Design Of Ionic Liquid-based Electrolyte For High Safty Lithium/lithium-ion Batteries And Its Properties

The development of human society has accompanied by an increase in energy consumption per unit of individual.The development of high-energy-density secondary batteries is an important solution for the current energy and environmental issues,especially for the current‘Big Cities’problems in China.Lithium/lithium-ion batteries are considered to be the most promising secondary batteries to aready commercialized and commercialized,respectively.At present,security issue is one of the bottlenecks restricting its further development,especially in the field of electric mobility.An important reason for the frequent safety problems in the field of electric vehicles is the common use of carbonate-based organic electrolytes with flammability.Therefore,the development of electrolytes for high-safety lithium/lithium-ion batteries are of great significances.In recent years,ionic liquid-based electrolytes possess excellent properties such as no flash point,difficulty in volatilization,and strong electrochemical stability have attracted attentions.In here,the ionic liquid electrolytes solvent system is used as the research objects,and two types of ionic liquids of pyrrolidine and piperidine with high electrochemical stability and wide electrochemical window are designed and prepared.This ionic liquid is used as solvent,additive or plasticizer.The agent developes a number of electrolytes and studies its performance in lithium/lithium-ion batteries.This research mainly includes the following aspects:1.By introducing an oxygen atom into the pyrrolidine cation side chain,an ether-functionalized ionic liquid 1-methyl-1-ethylether bisfluoromethanesulfonimide salt(PYR_1（2o2）TFSI)is obtained,and an PYR_1（2o2）TFSI-LiTFSI electrolyte is prepared,which possesses low viscosity and excellent ion transport capacity,and the electrolyte is applied to Li/LiFePO₄（LFP）batteries.The LFP battery is used as the test battery,and PYR_1（2o2）TFSI-LiTFSI is used as an electrolyte.The experimental results show that the charge/discharge performances of LFP batteries are significantly enhanced both at room temperature and high temperature（60 ^oC）.The Electrochemical Impedance Spetroscopy（EIS）and chronoamperometry of PYR₁₄TFSI-LiTFSI and PYR_1（2o2）TFSI-LiTFSI electrolytes show that the ionic conductivity at room temperature are 1.18×10^-3 S cm^-1 and 1.40×10^-3 S cm^-1,respectively,and the lithium-ion migration numbers are 0.115 and 0.222,respectively.2.In order to overcome the high viscosity of ionic liquid,we have added appropriate amount of dimethyl carbonate（DMC）organic solvent blending in PYR_1（2o2）TFSI-LiTFSI electrolyte to improve the performances of the electrolytes,and investigated the ratio between organic solvent and PYR_1（2o2）TFSI.The results show that when the volume proportions of dimethyl carbonate（DMC）is 20%,the PYR_1（2o2）TFSI/DMC（8:2）-LiTFSI electrolyte possesses the best electrochemical performance,and the ionic conductivity and lithium-ion migration number reaches 3.02×10^-3 S cm^-1 and 0.378,respectively.We have applied it in LFP batteries,the specific capacity of LiFePO₄ can retain 116 mAh g^-1 at room temperature at 1 C.In addition,the specific capacity of the battery can reach 160 mAh g^-1（0.1 C）at high temperature（60 ^oC）and it exhibits excellent cycle performance.3.To overcome the leakages of liquid electrolytes and the risk of dendrites,we have designed and prepared a polypyrrolidinium ionic liquid（PIL）gel electrolyte with PYR_1（2o2）TFSI as plasticizer,and the effects of the proportion of plasticizer on the electrochemical performance of the electrolyte are investigated.The results show that when the mass ratio of PIL/PYR_1（2o2）TFSI is 1:1,the electrolyte exhibits the best combinations of mechanical properties and electrochemical properties.Comparing with commercial liquid electrolyte,the gel electrolyte has high thermal stability and lithium-ion migration number（0.3at room temperature,0.674 at 60 ^oC）.Meanwhile,the capacity of LFP battery with this electrolyte has no decline but a slight increase after 100 cycles at 0.1 C at 60 ^oC.The reversible specific capacity of LiFePO₄ is maintained at about 149 mAh g^-1,and the discharge specific capacity of 95 mAh g^-1 can be maintained even in the case of 1 C large rate galvanic test.These results show that the comprehensive performance of the ionic liquid-based gel electrolyte is significantly better than that of the commercial electrolyte,even reaching a practical level.4.Considering the high cost of ionic liquids as the main solvent and the practical problems of metallic lithium anodes.We have designed the unique ionic liquid Pp₁₃FSI as a lithium battery electrolyte additive to improve the lithium negative interface environment through the synergistic effects of cation and anion in the ionic liquid Pp₁₃FSI.The results show that when the adding amount of Pp₁₃FSI is 1 wt%,the Li/Cu battery test exhibits excellent coulombic efficiency and cycle stability.After analyzing the lithium electrode after the cycle test,it is found that adding an appropriate amount of ionic liquid can significantly improve the lithium metal electrode.X-ray photoelectron spectroscopy（XPS）and in-situ charge-discharge observation analysis confirms that Pp⁺₁₃ promotes the formation of the rough solid electrolyte interface（SEI）film on the surface of lithium,which attribute to the cations decomposed on the surface of lithium metal anode to produce Li₃N,and further effectively inhibit lithium electrode dendritic growth and slowing down the formation of dead lithium.The research results provide an effective solution for the practical application of lithium metal secondary batteries.This thesis has developed a number of high-performance ionic liquid-based electrolytes for lithium/lithium-ion batteries.Through a series of tests and characterizations,the mechanism of action of ionic liquids is research,these provide practical experience and theoretical guidance for the development of high specific energy and high safety lithium/lithium-ion secondary batteries.