Study On Hybrid Electrolytes Containing Ionic Liquids For Lithium Second Batteries

The energy density of lithium-ion batteries using graphite negative electrode has approached its theoretical upper limit(250 Wh kg^-1).Researchers are paying more and more attention to lithium metal batteries（LMBs）in order to realize the high energy density(500 Wh kg^-1)of the lithium ion battery.Lithium metal has the lowest negative potential（-3.04 V vs standard hydrogen electrode）and theoretical specific capacity of3860 mA h g^-1,which far exceeding the traditional graphite negative electrode(372 mA h g^-1).Lithium batteries with liquid electrolytes and separator have safety risks such as combustion and explosion.Solid electrolytes can replace liquid electrolytes and separator to conduct ions and isolate electrons,respectively.In the meantime,it can avoid the problem of volatility and leakage of liquid organic electrolyte.Therefore,the development of solid electrolytes to replace traditional flammable liquid electrolytes is an important way to fundamentally improve the safety of lithium batteries.Polymer solid electrolytes are suitable for large-scale manufacturing with good flexibility and features of easy molding.Besides,they are non-flammable and have good contact with electrode.However,room temperature ionic conductivity of conventional polymer electrolyte is low(for example,room temperature conductivity is usually<10^-6Scm^-1 for poly（ethylene oxide）（PEO）electrolytes).In addition,its poor electrochemical stability（<3.9V）,and weak mechanical strength restrict their application in lithium battery.The solid-state electrolyte of a single component has been difficult to meet the actual application needs of the lithium battery,introduce other components,and realize the functional hybrid of each component,which is an effective strategy for improving solid electrolyte properties.Inorganic coponents can boost mechanical property of polymer.Ionic liquids have high ionic conductivity and wide electrochemical window.The addition of inorganic component and ionic liquid can boost the ionic transport,electrochemical stability and mechanical property of polymer electrolytes.Organic-inorganic hybrid electrolytes can combine the advantages of organic and inorganic components,which is an effective strategy to improve electrolyte properties.Polyaddition method is a convenient method to integrate different components without the necessity of harsh reaction conditions.In this paper,a class of hybrid electrolyte is prepared by introducing ionic liquid and/or inorganic components into the polymer electrolyte by the method of polyaddition,and its physical and chemical properties and electrochemical properties have been systematically investigated.Meantime,this type of hybrid electrolyte was assembled into a lithium metal button cell and a preliminary study on charging and discharging of cell was carried out.The main research contents are as follows:（1）Imidazolium-based ionic liquids（ILs）were firstly prepared by quaternization reaction.Then a series of organic-inorganic hybrid polymers were synthesized using ILs,polyethylene glycol（PEG）,polyhedral oligomeric silsesquioxane（POSS）and an isocyanate-containing compound as raw materials,accompanied by being mixed with lithium bis（trifluoromethanesulfonyl）imide（LiTFSI）to prepared the hybrid electrolyte（POSS-IL（EG））.It exhibits excellent thermal stability（>240℃）,low glass transition temperature（-46.8℃）,high ionic conductivity(0.97×10^-4 Scm^-1 at 25℃),and the ion transport performance can be easily tuned by adjusting the composition.At 60℃,it can not only achieve high ionic conductivity of 2.8×10^-4Scm^-1,but also shows good electrochemical compatibility with lithium electrodes in the galvanostatic cycling test at60℃.（2）UPy-diol contains quadruple hydrogen bonds,and the non-covalent interaction based on quadruple hydrogen bonds can give the material self-healing properties.A series of organic-inorganic hybrid polymers were synthesized using ILs,PEG,POSS,an isocyanate-containing compound and UPy-diol as raw materials,and mixed with LiTFSI to prepare a self-repairable hybrid electrolyte（PIEU）.The performance of the electrolyte was investigated by changing the molar ratio of POSS and UPy-diol.It exhibits excellent thermal stability（up to 275℃）and low glass transition temperature（-45.5℃）.In addition,it can self-heal its incisions quickly at ambient temperature（within 100 minutes）.The electrolyte exhibits high ionic conductivity(0.34×10^-4Scm^-1 at 25℃),and its ion transport performance could be easily tuned by adjusting the composition.At 60℃,it can not only achieve high ionic conductivity of 1.4×10^-4Scm^-1,and lithium ion transference number can reach 0.43,but also shows excellent long-term（>800 h）electrochemical compatibilities with lithium electrodes in the galvanostatic cycling test.The Li/Li Fe PO₄ battery assembled with the electrolyte showed good cycle performance.（3）A crosslinked hybrid electrolyte（SIPH-x,x represents mole ratio of EO/Li,y represents the mass percentage of TEGDME）was prepared by mixing ILs,PEG,bis（2-hydroxyethyl）disulfide,a trimer of hexamethylene diisocyanate,LiTFSI and tetraglyme（TEGDME）simultaneously and dispersing in tetrahydrofuran via polyaddtion method.It has excellent thermal stability（up to 170℃）,low glass transition temperature（-76.6℃）,and self-healing performance,exhibiting high ionic conductivity(2.67×10^-4Scm^-1 at25℃),high lithium ion transference number（0.43）,good electrochemical oxidative stability（4.3 V（vs Li⁺/Li））and excellent compatibility with lithium metal.Besides,the Li/Li Fe PO₄ battery assembled with the electrolyte showed excellent cycle performance.