Preparation And Properties Of High-performance Modified Membrane With One-sided Coating Of N-graphene Powder On Polyolefin Separator

Lithium-ion batteries are widely used in electronic equipment and electric vehicles due to their high operating voltage and energy density,low self-discharge,long cycle life,low capacity loss,no memory effect and no pollution.As a microporous polymer functional separator,the separator has the function of isolating the positive and negative electrodes,preventing the contact between the positive and negative poles and causing short circuit;and the microporous structure due to its many tortuous passages.The ions in the electrolyte flow freely in the micropores,so that a loop is formed between the positive and negative electrodes,and electrons of the external loop form an electric current,which is utilized for the electric device.However,the traditional commercial polyolefin separator has the disadvantages of low thermal melting temperature,poor heat resistance and thermal obturator performance,insufficient electrolyte absorption and wettability,high diaphragm interface impedance and low ionic conductivity,and the main influence of battery separator performance.The factors include the internal resistance of the lithium ion battery(such as the surface resistance of the diaphragm),the ionic conductivity,the interface structure of the lithium ion battery,and the electrolyte retention of the electrolyte,thereby directly affecting the charge and discharge rate,cycle life and safety and stability of the lithium ion battery.Wait.Therefore,preparing a separator with excellent performance is the key to improving the battery performance and electrochemical performance of a lithium ion battery;In this paper,the surface modification of the polyolefin separator is carried out by surface impregnation and coating to prepare the modified membrane,thereby improving the battery performance of the separator.With electrochemical performance.The plan is as follows:(1)Comparing the thickness of the modified separator prepared by different nitrogen-containing graphene powder/binders and the thickness of different modified membranes with the properties of commercial PP/PE/PP separators The best experimental scheme is the mass ratio m(nitrogen-containing graphene powder): m(sodium alginate): m(Tween-60)= 30:1:1,thickness is 60 ?m,which is referred to as the separator N-Grephene-60@ PP/PE/PP;The results confirmed that the polyolefin separator PP/PE/PP surface coated with nitrogen-containing graphene powder prepared modified membrane N-Grephene@PP/PE/PP than commercial PP/PE/PP separator It has better thermal closed cell performance,heat resistance,electrolyte wettability,higher ionic conductivity,lower diaphragm interface impedance and ion diffusion resistance,and its assembled lithium-ion battery has better charge and discharge capacity and rate performance and safe life.(2)Among them,N-Grephene-60@PP/PE/PP modified separator has the best electrochemical performance and battery performance.The obtained test result parameters are as follows: ionic conductivity is 3.6 m S·cm-1(PP/PE/PP is only 0.72 m S·cm-1),lithium ion diffusion resistance is 1.593 ?,and membrane surface impedance is 6.226 ?,nearly 10 times lower than commercial PP/PE/PP(PP/ PE/PP diaphragm surface impedance is 67.87?,lithium ion diffusion resistance is 2.011?);electrolyte has good wettability,electrolyte absorption rate is 105.4%,(PP/PE/PP is only 59.3%),thermal closed cell performance and heat resistance are better;The temperature is preferably high(170.5°C),the first discharge specific capacity of the battery is up to 137 m Ah g-1(0.5C)and the capacity retention rate of the charge and discharge cycle is 94.6%,and the first charge-discharge specific capacity efficiency is 99.16.%,(The commercial charge/discharge ratio of the commercial PP/PE/PP separator battery is 88.8%,and the first discharge specific capacity is 133.7m Ah g-1(0.5C)capacity retention rate is 89.1%);And N-Grephene-60@PP/PE/PP lithium-ion battery cycle charge and discharge 300 times capacity retention rate of 94.2%(PP/PE/PP is 77.1%);N-Graphene-60@PP/PE/PP has higher ionic conductivity,better thermal obturator performance,heat resistance and lower diaphragm interface resistance,and its assembled lithium-ion battery has higher charge and discharge Capacity,better rate performance,cycle life and safety.(3)N-Grephene@PE was prepared by surface coating of nitrogen-containing graphene powder on different polyolefin separators(such as wet PE),and the nitrogen-containing graphene powder-modified polyolefin separator was confirmed once again.The modified separator prepared by PP/PE/PP or PE)has better battery performance and electrochemical performance.The results show that N-Graphene@PE has a higher ionic conductivity of 2.27 m S·cm-1(more than 4 times higher)and a lower membrane surface impedance(1.773 ?),it is more than four times lower than commercial wet PE.while the surface impedance of the wet PE membrane is It is 9.36 ? and the ionic conductivity is only 0.44 m S·cm-1;and it has better charge and discharge capacity,cycle stability,rate performance and cycle life.(4)And through the comparison of the battery performance of the nitrogen-containing graphene modified membrane and the pure graphene modified membrane,the role of nitrogen in the nitrogen-containing graphene is confirmed,and the nitrogen present form of the nitrogen-containing graphene is substituted doping,that is,C in the sp2 configuration is replaced by an N atom.Since the N atom has an atomic radius similar to that of the C atom,the graphene can be doped as an electron donor in a substituted manner,and the resulting N-doped graphite is diluted to exhibit many excellent properties,such as opening the band gap and adjusting The conductivity type changes the electronic structure of graphene,increases the free carrier density of graphene,thereby improving the conductivity and stability of graphene,and increasing the active site of metal particles adsorbed on the surface of graphene.