Preparation And Modification Of Polyethylene Wax And Its Application In Lithium Ion Batteries

Internal short circuit is the main cause of the fires and explosions in lithium-ion batteries.Polyethylene wax?PEW?was added into the battery as the safety additive due to its high resistivity and thermal melting property in this paper.When the temperature of batteries rise up to 100 ^oC,the PEW melts and covers on the surface of electrode material,the internal reactions were cut off and the electrochemistry performance of li-ion batteries was degraded due to the melting PEW,therefore,the safety performance of li-ion batteries was improved at high temperature.Three contents of this work are showing as follow:?1?PEW with different particle sizes were prepared by solvent evaporation method.when the emulsifying speed was 2000 r/min,the particle size of PEW was about 18?m,the 6?m PEW was prepared at 4000 r/min.Then,the 6?m PEW was added to LiFePO₄electrode as the safety additive.The discharge capacity of LiFePO₄ batteries with 5 wt%PEW decreased to 94 mAh/g.The discharge capacity of LiFePO₄ batteries with 10 wt%PEW decreased to 59.8 mAh/g.The discharge capacity of LiFePO₄ batteries degraded seriously due to the PEW with high resistivity.?2?It has a bad effect on the electrochemical properties of Li-ion batteries when PEW was added to LiFePO₄ electrode directly.Therefore,the PEW with different surface densities were coated on the surface of electrode materials.Comparing by SEM,The 5?m thermal response coating was formed on the surface of electrode when the surface density was 0.92 mg/cm²,and a dense thermal closure layer was formed on the surface of the electrode after it was treated at 120^oC.According to the electrochemical performance test,the first discharge capacity of LiFePO₄ batteries with 0.92 mg/cm² were 134.9 mAh/g at0.5 C.The first discharge capacity degraded to 35.3 mAh/g after the electrode sheet with different surface density PEW were treated at 120^oC.Specially,the specific capacity of LiFePO₄ batteries with 0.92 mg/cm² PEW degraded to 10 mAh/g after 16 cycles,and its specific capacity rapidly attenuated to 10 mAh/g and it was close to 0 in the subsequent cycles.?3?Due to the poor conductivity of PEW,a positive temperature coefficient composite?PANI-PEW?was prepared by polymering amount of PANI particles on the surface of PEW.According to the conductivity test,the conductivity of PEW was less than1×10^-1616 S/m at room temperature,and the conductivity of PANI-PEW incresed to1.08×10^-33 S/m.According to the differential scanning calorimetry test and thermal gravimetry test,PANI-PEW still can melt and didn't decompose during 60¹20 ^oC.According to the electrochemical performance test,the first discharge capacity of LiFePO₄batteries with 15 wt%PANI-PEW was 123.1 mAh/g at 0.5 C.The discharge capacity degraded to 35.3 mAh/g after the electrode sheet with 15 wt%PANI-PEW were treated at120^oC.Specially,the discharge capacity of LiFePO₄ batteries with 15 wt%PANI-PEW degraded rapidly and it closed to 0 after the 12 cycles.