Flame-retardant Solid-state Li-Ion Conductors Based On P-Containing Polymers

Commercial lithium-ion batteries mainly use liquid electrolytes.Organic liquid electrolytes easily decompose and explode at high temperatures,which affect the safety performance of lithium-ion batteries.Besides,organic liquid electrolytes limit the electrochemical window of lithium-ion batteries.And lithium dendrites will occur during long-term charge and discharge,which causes safety issues of lithium-ion batteries.Therefore,solid electrolytes used in lithium ion batteries are expected to solve the above-mentioned problems,and thus have received widespread attention.However,compared with liquid electrolytes,existing solid electrolytes have lower ionic conductivity,poor mechanical properties and poor processability,which restrict the application of lithium ion solid electrolytes.In this dissertation,a phosphorus-containing ionic polymer with rigid skeleton has been synthesized by polycondensation.Lithium bis(trifluoromethanesulphonyl)imide(LiTFSI)was blend into it to achieve fast lithium ions conduction at room temperature.Through the optimization of material composition and synthetic routes,the ionic polymer with the best conductivity was determined as the product of polycondensation of tetrakis(hydroxymethyl)phosphonium salt and 4,4'-diaminodiphenyl sulfone.The electrochemical and flame-retardant properties of this ionic polymer were further studied.The research results of this article are as follows:1.By polycondensation of tetrakis(hydroxymethyl)phosphonium salt and five aromatic diamines,we have synthesized five phosphorus-containing ionic polymers with rigid skeleton.LiTFSI was blend into ionic polymers to get composite electrolytes.By testing the conductivity of the five composite electrolytes,the highest conductivity was determined to be(LiTFSI)2-TDS.(LiTFSI)2-TDS was prepared by poly(tetramethylolphosphonium-4,4'-diaminodiphenylsulfone)trifluoromethanesulfonimide(TDS)doped with LiTFSI.(LiTFSI)2-TDS was characterized by Raman,FTIR,solid nuclear magnetic,elemental analysis,etc.The characterization proves that TDS has a rigid skeleton structure.The positive charged phosphorus on the skeleton can bind TFSI negative ions,and the sulfone groups on the skeleton can bind lithium ions.So the Li-TFSI ion pairs are separated effectively,which realizes efficient transmission of lithium ions.Without any solvent,(LiTFSI)2-TDS electrolyte shows an ion conductivity of 3.16×10-4 S cm-1 at 35 ? and lithium transference number of 0.68,which is superior to most polymer-based solid lithium ion conductors.2.Further study on the electrochemical performance of solid electrolyte(LiTFSI)2-TDS shows that the activation energy of ion conduction is 0.36 eV,and its electrochemical window is as high as 4.4 V(0.7-5.1 V).After 325 plating-stripping cycles(325 hours),the interface contact between the electrode lithium foil and the electrolyte(LiTFSI)2-TDS is improved.The device performance is quite stable.In the combustion test,this solid electrolyte exhibits good flame retardancy with a limit oxygen index of 32%.During combustion,toxic gases such as nitrogen oxides and carbon monoxide cannot be detected in the combustion gas phase.(LiTFSI)2-TDS has good safety performance.