Preparation And Modification Of UHMWPE Lithium-ion Battery Separator Using Thermally Induced Phase Separation Technique

In this paper,ultra-high molecular weight polyethylene(UHMWPE)was employed as matrix resin and polybutene(PB)or liquid paraffin(LP)as a diluent for fabrication of UHMWPE lithium-ion battery separator by thermally induced phase separation(TIPS)method.The effect of diluent[including liquid paraffin(LP)and PB]and polymer(including UHMWPE molecular weight and concentration)on rheological properties and phase separation of UHMWPE/diluent systems were investigated by means of attenuated total reflection flourier transformed infrared spectroscopy(FTIR-ATR),melt flow rate(MFR),rotational rheometer,differential scanning calorimeter(DSC)and optical microscopy during the TIPS process.The formulates of UHMWPE/diluent systems were optimized.Basically,the UHMWPE membrane and various Mica-modified UHMWPE membranes were prepared via TIPS method.The effect of Mica content on the properties(such as micro-morphology and structures,gas permeability,thermal shrinkage and electrochemical properties,etc.)of UHMWPE separators were discussed.The results show that the critical transition concentration of UHMWPE/LP and UHMWPE/PB system are both about 20 wt%,and the crossover concentrations of crystallization rate for UHMWPE/LP and UHMWPE/PB system are 25 wt%and 30 wt%,respectively.Besides,the mechanism of phase separation for UHMWPE/LP and UHMWPE/PB system during the TIPS process are both solid-liquid phase separation(SLPS)caused by UHMWPE crystallization.The UHMWPE/PB system(UHMWPE/PB=20 wt%/80 wt%)is the optimize condition for the fabrication of UHMWPE membrane owing to its optimum rheological properties and rate of phase separation.In addition,the thermal stability of UHMWPE separator is promoted with increasing the concentration of Mica,whereas,the electrochemical properties get decreased.In the test of cycle performance with 0.2C rate,the discharge capacity of UHMWPE separator in the first 10 cycle reach up to 153.7 mAh/g,which is heigher than that of commercial PE separator(144 mAh/g).The discharge capacities of others are all less than that of commercial PE separator and decreases with increasing of Mica concentration.Therefore,to keep the balance between thermal stability and electrochemical properties of the separator,Mica content is selected to be 1 wt%to modify the UHMWPE separator to promote the thermal stability without decrement of the electrochemical properties.