【作者】 汪洋；

【导师】 李海龙；

【作者基本信息】 华南理工大学 ， 制浆造纸工程， 2020， 博士

【摘要】 无纺布隔膜耐热性优异,能够提高隔膜在电池生命周期内的结构稳定性,进而提升电池安全性。隔膜可以通过孔径、孔隙率和电解液亲和性来影响通过隔膜的锂离子传输速度和均匀性,进而影响活性材料的衰减程度。设计安全的电池隔膜的挑战是在结构稳定性和均匀快速的离子传输性之间进行权衡。天丝纤维是一种具有皮芯层结构的再生纤维素纤维,具有可原纤化特性,通过打浆处理可得到直径为十几纳米到2μm的原纤。本论文采用湿法造纸无纺布工艺制备了基于天丝原纤化纤维的单层和多层隔膜,明确了纤维几何尺寸和隔膜多层结构对无纺布隔膜结构和电池性能的影响特点,为无纺布隔膜的开发和设计提供技术支撑,主要研究结果如下:(1)原纤化纤维隔膜中粗纤维周围存在尺寸较大的孔隙。通过打浆工艺降低了粗纤维的数量,但很难将粗纤维完全去除。粗纤维的多阶分支结构增加了粗纤维与原纤分离的难度。通过多级多段筛分工艺逐级筛除粗纤维,降低了纤维最大直径的同时获得较高的得率。(2)研究不同原纤化纤维的隔膜孔隙结构和电池性能,并与聚乙烯(PE)隔膜比较,明确了纤维几何尺寸和隔膜孔隙结构对电池性能的影响。天丝纤维经不同打浆转数处理后,原纤化纤维隔膜孔隙结构发生了明显的变化,但较大的孔径(平均孔径在200 nm以上)使得不同打浆转数原纤化纤维的隔膜离子阻抗和电池性能差异并不明显。30℃和5C下,PE隔膜初始比容量较低的主因是较小的孔径增加了电解液过电位。-10℃和0.2C下原纤化纤维隔膜前期循环容量急剧降低的主因是孔隙通道不均匀引起的石墨表面不均匀镀锂。(3)利用聚对苯二甲酸乙二醇酯(PET)纤维和天丝原纤化纤维,结合湿法造纸无纺布工艺和陶瓷涂覆工艺,制备了基于天丝原纤化纤维的多层隔膜。当未拉伸PET纤维与原纤化纤维一起热压时,未拉伸PET纤维会在毛细力作用下向原纤化纤维间迁移堵塞孔隙,使电池性能恶化。由此确定PET纤维单独热压成无纺布后再于其上成形原纤化纤维为两种纤维的最佳组合形式。(4)调节多层隔膜与电极的接触面,发现PET无纺布侧与石墨表面紧密接触时其较大的堵孔区域会增加离子到达石墨表面的不均匀性。多层隔膜的离子阻抗主要由PET无纺布和陶瓷层提供。天丝原纤化纤维作为中间层,其尺寸和定量控制着陶瓷颗粒在隔膜厚度方向上的分布,进而影响隔膜的孔径和电池性能。与商品PET陶瓷隔膜对比发现,孔隙通道均匀性取决于陶瓷颗粒填充PET无纺布孔隙的致密程度。更多还原

【Abstract】 The nonwoven separators have excellent heat resistance,which can improve the structural stability of separators in the life cycle of batteries,thus improving the safety of batteries.The pore size,porosity and hydrophilicity of separators can influence the transport speed and uniformity of lithium ion through the separators,thus affecting the deterioration of active materials.The challenge of designing the safe separators is to establish a balance between structural stability and uniform and rapid ion transport characteristics.Lyocell fiber is a regenerated cellulose fiber with a skin-core structure.Frbirls with a diameter range from tens of nanometers to 2 μm can be obtained by the beating process due to the fibrillation tendency of Lyocell fiber.In this paper,the monolayer and multi-layer separaotors based on Lyocell fibrillated fibers were prepared by the wet-laid papermaking nonwoven process.The effect of fiber geometry size and multi-layer structure of separators on the structure and cell performance of nonwoven separators is clarified,providing technical support for the development and design of nonwoven separators.The main research results are as follows:(1)The large-sized pores exist around the coarse fibers in the fibrillated fiber separators.The number of coarse fibers is reduced by the beating process,but it is difficult to remove the coarse fibers completely.The multi-order branching structure of coarse fibers increases the difficulty of separating coarse fibers from fibrils.The coares fibers are removed orderly by the multi-level and multi-stage screening process,which can reduce maximum fiber diameter and have a high yield.(2)The pore structure and cell performance of separators with different fibrillated fibers are investigated to clarify the effect of fiber geometry size and pore structure of separators on cell performance,as compared to the polyethylene(PE)sepratator.The pore structure of fibrillated fiber separators changes significantly after Lyocell fibers are treated with different beating revolutions,but the difference of ion impedance and cell performance of separators is not obvious due to the larger pore size(average pore size of above 200 nm).At 30℃ and 5C,the main reason for the lower initial specific capacity of PE separator is that the smaller pore size increases the electrolyte overpotential.The main reason why the capacity of fibrillated fiber separators decreases sharply in the early cycle at-10℃ and 0.2C is the uneven lithium plating on the graphite surface caused by the inhomogeneity of pore channels.(3)The multi-layer separators based on Lyocell fibrillated fibers are prepared by using Polyethylene terephthalate(PET)fibers and Lyocell fibrillated fibers,combining with wet-laid papermaking nonwoven process and ceramic coating process.When the unstretched PET fibers and the fibrillated fibers are bonded by hot calendering,the unstretched PET fibers can migrate to the pores of fibrillated fibers under the capillary force and block the pores,which results in poor cell performance.It is the best combination of the two kinds of fibers that PET fibers are hot-pressed into non-woven fabric and then formed with a layer of fibrillated fibers on it.(4)By adjusting the contact surface between the multi-layer separators and the electrodes,it is found that when the PET non-woven side is in close contact with the graphite surface,the larger area of pore closure increases the inhomogeneity of ions reaching the graphite surface.The ionic impedance of the multi-layer separators is mainly provided by the PET nonwoven fabric and ceramic layer.As an intermediate layer,the size and basis weight of Lyocell fibrillated fibers control the distribution of ceramic particles in the thickness direction of separators,thus affecting the pore size of separators and cell performance.Compared with the commercial PET ceramic separator,it is found that the homogeneity of pore channels depends on the density degree which the pores of PET nonwoven fabric are filled with ceramic particles.更多还原