Preparation Of Polyimide Nanofibrous Composite Separators And Its Application In Lithium Metal Batteries

With the rapid development of electric vehicles,grid energy storage and portable electronic devices,the limited energy density of traditional lithium-ion batteries can hardly satisfy the increasing need of high-performance energy storage systems.Lithium metal batteries（LMBs）are considered as one kind of the most potential battery systems because of the low density,low negative electrochemical potential and high theoretical specific capacity.However,the commercialization of LMBs is still confronted with intractable issues.Irregular dendrite growth owing to the uneven deposition/stripping of lithium will connect the anode and cathode,resulting in inner short circuit,thermal runaway,and further serious security risks in the batteries.To solve problems mentioned above,all kinds of ways have been applied,consisting of the construction of three-dimensional（3D）lithium anode,the optimization of electrolyte,as well as the introduction of functional separators and artificial solid electrolyte interfaces（SEI）.As an important part of LMBs,the separator can not only regulate the transmission and deposition of lithium ions（Li⁺）,but also physically establish a mechanical barrier between the anode and cathode,which plays a crucial role in improving the safety and electrochemical performance of batteries.Nevertheless,commercial polyolefin separators are still afflicted with low porosity and weak liquid electrolyte affinity,resulting in the uneven deposition of lithium and the growth of lithium dendrites.In addition,due to its inferior thermal stability causing by the inherent low melting point of polyolefin,the separator composed of polyethylene or polypropylene shrinks significantly as the internal temperature of the battery rises to its melting point,which easily results in the short circuit risks.Therefore,developing functional separators possessing superior flame-retardant properties and thermal resistance is of vital importance.As a typical high-performance engineering polymer,polyimide（PI）is considered as an ideal candidate for battery separator applications owing to its low dielectric coefficient,superior chemical stability,mechanical properties and excellent thermal stability.Therefore,combining polyimide materials with electrospinning technique,this thesis has designed fluorinated-polyimide nanofibrous composite separators and further optimized the composition and structure,which provides a new modification idea for the application of functional separators in high safety LMBs.The research contents and results are summarized as follows:（1）A novel fluorinated-polyimide（F-PI）based composite nanofibrous separator with high ionic conductivity and well-distributed pore structure,has been prepared by a facile electrospinning technique of the composite solution of poly（vinylidene difluoride）（PVDF）and fluorinated polyamic acid（F-PAA）,followed by the subsequent imidization treatment.On one hand,the F-PI component with abundant polar groups offers a negative electric environment with a high concentration of active sites,which effectively promotes the rapid Li⁺transfer.On the other hand,the PVDF component,serving as both the physical linker between F-PI nanofibers and the regulator of homogenized pore size,can provide higher mechanical properties for the composite separator and realize more even lithium deposition on the anode surface simultaneously,so that the battery can remain stable in the long-term deposition and stripping process.Moreover,the trifluoromethyl（-CF₃）groups is profit for increasing the free volume of polyimide,thereby enhancing the thermostability and flame retardancy.Consequently,the symmetric cell with the composite nanofibrous separator finally achieves a steady cycling within 2400 h at a current density of 1 m A cm^-2and an areal capacity of 1 m Ah cm^-2.Even under the condition of 60°C,the cell with the separator exhibits an outstanding cycle stability in 1000 h with a low voltage polarization of about 15 m V,which is much better than the cell with commercial Celgard.Therefore,the flame-retardant fluorinated-polyimide based composite nanofibrous separator with high ionic conductivity and well-distributed pore structure holds great potentials for applications in high-energy and wide-temperature LMBs.（2）Through a simple dipping strategy,we coated 2%polyamide acid aqueous solution on both sides of the fluorinated-polyimide electrospun nanofibrous separator（F/PI）.Combined with freeze drying and subsequent imidization,a homogenous-reinforced fluorinated nanofibrous separator with sandwiched aerogel coatings has been prepared.On one hand,the F/PI nanofibrous layer with excellent three-dimensional porous structure and a large number of active sites,effectively promotes the rapid transfer of Li⁺.On the other hand,characteristics of the aerogel coating results in the improvement of porosity and electrolyte retention of the composite separator,which further reduces the internal resistance of LMBs and improves its electrochemical performance.In addition,the introduction of aerogel coating has effectively solved the uneven pore size distribution caused by electrospinning technique,providing orderly channels for the rapid Li⁺transmission and significantly improved mechanical strength for the separator.As a result,the Li|Li cell using the separator can achieve a stable cycle-life of more than 600 h at a current density of 3 m A cm^-2and an areal capacity of 1 m Ah cm^-2,with the lowest interface impedance of 2.7Ωafter 100 cycles.Therefore,the homogenous-reinforced fluorinated nanofibrous separator with sandwiched aerogel coatings overcomes the shortcomings of uneven pore size distribution and poor mechanical strength brought by electrospinning while retaining the high porosity and electrolyte wettability,thus providing a novel modification idea for the application of functional separators in high-performance LMBs.