Polydopamine Modification Of Polyolefin Separator And Preparation And Performance Study Of Ceramic/PVDF-HFP Separator

With the rapid development of electric vehicle and hybrid electric vehicle, thestate-of-art requirement of lithium-ion battery, metal-air battery, fuel cell and their materialsare proposed. Safety question of lithium-ion battery restrains their development andapplication although their combustion and exploration rarely happened. Separator with richpore structure, as a safe compartment, blocks the direct contact of cathode and anode activematerials and transports the lithium ions. At present, the commercial lithium-ion batteryseparators are made of PP, PE or PP/PE/PP. The disadvantages of them are poor electrolyteuptake ability which shows a worse C-rate discharge capacity and poor heat resistanceperformance which gives rise to short circuit problem. When the temperature is up to135℃,the pores of separator shut down and the battery does not work. When the temperature is upto165℃, the PP separator shrinks dramatically and the same time the battery is intenseexothermic because of the direct contact of the cathode and the anode.In order to improve electrolyte uptake ability and heat resistance performance ofseparator, polydopamine modification was used to improve the uptake ability of separator bysolution-immersion method and a series of ceramic/PVDF-HFP separators were prepared toimprove heat resistance characterization by phase inversion method. In addition, non-wovenPET ceramic/PVDF-HFP separators were prepared to improve the comprehensive properties,especially flame retardant performance. PVDF-HFP fiber separator and co-axialPET/PVDF-HFP fiber separator with core/shell structure were prepared by electrospinningmethod and the performances of separator and battery are studied here. The main contentsand results are as follows:1. Polydopamine immersion modification of polyolefin separator is used to improve thewetting ability between polyolefin separator and electrolyte and C-rate discharge performance of battery. It shows that polydopamine particles with large specific surface areaand uniform size are easy to obtain in alkaline conditions and electrolytes are easy to spreadout on the polydopamine modified surface because the lower contact angel. Ionicconductivity of polydopamine modified PP separator is up to3.08mS/cm, is2.4times of PPseparator. The polydopamine modification effect of PP separator is better than that of PEseparator. The discharge capacity of cell with polydopamine modified PP separator is270%times than that of PP separator at the discharge rate of3.0C.2. Ceramic/PVDF-HFP separator made by phase-inversion method has betterheat-resistant performance than that of polyolefin separator which is good for safeperformance of battery. Here, the shrinkage of ceramic/PVDF-HFP separator is less than5%(200℃,1h). PVDF-HFP separators doped with Al2(SO4)3and Al2O3have stableelectrochemical window and they can be widely used in lithium ion battery as separators.Ceramic particles can improve ionic conductivity of PVDF-HFP separator. The batteryshows the best C-rate discharge performance when ASO/PVDF-HFP with15wt.%ASOcontent is used as separator. Pore-forming agents have a significant influence on porestructure and surface morphology of separators. N-butyl alcohol and water are goodpore-forming agents for PVDF-HFP separators3. In addition, polydopamine modified Al2O3particles with core-shell structure arefabricated by using dopamine Tris solution. Al2O3/PVDF-HFP separator is made byphase-inversion method, and here Al2O3is modified by polydopamine. The concentration ofdopamine Tris solution has a direct influence on the thickness of the polydopamine(shell)and porosity of separator. The lower the concentration is, the higher the porosity of separatorand discharge capacity of battery will be. Al2O3and SiO2particles can improve theheat-resistant performance of PVDF-HFP separator. With the increase of ceramic content,heat-resistant performance is improved, but tensile strength and break elongation aredecreased. The proportion of ceramic in PVDF-HFP separator can not be too high. Becauseif its proportion is too high, absorption ability and C-rate discharge performance will bedecreased.4. Five kinds of separators, such as SiO2-PET, LLTO-PET, Al2O3-PET, YSZ-PET andBaTiO3-PET, were fabricated by using PET nonwowen fabrics as substrate, PVDF-HFP asbinder. It shows that nonwoven PET has an excellenct flame retardant performance and goodmechanical and heat-resistant performance. Five kinds of separators all have goodcompatibility and wettability with many kinds of electrolytes. The cells with these separatorshave good C-rate discharge capacity and cycle performance, especially SiO2-PET separator.5. PVDF-HFP separators are prepared preliminarily with needle electrospinning method and high-voltage electrostatic shuttle electrospinning method. The advantage of needleelectrospinning method is faster spinning speed and thinner diameter of fiber which will beeasily produced. And the high-voltage electrostatic shuttle electrospinning method can avoidthe blocking problem. At the same time, PVDF-HFP/PET fiber separator with core-shellstructure is successfully prepared by coaxial electrospinning with fuse-protection function.6. In addition, PVDF-HFP separator doped with graphene oxide (GO) was prepared byphase inversion method. Experimental results show that GO can improve the heat shrinkagecharacteristics of PVDF-HFP separator and significantly improve the discharge performanceof battery. It shows that the discharge capacity of GO/PVDF-HFP separator is up to85mAh/g,3.4times to that of PVDF-HFP separator at the discharge rate of4.0C.