liquids were used as solute salts to replace some lithium salts in concentrated PC and EC based electrolytes.The physical and chemical properties of electrolytes with different molar substitutions were studied.The experimental results show that the coordination number of Li ions in solution can be adjusted by substituting part of lithium salt with PP13*TFSI ionic liquid,and the formation mechanism of SEI film on graphite electrode surface can be changed.From SEM,TEM and FT-IR measurements,it can be seen that the performance of SEI film is greatly improved under the appropriate molar substitution amount,and the compatibility of PC electrolyte with graphite electrode is improved.The first Coulomb efficiency and long-term cycle performance are greatly improved.The optimum molar ratio is LiTFSI:PP13*TFSI=2:1 in 3M PC-based electrolyte and LiPF6:PP13*TFSI=3:1 in 1.8M EC-based electrolyte.More importantly,because the dielectric constant of EC is larger than that of PC,and the lithium salt anion(PF6-)and ionic liquid anion(TFSI-)used in EC are different,the SEI film formation mechanism on graphite electrode surface is also different.In PC-based electrolyte,the electrolyte systems 3 mol L-1 LiTFSI+PP13*TFSI(3:0,3:1,2:1,1:1,1:2)/PC are prepared,and the anion is only TFSI-.The addition of ionic liquids mainly affects the coordination number of Li ions,so the ratio of one electron and two electron reduction of solvent molecules is mainly regulated in the formation of SEI films.PC has low polarity and weak interaction with ionic liquid cations,so there is competitive migration between ionic liquid cations and solvated lithium ions.Because of the slow migration speed of solvated lithium ions,ionic liquid cations preferentially reach the electrode surface,which also hinders the strong reaction of solvent molecules on the electrode surface.In EC-based electrolyte,the electrolyte system 1.8 mol L-1 LiPF6+PP13*TFSI(1:0,4:1,3:1,2:1)/EC was prepared.Considering the larger polarity of EC,there are the interaction between EC molecules and the cations of ionic liquids,there was no competitive migration between the solvated Li ion and the cations of IL in the electrolytes with IL.More importantly,the introduction of ionic liquids changed not only the coordination number of lithium ions but also the coordination number of PF6-anion,so it is dominated to adjust the content between LiF and Li2CO3 in the formation of SEI film,which is from the reduction decomposition of solvent molecules and anions on the surface of the electrode.Secondly,considering the dual properties of room temperature ionic liquids,we use LiODFB as the solute salt,PP13*TFSI as the main solvent is mixed with linear carbonate solvent DMCS and the volume content of ionic liquids in the solvent is as high as 80%.The role of DMC is to increase the solubility of LiODFB in the electrolyte.The compatibility with LiFePO4 cathode is studied.We have prepared the electrolyte systems with different LiODFB concentration and different PP13*TFSI/DMC volume ratios.Among them,the electrochemical properties of three electrolyte systems,0.1 mol L-1 LiODFB/PP13*TFSI,0.4 mol L-1 LiODFB/PP13*TFSI-DMC(8:2),0.4 mol L-1 LiODFB/DMC,were studied and compared in detail.DSC and TG analysis showed that the thermal decomposition temperature of 0.4 mol L-1 LiODFB/PP13*TFSI-DMC(8:2)electrolyte system was lower than that of 0.1 mol L-1 LiODFB/PP13*TFSI,but the thermal decomposition peak became smaller,which indicated that the decomposition amount of solvent component decreased slightly with the increase of unit temperature.In terms of electrochemical performance,the addition of DMC increases the solubility of LiODFB in ionic liquid,and the first coulomb efficiency and long-term cycling performance of LiFePO4 electrode are the best in the electrolyte of 0.4mol L-1 LiODFB/PP13*TFSI-DMC(8:2),which indicates that the performance of SEI film is the best and the electrochemical performance of electrolyte is the best.The main reason is that the addition of DMC increases the solubility of LiODFB salt,i.e.the concentration of anionic ODFB-in electrolyte.XPS analysis showed that the increase of ODFB-concentration resulted in the change of the reaction products of electrolyte on the surface of LiFePO4 electrodes.In addition,SEM and TEM measurements showed that the effect of the two anions jointly participating in the film formation was better than that of either of them alone.The film thickness became thinner,and the film was relatively complete after 30 cycles.Moreover,the electrolyte of 0.4 mol L-1 LiODFB/PP13*TFSI-DMC(8:2)also exhibits good cycling performance at high temperature of 60?.Finally,PC-based electrolyte systems,such as 1.2 mol L-1 LiPF6/PC-DMC(3:7),1.2 mol L-1 LiPF6/PC-DMC(3:7)-3%FEC,1.2 mol L-1 LiPF6/PC-DMC(3:7)-3%FEC-3%LiODFB,1.2 mol L-1 LiPF6/PC-DMC(3:7)-3%LiODFB,were employed to work at 5V high potentials.LiNi0.5Mn1.5O4 material was prepared as cathode electrode.The results confirmed that PC-based electrolyte without any modify exhibits more excellent electrochemical properties than that in EC-based electrolyte reported in literature at high potentials,i.e.1.2 mol L-1 LiPF6/PC-DMC(3:7).PC solvent is promising substitution for EC at high voltage.