Research On Functionalized Composite Membrane Based On Ionic Liquids

As is well-known,lithium-ion batteries have the advantages of high energy density,long cycle life,no memory effect and environmental friendliness.However,safety,high efficiency,and fast charging batteries will become the necessity of application in future large-scale equipment with the increasing demand for energy consumption.Accordingly,the electrolyte should have high ionic conductivity,well lithium ion mobility,good thermal stability and good compatibility.At present,lithium ion batteries mainly face the following problems:low migration of lithium ions,poor interface compatibility,electric field distribution is uneven,as well as the growth of lithium dendrites.More,the traditional liquid electrolyte has disadvantages such as easy to leak,volatility,inflammability,poor security,etc.In order to meet the demands for safety,high efficiency and fast charging,the new electrolyte should have high room temperature ionic conductivity(>10-3 s/cm),lithium ion migration number approaching 1,wide electrochemical window,wide temperature range,good thermal stability,high safety and easy to forming stable interface.Here,through simple coaxial electrospinning,we introduced ionic liquid PPCl and inorganic nanoparticles Li2SiO3 into the core and shell parts of PVDF-HFP coaxial nanofibers,respectively,to obtain the porous electrolyte membrane with modified inorganic/organic components.PPCl,is fire-resistant,non-volatile,and contributes to formation of stable SEI in the battery,thus the safety and interface stability of the electrolyte is enhanced.Inorganic nanoparticles Li2SiO3 can inhibit the decomposition of LiPF6 and remove HF,to inhibit the dissolution of transition metal ions,thus improving the cycle life and capacity retention of lithium ion batteries.Meanwhile,this electrolyte membrane has advantages like rich porous structure and high electrolyte absorptivity.Therefore,the core-shell-structure nanofiber membrane is of great value in the application of fast charging lithium batteries.The main research contents as follows:(1)The ionic liquid PPCl was synthesized by grafting with chloropropylsiloxane and piperidine under nitrogen atmosphere.According to the hydrophobic properties of TFSI-and FSI-,PPTFSI and PPFSI ionic liquids were prepared by anion exchange of LiTFSI and LiFSI with ionic liquid PPCl in deionized water.(2)From the aspects of spinning solution concentration,spinning voltage,injection speed,spinning temperature,spinning distance,the preparation process of nanofibers membrane and influence factors were studied.The technological conditions of various spinning solution containing two different molecular weight of PVDF-HFP polymer,lithium silicate nanoparticles,doped ionic liquids polymer,and core-shell-structure nanometer fiber were optimized for the spinning process.The suitable spinningprocess parameters of different polymer solutions were selected.(3)Electrolytes were characterized by field emission scanning electron microscope(SEM),transmission scanning electron microscope(TEM),contact angle test,electrolyte absorptivity,porosity and thermal shrinkage test.The improved core-shell structure nanofiber membrane has high porosity,high electrolyte uptake,good thermal stability,well interface compatibility and high safety.(4)Various electrochemical performance tests were conducted on different electrolytes,including electrochemical impedance spectroscopy(EIS),chronoamperometry(CA),cyclic voltammetry(CV),linear sweep voltammetry(LSV),cyclic performance,rate performance and stability of symmetrical lithium battery.It is proved that core-shell-structure nanofiber membrane has high conductivity and shows good rate performance in NCM622?Li half-cell systems.At 20 C,this composite electrolyte shows discharge capacity 13 times higher than that of polypropylene(PP)membrane,with cycling stability.At 3 C,its discharge capacity is 6.2 times that of the traditional commercial separator after 300 cycles,and 3.3 times at 5 C.On the electrode surface,it can form stable interface membrane to reduce the impedance interface.After 30 days,its interface impedance was only 30%of that of the traditional commercial polyolefin membrane.Thus,the improved electrolyte greatly inhibits the growth of lithium dendrite and improves the battery safety,meeting the requirements of safety,high efficiency and fast charging for lithium ion batteries.