Preparation Of Polyimide-based Membranes And Their Applications For Lithium Ion Battery

Currently,the most widely used separators are commercial polyolefin microporous membranes due to their excellent mechanical strength,electrochemical stability and low cost.However,they show some inherent drawbacks,such as inferior thermal stability,poor electrolyte wettability and low porosity,which are not conductive to the performance of lithium ion batteries?LIBs?,especially the safety of electric vehicles.To solve above problems,in this paper,two high performance FPI separators were developed by electrospinning and phase separation,respectively.Next,in order to fundamentally solve the safety problems of liquid LIBs,one all-solid-state polymer electrolyte based on FPI was developed in this paper.This research will provide a new idea for developping polymer electrolytes,which used in next-generation all-solid-state LIBs.This Paper has mainly done the following research:?1?A robust fluorinated polyimide?FPI?nanofibers membrane was prepared by an electrospinning technique and subsequent thermo-crosslinking process.Compared to pristine FPI nanofibers membrane,the thermally crosslinked FPI nanofibers membrane possesses considerable mechanical strength?i.e.,31.7 MPa?,small average pore size?i.e.,1.14?m?and narrow pore-size distribution(?i.e.,0.91-1.36?m?and shows enhanced performance to prevent the growth and penetration of dendritic lithium.In addition,the FPI nanofibers separator has great electrolyte wettability,thermal stability,flame retardancy,ionic conductivity,electrochemical oxidation limit and low interfacial resistance.And the battery assembled with FPI nanofibers membrane with FPI binder exhibits enhanced cycle performance and rate capacity.?2?A kind of FPI flexible membrane as separator for LIB was prepared via phase inversion method.The FPI separator maintains intrinsic dimension even after heating at200°C and displays a sponge-like structure with abundant interconnected pores and a dominant porosity?67%?.In addition,the FPI separator shows desired electrolyte wettability,validated by contact angle tests?16.2°and 46.8°for FPI membrane and PE separator,respectively?and electrolyte uptake tests?420 and 132%for FPI membrane and PE separator,respectively?.The LIB with FPI separator as separator exhibits nice ionic conductivity(0.92mS cm^-1)than that with PE separator(0.30 mS cm^-1),and therefore affords better cycling stability and rate capability.?3?A series of FPI/SiO₂-IL-PEGS polymer electrolytes were prepared by compounding nanosilica fluid?SiO₂-IL-PEGS?,LiClO₄ and FPI using solution casting technique.Among them,the 300%SiO₂-IL-PEGS polymer electrolyte still affords a strength of 1.45 MPa and an elongation at break of 49%,which can maintain good self-supporting ability,flexibility and elasticity.All FPI/SiO₂-IL-PEGS polymer electrolytes exhibit good thermal stability without decomposition before 275°C,and their ionic conductivities are all much higher than pure FPI polymer electrolyte.The ionic conductivity of the FPI/SiO₂-IL-PEGS polymer electrolyte increases with the increasing SiO₂-IL-PEGS.However,when the proportion of SiO₂-IL-PEGS exceeds 300%,the ionic conductivity of the polymer electrolyte decreases.