Preparation And Performance Of High Safety Polyimide Based Lithium Ion Battery Separator

Lithium-ion batteries own the advantages of high power,good cycle performance,high energy density and long cycle life,and are widely used in electronic products,new energy vehicles and energy storage systems,which are rapidly developing in the direction of high energy density and high security.Security accidents caused by thermal runaway of lithium-ion batteries have become a stumbling block to the popularization of new energy in recent years,and the mechanical and thermal stability of the separator dominated by the internal short circuit is the main cause of its thermal runaway.In this thesis,a multi-stage structure of PI-based lithium battery separator was constructed adopting high performance polymer-polyimide（PI）with excellent mechanical properties and heat and flame resistance as the separator material,and the morphological structure and mechanical properties,thermal stability performance,porosity,absorption rate and electrolyte hydrophilicity of the separator prepared by different processes were studied.The separator was assembled into a button cell to test the ionic conductivity of the separator,the electrochemical stability window and the charge/discharge cycle stability performance and discharge rate capability of the cell,and compared with the commercial Celgard separator.The paper first adopts an industrialized easily solution blow spinning technique combined with thermal imidization in order to achieve continuous preparation of PI nanofiber separator.Secondly,the mechanical properties of PI nanofiber separator are improved by means of montmorillonite intercalation modification and in situ polymerization,and the controllable preparation of PI/PVDF/BM composite separator with thermal shutdown function is achieved via polyvinylidene fluoride（PVDF）non-solvent induced phase separation method combined with surface boehmite（BM）coating method,which is dedicated to give high safety performance to lithium-ion battery separator.Finally,polyhedral oligomeric sesquisiloxane（POSS）cross-linked PI aerogel with rigid backbone and bacterial cellulose（BC）composite separator with high mechanical strength were constructed,and their physicochemical and cycling multiplier performance of assemble buckle batteries were also initially explored.The main study and results are as follows:（1）The low-temperature solution polymerization of 3,3’,4,4’-biphenyltetracarboxylic dianhydride（BPDA）and p-phenylenediamine（PDA）and 4,4’-diaminodiphenyl ether（ODA）as monomers was used to establish the synthesis process of high-viscosity polyamide acid（PAA）spinning solution,and the optimal solution blow spinning process for the preparation of PAA nanofibers was optimized with the morphology and diameter of PAA nanofibers as the investigation criteria.The process parameters of PAA nanofibers were investigated,and the thermal imidization process of PAA nanofibers was explored to produce high strength PI nanofiber membrane,and their physicochemical properties as lithium battery separators and the series performance of assembling into button batteries were studied.The results showed that the highest intrinsic viscosity of PAA was obtained with a molar ratio of 1.015:1 of dianhydride and diamine（equimolar mixture of PDA and ODA）,a reactant mass fraction of 13 wt%,a reaction temperature of 0℃and a reaction time of 4 h.The concentration of PAA solution had the most significant effect on its solution jet spinning spinability and the morphology of the produced PAA nanofiber membrane.The average diameter of PAA nanofibers increased with increasing solution concentration,increased with increasing solution extrusion rate,decreased with increasing draft wind pressure,increased with increasing inner diameter of the spinneret,and decreased with increasing receiving distance.The optimal PAA solution jet spinning process is:spinning solution concentration 8 wt%,extrusion rate 2.4 m L/h,draft wind pressure 0.08 MPa,inner diameter of the spinneret 0.4 mm,receiving distance 20 cm.The PAA nanofiber membranes were heat treated at 250°C for 1 h,and then slowly ramped up to340°C for 1 h for thermal imidization,the resulting PI nanofiber membranes had good morphology,highest tensile strength,and best thermal stability performance.The average diameter of the obtained PI nanofibers is 209.1 nm,and the average pore size of the fiber separator is 1.8μm,with high porosity（87.3%）and electrolyte absorption rate（479.1%）,tensile strength of 13.02 MPa,and the separator does not undergo any shrinkage after heat treatment at 180℃for 1 h.It shows excellent thermal stability performance,the high security of the battery under extreme conditions such as high temperature,abuse or thermal runaway is enhanced.The physical and chemical properties of the PI nanofiber membrane were further compared with commercial Celgard 2400:the prepared PI nanofiber separator exhibited higher ionic conductivity(1.73 m S cm^-1)and wider electrochemical stability window,and the assembled Li/Li Co O₂ cell showed better discharge rate capability and cycling stability performance.