| 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 ?m 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. |
PDF Full Text Request