Modification Of Polyolefin Separators Based On Graphene Oxide In Lithium Battery

With the rapid development of electric vehicle and hybrid electric vehicle,the higher requirements of lithium-ion battery,lithium sulfur battery and their materials are proposed.The separator as a key component of the battery is closely related to battery's properties,such as electronic properties and safety problem.Accordingly,the researchers have carried out extensive and in-depth research on separator materials.Currently,microporous polyolefin membranes are the most widely used separator materials owing to their excellent processability,chemical stability,and low cost.However,they have poor electrolyte wettability,due to hydrophobicity and low polar of polyolefins'surfaces,which lead to poor C-rate capability and cycle performance of the Li-ion batteries.In addition,these separators exhibit large thermal shrinkage at high temperature,which often induces internal short circuit and could eventually lead to fire or explosion of the cells.Aim to conquer these challenges,we have prepared GO coated separators and studied the effects of kinds of preparation conditions,such as coating method,the oxidation degree of GO,types of adhesives and the thickness of layers,on performances of Li-ion batteries.Next,the effect of GO layer on Li-ion batteries was also studied based on optimal preparation conditions.The results show that the GO coated separator with the best performance is obtained with highest oxidation degree of GO,by a quadruple film applicator,with 1?m thickness and using PU as adhesive.We find this GO coated separator remarkably enhances the C-rate capacity and high-rate cycle performance for Li-ion batteries?LIBs?,compared to theAl₂O₃ coated PE and pristine PE separator.Morever,after coating,the thermal shrinkage of the separator at 130?decrease greatly,from 44%to12%.Meanwhile,the mechanical properties and shutdown temperature of the GO coated separators almost do not change.Due to its ultra high theoretical energy density,and sulfur is cheap,abundant and environmental friendly,the Li-S battery holds great promise for serving as next generation high energy density batteries.However,the insulating nature of sulfur and“shuttle effect”cause low active material utilization,leading to poor electrical performance of the Li-S battery.There has been an article reported that the oxygen-based functional groups on GO acted as ion-hopping sites of positively charged species?Ll⁺?and rejected the transportation of negatively charged species(Sn^2-)due to the electrostatic interactions,which means that GO can inhibit the“shuttle effect”.So,the GO coated separator was employed in Li-S battery and we studied the effect of oxidation degree of GO on battery as well.The results revealed that the residual graphitic structures in weakly oxidized GO can donate electrons to improve polysulfide utilization while the enriched oxygen functional groups can inhibit“shuttle effect”.Besides,we prepared Al₂O₃ coated separator and GO/CNT coated separator.The electrochemical properties of the cell using the GO coated separator are better than the cell using the Al₂O₃ coated separator and worse than the cell using the GO/CNT coated separator.Although Al₂O₃ coating layer can enhance the wettability of the separator,it cannot suppress polysulfide shuttle effect.Therefore,its rate performance and cycle performance are the worst.Carbon nanotubes?CNTs?possess excellent electrical conductivities and they can trap polysulfide.The addition of CNT and cooperation with GO make the electrochemical performance of the GO/CNT coated is the best.