Study On The Performance Ofionic Liquids As Electrolytes For Lithium-ion Batteries

Ionic liquids have the advantages of high conductivity,low melting point,wide electrochemical window,almost non-volatile,non-combustible,and high thermal stability,and have shown good application prospects in the field of battery electrolytes.In this paper,1-butyl-3-methylimidazole hexafluorophosphate(BMIMPF6)and 1-ethyl-3-methylimidazole dicyandiamide salt(EMIMDCA)were synthesized,and the two ionic liquids were prepared based on electrolytes and polymer electrolytes,their performance as lithium-ion battery electrolytes was investigated.The BMIMPF6 and EMIMDCA ionic liquids were synthesized using a two-step method.They have a low melting point,a wide liquid range,and a low viscosity.Among them,the viscosity of EMIMDCA at 25? is 21cP;their electrical conductivity at normal temperature is 2.34×10-3S/cm and 23.3×10-3S/cm,the change of conductivity with temperature conforms to the Arrhenius equation;their electrochemical windows are 4.4V and 3.3V,respectively,and the electrochemical stability is high.These two ionic liquids are potentially excellent electrolyte materials.LiClO4+BMIMPF6 and LiClO4+EMIMDCA electrolytes were prepared.The addition of lithium salt increased the electrolyte viscosity and reduced the conductivity,but in the range of lithium salt concentration examined,the conductivity was all 10-3S/cm of the order of magnitude.An electrolyte with a lithium salt concentration of 0.8mol/L was selected to assemble a Li/LiCoO2 battery for testing.It was found that the electrolyte has good compatibility with the positive electrode material,after 20 charge and discharge cycles,it can still maintain a higher battery capacity.Then,N,N-dimethylformamide was selected as the solvent and polyvinylidene fluoride hexafluoropropylene(PVDF-HFP)as the matrix,and the solution casting method was used to solidify the LiClO4+BMIMPF6 and LiClO4+EMIMDCA electrolytes—a white,translucent,self-supporting membrane-like solid electrolyte.The two solid electrolytes obtained are non-flammable,have microporous structures on the surface of the membrane,are resistant to bending and stretching,have good mechanical properties.The content of ionic liquids has an important influence on the electrochemical performance of solid electrolytes.The introduction of ionic liquids reduces the crystallinity of the polymer matrix,increases the amorphous area,and improves the conductivity of solid electrolytes.For example,GPE-5 solid electrolytes(EMIMDCA:LiClO4:PVDF-HFP=6:1:3,mass ratio)conductivity is 0.184×10-3S/cm,but excessive ionic liquid will reduce the mechanical properties of the membrane.Finally,two solid electrolytes,GPE-2(BMIMPF6:LiClO4:PVDF-HFP=6:1:3,mass ratio)and GPE-5,were selected to assemble into Li/LiCoO2 battery.It was found that the electrolyte is compatible with the cathode material well,after 20 charge and discharge cycles,it can still maintain a higher battery capacity.The electrolyte or solid electrolyte based on ionic liquid prepared in this paper establishes a certain experimental and theoretical basis for promoting the application of ionic liquid in the electrolyte of lithium ion batteries.