Research On High Voltage Organic Electrolyte For Supercapacitors And Lithium Ion Batteries

In order to improve the energy density of supercapacitors and lithium-ion batteries,this paper systematically studies the corresponding high-voltage electrolyte system.The ionic liquid EMIBF4(EMI)combined with propylene carbonate(PC)and 1,2-dimethoxyethane(DME)was selected for supercapacitors(EDLCs).Study the influence of auxiliary solvents on the physical and chemical properties of ionic liquids,and the influence of new electrolyte systems on capacitor capacitance.Introduce two fluorinated ethers,OTE and TFEE,for high-voltage electrolyte of lithium-ion batteries.Study the effect of the synergy between fluorinated solvents on the physical and chemical properties of the electrolyte and the electrochemical properties of lithium-ion batteries(LIBs).The main research results and conclusions obtained are as follows:(1)Compared with pure EMI,PC or PC-DME-based electrolyte has higher ionic conductivity and can provide better power characteristics,but at the expense of a stable potential window.When the cut-off voltage is 3 V,the energy density of a supercapacitor containing activated carbon and EMI-PC-DME electrolyte at 900 W/kg is 38.5 Wh/kg.(2)Studies have found that the electrolyte composed of OTE and fluoroethylene carbonate(FEC)has an oxidation stability of more than 5 V(vs.Li+/Li).At a cut-off voltage of 5 V,after 200 cycles,the capacity retention rate of the LiNi0.5Mn1.5O4/Li battery is as high as 92%,which is greater than the conventional electrolyte and the "monofluorinated"electrolyte with only OTE added.SEM,TEM and XPS tests show that in the "double-fluorinated" electrolyte,there is a uniform and F/P-rich surface film in the circulating LiNi0.5Mn1.5O4 cathode,which may be due to the synergy of the two fluorinated solvents under high voltage Effect.(3)Simulation calculations show that in the "perfluorinated" electrolyte composed of TFEE and two fluoroethers,the introduction of TFEE significantly changes the lithium-ion solvation structure and affects the lithium ion desolvation process.In addition,the fluorinated solvent has a lower HOMO value,which can synergistically improve the oxidation stability of the electrolyte.In the high-voltage LiNi0.5Mn1.5O4 cathode,the perfluorinated electrolyte(1 mol LiPF6/FEC+FEMC+TFEE)has a capacity retention rate of about 85%after 300 cycles at a cut-off voltage of 5 V,which is far higher than other electrolyte.At a cut-off voltage of 4.45 V,the capacity retention rate of the LCO/MCMB battery with FFT is about 84%after 100 cycles.SEM,EIS and XPS tests show that the new electrolyte system FFT will not only form a uniform CEI film on the surface of the positive electrode,but also preferentially participate in the formation of the SEI film on the surface of the negative electrode.The oxidative stability of the battery is enhanced,the interface impedance is reduced,and the irreversible reaction between the electrolyte and the electrode is inhibited from continuing to occur during a long cycle.