| As times advance,more and more new energy vehicles are being used,which brings about the huge challenge of energy storage batteries in terms of performance,such as high energy density,long-term cycle stability,and high safety requirements.The separator,known as the"third electrode"of the battery,is responsible for maintaining the safety of the power battery and the critical function of transporting lithium ions,despite not being involved in the power battery reaction.Therefore,the design of high-performance separators is crucial in the development of high-power batteries.The most widely used commercial separator is the inorganic ceramic-modified polyolefin separator prepared by organic or water-based coating.Among them,oily coatings use very expensive and toxic solvents.Traditional water-based coatings have poor adhesion between the obtained coating and the substrate due to the high surface energy of the slurry,making detachment and affecting the battery cycling performance likely.Additionally,the preparation cost of the substrate polyolefin film is high,and the process is complex.By comparison,nonwoven fabric-based separators have recently received more attention due to their higher cost-effectiveness.Based on this idea,this paper aims to use melt-blown polypropylene nonwoven fabric(PP)as the skeleton support structure and prepare a composite separator by using water-based coating after modifying the nonwoven fabric substrate with polydopamine.The specific research contents are as follows:Firstly,we used polypropylene nonwoven fabric as the substrate and modified it with dopamine to obtain polydopamine-coated polypropylene nonwoven fabric(PDA-PP).Then,natural polymer sodium alginate was used as the adhesive,and inorganic nanoparticles boehmite(γ-Al OOH)were introduced onto the surface of the modified polypropylene nonwoven fabric by water-based coating to obtain the desired composite lithium-ion battery separator.The study found that the hydrophilicity of the polypropylene nonwoven fabric was significantly enhanced after dopamine treatment compared to the original polypropylene nonwoven fabric.Composite separators with different proportions of boehmite showed significant differences in electrochemical and mechanical properties,among which the composite separator C-PCS with the highest boehmite content exhibited the best performance.Next,N-methyl-2-pyrrolidone(NMP)was introduced as a non-solvent into the water-based slurry of boehmite,and porous composite separator(PCS)was prepared by the above process.Among them,NMP has a high boiling point,which will phase in the drying process of the coating to form micro-droplets and eventually volatilize to form pores.The results show that the porosity of the PCS composite separator(61.4%)is greatly improved compared to that of the C-PCS composite separator(50.9%),and the permeability of the PCS separator(24 s/100 cc)and the electrolyte absorption capacity(165%)are both improved compared to that of the C-PCS,resulting in a better Li~+transference number(0.58).However,the PCS separator also has its limitations,with relatively limited improvement in mechanical properties.Finally,we replaced the inorganic nanoparticles of boehmite with halloysite nanotubes(HNTs),and prepared the required composite battery separator,referred to as PHS,using the water-based coating method and the dopamine-modified polypropylene nonwoven fabric.The hollow structure of HNTs can bring more voids and their surface-rich groups can enhance the absorption of the electrolyte by the separator.In addition,the high aspect ratio of HNTs is expected to improve the mechanical strength of the separator.The research found that compared with commercial Celgard 2400 separator and PCS composite separator,the PHS composite separator showed stronger electrolyte wettability and absorption.Compared with the boehmite composite separator mentioned above,this halloysite nanotube composite separator had a better tensile strength(12.85 MPa). |
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