Study On The Properties Of Boehmite Coated Polyolefin Composite Separator

Lithium-ion batteries are widely used due to their advantages such as high energy density and long cycle life.As one of the four main components of lithium-ion batteries,the structure and performance of the separator are very important.In current lithiumion batteries,most of the separator materials are polyethylene(PE)or polypropylene(PP)polyolefins.Polyolefin separators have many advantages such as good mechanical properties,stable electrochemical properties,and so on.However,its shortcomings are also very prominent,that is,the poor wettability of the separator to the electrolyte affects the electrochemical performance of the battery,and the poor thermal stability of the separator restricts the safety of the battery,and even causes spontaneous combustion or explosion in extreme cases.In this thesis,polyethylene(PE)separator is used as the base separator,and boehmite particles,water and adhesives are mixed to form a water-based slurry.The PE separator was coated and modified by the dip coating method,and the influence of the boehmite coating on the physical and chemical properties of the separator and the electrical properties of the battery made of the separator was studied.At the same time,the contribution of the hydroxyl group(-OH)in boehmite to the coating effect,and the influence of the particle size and morphology of the boehmite particles on the coating effect are studied.The research has reached the following conclusions.1.The influence of boehmite coating on separator performance.The boehmite coating greatly improves the wettability of the separator to water,and the contact angle is reduced from 109.3° to 15.2°.Because the electrolyte and water have the same polarity,the boehmite coating greatly improves the wettability of the separator to the electrolyte.Correspondingly,experiments have found that although the porosity of the separator coated with boehmite has decreased,its electrolyte uptake and lithium ion conductivity have increased.The boehmite coating slightly improves the thermal stability of the separator at high temperatures,but it does not significantly improve its mechanical properties.2.The influence of boehmite coating on battery performance.Compared with the half-cell containing the PE separator,the battery electrical performance of the separator containing the boehmite coating has been significantly improved.The rate performance results of 5 cycles at 0.2C,0.5C,1C,2C,3C,5C,and 8C show that the half-cell assembled with boehmite-coated PE separator has a high discharge specific capacity at each current density.And with the current density increases,the gap between the two is more obvious,indicating that the boehmite-coated separator has more advantages in high-power charging and discharging.The capacity retention of the half-cell containing the PE separator is 84.9% after 100 cycles at 8C,while the half-cell assembled by the boehmite-coated PE separator is as high as 92.9%,and shows better cycle stability.3.The influence of-OHs in boehmite.After calcination at a certain temperature,the-OHs in the boehmite particles are removed,and the particle size and morphology of the obtained powder product will not change.Through experiments,it is found that the wettability of the separator coated with the particles without-OHs is similar to the separator coated with boehmite particles,indicating that the-OHs in boehmite has no special contribution in wetting between the coated separator and the electrolyte.Through the electrical performance test,it is found that the rate performance and cycle performance of the half-cell assembled with the separator applied with this particle coating are equivalent to the half-cell assembled with the boehmite-coated separator.4.The influence of boehmite particle size.The performance of boehmite-coated separators with four powder particle sizes and half-cells assembled with them were studied.The particle size distributions are BG-613(D10 0.812 μm,D50 1.382 μm,D902.315 μm)and BG-611(D10 0.455 μm,D50 0.798 μm,D90 1.338 μm),BG-601(D100.35 μm,D50 0.50 μm,D90 0.67 μm)and BG-200-S(D10 0.063 μm,D50 0.118 μm,D90 0.224 μm).It is found that the half-cell assembled with BG-601 coated separator has the best rate performance.The particle size has no significant effect on the mechanical properties of the coated separator.5.The influence of boehmite particle morphology.The performance of boehmite nanowhiskers and boehmite nanoparticle coated separators with similar particle size and half-cells assembled from them were studied.The study found that the mechanical properties of the boehmite nanowhisker coating separator is slightly better than that of the nanoparticle coating separator,but the morphology has no significant effect on the thermal stability of the coating separator.The porosity of the boehmite nanowhisker coated separator is higher than that of the boehmite nanoparticle coated separator,and even higher than that of the PE separator.This makes the half-cell assembled by the boehmite nanowhisker coated separator have the best electrical properties.