Study And Optimization On LiPF₆-PP₁₃TFSI/EC+DEC Electrolytes

Ionic Liquids, known as the room temperature molten salt, aremade up of specific cations and anions. They exhibit the liquid stateat room temperature or near the room temperature with a low vaporpressure, high conductivity, good thermal and electrochemicalstability. When they are used as the lithium salt solvents in Li-ionbattery electrolytes, the combustion, explosion and other safetyhazards, due to the use of the flammable and volatile organic solvents,may be eliminated. However, there also exit some problems such aspoor compatibility with solid electrodes, ineffective SEI films, poorlithium ion migration when the ionic liquids are used as solvents in theelectrolyte. But it is possible to improve the above-mentionedproblems when a certain amount of organic solvents with lowviscosity are added into the ionic liquid/lithium salt system. Therefore,we investigate the solvent ratio and lithium salt content on theLiPF₆-PP₁₃TFSI/organic solvent （EC: DEC=1w:1w） electrolytes.Moreover, the electrochemical characteristics and mechanisms about the mixed electrolyte are analysed.1. Analysis and optimization on LiPF₆-PP₁₃TFSI/EC+DEC electrolytewith different ratio of solventsFirstly, we analysis and evaluate the flammability and theelectrochemical stability of the mixed electrolytes, whoseconductivities and lithium deposition-dissolution efficiencies are alsocharacterised. Then, the assembled batteries and theelectrolyte/electrode interfacial properties are evaluated usinggalvanostatic charge-discharge method, cyclic voltammetry,electrochemical impedance spectroscopy and scanning electronmicroscopy-energy dispersive spectrometry technology. Theperformance of carbon microbeads （CMB） anode in the60wt.%PP₁₃TFSI-containing mixed electrolyte shows151.8mAh/g capacitiesand a coulombic efficiency of90.0%during the fifth cycle.2. Effect of lithium content on the mixed electrolytes and itsinfluencing mechanismThe performances of CMB/Li batteries using the mixedelectrolytes with0.2,0.6,1.0mol/kg LiPF₆have been compared. Theresults indicate that CMB/Li battery with0.6mol/kgLiPF₆-PP₁₃TFSI/EC+DEC has better interfacial properties and shows asmallest value of interface impedance after10cyclic voltammetrycycles. At the same time, the fifth charge-discharge capacities about CMB electrodes are210mAh/g and200mAh/g, respectively.Furthermore, IR spectra shows that, with the increase of LiPF₆contents, the continuous broadening of-SO2absorption and the smallred shift of-SO2peak can be observed, and the addition of LiPF₆produces a new absorption, which is associated to the S-N-S and-CF3groups in the region of700-500cm-1. This implies that there is astrong interaction between Li+and TFSI-, which may be the rootreason decreasing lithium ion transfer number with the growth oflithium salt content, and thereby affect the performances of thebatteries.