Preperation Of Modified PVDF And Its Application In Li-ion Batteries

Although the binder accounts for a small proportion in the lithium ion battery system,it ensures that the positive and negative active materials can be stably attached to the current collector,and is one of the key materials constituting the lithium ion battery system.Polyvinylidene fluoride(PVDF)is one of the most widely used binders in the industry.However,the interaction force between PVDF and the electrode material is van der Waals force.In the process of use,due to insufficient bonding strength,the electrode particles are easily detached over time,resulting in capacity decay,for lithium ion batteries.The safety performance and service life have a great impact.In addition,the volume expansion of PVDF in the electrolyte reduces the bond strength between the electrode material and the current collector,and the resistance increases,which poses a great hidden danger to the overall performance of the lithium ion battery.The study of this paper is mainly to modify the PVDF binder,improve the bond strength of the binder,enhance the stability of the fluid interface,and improve the cycle life and electrochemical performance of the lithium ion battery.Physical and chemical properties of materials are determined by modern methods such as peel strength test,X-ray diffraction analysis,charge and discharge performance measurement,X-ray energy spectrum analysis,scanning electron microscopy,contact angle measurement,thermogravimetric analysis,AC impedance spectroscopy,and cyclic voltammetry.The main findings are as follows:(1)The PVDF binder was modified with 3-aminopropyl triethoxysilane(KH550),?-glycidyl ether oxypropyl trimethoxysilane(KH560),?-methacryloxypropyl trimethoxysilane(KH570)and ?-mercaptopropyl trimethoxysilane(KH590)respectively,and the peel strength of the positive paste coated on aluminum foil collector was tested.The results showed that the peel strength of the modified binder was improved.Among them,KH550(addition 3%)and KH792(addition 3%)had the most obvious effect on the peel strength,which increased 10.7 times and 12.2 times,respectively.(2)The application of modified PVDF binder to the research results showed that: in the voltage range of 2.5~4.3V and current density 1C,the sample modified with KH550(added by 2?L)has the first coulombic efficiency of 81%,the initial discharge capacity is153 mAh·g-1,and the KH792(addition amount is 2?L)has the first coulombic efficiency of 83%,the initial discharge capacity is 164mAh/g.The modified sample was superior to the unmodified sample(first coulen efficiency 81%,initial discharge capacity 147mAh/g).In the 2.5~4.3V voltage range and different currents,the sample modified with KH550(added 2?L)has the best performance.The capacity of the first cycle discharge is164mAh/g(0.2C),160mAh/g(2C),151mAh/g(5C),146mAh/g(8C),and 128mAh/g(20C),159mAh/g(0.2C),respectively.According to the analysis of AC impedance spectroscopy and cyclic voltammetry,the charge transfer impedance of KH550 modified PVDF binder and KH792 modified sample battery was reduced,and the redox peak position differencewas small.(3)The samples of kh550 modified binder and KH792 modified binder were used Di(2-ethylhexyl)phosphate(P204)to prepare the optimized KH550 modified binder and the optimized KH792 modified binder.According study,Under the voltage range of2.5~4.3V and current density 1C,the first coulombic efficiency of the modified KH550 modified binder was 85%,and the initial discharge capacity was 166mAh/g.And the modified KH792 modified binder was 83%,and the initial discharge capacity was164mAh/g.Under the voltage range of 2.5~4.3V and different currents,the first cycle discharge capacity of P204 optimized KH550 modified samples was 168mAh/g(0.2C),163mAh/g(2C),155mAh/g(5C),152mAh/g(8C),132mAh/g(20C),and 165mAh/g(0.2 C).It was found by AC impedance spectroscopy that the charge transfer impedance of the optimized KH550 modified sample decreased and the lithium ion diffusion coefficient became larger.