Design And Construction Of New Electrolyte And Separator Base Materials

The realization of the"carbon peak,carbon neutral"goal requires the development of new energy material systems.Lithium-ion batteries(LIBs)are important electrochemical energy conversion and storage devices due to the advantages of low self-discharge,high specific energy density and zero memory effect.They are widely used in daily life,including but not limited to consumer electronics,medical care Equipment,electric vehicles(EV)and large power supplies.As an inactive material,the separator not only insulates the positive and negative electrodes and prevents internal short circuits in LIBs,but its microstructure can further affect the transmission behavior of lithium ion(Li⁺)inside the battery,thereby further affecting the electrochemical behavior of the battery.Polyolefin polymers have become the main commercial separator material due to their excellent electrochemical stability,good mechanical strength and low price.However,the high crystallinity,non-polar molecular structure and lower melting point temperature of polyethylene(PE)and polypropylene(PP)lead to poor electrolyte wettability and lower thermal stability of polyolefin separators,thereby reducing the uniformity of the Li⁺flow in the liquid electrolyte at the interface increases the risk of internal short circuits due to thermal shrinkage.Solid-state lithium batteries(SLBs)have become an important development direction for the next generation of lithium batteries.The high energy density(3860 m Ah g^-1),light weight density(0.534 g cm^-3)and low reduction potential(-3.04 V vs standard hydrogen electrode)of lithium metal are considered to be the most ideal negative electrode material for the next generation.However,the highly reactive lithium negative electrode is prone to side reactions with traditional liquid electrolytes.On the other hand,liquid electrolytes are toxic,flammable,and easy to leak,which further increase the safety risks of LIBs.Therefore,the use of solid electrolytes(SSEs)can effectively improve the safety of the battery and further increase the energy density of the battery.Polymer solid electrolytes have the advantages of flexibility,easy processing and molding,and high interface stability,which have been extensively studied in recent years,but they also have shortcomings such as low electrical conductivity and insufficient mechanical strength.In view of this,through the combination of experiment and theoretical simulation calculation,this dissertation will prepare a series of separators and polymer solid electrolytes with significantly enhanced physical and chemical stability from the perspective of material structure design.The influence of the new material on lithium-ion transport dynamics,interfacial lithium deposition uniformity and battery safety at high temperature was also explored,providing ideas for the development of the next generation of new separator and electrolyte.The main research contents of this dissertation are as follows:(1)An integrated negative electrode/separator structure was designed and studied using poly-vinylidene fluoride hexafluoropropylene(PVDF-HFP)copolymer as the substrate.The structure is based on a copper oxide@graphene(CuO@graphene)anode and a PVDF-HFP composite polymer membrane doped with active ceramic material CuO.The results show that PVDF-HFP composite membrane doped with inorganic ceramics has significantly improved thermal stability.Ceramic particles in the polymer substrate can inhibit the PVDF-HFP chain movement,thus inhibiting the crystallization of the polymer and reducing the crystallinity of the PVDF-HFP membrane.The increase of amorphous zone of PVDF-HFP/CuO complex further improved the liquid absorption rate of the membrane.Theoretical calculations show that the enhanced strong interaction between CuO@graphene and PVDF-HFP/CuO membranes significantly shorten the Li⁺transport path,accelerate the electron transport,and alleviate the defect of large volume change of negative active oxide in electrochemical reaction.Based on this structure,LIBs still have an ultra-high reversible capacity(637.2 m Ah g^-1)and a capacity retention rate of 99%after 100 cycles at 0.5 C.(2)In order to further reduce the mass of non-active materials in LIBs and improve the energy density of batteries,a double-polymer composite separator(UP3D)rich in PVDF-HFP electrolyte with strong affinity was prepared by immersion method based on ultra-thin 3?m porous polytetrafluoron(PTFE)film.The UP3D composite membrane,with a thickness of only 11?m and an ultra-high porosity of 74%,reduces the Li⁺transfer resistance by more than 4 times(2.67 m?mm^-1)and achieves a high Li⁺flux transfer capacity(22.7 m A cm^-2).Experimental results and theoretical simulation show that UP3D separator significantly reduces the lithium-ion concentration gradient between anode and cathode during charge and discharge,and increases the average Li⁺concentration level on the surface of lithium metal,thus achieving the homogenization of Li⁺deposition.No obvious dendrite growth was observed at 1 m A cm^-2 current density for 320 h.The half-battery based on the positive electrode of lithium iron phosphate(LiFePO₄)achieved a capacity retention of 90%after 1000 cycles at current density of 2 C.This work provides a direction for the development of high energy density and long-life LIBs.(3)A binary solid polymer electrolyte(SPE)with lithium wetting and flame retardant was designed by combining polybenzimidazole(PBI)polymer to strengthen?-phase poly vinylidene fluoride(PVDF)polymer matrix to construct a safe room temperature solid battery.Phase field simulation and density functional theory calculations show that the negatively charged rigid straight chain PBI molecule has excellent affinity for lithium bitrifluoromethanesulfimide(LiTFSI)and has overlapping electron density with the lithium anode interface,which accelerates the conduction of Li⁺at room temperature and homogenizes the deposition of Li⁺on the SPE/Li metal interface.High conductivity of3.7×10^-4 S cm^-1 is achieved.Dendrite testing at 0.1 m A cm^-2 current density was stable for 2000 h.The proton site NH-of PBI formed multiple hydrogen bonds with PVDF,and the thermal shrinkage of SPE decreased by 40%at 300 ?.X-ray photoelectron spectroscopy(XPS)and solid-phase nuclear magnetic resonance spectroscopy(NMR)confirmed that part of the-CH₂-CF₂-structure in the PVDF chain segment was converted into non-combustible-CF₂-CF₂-groups during combustion.This work defines an effective strategy for achieving dendrite-free,room-temperature high-capacity solid-state batteries.(4)AIMD was used to simulate the dissolution process of polymer chain in solvent.The solvent molecules of low molecular weight acetone and N-methylpyrrolidone(NMP)diffused into the PVDF-HFP polymer chain at a high speed,and gradually unentangled,expanded and dissolved the polymer chain.The results showed that the proportion of polar?phase increased from 59%to 81%after recrystallization of PVDF-HFP induced by high polarity NMP compared with low polarity acetone.Recrystallization of PVDF-HFP induced by low-polarity acetone increased the proportion of non-polar?phase from19%to 41%compared with high-polarity NMP solvent molecules.Differential scanning calorimetry(DSC)showed that PVDF-HFP had a higher melting point to improve the thermal stability of the membrane.It provides a new idea for the preparation of high safety separator.