Study On Modification Of Polyolefin Separator For Lithium-ion Batteries

The separator which is one of the most important components of lithium-ion battery is usually placed between the anode and cathode and does not directly participate in the electrochemical reaction.However,its structure and performance have an important impact on the performance of the battery.The main function of the separator is to prevent the physical contact between the electrodes and act as a reservoir of the electrolyte to achieve ion transmission.The traditional separators are usually polyolefin materials,which have excellent mechanical strength and good electrochemical stability.However,its low thermal stability and poor electrolyte wettability greatly limit the electrochemical performance of lithium-ion battery,especially the high temperature safety performance.Aiming to these problems,the PP/PE/PP separator was used as the substrate and modified by direct surface coating and atomic layer deposition(ALD)respectively to improve the wettability and thermal stability of the separator and the electrochemical performance of the battery.The main research contents are as follows:(1)The ZIF-67 and carbonized-derived Co/N-C were successfully coated on the PP/PE/PP separator by using a blade coating.In comparison to ZIF-67,the Co/N-C modified separator shows higher wettability(contact angle:20.4°)and electrolyte absorption(140%).At 150?the thermal shrinkage of Co/N-C modified separator is only6.6%(Pristine:33.3%,ZIF-67 modified:13.3%).The ionic conductivity of Co/N-C modified separator increases to 1.501 ms cm^-1(Pristine:0.918 ms cm^-1,ZIF-67 modified:1.476 ms cm^-1),and its decomposition potential increases to 4.91 V(Pristine:4.48 V,ZIF-67 modified:4.80 V).Particularly,the Co/N-C modified separator provides significantly enhanced cycling stability with a capacity retention rate of as high as 95.3%at 1C after 100 cycles.(2)Two oxides of Al₂O₃ and Zn O were deposited on the surface of the PP/PE/PP separator by ALD technology.The vapor-phase ALD precursor enters the mesh porous separator,which can achieve uniform coating of the deposited layer and precise control of the thickness.Studies have shown that the separator deposited with Zn O has the best contact angle(37.8°)and electrolyte absorption(135%).Particularly,at high temperature(150°C),compared with the traditional direct coating method,the separator deposited with Al₂O₃ and Zn O basically has no thermal shrinkage(?1.5%),which significantly improves the safety performance of the battery.In addition,the Zn O-deposited separator also showed the best results in the electrochemical test.After 100 cycles of the Zn O@PP/PE/PP battery separator,the cycle capacity retention rate at 1C is as high as 98.2%.