Enhance Modification Research Of PVdF-HFP Based Separators For Lithium-Ion Battery

Rechargeable lithium-ion batteries(LIBs) have been considered as one of the most promising energy storage devices for modern portable electronics, hybrid electrical vehicles since their initial commercialization by the Sony Corporation in 1991. As an essential part of LIBs, separators play an important role to prevent physical contact of the anode and cathode whereas allowing the free transport of ions. PVdF-HFP exhibit favorable properties including high polarity and good chemical inertness and stability.PVdF-HFP has low degree of crystallinity due to the addition of hexafluoropropylene(HFP), high mechanical flexibility and good compatibility with respect to electrodes. In spite of this, there are still some drawbacks to be resolved, such as non-ideal ionic conductivity, poor mechanical strength, large thermal shrinkage and unstable C-rate performance, which have prevented their wide practical application. This article will adopt the following methods to reinforce performances of PVdF-HFP based gel polymer electrolyte membrane.(1) We have prepared two kinds of composite separators based on PP nonwoven and PVdF-HFP, one is blending-type separator(CPE) with PMMA, and the other is a sandwich-like separator with the introduction of PMMA nanoparticles on the PVdF-HFP surface(nano-CPE). Results showed that nano-CS displayed higher ionic conductivity(1.846 mS cm-1) than CPE(0.2)(1.572 mS cm-1). In addition, the LiFePO4/Li half-cell assembled with nano-CPE displayed the best discharge capacity,152 mAh g-1at room temperature. These results demonstrated that the well developed interstitial voids formed between PMMA nanoparticles will not change the tortuosity of matrix and can effectively absorb liquid electrolyte with capillary force, thus to improve the comprehensive performances of this separator.(2) In last system, the modification method enhanced the thermal stability and electrochemical performances of the separators for LIBs. Nevertheless, the mechanicalstrength of these separators still needs to be enhanced and so we introduce the gel structure. We used well-dispersed organic-inorganic nanoparticles, V-POSS, as crosslinker to fabricated gel structure with PEGMEMA and MMA. Besides,interpenetrating polymer networks(IPN) were formed between PEGMEMA and MMA chains and PVdF-HFP polymer chains. All gel polymer membranes were stable up to5.0 V and got a very high decomposition temperature above 200 oC. Among them,GPE60 had the highest ionic conductivity(1.51 mS cm-1) and an acceptable tensile strength(11 MPa). Cells assembled with GPE60 displayed the best discharge capacity,150 mAh g-1 at room temperature.(3) In order to further strengthen the electrochemical performances of GPEs in last section.PEG600 was chosen to be pore-foaming agent. When the PEG was washed out, many pores were formed in polymer membrane. Results showed that P10 separator had the highest ionic conductivity(1.94 mS cm-1) and an acceptable tensile strength(10 MPa).Besides, cells assembled with P10 separator displayed the best discharge capacity, 156 mAh g-1, at 0.2 C, 112 mAh g-1 at 0.4 C, respectively, at room temperature. These results demonstrated that PEG as pore-foaming agent may enhance the C-rate capability of polymer electrolyte membrane.