High Temperature Resistant Lithium Ion Battery Separator Based On Polyether Ether Ketone(PEEK)

Lithium-ion batteries are widely used in mobile devices,electric or hybrid electric vehicles and energy storage systems.In lithium-ion batteries,the separator mainly acts as a barrier between positive and negative electrodes to prevent short-circuit and provides a path for lithium ion transmission,which is closely related to the capacity and safety performance of the batteries.At present,the commercial lithium-ion battery separators mainly consist of polyolefins.The hydrophobicity and structure of the materials lead to low porosity,which leads to low electrochemical performance of lithium-ion batteries,poor thermal stability and poor high temperature safety of batteries.In order to solve the two core problems of poor wettability and low thermal stability of commercial separators,this thesis focuses on a new thermally stable polyether ether ketone(PEEK)polymer.PEEK has good mechanical properties,excellent thermal stability and stable electrochemical performance.Since PEEK is solvent resistant,it is onlysoluble in concentrated sulfuric acid and methane sulfonic acid.The research covers the preparation method,structure control and surface modification.A series of porous membranes and composite membranes with excellent pore structure and high temperature resistance were systematically investigated.In chapter three,PEEK membranes with different pore structures were prepared by vapor-induced phase separation method.The effects of concentrated sulfuric acid(SA)in the solvent of casting solution were systematically studied.At low SA content,the solubility of the mixed solvent to PEEK and the water vapor absorption rate decrease.This is advantageous for solid-liquid separation and crystallization formation.When the SA/MSA ratio is changed from 15/75 to 0/90(wt.%/wt.%),the porosity,pore size,electrolyte absorption rate and conductivity of the obtained PEEK separator increased first and then decreased.The separator of the best structure was observed at a ratio of SA/MSA ratio of 8/82.The PEEK separator exhibits an electrolyte absorption rate 5 times higher than that of the PP separator.In addition,the porosity and conductivity of the PEEK separator are superior to those of the commercial PP.PEEK has high thermal stability without obvious shrinkage at200 ~oC.In order to further explore the effect of the pore structure of the membrane on the performance of the battery,we used a immersion phase separation and vapor-induced phase separation method to prepare a PEEK membrane with finger-like structure.The PEEK membrane(S-PEEK/PVP)with sponge-like structure by adding a polyvinylpyrrolidone(PVP)pore-forming agent was also obtained for comparison.The memrbanes were characterized by SEM,porosity,electrolyte absorption rate,and battery cycle performance and rate performance test analysis.The results indicated that the S-PEEK/PVP separator had an interconnected pore structure with improved electrolyte wettability by PVP.The charge and discharge capacity of the battery with sponge-like structure was remarkably improved.The chapter four,PEEK/PP double-layer composite lithium ion battery separator was prepared to improve the high temperature resistance,electrolyte wetting performance and battery charge and discharge capacity of commercial PP film.In order to further improve the dimensional stability of commercial separators at high temperatures,the authors suggest that in addition to the single-sided coatings studied in this paper,commercial PP separators can be further modified by double-sided coating.