（2）PI/OMMT composite separators were prepared by combining in situ intercalation polymerization and solution blow spinning technique.The X-ray diffraction（XRD）results of the study showed that the layer spacing of the organic montmorillonite（OMMT）was increased from1.518 nm to 2.226 nm after cetyltrimethylammonium bromide（CTAB）intercalation modification.Characterization by infrared spectroscopy（FTIR）,thermal stability performance（TGA）and transmission electron microscopy（TEM）together indicated the successful intercalation modification of Na-MMT by CTAB,which achieved good dispersion and exfoliation of OMMT in PI matrix.The addition of OMMT increased the apparent viscosity of the PAA/OMMT composite spinning solution and the average diameter of the produced PI/OMMT fibers,and improved the thermal stability and mechanical properties of the PI/OMMT composite separator.The thermal decomposition temperature of the composite separator showed a trend of increasing and then decreasing with the increase of OMMT addition,in which the highest thermal stability of the composite separator was achieved with the addition of 7 wt%OMMT,and the tensile strength and elongation at break also reached the maximum value of 26.23 MPa and 15.9%,respectively,and the tensile strength was improved by 101.46%compared with the pure PI separator.The addition of OMMT slightly decreased the porosity of the composite separator,but its good affinity with the carbonate group in the electrolyte increased the surface wetting performance,liquid absorption rate and ionic conductivity of the PI/OMMT composite separator.The battery assembled by PI/OMMT composite separator showed excellent cycling stability and rate capability,and the discharge specific capacity of the battery assembled with composite separator at 7 wt%OMMT additionwas 118.5m Ah g^-1 after 100 cycles of testing at 0.5 C/0.5 C charge and discharge rate,with a capacity retention rate of 87.5%,while after 100 cycles of the same,The discharge specific capacity retention rate of Celgard 2400 and pure PI separator after 100 cycles was 73.4%and 86.4%respectively.（3）To further improve the thermal safety performance of Li-ion batteries,a PI/PVDF/BM composite separator with thermal shutdown function and excellent thermal stability was prepared via coating polyvinylidene fluoride（PVDF）solution combined with non-solvent-induced phase separation（NIPS）on the surface of PI nanofiber membrane,and then coating boehmite（BM）on the surface of PVDF membrane to further improve the thermal safety performance of Li-ion batteries.The effects of PVDF concentration,porogenic agent PVP addition,solidification bath concentration and solidification temperature on the pore structure,porosity and mechanical properties of the produced PVDF porous membrane were investigated.The results showed that when the PVDF concentration in the film-making solution was 12 wt%,the PVP addition was 5%,the volume ratio of DMAc to water in the solidification bath was 70:30,and the solidification temperature was 25°C,the PVDF porous film was a homogeneous full honeycomb porous film with the tensile strength,porosity and average pore size of 3.62 MPa,63.1%and 0.142μm,respectively.The tensile strength of the produced PI/PVDF/BM composite separator is 7.62 MPa,and the electrolyte contact angle on the BM side is almost zero,showing an extremely desirable hydrophilic performance.the introduction of PVDF film layer endows the PI/PVDF/BM composite separator with thermal closure function,and its thermal shut-off temperature window is 170～300℃,and there is no obvious thermal shrinkage after 0.5 h of treatment at 180℃.The PI/PVDF/BM composite separator exhibited higher porosity（75.4%）,electrolyte absorption（277.6%）and ionic conductivity(1.17 m S cm^-1)compared to the commercial Celgard 2325separator.The battery assembled by PI/PVDF/BM composite separator exhibited a discharge specific capacity of 122.6 m Ah g^-1 and a capacity retention rate of 85.2%after 100 charge/discharge cycles at 0.5 C/0.5 C,showing a superior cycling stability.PI/PVDF/BM composite separator combines a more desirable thermal shutdown temperature and a higher film breakage temperature,significantly enhancing the thermal safety performance of lithium-ion batteries.（4）The construction of OAPr S-PI/BC composite separator was initially explored by laminating an octa-aminopropyl caged sesquisiloxane（OAPr S）cross-linked PI（OAPr S-PI）aerogel rigid backbone with a high mechanical strength bacterial cellulose（BC）film with highly interconnected pores.The results of nuclear magnetic study confirmed the successful synthesis of the octahedral cage structure OAPr S·8HCl.The OAPr S-PI aerogel membrane presents a three-dimensional network structure with highly interconnected pores,yet the tensile strength is only 3.26 MPa.The tensile strength of the OAPr S-PI/BC separator made by laminating with the thinned BC membrane is increased to 33.67 MPa,and the high strength of the composite separator and the rigid pore structure of its aerogel layer are expected to effectively inhibit the growth of lithium dendrites.The porosity,electrolyte absorption and ionic conductivity of OAPr S-PI/BC composite separator were79.3%,491.6%and 1.49 m S cm^-1,respectively,and the electrolyte retention rate was significantly higher than that of Celgard 2400 separator,and the batteries assembled with this composite separator showed higher discharge specific capacity and rate capability at the same charge/discharge rate.