| Separator is an important part in lithium ion battery,Poly(vinylidene fluoride) (PVDF) is a kind of suitable material to be used as separator due to its excellent thermal stability,mechanical properties,chemical and electrochemical stability.At present,the main technology to prepare separator for lithium ion battery is microporous matrix phase inversion-activation method explored by Tellcordia Co.Ltd. This technique is composed of two steps:the preparation of microporous matrix and activation.In previous studies,microporous matrix was usually obtained by immersion precipitation process.This technique is not only difficult to control porous structure of membrane,but also not excellent mechanical properties and wide pore size distribution.The main advantages of thermally induced phase separation process (TIPS) are easily porous structure control,good mechanical properties and narrow pore size distribution.However,little researches have been carried out to apply microporous membranes prepared via TIPS process in separator of lithium ion battery. In this work,PVDF and PVDF-HFP microporous membranes,PVDF/PMMA and PVDF/PEO-PPO-PEO blend porous membranes were prepared via TIPS process and (separator/polymer electrolyte) systems based on microporous membranes were prepared by activation.Ethylene carbonate,triacetin,sulfolane and dibutyl phthalate were introduced to research the effect on porous structure and properties of microporous membranes.The result indicated that solid-liquid phase separation occurred for polymer/diluent system during TIPS process.Sulfolane is a good candidate for PVDF,because not only microporous membranes with good mechanical properties,but also PVDF-based blend membranes can be obtained using sulfolane as the diluent.PVDF microporous membranes were prepared and the influences of polymer molecular weight,polymer concentration,quenching condition and extractant on the aggregation structure,porous structure and mechanical properties were studied. (Separator/polymer electrolyte) system was obtained by activation.How the aggregation structure and porous structure influence ionic conductivity and electrolyte uptake were investigated.It was found that microporous membranes with spherulites prepared at certain condition processed good mechanical properties and well-interconnected porous structure.Ionic conductivity was beyond 1.0×10-3S/cm at 20℃,electrochemical stability was in the range of 0～4.7 V(vs Li+/Li ),logσand 1/T showed a linear variation and followed Arrhenius-type behavior.The contribution of amorphous domains to electrolyte uptake and ionic conductivity is in the range of 33～44%and 7.4～19.3%,which means that the transfer of carrier ions is mainly through pores filled electrolyte.PVDF-HFP was used to decreases the crystallinity of PVD,PVDF-HFP prepared via TIPS process.Polymer concentration and quenching condition were used to control the aggregation structure,porous structure and mechanical properties of microporous membranes.(Separator/polymer electrolyte) system was obtained by activation.The effects of aggregation structure and porous structure on ionic conductivity and electrolyte uptake were investigated.The experimental results showed that compared with PVDF system,PVDF-HFP system has higher ionic conductivity(2.93×10-3S/cm,20℃) and better electrolyte maintenance,logσand 1/T followed VTF equation.The contribution of amorphous domains to electrolyte uptake and ionic conductivity is in the range of 50～60%and 30～40%,which means that the transfer of carrier ions within the gel phase should not be ignored.Considering the good compatibility between PMMA and PVDF,the addition of PMMA will decrease the crystallinity of PVDF and increase electrolyte uptake. PVDF/PMMA blend microporous membranes were prepared via TIPS process.The effects of polymer weight fraction and quenching condition on the aggregation structure,porous structure and mechanical properties of microporous membranes were studied.The experimental results revealed that the addition of PMMA decreased the crystallinity,mechanical properties and increased porosity.Compared with PVDF microporous membrane,after activation,the blend membranes processed higher ionic conductivity(2.59×10-3S/cm,20℃) and better electrolyte maintenance.The electrochemical stability was in the range of 0～4.5V(vs Li+/Li).Due to good compatibility between PVDF and PPO segment,and good affinity between PEO and liquid electrolyte,PVDF/PEO-PPO-PEO blend microporous membranes were prepared via TIPS process.The effects of polymer weight fraction and quenching condition on the aggregation structure,porous structure and mechanical properties of microporous membranes were studied.The experimental results revealed that spherulites became regular with the addition of PEO-PPO-PEO. It was found that secondary crystallization existed in the process of non-isothermal crystallization and enhanced by increasing PEO-PPO-PEO content,which meant that there existed interaction between the two polymers.Compared with PVDF microporous membrane,after activation,the blend membranes processed higher ionic conductivity(2.59×10-3S/cm,20℃) and better electrolyte maintenance.The electrochemical stability was in the range of 0～4.5V(vs Li+/Li).logσand 1/T followed Arrhenius-type behavior.The cell measurement revealed that the charge-discharge efficiency and discharge capacity of PVDF-HFP,PVDF/PMMA and PVDF/PEO-PPO-PEO system were better than that of PVDF system.
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