Preparation and evaluation of a separator with an asymmetric structure for lithium-ion batteries

The different electrochemical characteristics stemming from the cathode and anode of lithium-ion batteries may have different effects on the surface properties of contacted separators. Herein, a series of composite separators with an asymmetric porous structure are fabricated by a one-step phase inversion method coupled with the settling of SiO2 nanoparticles due to gravity within the poly(vinylidenefluorideco-hexafluoropylene) (PVDF-HFP) polymer matrix. The unique asymmetric porous separators are composed of a PVDF-HFP-rich layer with high porosity on the front surface, which is in contact with the air, and a SiO2-rich layer on the back surface, which is in contact with the substrate. Besides their excellent thermal stability and high safety towards fire, these asymmetric porous separators result in a high discharge capacity, enhanced cycling performance and excellent rate capability when assembled with a LiNi0.5Mn1.5O4 cathode in coin cells, indicating that the asymmetric porous composite separators could be a promising candidate to improve the performance of lithium-ion batteries.