Effects Of Different Chain Extenders On Preparation And Properties Of Cellulose Acetate Cross-linked Separators

A separator is one of the key components of the lithium-ion battery,which can allow lithium ions to migrate between the positive and negative electrodes,avoiding short circuit due to the passage of electrons at the same time.The separator structure greatly affects the electrochemical performance,cycle life and safe use of lithium-ion batteries.At present,the dominant separator products in the market are polyolefin membranes,such as polypropylene(PP)and polyethylene(PE).However,they all had inherent defects such as poor wettability,poor rate performance and serious shrinkage at high temperature.Gel polymer electrolyte(GPE)is one of the good membranes that have excellent electrolyte affinity,electrochemical performance,thermal stability and safety.And it has potential value to replace polyolefin membranes.In this paper,the cellulose acetate with highly polar groups and good thermostability was used as the main material of GPE membranes.The cross-linked gel polymer electrolyte membrane was prepared by chain extension reaction of hydroxyl groups on cellulose segments.Chain extenders of different structures and end groups were selected to study their effects on the formation of a cross-linking structure and its properties.The specific research contents and results were as follows:(1)Hexamethylene diisocyanate(HDI)was extended on the molecular chain of cellulose acetate(CA)with hydroxyl group by the condensation reaction.The results showed that:The chain extension reaction of cellulose acetate(CA)was accompanied with the cross-linking reaction.HCA membranes were relatively brittle.With the increased of HDI ratio,the crosslinking degree of membranes gradually increased.Among them,HCA70(NCO/OH=0.70)membrane had relatively balanced performance,with liquid absorption rate of 203.7%,tensile strength of 30.71 MPa and ionic conductivity of 1.55 mS cm-1.The rate discharge performance of the half cell could reach 60%at 4.0 C rate(81 mAh g-1).(2)Aiming at the brittleness defect of HCA membrane,we introduced the molecular structure of CH2CH2O ether,that is,tetraethylene glycol(TEG)was used to crosslink the product of cellulose acetate grafted with isophorone diisocyanate(CA-IPDI)to prepare the crosslinked membrane(TCA membrane)containing ether structure.The results showed that:Compared with HCA membranes,TCA membranes not only had better elongation at break,but also had good electrochemical stability.Among them,TCA15 membrane had the best comprehensive performance,with elongation at break of 32.35%,liquid absorption rate of 197.7%and ionic conductivity of 1.55 mS cm-1.The rate discharge performance of the half cell could reach 70%at 4.0 C rate(92 mAh g-1).(3)Three oligomer chain extenders with molecular weight of 1000,namely hydroxyl terminated polyethylene glycol(PEG),diaminopolyethylene glycol(NH2-PEG-NH2)and aminopropyl terminated polydimethylsiloxane(NH2-PDMS-NH2),were selected to react with CA-IPDI in order to study the effects of the different end groups and different segment structures on the crosslinking formation and membrane properties of cellulose acetate.The results showed that:The membrane(C1OH membrane)prepared by hydroxyl polyethylene glycol(PEG)hasn’t formed the cross-linking structure,but the membranes(C2NH2 and C3NH2)obtained by diaminopolyethylene glycol and aminopropyl terminated polydimethylsiloxane had good cross-linking structure.It showed that the reaction activity of hydroxyl decreased with chain length,and was significantly lower than that of corresponding amino group.The oligomer chain structure had a great influence on the wettability and electrochemical properties of membrane electrolyte.Among them,the performance of C2NH2 membrane was more excellent,the liquid absorption rate is 282.4%,the ionic conductivity is 2.04 mS cm-1,and the electrochemical stability window can reach 5.5 V.The discharge ratio and specific capacity is 115 mAh g-1 at 4 C and could reach 74.1%of the initial 0.2 C.The long chain crosslinking agent with appropriate structure could improve the ionic conductivity and electrochemical properties of membranes,while the chain length is too long to crosslink cellulose acetate.The defect can be solved by high activity end group(-NH2)reaction when our giving consideration to crosslinking reaction and membrane properties.