Fabrication And Properties Of Separators Based On Poly(Vinylidene Fluoride) And Its Copolymers For Lithium-ion Batteries

In recent years,the large-scale application of new-energy vehicles and portable electronic equipment has strongly promoted the development of lithium-ion batteries.At the same time,higher requirements have been put forward for lithium-ion batteries in terms of capacity,lifespan and safety.Separator,which has been an indispensable part of lithium-ion batteries,plays a vital role in maintaining the stability and safety of cells.Nevertheless,traditional commercial polyolefin separators have a poor wettability against commercial liquid electrolyte and are easy to shrink when exposed to heat,which cannot meet the high performance and high safety requirements of lithium-ion batteries.Poly(vinylidene fluoride)(PVDF)and its copolymers have been the hotspots in the field of separators for lithium-ion batteries owing to their good wettability and thermal stability.In this paper,the three different copolymers of PVDF,poly(vinylidene fluoride-cochlorotrifluoroethylene)(PVDF-CTFE),poly(vinylidene fluoride-co-trifluoroethylene)(PVDF-Tr FE)and poly(vinylidene fluoride-co-hexafluoropropene)(PVDF-HFP),were systematically integrated and separators based on PVDF and its copolymers were fabricated through the simple solution casting method.By comparing the physical and electrochemical properties of different PVDF-based separators,it was found that PVDFTr FE separator displayed a high priosity of 84.5%,a favorable electrolyte uptake(115.4%),strong affinity with electrolyte,and high ?-phase content,contributing to the best cycling and rate performance among all PVDF-based copolymers.Additionally,lower cell polarization made it easier for PVDF-Tr FE separator to mitigate the lithium dendrites.Compared with other PVDF-based copolymers,it is no doubt that PVDF-Tr FE is more suitable for the preparation of lithium-ion battery separators.However,PVDFTr FE separator has poor mechanical properties(14.5 MPa)and low melting point(?120 ?),which may not be conductive to the safety and stability of lithium-ion batteries used in high temperature environments.To make up for these shortcomings of PVDF-Tr FE sperator,the polyimide/poly(vinylidene fluoride)(PI/PVDF-Tr FE)composite membrane was prepared by developing a thin PVDF-Tr FE layer on the basis of PI membrane with excellent thermal properties.The composite separator has excellent tensile strength(19.4 MPa),high priosity(75.8%),and good wettability with commercial electrolyte.In addition,the introduction of PI layer greatly improves the thermal stability of the composite membrane,and the thermal shrinkage at 180 ? for 1 h is only 4%.Cells assembled with this composite separator exhibit better cycle performance than the commercial separator Celgard2500 in the high temperature of 100 ? and 80 ?,and can still maintain the excellent charge-discharge performance after a sudden high temperature test,which guarantees the safety and stability of high-temperature lithium-ion batteries.