Prearation And Properties Of Cross-linked Separator Based On Cellulose Acetate Lor Lithium-ion Batteries

Lithium-ion battery(LIB)is the portable energy storage device that has been widely used,and it has the advantages of light weight,high energy density,long cycle life and wide voltage range.As an important part of the lithium-ion battery,the separator has two main functions of isolating the electrodes and providing channels for li-ions.Therefore,the separator plays an important role on the performance of the lithium-ion battery.Among the separators,the gel polymer electrolyte separator(GPEs)is a research hotspot in current,because it has a strong affinity with the electrolytes and can store the electrolytes effectively.Meanwhile,the GPEs has considerable safety and electrochemical performance.In this paper,the natural polymer cellulose acetate was used as matrix material,together with the modified nano-SiO2 and poly(ethylene glycol)dimethacrylate(PEGDMA)formed the composite membrane with cross-linked structure by thermal initiation of radical polymerization.Furthermore,the size effect of inorganic cross-linker nano-SiO2 on the performance of separator was also studied.There still existed the problem that the strength of the prepared membrane would get worse after absorbing the electrolytes,therefore,for improving the strength of the gel polymer electrolyte separator,the cellulose non-woven was used as a supported layer and the enhanced separator with sandwich structure was prepared.The research content could be divided into the following three parts:(1)The modified cellulose acetate(M-CA)with double bonds in the side group was prepared by modifying the cellulose acetate with methacryloyl chloride,and the modified nano-SiO2(M-SiO2)with vinyl groups on the surface was prepared by modifying nano-SiO2 with vinyltrimethoxysilane(VTMO).Then,the matrix M-CA and cross-linker M-SiO2 together with monomer PEGDMA were used to form a cross-linked structure by thermal initiation of radical polymerization,and the cross-linked composite membrane based on cellulose acetate was prepared by removing the solvent.The effects of the components on the performance of the cross-lined membrane were discussed.The physical performance and electrochemical performance of membranes and the performance of assembled cells were characterized.The results showed that adding M-SiO2 and PEGDMA could help the membrane to form stable cross-linked structure.The membrane showed good thermal stability and had not shrinkage under the temperature of 200? for 0.5 h,which could attribute to the cellulose acetate and SiO2.And the C-CAS15 membrane contained 15 wt%M-SiO2 possessed better comprehensive performance,the electrolyte uptake was up to 248%,the mechanical strength was up to 14.30 MPa and the membrane still kept certain toughness,the ionic conductivity was 1.54 mS cm-1.Meanwhile,compared with the cells assembled with PE separator,the cells assembled with C-CAS15 separator showed better cycle charge-discharge performance and C-rate discharge performance(2)On the basis of the cross-linked composite membrane containing cellulose acetate,the different nano-SiO2 particles were used to study the effect of the nano-SiO2 size on the performance of membranes.The results showed that the diameter of the cross-linker nano-SiO2had big effect on the electrolyte uptake and ionic conductivity of the membranes,but it didn't nearly affect the thermal and electrochemical stability of the membranes.Among the cross-linked membranes,the membrane(CAS50)with the SiO2 diameter 50 nm had the better performance.The CAS50 membrane had high electrolyte uptake of 345%and ionic conductivity of 2.38 mS cm-1,and the tensile strength was up to 18.71 MPa,meanwhile,it had the lower interface impedance between the membrane and the electrodes.In addition,the cells assembled with the CAS50 membrane exhibited more considerable cycle charge-discharge and C-rate discharge performance(3)The mechanical strength of the above membrane would get worse after forming the gel polymer electrolyte with the membrane absorbing the electrolytes.Therefore,the cellulose non-woven which had good compatibility with cellulose acetate could be used as supported layer to strengthen the gel polymer electrolyte.The strengthened membrane(CCAS)with sandwich structure was prepared.The PP non-woven supported membrane(PCAS)was used for contrast.The results showed that the cellulose non-woven not only improved the mechanical strength of the gel polymer electrolyte effectively,but also could be good for the absorption and storage of electrolytes,and further improved the ionic conductivity of membranes.And the PP non-woven improved the mechanical strength,but it had the poor affinity to the electrolytes and poor thermal resistance and decreased the other properties of the membranes.