| On the background of the significant strategic decision of"carbon peak and carbon neutrality,"advanced energy storage systems and power battery fields are encountering unprecedented development opportunities.Lithium-ion battery has become the research core of future advanced energy storage systems and power batteries due to the high energy density,no memory effect,and long cycle life of lithium-ion batteries.As an important component of lithium-ion batteries,a separator plays a critical role in ensuring battery safety and improving electrochemical performance of battery.Currently,commercial separators are typically made of polyolefin polymer and prepared using a stretching technology,exhibiting poor electrolyte wettability,low porosity,poor heat resistance,and severe thermal shrinkage because of high residual internal stress.These drawbacks of commercial separators seriously hinder the further improvement of the performance of lithium-ion batteries and cause frequent battery safety issues.Therefore,developing a new type of functional separator is of great significance for improving the performance and safety of lithium-ion batteries.As a new type of high-temperature-resistant and self-extinguishing engineering plastic,polyarylene ether nitrile（PEN）own prominent comprehensive properties.The high number of polar groups（nitrile groups,ether bonds）in PEN molecular structure is conducive to improving electrolyte wettability,PEN has many advantages as separator materials.However,there have been no reports on its application in lithium-ion battery separators.In view of this,through surface/interface engineering,micro/nanostructures and multifunctionality construction,this dissertation prepares a series of new lithium-ion battery separator based on material structure design.PEN is used as the polymer matrix of the separator,and non-solvent-induced phase separation method is adapted to construct pore structures of PEN separators.The influence of pore size and interfacial physical/chemical structure of separators based on PEN materials on the lithium-ion transport kinetics,lithium dendrite growth,and high-temperature battery safety is thoroughly explored,providing a theoretical basis and technical ideas for the development of new separators to enhance the safety and electrochemical performance of lithium-ion batteries.The main research contents of this dissertation are as follows:1.The influence of non-solvent and porogen types on the pore structure and performance of the prepared PEN separator is investigated.The intrinsic formation mechanism of porous PEN membranes using non-solvent-induced phase separation method is investigated,the regulation of the morphology and structure of porous PEN membranes is achieved.The optimal conditions for the preparation of porous PEN membranes are screened out through the study of the performance of PEN separator.The spongy-like porous PEN membrane prepared under the optimal conditions is more suitable for lithium-ion battery separator,and its porosity,electrolyte absorption,ionic conductivity,heat resistance and electrochemical stability are higher than those of commercial PP separator.The Li Fe PO4/Li battery based on PP and PEN separators exhibits outstanding battery performance at both room temperature and high temperature.The first specific discharge capacity of battery is 146.8 m Ah·g-1,which can still maintain a specific discharge capacity of 130.3 m Ah·g-1 after 100 cycles.2.A hydrophilic and cross-linking strategy is proposed here to modify and enhance thermal resistant and flame-retardant porous PEN separator.The optimal experimental parameters are obtained by adjusting the concentration and reaction time of the deposition solution.The introduction of the hydrophilic and dual-crosslinking coating further increases the number of polar groups（such as cyano,amino,and hydroxyl）on the surface of the PEN membrane,endowing the composite membrane with super electrolyte affinity（the contact angle is 0°）.Moreover,the introduction of polydopamine-polyethyleneimine（PDA-PEI）cross-linked structure significantly improved the mechanical properties of the porous PEN membrane,the tensile strength of PEN separator increases from 9.1 MPa to24.0 MPa after coating of PDA-PEI structure.Electrochemical results show that the composite separators demonstrate good lithium metal interface compatibility,high ionic conductivity(1.5 m S·cm-1),and lithium ions migration number(tLi+=0.52).Compared with PP and PEN membranes,the lithium ions battery based on PEN@PDA-PEI separator exhibit more stable cycling performance and superior high-rate charge-discharge capability.3.A continuous rigid MOF layer with anionic properties is in-situ grafted on both sides of porous PEN membranes pre-modified with polydopamine（PDA）（PEN@PDA）,which leads to sandwiched MOF/PEN@PDA/MOF multifunctional separators.The most appropriate reaction conditions for obtaining uniform and defect-free ZIF-67 layers are identified by optimizing the solvent composition and reaction time of MOFs precursor solution.The growth mechanism of ZIF-67 layers on the PEN polymer membranes is investigated by means of physical and chemical characterization.The density function theory calculations and experimental results indicate that the ZIF-67 layer of MOF/PEN@PDA/MOF multifunctional separator can be used as a"Li-ion guides"with excellent affinity for anions（PF6-）,which extends the“Sand’s time”of lithium dendrite nucleation and contributes a high Li+transfer number of 0.81.Both of Li-Cu half-cell and Li-Li symmetric cells tests show that the functional separators stabilize the lithium-metal electrode interface.Furthermore,the Li Fe PO4/Li batteries based on functional MOFs/NA/MOFs separators display stable cycling performance even at 90°C.4.A thermoregulating PEN composite separator is designed through the combination of high temperature resistant PEN porous membrane and paraffin wax（PW）phase-change materials with battery thermal management potential.Paraffin wax is firstly poured into hollow nanotubes（HNTs）by vacuum impregnation method,then PDA is coated on the surface of the HNTs-PW to obtain the phase change composite filler of HNTs-PW@PDA.PDA is used to improve dispersion of HNTs-PW@PDA fillers in polymer solution.A series of phase change PEN composite separators are prepared though combing HNTs-PW@PDA with PEN via a NIPS method.The addition of phase change composite fillers greatly improves the porosity,electrolyte absorption rate,ion conductivity and thermal dimensional stability of the separator（thermal shrinkage rate of1.6%,210°C for 30 minutes）.Under abuse conditions,the generated heat in batteries stimulates the melting of the encapsulated PW,which absorbs large amounts of heat without creating a significant rise in temperature of battery.Compared with PP and PEN separators,the phase change composite separator can delay the temperature rise,thus potentially improving the safety of lithium-ion batteries.The cyclic voltammetry curves show that the introduction of phase change fillers does not affect the electrochemical performance of the PEN composite separator.Battery performance indicate that Li Fe PO4/Li batteries based on phase change composite separators can work normally at100°C. |
