Researches On The Preparation And Surface Coating Technics Of Polyolefin Microporous Membrane For Lithium Ion Batteries

As an important part of lithium-ion battery,separator is one of the key factors determining the performance,especially the safety of the battery.Polypropylene?PP?and polyethylene?PE?are widely used materials because of their excellent film forming characteristic and chemical stability.Because of the foreign technology blockade,it is very difficult to develop our own technology in China.Although the domestic separator products have made great progress in recent years,most of them are concentrated in the low and medium-end market.There still lack of a key technology breakthrough on the preparation and surface coating technics.Therefore,it is essential to carry out the related basic research for application,which will provide basic theory and technological support for the improvement of separator performance.1.Porousness is one of the key requirements for separator.And higher the content of?crystals is critical for pore forming and pore size distribution.In general,nucleating agent can regulate the PP crystallinity.In this paper,through theoretical calculation and experimental analysis,the nucleating agent was optimized and the processing temperature range was determined.The isothermal and non-isothermal crystallization kinetics of PP modified by nucleating agents such as nanometer calcium carbonate,N,N'-dicyclohexyl-2,6-naphthalenedicarboxamide?TMB?,Sodiumbenzoateand benzene-1,2-dicarbo-xylic acid were determined combining differential scanning calorimeter?DSC?.Result shows that at a crystallization temperature of 127°C,TMB modified PP has the highest crystallization rate and?crystal content.2.The PP microporous membranes were produced by biaxial stretching technic with TMB as?crystal nucleating agent.Effects on the average pore size,pore size distribution and porosity of the TMB concentration,drawing temperature and ratio were discussed.When the TMB content reaches 0.5 wt%,the highest?crystal content was got,which is benefit for getting a membrane with high porosity.At a fixed drawing ratio,the average pore size and permeability of the PP membrane become larger with increased drawing temperature.At a fixed tensile temperature,the average pore size and porosity first increase and then decrease with increased drawing ratio.Therefore,by adjusting the drawing temperature and drawing ratio,the microporous membrane can be obtained with different porosity and average pore size.3.The microporous PP membranes were produced by biaxial stretching with TMB and Al₂O₃ as composite nucleating agent.Effects on the average pore size,pore size distribution and porosity of Al₂O₃concentration,drawing temperature and ratio were investigated.Results show that at the same stretching temperature and ratio,the porosity and average pore diameter are higher than that with TMB.With 0.4wt%Al₂O₃,porosity of the micro-porous membrane was the highest.At a fixed drawing ratio,the average pore size and permeability of the PP membrane become larger with increased drawing temperature.At a fixed tensile temperature,the average pore size and porosity first increase and then decrease with increased drawing ratio.Therefore,by adjusting the drawing temperature and drawing ratio,the microporous membrane can be obtained with different porosity and average pore size.The discharge capacity of the coin cell using commercially PP maintains 86%of its initial capacity after 100 cycles,while that for the 0.5T and 0.5T-0.4Al were 89.54%and 87.25%,respectively.4.Al₂O₃ and Teflon?PTFE?were coated on both sides of porous polyethylene?PE?membranes.The effects on the permeability,thermal shrinkage,pore size distribution and porosity of Al₂O₃ content were discussed.Compared with the original PE separator,the thermal stability of the composite separators improves greatly.When the Al₂O₃contentwas 47 wt.%,the porosity of the composite membrane was 44.8%and the absorption rate was up to 199.1%.Graphite/composite microporous membrane/lithium plate were assembled into battery.After 100 cycles at 0.1C and 0.5C,the capacity retention rate was 88.4%and 75.9%,respectively.Especially at 0.5C ratio,the capacity retention rate was higher than that of the PE film?62.4%?.5.A new kind of BaSO₄ composite PP separator for LIB was prepared using styrene butadiene rubber?SBR?as binder.The effects of SBR content on the permeability,thermal shrinkage,pore size distribution and porosity of the membrane were discussed.Results showe that the porosity of the composite membrane was 54.2%when the addition of SBR was 5%of the quality of BaSO₄.Lithium manganate/composite microporous membrane/lithium plate were assembled into battery.After 100 cycles at 0.1C,the capacity retention rate was 94.9%.And the multiplier performance of the composite film also improved.