| Lithium-ion battery is so widely used that more and more attention is attracted to itsresearch and application. As a component of lithium-ion battery, separator physicallyseparates the positive and negative electrodes while the free flow of lithium ions in liquidelectrolyte passes through its microporous structure, thus separator plays an important role inincreasing battery capacity, improving safety of battery and enhancing cycling performance.Polyethylene (PE) is widely used to fabricate commercial separators for lithium-ion batteriesdue to its good mechanical property, chemical stability, thermal stability and thermalshutdown capability. There are two kinds of processes to fabricate polyethylene separators,dry process (melt-stretching) and wet process (thermally induced phase separation). Porositiesof separators made by dry process are relatively lower but pore sizes of them are largercompared with wet process. Given this context, polyethylene separators for lithium-ionbatteries were fabricated by wet process in this paper. At first, Polyethylene/paraffin oilnascent membranes were prepared by melt extrusion, and then nascent membranes wereoriented biaxially at a certain temperature until paraffin oil in membranes was extracted toobtain microporous membranes. After that, microporous membranes were heat-treated at ahigher temperature to achieve good mechanical performance. Morphologies, pore sizes,porosities and mechanical properties of separators were investigated.(1) Nascent membranes were extracted to test true contents of polyethylene inmembranes, and the result showed there was a slight difference between polyethylenecontents in nascent membranes and feed ratio. Properties of polyethylene/paraffin oil nascentmembranes were investigated. Melt flow rates of neat PE and PE/paraffin oil nascentmembranes were compared，which showed that existence of paraffin oil improved fluidity ofPE and decreased temperature of extrusion. Viscosity curves of polyethylene/paraffin oilbinary system were obtained by rotational rheometer. Viscosity of polyethylene/paraffin oilbinary systems decreased with increase of temperature and shear rate. Viscosity curves ofpolyethylene/paraffin oil binary system were fitted by Ostwald-de Wale equation andCarreau-Yasuda model. Ostwald-de Wale equation failed to fit viscosity curves with Adjust r2 below0.98but Carreau-Yasuda model succeeded to fit viscosity curves with Adjust r2above0.998. n and a defined by Carreau-Yasuda model decreased with increase of molecular weightof polyethylene and increased with decreased with increase of temperature. Phase separationof polyethylene/paraffin oil binary systems were observed by optical microscope (OM) androtational rheometer. Only solid-liquid separation was observed. The temperature ofsolid-liquid separation was obtained by differential scanning calorimeter (DSC). Whencooling rate increased, phase separation temperature decreased. Besides, phase separationtemperature differed when molecular weight of PE changed. Moreover, temperature ofextruder die had a significant influence on thickness of nascent membrane. The highertemperature was, the thinner membrane would be.(2) Crystallization kinetics of polyethylene/paraffin oil nascent membranes wereinvestigated. Avrami equation was adopt to analyze isothermal crystallization kinetics and didwell in interpreting isothermal crystallization behaviors. Values of n in Avrami equationhovered at2.1and changed little with increase of crystallization temperature Tc, indicatingthat crystallization structure of polyethylene was a combination of two-dimensional structureand three-dimensional structure. lgZ increased with decrease of crystallization temperature,meaning that crystallization rate became faster at lower temperature. Effective activationenergy (ΔE) of isothermal crystallization was calculated by Friedman method. The result ofΔE showed that isothermal crystallization rate at the beginning of crystallization was moresusceptible to temperature. Jeziorny method and Mo method were applied to analyzenon-isothermal crystallization. Values of n in Jeziorny method were between2and3, same asisothermal crystallization. Besides, while cooling rate increased, lgZcincreased and n stayedalmost the same. That is, crystallization rate became faster but crystallization structure did notchange much. Values of F(T) in Mo method increased gradually with relative crystallinitygrowing from10%to90%, which demonstrated that it was harder and harder forcrystallization while crystallization proceeded. Effective activation energy of non-isothermalcrystallization was calculated too, showing that non-isothermal crystallization rate at thebeginning of crystallization were more susceptible to temperature, same as isothermal crystallization. Crystalllization rate decreased with increase of molecular weight ofpolyethylene. Under isothermal condition, ΔE of nascent membranes decreased with increaseof molecular weight of polyethylene; under non-isothermal condition, ΔE of nascentmembranes with different molecular weights of polyethylene were very close to each other.XRD patterns were obtained by X-ray diffractometer (XRD). Interplanar spacing and averagecrystallite sizes were calculated from XRD patterns, which showed that crystalline structuresof nascent membranes with different molecular weights of polyethylene were almost the sameand cooling rate had a slight influence on crystalline structure of nascent membranes.(3) Morphologies and mechanical properties of polyethylene separators wereinvestigated. Pore sizes of polyethylene separators were between30and50nm. Polyethyleneseparator with a molecular weight of5×105possessed the largest pore size and the lowestporosity. Air resistance of polyethylene separator decreased with increase of molecular weightof PE, increased with increase of thickness of separator. Puncture strength of polyethyleneseparator increased with increase of molecular weight of PE, increased with increase ofthickness of separator, and puncture strength was larger than300g. Tensile strength ofpolyethylene separator increased with increase of molecular weight of PE, changed little withincrease of thickness of separator, and tensile strength was larger than160MPa (MD) and108(TD). Shrinkage of polyethylene separator decreased with increase of molecular weight of PE,and shrinkage at90℃for2h and at105℃for1h was smaller than5%. |
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