Construction Of Three-dimensional Cross-linked Network Separator And Binder Applied In Lithium-ion Battery

With the development of new energy technology and electric vehicles,the technical difficulties of high-power lithium-ion batteries in terms of safety,cycle stability and high current performance are gradually increasing.Separators and adhesives as polymer materials inside the battery are extremely important for the performance of the battery under high power conditions.Traditional polyolefin separators are no longer able to meet the current requirements of high-power lithiumion batteries due to their own defects such as poor temperature resistance and low porosity.Non-woven separators prepared using gel coatings such as polyvinylidene fluoride(PVDF)to optimize the pore size of high temperature resistant non-woven fabrics have become an important research direction in recent years.In addition,PVDF is also a common binder for battery electrode materials due to its good electrochemical stability.However,such conventional coatings or binders prepared from a single polymer are very susceptible to peeling or causing collapse of the pore structure under prolonged electrolyte wetting,which ultimately affects the long-term cycling stability of the battery.Gel electrolytes prepared from three-dimensional cross-linked network polymers are of great interest due to their unique advantages in terms of liquid retention and structural stability.For this reason,their using as separator coatings or binders will be of great importance for the improvement of battery performance.Polyamide-amine(PAMAM)dendrimers have a spherical topology,internal cavities for encapsulation,repeating units that can interact with the electrolyte and peripheral groups that can easily react.Therefore,the construction of three-dimensional cross-linked network coatings or bonding agents using PAMAM as a unit will provide good hydrophilicity and structural stability for separators or electrodes.Based on this,this paper achieves the construction of a cross-linked network through the reaction between the amine group at the periphery of PAMAM and the C-F bond in the gel electrolyte PVDF,and uses it for the modification of non-woven separators and electrode material binder to explore the effect of the introduction of PAMAM and the design of the cross-linked network on the battery performance.The main research content and results of this thesis are as follows:Firstly,we successfully prepared a series of new lithium-ion battery composite separators by introducing different generations of PAMAM dendrimers and polyvinylidene fluoride-hexafluoropropylene(PVDF-HFP)to construct a threedimensional cross-linked network coating by chemical reaction,and applying the coating to the surface of PPS nonwoven fabric.The composite separators were characterised using rheology and infrared spectroscopy to demonstrate the successful construction of the cross-linked network.Through testing of the separator performance,the introduction of PAMAM was found to improve porosity,electrolyte absorption,wetting ability,ionic conductivity,interfacial compatibility,thereby weakening polarisation in the battery and improving battery performance.In particular,crosslinked composite separators constructed from fourth generation PAMAM dendrimers can increase the discharge specific capacity of the battery to a maximum.In addition,it has been found that the cross-linked composite separators have excellent mechanical properties and cycling stability.These effects are particularly evident when the PAMAM dendrimers are at a higher generation.Secondly,this chapter successfully prepared a new binder for lithium-ion battery cathode materials by introducing fourth generation PAMAM dendrimers and PVDFHFP.The effect of the introduction of PAMAM dendrimers on lithium-ion battries was also investigated.After a series of tests and characterisations,the three-dimensional cross-linked network binder was found to have better bonding ability and electrolyte affinity.The binder improves the conductivity of the electrodes through closer contact between the electrode materials and the good electrolyte affinity reduces the interfacial impedance of the cell.These behaviours ultimately give the cell better multiplier performance.The three-dimensional cross-linked network bonding agent keeps the electrode surface morphology flat and smooth during cycling,thus allowing the discharge specific capacity of the battery to remain 98% after 250 cycles.This 3D crosslinked binder will therefore provide a new idea and solution for the design of next generation binders.