Fabrication And Modification With Polydopamine Of Lithium-ion Battery Separator Based On Poly (M-Phenylene Isophthalamid)

Separator,as one of the indispensable components of lithium-ion batteries,has a crucial impact on battery performance and safety.Selecting and developing high-performance separator materials is the core to achieve long-term application and reduce costs of batteries.This study used poly(m-phenylene isophthalamid)(PMIA)as the heat-tolerant separator material and aimed at preparing a porous separator with uniform and sponge-like structure for optimizing the electrochemical performance of batteries.The influence law and theoretical strategy were provided for the preparation of crystalline polymer separators by through adjusting the thermodynamic and kinetic factors during the non-solvent induced phase separation technology.And the separator was modified by mussel-inspired polydopamine(PDA)to prepare the PMIA-PDA separators with enhanced wettability.The lithium ion migration was strengthened by adjusting the deposition amount of PDA,thus ehancing the electrochemical performance of separators and cyclic stability and rate performance of batteries.The details were summarized as follows:(1)Based on the ternary phase diagram of crystalline polymers and the technology of non-solvent induced phase separation,the influence of thermodynamic factors(polymer concentration,co-solvent content,type and content of non-solvent additives)and kinetic factors(component and ratio of coagulation bath,film-forming temperature)on the structure,porosity and contact angle toward carbonate electrolyte of separators were systematically investigated.The optimal parameters for fabricating PMIA separator were as follows:The concentration of polymer,co-solvent(Li Br),additive(glycerol)in the casting solution was 17 wt%,3 wt%and 10 wt%,respectively.The temperature of the casting solution and the coagulation bath were both 25°C.And two-stage coagulation bath were adopted,including immersing in the first-stage coagulation bath of glycerol/DMAc(30:70 v/v).Finally,PMIA porous separators with uniform and sponge-like structure were successfully prepared after being soaked in deionized water for 24 h.The PMIA separator exhibited better heat shrinkage resistance(only 3.1%at 200?),higher porosity(54.5%),and better electrolyte uptake(124.5%).The PMIA separator in Li Co O₂/Li battery also presented good cyclic stability with the capacity retention of 78.0%after 50 cycles at 0.2 C.(2)Inspired by the structure of mussels and their secreted bonding proteins,PDA modification is applied on the surface of the PMIA separators to fabricate PMIA-PDA separators.The introduced amino,quinonyl,and catechol groups in PDA layer could enhance the wettability of the resultant PMIA-PDA separator toward electrolyte.Specially,the pyrrole nitrogen in PDA is lithium-philic,which was conducive to form a stable solid electrolyte interphase(SEI)membrane and improve the cyclic stability of battery.The effect of PDA deposition amount on the thermal stability,mechanical properties,wettability toward electrolyte,ion transport resistance and battery performance were studied.The experimental results showed that the comprehensive performance of the separator was the best,when the deposition time was 4 h.The PMIA-PDA-4 separator maintained the original thermal stability(thermal shrinkage of 5.2%at 200°C)and exhibited robust tensile strength of 25.4 MPa.The contact angle toward liquid carbonate electrolyte of the thermal-treated PMIA-PDA separator could maintain at 26.1°,which decreased by 42.7%compared with the pristine PMIA separator,which effectively promoted the migration of lithium ion.The corresponding ion conductivity and lithium ion transference number were 0.86 m S/cm and 0.65,respectively.Consequently,the battery performance tests showed that the cyclic performance of PMIA-PDA separators at 25?and 60?was significantly improved.In the Li Co O₂/Li battery system,the discharge specific capacity was 137.6 m Ah/g after 50 cycles at 0.2 C wih the corresponding capacity retention rate of 95.8%.When the rate reaches 2 C,the specific discharge capacity is 90.9 m Ah/g.