Research On High-performance Lithium-ion Battery Separator Based On Polyvinylidene Fluoride-hexafluoropropylene

The key to sustainable energy development is to research and develop new energy materials and devices to replace traditional energy based on petrochemicals.Due to their long cycle life and low self-discharge,lithium-ion batteries contribute important applications in many fields,such as electric vehicles,consumer electronics,and backup power supplies.In order to meet the increasing demands of these applications,the overall performance of lithium-ion batteries still needs to be improved.The separator conducts lithium ions and blocks electrons in a lithium-ion battery.Its thermal stability,mechanical strength,and surface properties are closely related to the safety,stability,and energy density of the battery.Commercial polyolefin separators show low high-temperature stability,are easy to burn,and have poor electrolyte wettability,causing energy loss and arising safety hazards.Optimizing the structure and properties of the separator,improving the thermal stability and mechanical strength of the separator,overcoming the alteration at the electrode/electrolyte interface,suppressing side reactions,and developing a safe and energy-dense battery system are key scientific issues in the energy field.This article is based on the research of polyvinylidene fluoride-hexafluoropropylene(PVdF-HFP)polymer material that owns higher thermal stability and better electrolyte wettability.The objective is to prepare high-performance lithium-ion battery separators to achieve high-safety and high-capacity lithium-ion batteries.In Chapter 3,a surfactantfree PVdF-HFP/acetone colloidal solution was prepared,and a PVdF-HFP nanostructured separator with a hierarchical structure and high mechanical strength was efficiently synthesized by electrophoretic deposition technology.Compared with the conventional phase-inversion separator,the tensile strength and flexibility of the EPD PVdF-HFP separator are increased by 2?4 times.Furthermore,the PVdF-HFP separator is directly electrodeposited on the graphite anode to form an integrated separator-anode structure,which improves the separator/electrode interface.Meanwhile,its thermal stability is 3?10 times higher than that of the PVdF-HFP and commercial PP separators,greatly enhancing battery safety.In a half-cell,the separator-anode structure delivers stable electrochemical performance,high rate performance,and long-term battery cycling.In Chapter 4,based on the multi-level compound mechanism between PVdF-HFP and garlic components,organic sulfides and inorganic salts,a garlic component/PVdFHFP composite separator was designed and prepared.The organic sulfides and inorganic salts in the garlic powder modify the chemical environment of the electrolyte,in situ forming a multi-level composite solid electrolyte phase interphase layer,which improves the flexibility of the solid electrolyte interphase and electrolyte wettability and inhibits side reactions.Meanwhile,a large number of anions are immobilized in the separator structure,which increase the cation transference number to 0.9,promote lithium-ion migration,reduce ionic concentration gradient,and inhibit lithium dendrite growth.With the merits as mentioned,a lithium metal anode realizes long-term cycling stability of 3000 cycles.In this paper,advanced separator preparation and modification methods are studied based on the PVdF-HFP polymer material,which effectively improves the safety and electrochemical performance of lithium-ion batteries,and provides effective concepts for the application and exploration of advanced lithium-ion battery separators.