Abstract:As a new type of "green solvent", due to its low vapor pressure, thermal stability and good safety, environmental friendly, ease to be designed and other unique characteristics, ionic liquids (ILs) can be advantageous over the traditional organic solvents. The application of ILs for the electrolyte of the lithium-ion battery is expected to solve the problem of its high temperature safety. However, because of high viscosity, high reduction potential and poor stability of carbone anode material, there are still a lot of basic research work to be done. In addition, study on the corrosion behavior of ILs can be extended as a new green inhibitor in the industrial field.This work was focused on how to solve the high viscosity and high reduction potential of iminazole ILs in lithium-ion secondary battery and investigate the corrosion inhibition performance of benzotriazole ILs. The main points in this research can be summarized as following.To reduce the viscosity of pure1-butyl-3-methyl-imidazolium tetrafluoroborate (BMIMBF4), y-Butyrolactone (GBL) organic diluent was adopted. Viscosity and ionic conductivity of the binary mixture in the presence and absence of two lithium salts, lithium difluoro (oxalato) borate (LiODFB) and lithium tetrafluoroborate (LiBF4), were systematically studied. The results indicated that the mixture of BMIMBF4/GBL in volume fraction of30/70containing lithium salt exhibited the highest conductivity. Effect of temperature on ionic conductivity of four electrolytes was in the order of BMIMBF4+1mol·L-1LiODFB>BMIMBF4+1mol·L-1LiBF4>BMIMBF4/GBL (30:70)+1mol·L-1LiODFB>BMIMBF4/GBL (30:70)+1mol·L-1LiBF4.Electrochemical impedance spectroscopy, cyclic voltammograms and charge-discharge performance testing results showed that interface stability between BMIMBF4and Li metal could be improved by using LiODFB, two membrane forming additives including vinylene carbonate (VC) and fluoroethylene carbonate (FEC). The cell of LiFePO4/Li in BMIMBF4/GBL(30:70)+1mol·L-1LiODFB electrolyte had the best charge and discharge performance. At0.1C rate, the discharge capacity of the first and20th cycle was138.8and138.3mAh·g-1at room temperature, respectively. At60℃the cell had the initial discharge capacity of149mAh·g-1and20th cycle discharge capacity of161.1mAh·g-1. LiFePO4/Li in VC modified BMIMBF4/GBL(30:70)+l mol·L-1LiBF4electrolyte showed the better charge and discharge performance than that of FEC modified. The LiFePO4containing VC delived the initial capacity of123.1mAh·g-1and20th cycle discharge capacity of129.1mAh·g-1at room temperature. While the discharge capacity of the first and20th cycle was150.5and143.2mAh·g-1at60℃, respectively.The results from electrochemical impedance spectroscopy, potentiodynamic polarization curves and weight loss method consistently displayed that three kinds of benzotriazole ILs, namely1,3-diethylbenzotriazole bromide (C2BtEtBr),1-butyl-3-ethylbenzotriazole bromide (C4BtEtBr),1,3-dibutyl benzotriazole bromide (C4BtBuBr), could inhibit the corrosion of15Mn steel in5wt.%HCl solutions and indicated that they were mixed-type inhibitors. The inhibition efficiency increased with the increase in the concentration of inhibitor, whereas the inhibition efficiency decreased with the temperature increased. The inhibition efficiency of the same concentration was determined as in the order of C4BtBuBr>C4BtEtBr>C2BtEtBr. The adsorption model obeys Langmuir adsorption isotherm at303K.The associated activation energy of corrosion (Ea) and other thermodynamic parameters such as enthalpy of activation (△H), entropy of activation (△S), adsorption equilibrium constant (Kaads) and free energy of adsorption (△Gads) were calculated to elaborate the mechanism of corrosion inhibition. Higher values were obtained for Ea and△H in the presence of inhibitors indicating the higher protection efficiency for these inhibitors. There was also a parallelism between the increase in inhibition efficiency and the increase of Ea and AH values. On the other hand, AS values in the absence of the inhibitor were found to be negative, whereas AS values were changed to be positive with the increase of the concentration of the studied inhibitors, due to molecular hydrogen representing more orderly arrangement.Kads values were found to decrease with the temperature increased. The negative values of△Gads (-40kJ·mol-1<AGads<-20kJ·mol-1) indicated that the adsorption of the benzotriazole molecule is a spontaneous process. The inhibition effect of the compound on the corrosion can be attributed to the complex physic-chemical interaction between the cationic inhibitor molecules and the carbon steel surface, dominantly electrostatic interaction. |