| Polymer lithium-ion battery (PLB) is a novel second rechargeable system based on liquid lithium-ion battery. It has outstanding properties such as high energy density, long cycle life, discemable leakage, light weight and flexible shape. The PLB is attractive as power sources for mobile gear including cellular phones, PDAs, notebook computers and portable audiovisual equipment. Therefore, the demand of polymer electrolyte membrane is increasing. Poly(vinylidene fluoride)-based polymers have attracted more and more attentions due to their good chemical, electrochemical stability and good mechanical processability. However, the previous research was focused on the electrochemical properties of the polymer electrolyte membrane. Up to date, few reports related to the influence of preparation of membrane on its structure and properties. In this paper, we aimed at studying the relationship between the preparation technology and structure, properties of polymer electrolyte membrane.The influences of the different factors on the preparation of polymer microporous electrolyte membrane via the immersion precipitation method were investigated. For crystalline polymers, there were solid-liquid phase separation and liquid-liquid phase separation during membrane formation. The thermodynamic equilibrium phase diagram was determined by the cloudy point method and the solid-liquid demixing boundary was measured. The mass transfer kinetic processes of various membrane-forming systems were tested by light transmission experiment. It was found that when the membrane-forming systems or the preparation conditions were changed, the phase separation of casting solutions could shift from the instantaneous demixing to the delayed demixing. Combined with membrane structures, it could be obtained that instantaneous demixing resulted in the formation of membranes with a dense skin layer on top surface and "fingerlike" macrovoids in the sublayer, whereas the delayed demixing yielded membrane structure containing a dense skin layer and a "spongelike" pore sublayer. Furthermore, after the membranes were activated by liquid electrolyte, the conductivity of membrane containing fingerlike pores was higher than that of membrane with sponge pores.The effect of HFP content in PVDF-HFP on membrane structure and performance wasstudied by immersion precipitation. When HFP content increased from 0, 5 to 12mol%. the melting temperature of the resultant membrane reduced from 163, 155 to 143 °C. Likewise, the crystaliniry of membranes also reduced with HFP content increasing. The lower crystalinity favored the electrolyte absorption and swelling, so the higher the HFP content, the larger the porosity, uptake and conductivity of the resultant membranes.During the preparation of porous membrane by immersion precipitation, the choice of the pair of solvent and nonsolvent also was probed. When nonsolvent was water and solvent was DMAc, DMF, DMSO, NMP and TEP, the water content inducing the five casting solutions to precipitate decreased in sequence ofNMP/H2O/PVDF-HFP>DMAc/H2O/PVDF-HFP>DMF/H2O/PVDF-HFP >TEP/H2O/PVDF-HFP>DMSO/H2O/PVDF-HFP. When TEP was used as solvent, the membrane with sponge pores was obtained. However, the membranes were prepared with fingerlike and sponge pores in the entire sublayer supporting the dense top layer. The porosity, N2 flux, uptake and conductivity of these five membranes increased following TEP, NMP, DMSO, DMAc and DMF. When alcohol was used as nonsolvent and DMAc was used as solvent, the nonsolvent content at which the casting solution began precipitation increased following the order of MeOH |
PDF Full Text Request