The Modification Of The Polyolefin-based Separator For High Performance Lithium Ion Battery

Lithium-ion batteries(LIBs)are widely used in mobile phones,laptops and other portable electronic devices due to their high energy density,long cycle life,high operating voltage,low self-discharge,and no memory effect.With the application of LIBs in the emerging areas such as electric vehicles and energy storage systems,their performance need to be further improvement.Separator is an important component of a lithium-ion battery,which directly influences the cycle performance,rate performance and safety of the lithium-ion battery.At present,polyolefin-based separators remain dominance in commercial LIBs on account of their excellent mechanical properties,good electrochemical stability,proper low cost and thermal shutdown properties.However,the conventional polyolefin separators suffer from their intrinsic hydrophobicity and greatly limit the performance of lithium-ion batteries.Based on these issues,this dissertation focused on the modification of the polyolefin separators to improve their electrochemical performance.The main research contents and results are as follows:self-assembled of TA/PEI(tannic acid/polyethyleneimine)on PP separator.A thin and uniform TA/PEI layer was formed onto the surfaces of the PP separator through a simple assembly process without destroying its microstructure.The electrolyte affinity of the modified separators were significantly improved.The TA/PEI coated-separator possessed superior ionic conductivity(0.95 mS · cm-1)and lithium-ion transference number(0.44),indicating that the TA/PEI layer played a role in the lithium ions migration.The possible mechanism of the surface promoting lithium ion migration was discussed in this paper.As a result,cells with the TA/PEI-coated PP separator displayed superior cycle stability:the PP-2 separator have a discharge capacity retention of 90.9%,much higher than that of the bare PP separator(69.3%).Preparation and Characterization of the P(AA(Li)-co-BA-co-MMA)coated PP Separator.P(AA-co-BA-co-MMA)terpolymer was synthesized and the copolymer was successfully introduced on the surface of the PP separator by dip coating.The influence of the copolymer concentration on the performance of the coated separator was investigated.When the copolymer concentration is 2%,the modified separator achieves the best performance:the ionic conductivity of the MP2 is 0.87 and the discharge capacity retention is 92%after 200 cycles at 1 C.In addition,to investage the influence of the Li+ and H+ on the performance of the separator,P(AA-co-BA-co-MMA)were also used to modified the PP separator.The results show that the lithiated separator have better electrochemical performance and cycle stability.Thin poly(lithium acrylate-co-butyl acrylate)layer-coated polypropylene separator.poly(lithium acrylate-co-butyl acrylate)(P(AALi-co-BA))was synthesized and the thin P(AALi-co-BA)layer-coated polypropylene(PP)separator was prepared by a simple dip-coating process.This modification has less influence on the pore structure of original PP separator,while significantly improving the electrolyte wettability and uptake.The electrochemical performance of the modified separator was also been significantly improved.The ionic conductivity of the PP-2 separator is 0.84 mS cm-2,higher than that of the original separator of 0.77 mS cm-2.In addition,the modified separator shows lower interfacial impedance.Batteries assembled with the modified membrane exhibit good battery performance:the discharge capacity retention of the cell with the modified membrane is 94.5%after 200 cycles at 1 C,whereas the.cell containing the original PP separator of the battery is only 69.3%.