沉淀剂对聚乙烯表面涂覆聚偏氟乙烯锂离子电池隔膜结构及电化学性质的影响

Effect of Non-solvent on the Performance of Polyvinylidene Fluoride Coated Polyethylene Separator for Lithium-ion Batteries

为了降低目前商用聚乙烯锂离子电池隔膜在闭孔温度下的热收缩比率、同时提高复合隔膜的电化学性能,通过浸没沉淀法在聚乙烯(PE)隔膜两侧涂覆聚偏氟乙烯-共-六氟丙烯(PVDF-HFP)微孔涂层.通过改变强沉淀剂和弱沉淀剂的比例,研究混合沉淀剂与溶剂的溶度参数差异(Δδ)对于微孔涂层结构的影响.随着Δδ逐渐增加,固-液相分离产生的微孔结构逐渐扩展,涂层由致密表面结构向球状颗粒聚集的微孔结构转变.微孔涂层的结构对于复合隔膜的热收缩比率、电化学性能均有明显的影响.在沉淀剂中水含量40%、60%的条件下,成功地制备出了热收缩显著降低,电导率、循环性能和倍率性能等电化学性能均有所提升的三层复合隔膜.

For the purpose of improving the safety of lithium-ion batteries, polyvinylidene fluoride-co- hexafluoropropylene （PVDF-HFP） porous layers were coated onto both sides of polyethylene （PE） separators by immersion-precipitation process. The effect of differences in solubility parameters （Aδ） between solvent and composite non-solvent on the structure of the coating layer was studied by varying the strong non-solvent/weak non-solvent ratio in order to prepare a coating layer with porous structure,which is beneficial to reduce thermal shrinkage of composite separator and to improve electrochemical properties of the lithium-ion batteries. As Aδ between the solvent （DMAc） and composite non-solvent （ethanol/water composite non-solvent, different in ratio between ethanol and water） increases, the microporous structure of coating layer becomes more developed. Taking the conceptual image of phase diagram into consideration, this is suggested to be caused by different phase separation mechanisms. Ethanol, a weak non-solvent which is against the phase separation process,just develops a dense structure. As the content of strong non-solvent （water） increases, solid-liquid phase separation takes place and it finally develops a porous layer with spherical PVDF-HFP crystallites of roughly identical size. Obvious differences in thermal shrinkage and electrochemical properties of the composite separators were observed with different structures in the coating layers. Sandwiched separators with reduced thermal shrinkage, increased conductivity, enhanced cycle and C-rate performance were prepared by this immersion-precipitation process with ethanol/water ratios 3：2 and 2： 3.