Design And Comprehensive Performance Research Of Polyimide Aerogel Safety Battery Separator

With the rapidly accelerating demands for electric vehicles,energy storage grids,and portable electronic devices,lithium-ion batteries(LIBs)have emerged as an important energy storage device due to their high specific energy densities and long cyclic performances.However,numerous cases of fire and explosion have been reported for these high energy density LIBs,and this has triggered extensive concerns regarding their safety as primary energy storage device.Separator is an important component in LIBs,which is of great significance to ensure the safe and efficient operations of LIBs.Currently,polyolefin-based membranes have dominated the battery separator market due to their superior mechanical propertiesand low cost.Despite their market dominance,polyolefin separators typically suffer from poor thermal stability due to their low melting points and poor electrolyte wettability.Thermal instability of a separator may lead to internal short-circuit,and even resulting in thermal runaway(TR)accidents.As a result,to mitigate this hazard,tremendous efforts have been devoted to the development of new-type separators with excellent thermal stability and electrochemical properties.Polyimide aerogel combines the excellent properties of polyimide and aerogel,which has 3D structure,high porosity,high thermal resistance and good mechanical properties.However,it is still a major technical challenge to prepare polyimide aerogel films with great flexibility and strength,and there is a lack of a comprehensive study of polyimide aerogel safety battery separators.Therefore,the aim of this research is to develop polyimide aerogel separators for advanced lithium-ion batteries and improve the safety performance of batteries.The main researches in this thesis contain:(1)Preparation and analysis of high temperature resistant polyimide aerogel(PIA)separator.A new type of thermotolerant polyimide aerogel(PIA)separator was prepared by a simple sol-gel method,the steps contained diamine and dianhydride monomers condensing,cross-linking,coating and gelling,aging,solvent exchange and supercritical drying.At the same time,the thermal stability of the PIA separator was characterized and its pyrolysis reaction kinetics and intermolecular bond energies were deeply analyzed and studied.In addition,the pyrolysis products of the polyimide aerogel separator were tested and analyzed by combined thermogravimetric-infrared spectroscopy and synchrotron radiation vacuum ultraviolet photoionization mass spectrometry to investigate the pyrolysis mechanism of the PIA separator.(2)Electrochemical performance of PIA separators and thermal safety of batteries.Due to the high porosity and electrolyte wettability of the PIA separator,the Li-LiFePO4 coin cell using the PIA separator has excellent cycle performance and rate capability,with a capacity retention rate of 79.2%after 1000 cycles.In addition,The LiFePO4|Li metal batteries with PIA separators are extremely stable at 90℃(>300 cycles),and maintain stability even at 120℃(>50 cycles),showing its potential for application in high-temperature scenes.At the same time,benefiting from its high thermal stability,the PIA separator can improve the thermal runaway temperature of the LiFePO4-graphite pouch battery and reduce the risk of thermal runaway of the battery by more than 30%than that of the commercial Celgard 2400 separator’s.(3)Preparation and properties of flexible reinforced polyimide aerogel separator.Based on the previous study,the synthesis methods of the polyimide aerogel are greatly optimized in this work to obtain more an advanced polyimide aerogel separator.The co-precursor and cross-linking methods were used to obtain modified polyimide aerogel separator(M-PIA separator).Mixed diamine monomers,4,4’oxidianiline(ODA)and 2,2’-Dimethylbenzidine(DMBZ)contributed to the improvement of the total polymer concentration in the gelation solution and promoted the mechanical property of the prepared separator greatly.The aromatic triamine,1,3,5-triaminophenoxybenzene(TAB)was used to form the cross-linked structure,which can further enhance the flexibility and tensile strength of M-PIA separator.In addition,the prepared M-PIA separator contains rich polar functional groups,and,therefore,it exhibits a high affinity with polar electrolytes.As a result,the Li-LiFePO4 coin cell assembled with M-PIA separator showed stable rate capability and long-cycle performance,and the capacity retention rate after 1000 cycles can reach 88%.In the lithium deposition experiments,the deposition efficiency and stability of the M-PIA separator battery were both higher than those of the Celgard 2400 separator.In addition,after the lithium deposition,the M-PIA separator suppressed the growth of lithium dendrites on the copper foil due to its uniform pore distribution and high ionic conductivity,and no obvious dendrite lithium can be observed on its cross-section,confirming the promising prospects of lithium metal batteries and the potential to inhibit lithium dendrite growth.More importantly,the NCM523-graphite pouch battery assembled with M-PIA separator had high thermal stability,low heat release rate,and thermal runaway temperature up to 200℃.At the same time,the thermal runaway reaction mechanism of the pouch battery assembled with the M-PIA separator was explained by DSC analysis of each component of the pouch battery.Thus,based on the collective results,this work paves a new way for the fabrication of high-performing polyimide aerogel separators for LIBs which can reduce the thermal runaway risks,and it also provides new insights into the application of porous polyimide aerogel materials.