A Novel PBI Separator And Its Application For Lithium Batteries

In the past 18th century,electronics achievements changed the direction of industry development.Until the 20th century,membrane technology changed the routine of chemical engineering.Recently,membrane technology has closely related to human life.Lithium batteries have been widely used for electrochemical energy storage.They are also the key for electric vehicles.In lithium batteries,separators greatly affect the charge and discharge capacities,performance stability and battery safety though they are not the component participating energy conversion directly via charge and discharge.Separator plays a key role in improving battery performance and reliability.Development of high-performance separator is rather important for battery performance improvement.Polybenzimidazole material not only is widely used for fabrication of advanced fiber,but also will be applied in membrane technologies due to its high flexibility and strength,excellent thermal and chemical stability.A novel porous membrane fabrication technique is developed via flash freezing of PBI solution with liquid nitrogen and sequent freezing drying(LNF-FD).Compared with the solution casting(SC)technique,the LNF-FD technique achieves higher porosity without using traditional solid template.Compared with the non-solvent induced phase separation(NIPS)technique,the LNF-FD technique presents simple procedure without adding third solvent as a non-solvent pool.The obtained porous membranes are potential canidates for separator and separation applications,showing plentiful hierarchical pores and high porosity(76.38%).The fabricated porous PBI membrane(FPM5)is used as a separator for Li/LFP battery.The excellent thermal and chemical stability of the manmrane ensures the battery safety when operting at high temperatures or running out.The benzimidazole group and ether bond existing in PBI backbone chain enhance the affinity of mambrane with electrolyte.The LNF-FD technique contributes plentiful hierarchical pores and high porosity to the PBI membrane.The fabricated FPM5 acieves an electrolyte absorption rate as high as 1440.93%,reaching ion conductivity up to 0.45 mS cm-1,leading to the rapid balance among electrode-electrolyte-separator interfaces during charge and discharge of the Li/LFP battery.The SEI formation on surfaces of anode material is enhanced effectively.The FPM-based Li/LFP batteries exhibit excellent long cycle performance and high rate performance with capacity retentions of 91.41,94.38,and 97.99%at a rate of 1 C,2 C,and 5 C,respectively after 500 cycles,much better than that of the commercialized Celgard 2400-based Li/L FP batteries.Beside of the high flexibility and strength,excellent thermal and chemical stability,and electrolyte absorption capability,FPM demonstrates high capability to absorb lithium polysulfides,thereby effectively surppressing polysulfide shuttle to improve coulombic efficiency of Li-S battery.With increasing the PBI content in membrane solution for membrane fabrication,the FPM-based Li-S batteries present a coulombic efficiency as high as 97%,owing to the increase in ionic conductivity and absorption capability of lithium polysulfide for suppression of polysulfide shuttle.However,the increase in the PBI content leads to the decreased membrane porosity and electrolyte absorption capability,as well as the lower ionic conductivity,resulting to the increase in impedance and polarization.As a result,the battery shows lower cycle performance and rate performance.Through optimization of membrane compositions and membrane fabrication procedures,a porous PBI membrane(FPM10-250)with thickness of 80?m is fabricated via flash freezing of a membrance solution containing 10 wt.%of PBI before freezing drying.The assembled Li-S battery based on FPM10-250 shows improved coulombic efficiency of 94%and cycle performance with the capacity decay rates of 0.410%/cycle over 100 cycles at 0.2 C.According to the excellent performance of the fabricated porous-PBI membranes for Li/LFP and Li-S batteries,it is believed that they can be widely used in fuel cells,air batteries,supercapacitors,and so on,as the separator for energy conversion devices.In particular,the FPM will find broad applications in electric vehicles in which high safety and reliability are required.