Research On The Preparation And Properties Of γ-Al₂O₃ Composited Polyimide Separators For Lithium-Ion Batteries

Due to its high capacity,long battery life,and convenience,the lithium-ion battery（LIB）has become one of the most popular energy storage devices in recent years.However,LIB still has safety issues.Traditional polyolefin separators（PE,PP,and PP/PE/PP）have a low melting point and thermal shrinkage.When LIBs are used under extreme conditions,they are prone to deadly dimensional shrinkage,resulting in battery short circuits or even fire and explosion.Separator is a vital part of LIB.To alleviate the drawbacks of polyolefin separators,researchers have raised electrospun polyimide nanofiber membranes with high-temperature resistance,high chemical stability,and high porosity.It is considered a strong competitor to replace traditional polyolefin separators in the future.However,the development of electrospun polyimide nanofiber membranes is hindered due to deficiencies in mechanical properties and wettability.Among numerous efforts,we successfully fabricated the triple cross-linking structured polyimide nanofiber membranes（PI）in the previous study,which significantly improved the mechanical properties.In this paper,the triple cross-linking strategy improves the PI nanofiber membrane,and the introduction of inorganic aluminide further improves the wettability and electrochemical performance of the PI nanofiber membrane.The research work carried out in this paper includes:（1）An Al（OH）₃ precursor was prepared by solution precipitation and the hydrothermal method,and ceramic material containing theγ-Al₂O₃ phase was successfully prepared by calcination to 400°C.Then,theγ-Al₂O₃ composite PI nanofiber membrane（PI/γ-Al₂O₃）was prepared by impregnating theγ-Al₂O₃ ceramic particles into the electrospun PI nanofiber membrane.The mechanical properties,thermal properties,and wettability of the composited PI nanofiber membrane were compared with those of the pristine PI.The results show that theγ-Al₂O₃ phase is successfully introduced into a PI nanofiber membrane by XRD pattern analysis.In the further characterization,the composite nanofiber membrane has excellent mechanical properties and thermal stability;the tensile strength is higher than 60 MPa,and the mass loss 5%temperature is higher than 470°C(T_5%),and no obvious peak heat release flow rate was observed in the DSC analysis.In addition,the electrolyte was completely absorbed after 10 s of contact with the PI/γ-Al₂O₃ nanofiber membrane.（2）A novel core-shell structured PI anchoredγ-Al₂O₃ nanofiber membrane（PI@γ-Al₂O₃）was prepared by an in-situ complexation hydrolysis method.Then,the micromorphology,physical properties,and electrochemical properties of the pristine PI,PI/γ-Al₂O₃,and PI@γ-Al₂O₃ sample systems were compared.The results show that aγ-Al₂O₃ anchoring can be successfully and uniformly applied to the surface of PI nanofibers.The prepared PI@γ-Al₂O₃ nanofibers have excellent heat resistance and dimensional stability with no shrinkage at 200°C.In addition,the electrolyte contact angle of the PI/γ-Al₂O₃ nanofiber membrane is almost 0°.Due to its excellent wettability,electrochemical impedance spectra analysis showed that its ionic conductivity was 1.74 m S·cm^-1,and its interfacial impedance was 24Ω.In the test of a symmetrical lithium metal battery（Li⁺/Li）,the polarization voltage fluctuates little and the resistance to internal resistance increases strongly within the 400h cycle.Compared with the pristine PI and PI/γ-Al₂O₃ separators,Li Fe PO₄ half cells assembled with a PI@γ-Al₂O₃ separator show better C-rate performance and cycle stability.The specific discharge capacity is 115.8 m Ah·g^-1 at 5.0 C and 154.5 m Ah·g^-1 after 100cycles at 0.5 C,and the coulomb efficiency is up to 98%.（3）PI nanofiber membranes armored with an aluminide precursor were conducted with the same process at different calcination temperatures to obtain the PI nanofiber armored with different aluminide phases.According to different temperature variables（100°C,200°C,300°C,400°C,and 500°C）,the fabricated samples were labeled PIA-100,PIA-200,PIA-300,PIA-400,and PIA-500,and compared with the pristine PI nanofiber membranes.Using the FT-IR and XRD qualitative and quantitative analyses,the interior of PIA-100 and PIA-200 nanofibers is incompletely imidized,while the shell phase is Al（OH）₃.The almost complete imidization could be found in the PIA-300 nanofiber membrane,while the shell phase isγ-Al OOH.The imidization degree of the nanofiber membranes of PIA-400 and PIA-500 is complete,while the shell phase isγ-Al₂O₃.The higher the calcination temperature is,the better the wettability of the composite nanofiber membrane is.The initial contact angle of the PIA-500 nanofiber membrane with the electrolyte is 15.7°,and the electrolyte uptake is 243%.Compared with other samples,the PIA-400 separator has better comprehensive electrochemical performance,with 1.39 m S·cm^-1 and 32Ωinterfacial impedance.The Li Fe PO₄ half-cell with the PIA-300 separator has a higher C-rate performance with a discharge capacity of 112.2 m Ah·g^-1 at 5.0 C.In addition,the PIA-500 nanofiber membrane becomes unstable after the 90^th cycle,and thus it is not suitable for the long-term cycle of LIBs.